HVAC design complexity with custom masonry heater (cold climate)
When we set out to build a home, did we expect to spend 6 figures on HVAC, incl. the masonry heater? No, no we did not. We need to start HVAC soon but we haven’t chosen the installer. We had engineers do the design, but the architect coordinated this and I’m not so sure about it because the prices seem astronomical so I want to make sure it is reasonable and based on reality rather than arbitrary scenarios to satisfy building codes.
Our design includes a 5-ton Daikin ASHP with 3 AHUs for zone control. I asked and the engineer said we could go down to 4 ton (L0 1-ton, L1 2-ton, L2 1-ton) but we will lose efficiency in winter below -21ºC because we’ll use the backup electric. I understand they are designing for max. efficiency and low operating costs, but I think some nuance could have been missed and it might be very expensive to do further engineering, so I am just curious if anyone here has any thoughts/feedback!
I don’t think we are in the “super insulated” category but it seems pretty good. I think this is where we are:
– climate zone 6*
– R80 attic
– R38 above grade walls
– R20 basement ICF walls
– R24 slab
– Abundant windows but they are triple glazed, R5 and SHGC 0.27, I don’t remember u value.
– Aiming for 0.6 ACH50 but it might be tricky to hit this target – we will have vented kitchen exhaust, active makeup air duct, a chimney, and a vented air intake for the fire as per code… so we will do what we can.
– The house is a simple rectangle w/ small bumpout section
– L0 walkout basement ~ 1,176 SF w/ minimal glazing
– L1 main level ~ 1,144 SF w/ lots of glazing and a big central custom masonry heater*
– L2 upstairs ~ 1,069 SF w/ lots of glazing
– total interior floor plan = 3,389 SF
– mechanicals incl fully ducted ASHP, ERV and MAU to avoid depressurization from kitchen exhaust
Because of the vertical height w 3 levels + the masonry heater on L1, the zoned system makes sense to me and it seems more efficient using multiple AHUs instead of dampers for zoning? This means we have ceiling ductwork on 3 levels. I read Allison Bailes’ post about 5-ton never being the answer, so it seems he would recommend separate smaller outdoor condensers unless I misunderstood the whole article (which is possible). This sounds nice and resilient, but expensive.
I want to have a thermostat for each level because:
A) in the winter, we hope to primarily use the masonry heater (wood) for heating at least the main level. It will be super cozy, maybe sometimes overheated, so the ASHP might be minimally used in winter on L1, more for mild weather or summer cooling. Some heat will get upstairs through nearby stair opening.
B) We don’t want to waste energy heating/cooling the upstairs (bedrooms) during the day, or waste energy heating/cooling the basement and main floor while we are sleeping.
C) A good portion of L0 walkout is a big mechanical room, a big pantry/storage room, and then future living space which might only be used minimally in the near term as a gym area but eventually finished for rec space and guest room.
Yes, we must satisfy the building code but I don’t see why we would design the system as if we are not spending a fortune on a masonry heater, which we will use at the coldest time of year. The heater might give off ~ 20,000 Btu hourly, but it is a wild card until built and tested. Regardless, I would think we could slightly undersize the heating at least on level 1, although the backup electric heat might seem inefficient – I highly doubt we will use backup electric heat because we will use wood when it is extremely cold.
I was worried about oversized cooling – but it seems Daikin VRV can run on lower speeds so maybe a 4 or 5 ton would not be a problem in the summer? Maybe the ERV or heat pump can run on a dehumidification mode?
Does the design seem realistic based on what I’ve described? Is there a better way to achieve the same thing, effectively?
I need to see if there is updated energy modelling because the report I have was from before we reduced amount of glazing, increased attic insulation, and the engineer also set the air tightness to 1.0 instead of 0.6 – so I have no idea if the system was designed based on these incorrect details, or if it would even make much difference.
In short, I know almost nothing about this but our ASHP seems massive considering we are building what seems like a pretty good house which includes a wood fired masonry heater to provide a lot of warmth on the coldest winter days. Also, I might need to learn how to ask a proper question… but I don’t want to be concise only to hear “it depends” in response 😉
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A collection of one thousand construction details organized by climate and house part
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Without knowing the window sqft, it’s hard to know. But plugging some assumptions in, if your above grade walls are 10ft/floor and the footprint is roughly 20x 50, you’re looking at (20*2 + 50*2)*20 = 2800 sqft of wall. Say 20% is window at R = 5, you have 8,000 Btu/h of heat loss through the windows at -5F and about 5,000btu/h through the walls (if R 38 is the effective value, not the nominal value). The ceiling heat loss is negligible and the basement will be low too. You might well need only 2 tons.
So basically, get a real heat loss done. Doesn’t sound like your architect has completed this step, so has been wasting your time thus far in regards to HVAC selection. Three units is a fine solution. You might get away with just baseboard in the basement if the load is small enough. Or a single ductless head might suffice for the basement. That’s like $5-10k USD for a ductless head or a few hundred USD for some electric baseboard. Efficiency for low load spaces matters a lot less than you might think, so saving thousands for the basement upfront is probably a better financial decision.
A separate, potentially sticky decision, remains with the masonry heater. I think you’re in danger of having an extremely uncomfortable house if you place a gigantic, uncontrolled wood stove in it. I’d reconsider. If you want a flame in the house, an extremely small propane or gas fireplace might be more appropriate. Wood stoves just aren’t great choices for well insulated homes, sadly. If you google “flatrock passive”, there’s a GBA series about one such house that made wood work, sort of, but it was a hydronic system which would certainly jack your costs up.
Basically ditto to what Paul said.
If you've hired an engineer, I would think the first thing he would have done is a Manual J, which is the heating and cooling load calculation. Basically the thing to check is that all of the measurements and assumptions are correct, if they are then the output will be correct. That will tell you what size system you need.
I think you're going about it all wrong with thoughts like "We don’t want to waste energy heating/cooling the upstairs (bedrooms) during the day, or waste energy heating/cooling the basement and main floor while we are sleeping." In a house with 0.6 ACH50 and high insulation everywhere you're not going to get big temperature differentials inside the building envelope. Even if you did, heat would flow from the warmer parts to the cooler parts. So don't think of zoning as something that saves you energy, think of it as something that enhances comfort by allowing you to dial in the temperature to exactly where you like it.
I'm also skeptical of the masonry heater. Where are you getting the 20,000 BTU/hr? I would think 200,000 BTU/hr would be more plausible. Firewood is on the order of magnitude of 10,000 BTU/pound, I don't see being able to keep a fire going that only burns two pounds per hour. Twenty pounds per hour I'd buy. And once you get something like that heated up there's no way to turn the heat down. I expect your heat loss will come in around 40,000 BTU/hr which means that wood burner will roast you out.
Akos will probably be along shortly to tell you why three heads on one compressor is a bad idea, I'll leave that to him.
I think the idea is that you burn at a higher rate than the concrete and stone emits, so it “averages” out to about 20,000btu/h.
But still, there's no way to control it or turn it off. So if the house warms up to 85F the only thing you can do is open the windows until it cools off again.
I totally agree. It’s a bad fit.
Paul - I have an energy modelling report from the engineer with a bunch of calculations, however I am not sure if he updated the numbers for the final design because like I mentioned we made some minor improvements to the design for value engineering right after the first energy modelling was done, to reduce costs and heat losses slightly. I will review the report I do have and ask about that.
Interesting idea for the basement, I think I suggested this to the architect and she was not into the idea of entertaining new ideas, she just wanted to be done with me :) It does seem crazy to me, a non-expert, to use the same air handler in the basement as the one on the 2nd floor where the bedrooms are, they seem completely different and the load calcs vary widely. Our loads are listed by room so I can go and add up by level.
DContrarian - thank you, that makes sense re: zoning/comfort. It makes me wonder if we even need the individual AHUs?! I imagine this helps with efficiency for the ductwork, or something. Also the issue of the main floor being overheated from the fire vs upstairs needing heat. Engineer said for cost savings we could consider changing to a two-zone system and having the T Stat for the upper floor on L2 so the masonry heater doesn't influence the heating on L2. This would eliminate an air handler, T Stat, backup resistance coil, refrigerant line set and additional ducting for the extra zone. (*I am not sure exactly what this means, but I believe basement would be 1 ton with 3kW backup, L1/L2 combined 3 ton w/ 8kW backup and both connected to a 4 ton condenser outside.
Are you specifically familiar with masonry heaters? They are very, very different from wood stoves. I can dig into the details a bit more, it can be challenging because most resources are not English as these are common in Europe - Russia, Finland, Norway, Austria, Germany, etc. Overall, it has thousands of pounds of thermal mass so when you burn a full load of firewood, it burns hot and fast and then absorbs the heat before very very slowly releasing heat output over many hours. I have a hard time imagining 200,000 Btu/h but I hardly know what I am talking about 😂
Can you share the calculations? A new house with that level of insulation and 2000sqft above grade should be nowhere near five tons.
I’m familiar with the masonry heater concept (burn at full output, store excess heat) I just think they’re a very costly (financially, space, and carbon emissions, your labor) solution in search of a problem. The gasification boiler in the Flatrock passive series is also expensive, complicated, etc., but at least puts most of the heat into an insulated tank for some amount of control. It’s like being prescribed a twice daily medication and taking a month’s supply at once and hoping it averages out.
He's climate zone 5, so I assume his design temperature is around 0F, and at that temperature the heat pumps put out about half of their nominal capacity. So "five tons" probably means a design load of 35-40K.
He also wrote "I asked and the engineer said we could go down to 4 ton but we will lose efficiency in winter below -21ºC because we’ll use the backup electric." So at -21C -- -5F -- 4 ton nominal would suffice. So it may be even less.
I think there is difference between fast-acting drugs and time-release drugs… a masonry heater’s surface is warm to the touch, but not really hot enough to burn you, the heat is gentle radiance whereas hugging a steel wood stove will send you to the ER.
#8 DC, depends if cold climate heat pump or not. I was discounting 5 tons 20% to 4tons, which is still huge.
How about 4 tons nominal at -5F, derate by 20% gives 38K BTU/hr? That sounds plausible to me. Zone 5, 3400 SF on three levels?
I looked at NEEP.org at the Daikin models, all of the 5 ton units are rated around 30K BTU at 5F.
I may be missing something, I find the NEEP search function challenging.
A model number would be helpful.
Hey, I’m a she, not a he! 😂
Yes, cold climate heat pump - of course! Here is the submittal for the specific condenser, let me know if you want the AHUs. Daikin VRV-IV S:
https://backend.daikincomfort.com/docs/default-source/product-documents/residential/submittal/rxtq60tavju.pdf?sfvrsn=a8772e26_4
It comes in 3, 4 and 5 ton sizes. I lean toward 4 even just based on the fact that it has 1 fan instead of 2, so it is much smaller outside the house. But 5 ton is quieter, and the architect insisted on putting this immediately outside the window of the study where I will truly lose my mind if I can hear the condenser or feel the vibration.
OK, here's the NEEP page for that unit:
https://ashp.neep.org/#!/product/51530/7/25000/95/7500/0///0
It's only rated for 31,000 BTU/hr at 5F. I don't think that's going to be enough.
Not enough?! I see what you mean though, but it is not often below -20ºC so maybe that is fine? Again I think the loads might be slightly lower than these calcs, but I see without the masonry heater we’d really be using the backup electric. So maybe 5 ton is not so crazy after all? I definitely trust the engineers who go through and calculate more than any HVAC installer who uses a rule of thumb, the installers were all shocked about using a 5 ton unit for residential. Could just be the area, if they are not familiar with this type of home…
The load calculation should have included a design heating temperature. You can also find it here:
https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf
Look up your county and grab the number under 99% Heating Temperature.
Of course you are familiar with masonry heaters, I forgot I am now in the land of nerds! Definitely a hotly debated topic… I enjoyed this BS&Beer episode about masonry heaters, I wish they got more technical though and talked about real use cases in well insulated homes: https://youtu.be/y-EMYz-4iq4?si=RCRbedZwO2zg-oyb
At the very least, maybe it is a good investment in the name of science? We might roast alive but we can document it and share with others to warn them of the dangers of a masonry heater in this kind of house.
I am very into “smart” high-tech stuff, but it would be nice to have a way to heat and cook that is a bit more resilient. Not that I think the world is coming to an end, but theoretically if we go the direction we very well could at this point whether due to global threats or simply AI which seems most realistic - there is something comforting about being able to go outside and harvest your own firewood to heat your home. We are planning a bake oven on the kitchen side for cooking so that is also a free resource, as you can slow roast something all day instead of turning on an oven. I love technology but am concerned about the need for all the computerized mechanical systems with a screwed up world economy and fragile supply chains. Like, what happens if we can’t get our ASHP repaired? What happens when the air filter replacements are like $1k each? There is something special about wood fire and I simply think that masonry heaters are far more advanced than wood stoves so they don’t have the same issues. It will require getting used to living with, but for me personally I feel like if the worst problem is having to crack some windows open on a sunny winter day if it overheats - in the middle of the country, I can think of worst situations to be in. We have about a million operable windows so it will be good to have a reason to open them! Maybe my logic is flawed 😉
Attached below are some screenshots from our original report. Keep in mind the design has changed a tiny bit like no skylight and bit less glazing.
Question: should we expect the load calcs would change much with slightly less glass, and greater level of air tightness?
Oh, pretty bang on with your 40k Btu estimate - impressive!
Glass is the main driver of heat loss. The world’s most efficient window is not much better than an uninsulated wall.
An air filter for a ducted system is about $10!
I think we have too much glass, too late now though - windows arrive next week! We reduced a bit from what is shown in that 3D model. That being said, it seems like we don’t really need less heat loss so we can enjoy the big windows and still be cozy in the winter!
I’m thinking of these low pressure drop MERV-16 filters we were recommended, I think they were a couple hundred $ each. And for 3 units, that is a lot… I settled on MERV-13 but have not gotten quite far enough yet to know about the filter box and exact filter type. Definitely makes sense to go with something generic that accepts different filters. I can’t remember the MERV-16 I’m thinking of but it was from California and had its own proprietary filter box that only fits those filters. Seems like a terrible idea.
So air conditioning isn't really my thing, but I look at that report and see 1.2 tons of cooling, and a three head multisplit with five ton nominal capacity and I think you're going to have trouble with that. Hopefully one of our resident experts will show up to advise further.
As to the question of whether the masonry heater will influence the heat pump, the answer is no. The heat pump is controlled by a thermostat which is a pretty dumb device, it turns on when the temperature is below the set point and turns off when it's above. If the masonry heater drives the temperature above the set point the thermostat turns off.
>Yes, we must satisfy the building code but I don’t see why we would design the system as if we are not spending a fortune on a masonry heater, which we will use at the coldest time of year.
Your insurance company will insist that you have a thermostat-controlled heating system. They don't want to deal with frozen pipes.
Definitely, what I mean is that we need to meet the requirements for the building code and for insurance so we can show that at the end of the day the heating system can work automatically and keep the house a safe temp (even if this means using the backup electric which is really inefficient). However - how we actually use the system in real life is a different matter. On the coldest winter days, if we keep the main floor toasty with free wood from our land (yes, sometimes having to open windows if it overheats) - our energy consumption will be quite low for the ASHP because it could be not running at all on the main floor, and maybe it runs minimally or only at night upstairs and in the basement. Does that make sense? So if there is a separate Tstat on each floor, it can account for the temp differential between the roasting main living area and the cooler bedrooms upstairs.
For what it’s worth, I don’t mind getting a bit toasty with the radiant heat - wearing summer clothes and enjoying mid-70s temps sounds ok.
Oh, yes the heating design temp was -21.2ºC and cooling design temp 30.7ºC DB / 22.3ºC WB … quite the range, eh?
The Daikin has low capacity at low temps. Other brands can probably meet 100% of your heating needs at -21C, without electric resistance.
>Are you specifically familiar with masonry heaters? They are very, very different from wood stoves. I can dig into the details a bit more, it can be challenging because most resources are not English as these are common in Europe - Russia, Finland, Norway, Austria, Germany, etc.
I would look closely at those installations to see if any of them are in modern, well-insulated and highly-sealed buildings. That kind of heat distribution works best in high-loss buildings, you see it in different formulations like first-generation radiant floor heat and cast iron radiators. It turns out that what people mistakenly call "thermal mass" is really the effect of a highly conductive material, large surface areas and high heat losses, which necessitate the ability to pump out large quantities of heat.
In modern houses responsiveness becomes a much bigger factor because the heating loads are so low. On a winter day, the sun going behind a cloud can be the difference between needing full heat and zero heat. There's no way to turn off a big, hot object, all you can do is wait for all the heat in it to escape.
If you light a fire and burn some wood, some of the heat in that wood goes up the chimney, the rest goes into the house. Masonry is highly conductive and has so-so heat capacity, so it will pretty quickly warm to a uniform temperature. There is almost no insulation between the masonry and the air in the room, so heat will flow into the air. As the air warms, heat will flow out of the air into the building and ultimately out of the house. The air will continue to warm until the heat flowing into it is balanced by the heat flowing out of it.
The air has almost no heat capacity -- warming the air inside a house of your size from 73F to 100F takes about 17,500 BTU. Since your house is well-built -- congratulations on that -- it's not going to have a lot of heat loss, so the air is going to get almost to the temperature of the masonry. Which is probably going to be uncomfortably hot.
But don't take my word for it. Go to a site that focuses on wood heat, I recommend Hearth.com. Those guys take wood seriously. They spend a lot of effort trying to make wood burners that can somehow modulate the heat out into the house, so you don't boil and freeze. It's not easy.
Hmmm, as a contrarian surely you can understand… the more people don’t like the idea, the more I want to test it out and see what happens. My greatest downfall 😂 I can report back next winter.
At this point many elements are still flexible. Pretty sure we will do the masonry heater, but it’s possible we could adjust the design since it is custom. The mechanicals are flexible, and I do wonder if the multiple air units are a good idea so I’ll wait patiently for the resident experts to show up. It’s Friday night and I have COVID, I’ll be here. In the meantime, will check out hearth.com - thank you!
My 50th question to add to the thread - is there not some way we can recirculate the warm air from the main level to colder parts of the house? Sure, we have a ceiling fan in the living room but I wonder if there is a way to mechanically move air from the living room down to the basement, for example. If it’s roasting as you put it, the basement or 2nd floor might still be kind of chilly - the problem seems to be getting the air evenly distributed when its radiating from a central source on the main floor.
Izzza
The people I know who have had trouble with large masonry stoves were relying on them for all their heating, and found they either under or over heated the space based on changing weather, and the size of their fires. It was their slow response that proved problematic. The ones that seemed to work was when they were used to provide a base load that got supplemented when necessary with other heat sources.
In some ways that's similar to how many people around here use mini-splits.
I hope you feel better soon. I'm also home with Covid.
In a house that is ACH50 0.6 tight you're going to need mechanical ventilation. I'd go with an energy recovery ventilator with some sort of IAQ sensor so it only runs when humidity, CO2 or VCO's get above an acceptable level. So the obvious thought would be to integrate that with circulation, set it up so when it's not ventilating it's recirculating air within the house.
1 unit is a little cheaper, 3 is more redundant. You can make the 1 unit 1 zone or 3 zones and accept wider temperature variations across floors. Efficiency tilts towards 3 outdoor units. Just depends on your priorities.
Circulation from a centrally located heat source can be accomplished with holes cut between floors. That’s not radiant heating of course, just passive. Those could be fans too I suppose. If you used a gasification boiler, you could use air coils to transfer heat up and down, using the same ductwork and blower(s) as the heat pumps.
So Malcolm, in our case if we did smaller firewood loads for less output would this provide the base heating you’re referring to? Or by ensuring there is so much thermal mass compared to the firebox size, that the output is really limited and gradual. Then, if we want additional heat we could turn the ASHP on/off as required. Or, if we are in touch with the weather and can tell it is going to be very cold and not sunny, we could load extra firewood for more slow-released output.
Part of the appeal is the challenge of working with the fire and understanding weather patterns and the behaviour of the heater. Life gets a bit boring when everything is automated and fully controllable and predictable.
As for the air tightness - 0.6 is the goal but I’m not even sure if we can achieve that with so many penetrations in the envelope. Planning this Venmar ERV: https://www.venmar.ca/136-air-exchangers-venmar-avs-x-series-erv-259-cfm-75pc-sre-top-ports.html
Funny you should suggest using IAQ sensors, this is exactly what I wanted to do and nobody wanted to talk about it 😭 I figure I can try to set something like that up after everything is installed. It seems odd to use a whole home ERV 24/7 regardless of occupancy, for instance. I’d think if there is a recirc mode that will have some benefit while the masonry heater is toasty!
Perhaps covid has now reached my brain, but is there some kind of fan that could go in the floor assembly? Like, beside the masonry heater there is a little grille, a fan inside sucking air from L1, and under in the basement on the other side there is another grille with air blowing down to get that hot air into the basement? In theory could do the same thing above. We have open web joists, I imagine something like this would sit between or like an inline blower going through the joists. Does that make any sense?
-21C is not that cold, for example this Daikin unit delivers pretty close to nameplate capacity at that temp:
https://ashp.neep.org/#!/product/78773/7/25000/95/7500/0///0
Based on the discription of the place, it is probably in the ballpark to carry the whole place.
No point in zoning the basement, no energy saved there.
You can zone the 2nd floor, this can be done with a motorized damper with its own local thermostat. What to watch with this setup is you want the ducting larger on the main floor so when the damper is closed or partially closed, the main floor ducts need to handle the full CFM of the unit. A 2nd air handler is also possible, but in this case it should be on its own outdoor unit. This gives you much better turndown, BOM cost is about the same and some backup in case one unit fails. Does add a fair bit of cost though.
These cold climate heat pumps also don't just shut off in cold temperature, they will continue to deliver heat but at lower output, so even if the resistance heat needs to run, it will only need to provide the shortfall.
I grew up with masonry heaters, I can say there is nothing magical about them. The unit was warm to the touch by the morning but not enough to deliver any heat as the place would be freezing by then. If you want wood fired ambiance, I would go for a small stove that has a fresh air intake. Something with a long exposed two story stove pipe will do a good job of heating both floors.
I would be careful with trying to duct from the stove to other places. Most codes don't allow HVAC returns near a wood burner, don't know if dedicated ducting would fall into this category but feels like it would.
Thank you Akos
1. This alternate Daikin unit, what is the main difference from the VRV-IV S? Seems much better at cold temp, yes, but is it variable so it will be ok for low load? Will it be fine in the summer if we need only minimal cooling? Wondering if there are any drawbacks, otherwise I’d think the engineer would have included this instead since the cold weather performance is so much better! Is this the same unit, a 3 ton?: https://www.daikinac.com/content/assets/DOC/SubmittalDataSheets/SkyAir/FTQ36TAVJUD-RZQ36TAVJUA-Submittal-v1.pdf
2. Can you explain what you mean about not zoning the basement? How would you heat it? I see a scenario where the upper floor(s) temp might be quite warm, but the basement could still be cool because there are only windows on the east side. If we want to heat the basement without making it even hotter upstairs, how does that work?
3. For zoning the 2nd floor, I think I understand what you mean about the larger ducts - so it would be possible to separately control the level 1 and level 2 temps with a damper instead of another air handler to save $… would we be able to set the heating temp much lower on the main floor to allow the wood heat to be primary, but then set the temp warmer upstairs so the heat pump might heat the bedrooms at night if needed (without unnecessarily over-heating the main floor) ?
4. I see your point but can’t a girl just want a masonry heater 😂 Loading a fire more than once or twice a day sounds cumbersome. Wood stove emissions are also too high, even the “eco” ones. Wood stoves are also not super safe for young kids and I plan to have babies in the near future - a scalding hot metal box in the middle of the home doesn’t seem ideal. Wood stoves do not have bake ovens, so how am I supposed to slow roast stuff all day - a regular oven? Wood stoves are also more of a risk for backdrafting, and with vented kitchen exhaust nearby this is a risk (planning an active make up air though). What do we do with the hardware we already ordered from Finland? Yikes! I think the masonry heater will stay, but it is possible I could ask about the design as maybe we could make the firebox smaller and/or less thermal mass overall. Depends what the goal is. But overall if the worst problem is overheating while we are at home, I don’t really see the problem with letting some fresh country air in the house even if we have to lose a bit of the free energy from the wood heat.
5. I think you’re right about the duct near the stove, I assume you’re talking about depressurization/backdrafting risk?
“Free” doing a lot of work when the masonry heater costs so much and requires a daily fire or two :)
I feel like it’s totally worth it, we’ll break even in approximately 126 years 😜
Hey! Hopefully you’ll be around for that day. Carrying a bundle of wood daily is supposedly good for longevity.
1. I'm not familiar enough with the Daikin product line to comment on the difference. The one I linked to is a hyper heat modulating units, so it will turn down the same (in most cases better) than a VRV system. Their multi splits are geared for commercial places which much higher loads than a house. You will have to dig up the engineering manual and compare. In zone 5 you want something that is rated down to at least -15F (-26C), -22F (-30C) is even better. Usually the non-hyper heat units have much higher min operating temperature which you might reach during a cold spell.
2. There is no need for the basement to be on its dedicated zone, it can share the air handler with the main floor. The typical way here to heat a basement is a couple of takeoffs from the main floor trunk. Common to put adjustable dampers on these and close them down in the summer to avoid overcooling. Humidity is typically managed by a stand alone dehumidifier in the basement, nothing fancy, box store stuff.
3. What you propose is possible but you need something like a Honeywell zoning controller and thermostat interface module for the air handler (don't know what these are called on Daikin units or even if they exist). On other brands you loose a bit of modulation capacity when you use a standard wired thermostat but it still works well enough when properly configured. To have full heat/cool to the 2nd floor with the main floor off, you need the ducting for both floors to handle near full air handler airflow. Not a big deal, bit bigger ducting. You can save a fair bit of duct sizing by not shutting the zone off completely, so some heat/cool would still be delivered to either floor even if they are not calling for heat. This tends to work well in well insulated places and helps even out temperatures.
4. I have a small cottage that is heated by an oversized wood stove. The way we deal with it is once the place gets nice and cozy (usually 26C), we open windows. Not energy efficient but works well enough.
If you want heat from your masonry heater to make it upstairs, it should be located somewhat centrally in the house with a largish void between the two floors. The main stair case with a bit more open area around would work. If all the upstairs rooms have a door facing this open area, it will heat (and in most cases overheat) the 2nd floor without any additional mechanical complexity. Make sure to install a large ceiling fan here to control stratification.
5. Code is worried about back drafting an open combustion device. The pressure limit for most is 5PA which is an extremely small number (think wind pressure from a mild breeze). A large return without adequate matching supplies nearby can easily pull 5PA.
Even without nearby ducting this is a very hard problem in tight houses, most need to open a window whenever the fire is burning to avoid back drafting.
Ahh, thank you! It definitely seems this Hyper Heat one is more appropriate… this could be why the installers all thought the VRV was commercial.
I can ask the engineer if we might be able to just use bigger ducts and dampers so we don’t need the extra air handlers and related parts… I’m not sure what will fit as we have open web joists which might not fit larger ducts through the openings.
The heater is centred between kitchen, dining and living - the stairs are ~ 4 feet from the heater and it’s a bit of an opening so some heat would get up there.
We definitely need active make up air, that is already specified. I want 0 depressurization so we won’t take any chances with that.
We had to provide fresh air intake for the heater by code, it’s a long story but I don’t think this heater will suck air from the room the way a wood stove or even a typical “contraflow” masonry heater would. Long irrelevant story but the style we planned is a double chamber or double bell style, it doesn’t require much air because the gases aren’t forced, it is free gas combustion.
"Loading a fire more than once or twice a day sounds cumbersome."
Let's say you get ten hours of heat out of a loading. That's about 400,000 BTU, about 40 pounds of wood, a healthy armload.
Let's say your heater weighs 20,000 pounds. Masonry has a specific heat of about 0.5, so it's going to have a heat capacity of about 10,000 BTU/degree F. So to store 400,000 BTU you need to raise the temperature by 40F. So if it starts out at 70F it needs to go to 110F.
There are going to be practical problems with having a large object in your house that's that hot. First, 110F is hot enough to burn, it's not the toddler-friendly surface you're looking for. Second, because the air in the house has very little heat capacity and there's no insulation between the heater and the air, the air is going to get very hot as well.
It also turns out that you're going to have to store more than 400,000 BTU, because the heat loss of the house is going to be higher when the temperature inside is higher. So you actually need to have a higher temperature, and all of the problems are worse.
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As annoying as it is to have to scramble and change the plans, it's much better to change the plan before something is built than to build it and then have to change it.
I can't fault the architect or the mechanical engineer here. They didn't design the masonry heater, that's not what they were asked to do, and had they been they probably would have -- or should have -- said they had no competence to design such a thing. From the mechanical engineer's perspective, he's got to design the system so it works when this thing isn't running, so it doesn't really change his job, he can just ignore it. The architect's two favorite words are "by others," that means he just has to leave room for something in the plans, someone else is responsible for designing it.
The mason who builds this isn't going to be responsible for it either, he's just going to build what you tell him to build.
You need to find someone to design this thing. My recommendation would be to scale back your ambition somewhat; rather than trying to heat the whole house for a half a day on a load of wood, have something that is pleasantly warm, keeps the area around it cheerfully warm and doesn't impose a heavy energy penalty on the house when it's not burning.
The other thing I would look into is to be absolutely sure you're not going to have issues with permitting, inspection or insurance. In some parts of the country it has become very difficult to install wood-burning equipment, both because of air pollution requirements and because of fire safety. When I read that you had imported the hardware from Finland, my first thought was, "Is that UL-listed?" Because where I am they won't let you put it in your house if it's not.
6 figures for ASHP residential design in a build of this nature is indicative of inexperience in the situation. VRF in this situation is completely unnecessary and total overkill. Not to mention the minimum output levels of a system that size may result in serious inefficiencies not anticipated by the engineer.. We’ve specialized in whole-house ASHP (primary/sole source) design and installation for 12+ years. We work in Upstate NY zone 6. There are many design options. You are correct separate air handlers per floor is prudent. There are also many ductless options (can be in combination with ducted) that may be a good option depending on floor layout. ICF basements (assuming insulated slab 2” EPS min.) have such a minimal load, and rarely, if ever, need cooling. If there’s equipment load or a fitness room it may be needed…but the point here is the load is very small and you can treat this space separately either with radiant (a modulating mini-electric boiler that can be easily adapted to air to water as it becomes available) or separate ASHP system. It’s a different environment and is best treated with its own zone. If you’re building a tight envelope (code is 3 ACH) 4 ton is absolutely more than enough for the other floors. You do not need any electric backup! Bottom line is an accurate manual J can easily be calculated from your drawings. We install exclusively Mitsubishi equipment and their design software provides calculated adjusted capacity for your specific system at design temp and allows us to reconcile output with load. No guessing. Your load, all three floors is under 50k btu. I’ve done hundreds of load calcs and subsequent while-house installs over the years. There is waaaay too much information to give you in this response but you are correct to question this quote. Its ridiculous. Even if you did single zone ducted ASHP System for basement and a 3.5-4 ton branch box system for the other two floors you’re under 50k. This is a routine job and you have anything but a routine quote.
Dave
One of your replies states that ASHP are about 50% capacity at 0 deg F. THIS IS COMPLETELY FALSE! A Mitsubishi Hyper Heating system (single zone) is 100% rated heating capacity at -5 deg F. Multi zone systems are 100% rated capacity at 5 deg F. 50%? That’s ludicrous. Yes there are some heat pumps that are, but no company that has any expertise in the field would ever select that equipment for your application. In that same reply, it was estimated that the load was 35-40k, that is much more likely but again, a manual J must be done. That can be done in an hour or two as long as window/door schedule is present and drawing shows orientation of build (North). I don’t have time to read all the replies but this 50% capacity at 0 deg F statement is completely inaccurate and problematic for those who do not have the experience and education to know this is a half truth not applicable to your situation.
Dave
I think I'm the poster you're referring to. The equipment the original poster was quoted is specced at 57K BTU/hr at 47F, 31K BTU/hr at 5F.
https://ashp.neep.org/#!/product/51530/7/25000/95/7500/0///0
Thank you DC, yes I was referring to your statement “at that temperature the heat pumps put out about half their nominal capacity”. It was not clear that this was a reference to a specific make/model, it reads more as a general statement, apologies for the misinterpretation. Had the Daikin model and Neep data been referenced in that post it would have been evident. Understanding this was indeed an accurate reference to a specified system confirms this is a very poor design choice.
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I feel your pain, I hear people all the time make statements like "heat pumps stop working below freezing." Even people in the business who you'd think would know better. I'm in DC, which is pretty ideal heat pump territory -- winter design temp of 21F, just about equal heating and cooling load -- and still there's an enormous amount of fear-mongering.
DC- To clarify, your statement in #8 was prior to the equipment info being posted in #15 so it was in fact a general statement and was not referencing any spec at that time.
Also, addressing a separate issue - a cold climate ashp only has to meet certain efficiency standards at given temps (you can read the CCASHP spec on the NEEP site). It does NOT need to maintain a certain percentage of output at the 5 deg F rating, so manufacturers simply turn down the compressor to maintain efficiency and of course the low ambient output tanks. There is nothing wrong with this, they're not designing that particular equipment to be sole-source primary heat equipment and they don't market it as such...but it's still CCASHP NEEP listed. This is counterintuitive, one would think that a CCASHP would intrinsically be appropriate for cold climate primary type heating applications, but that is not the case. There are plenty of non- "hyper heating" heat pumps on the NEEP CCASHP list.
I have read through all of these posts, there is some excellent info here and the OP is fortunate to have the input.
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No one will ever care about your house as much as you do. As you go through the construction process keep that in mind.
We went through a similar process with our own home, eventually opting not to have a fireplace at all. Instead we did a nice stone outdoor fireplace. It's positioned so that you can see the fire from the living room window if you want to sit inside and watch the fire. It's nice to sit outside on cool evening.
Maybe talk to the mason about building the same thing, but outdoors?
It’s so sad because I’ve spent a million hours researching masonry heaters and made such a fuss about it during design, aghhh, but yes I think we are starting to lean in that direction (no fire at all). The ROI just isn’t there, we also thought it would cost like $15k not $40k!
Our construction is cost-plus and it seems pretty much everything was underestimated, so it would be nice to cut costs for a change!
Great idea about the outdoor fire. We do want to build an epic greenhouse someday, if we can ever afford to do that, so maybe a little masonry heater in there would be a good way to heat it and have a cool hangout area with a big heated bench! If we are not able to return the hardware or sell it I guess we can try repurposing it in the future.
Also, I only realized yesterday we are actually climate zone 6 so colder than 5! I thought it was the same as the growing zones and for that we are a 5.
I think that has been the most disappointing part of this whole process - that people really don’t care nearly as much as we do. For many people, they’re just doing their job. Good point!
Izzza,
"I think that has been the most disappointing part of this whole process - that people really don’t care nearly as much as we do. For many people, they’re just doing their job."
Is that really surprising? They are just doing their job. This isn't a once in a lifetime project for them, it what they do everyday. Like any other field, that doesn't excuse poor performance, but I'm stumped trying to think of a profession that gets more invested than their clients about the projects they work on.
To be fair, there are passionate and qualified people out there who care deeply about the quality of their work. Many of the people who frequent this forum enjoy sharing their expertise just from a sense of pride and interest in the subject.
And who among us hasn't felt the frustration of dealing with a client who doesn't really care about something we think is important?
Of course, in every human endeavor half of the people are below average.
At this point there's still some work to do to straighten out the HVAC even without the masonry heater. I would recommend starting a new thread asking what the broad outlines of the system should look like given what we already know: climate zone 6, design temperatures of -21.2ºC and 30.7ºC , loads of 41K and 14K BTU/hr, 3400 SF on three levels. Also ACH50 of 0.6.
See where the consensus lands in terms of number of compressors, ducted vs ductless, number of zones, and how to handle ventilation.
Izzza -
1. ASHP design - there are many options. Just remember, performance first. You want to be comfortable in all parts of the house and the system should perform efficiently. The system needs to be designed to carry the full load at design temp. Anything short of that will leave you with regret. You will not always be home to keep a fire burning (we'll get back to the Russian fireplace). The engineer you chose is probably designing only with what he/she has experience with and is comfortable with. It's typical. What they've selected is NOT a good choice for your situation. Your system should be around 100% output at 0 deg F and 75% at -13 deg F. This 5 ton system has less output at 0 deg F than a properly selected (hyper heating) 3 ton system will have. It's more costly and less efficient. A 4 ton hyper heating system is more than enough for this project.
2. Zoning=comfort. Yes, you CAN put the basement on a zone with main floor. Is it ideal from comfort standpoint? No, your instincts are very good and based on your intended use of that space it should be zoned. A single zone ductless (or ducted, depending on layout or how space is to be used) is a much better solution. You can also run this zone off of a multi zone system which reduces installation cost (but if you have a gym there, you would not be able to use AC if other zones are heating). It gives you direct control over an area with a very different load profile than the first floor. Floors should be zoned. Do they NEED to be? No...do you NEED snow tires? No....it's a question of best result and your satisfaction with the end result. We typically keep bedroom areas zoned in a group (or ductless, individual) away from Living areas for obvious reasons. It gives you the ability to control those spaces independently. This does not need to be a complicated task. You can save money by using ductless wall units or ceiling cassettes in basement and/or first floor (wall units are certainly most economical). Second floor is low load bedrooms, baths, and best to consolidate all those small loads with an air handler.
3. Masonry heater - it is apparent that you're in love with this. Based on that, go for it. Yes, it will overheat the space and yes it may be problematic, but if you don't do it seems evident you'll always wish you did. You want the challenge, I respect that. Take it on and learn from it and share what you learn. I would never recommend it as a practical solution but your happiness is what matters. I grew up with wood heat. You can build and maintain a small fire, if you're home. You can have a 1" bed of coals the size of a dinner plate that takes one small stick of wood every couple hours if necessary. Yes, this is a very large thermal mass it can and will absorb heat and radiate it at a low level. The more mass the more the heat dissipation. I have an outdoor wood fired pizza oven. It can be 800-900 degrees inside (cooking surface not air temp) and the outside is barely warm. I designed an ASHP systems for a brewery near Ithaca, NY that has a very large wood fired pizza oven indoors. It does not contribute as much to the cooling load as you would expect...granted it's an oven and not designed to heat the space, but it has a continuous wood fire inside and a serious amount of masonry on the outside. Mass is a great equalizer...but as you've been told repeatedly and correctly, this type of heating in a low load building is going to overheat the space at times. You seem like you want to be engaged and learn, you'll enjoy the interaction but expect issues and it may not be the experience you're hoping for. As you know, they also pull combustion air from the space they're installed in. You're concerned with makeup air for kitchen exhaust just keep in mind that air for the fire/draft needs to be provided, doesn't matter what type of heater or how it's designed. Fire needs air and chimney needs draft, physics. Woodburning appliances (except for direct vent) are problematic in tight envelopes. Crack a window...ugh
DC &; Malcolm - yes, you’re both right. I did, however, notice a reluctance for some to listen to me and potentially being a 30 yr old woman has something to do with it. I hold everyone to a high standard and I think that bothers those who want to coast and take advantage of clients who are too busy to pay attention. I pay very close attention to almost everything, and I don’t always like what I find. The project is not exactly what I dreamt it would be, but it’ll turn out pretty sweet nonetheless so I’m grateful for that when we are in the midst of a housing crisis here in Canada. And I have a new found passion for building science I never knew I needed 😉
Good idea to start a fresh thread and see what consensus is on the general system design. Engineer might kill me, or maybe he will be flattered by my passion for HVAC. I imagine most people are not home with covid on NYE so I’ll post this week maybe once we talk to our builder about the multitude of simultaneous crises.
Mitsuman - More great advice! So you are in favour of the additional air handlers, do you think it is a problem connecting to one big condenser or you agree with others that multiple smaller condensers would be better? You’re not wrong, I am “in love” with masonry heaters - the design is just too fascinating and the efficiency/low emissions is super impressive. Also, we do have a 6” fresh air intake for the firebox as per code and I really don’t mean to send anyone down this rabbit hole but the thing about the masonry heater we selected is that it is a different kind of combustion which does not rely on energy/draft to force the gases up the chimney instead of into the room. It is a double chamber or double bell design, not contraflow. I might have to post separately because I really am curious what the nerd consensus is on this one. I will refrain from going in depth but whether we end up with a heater or not, I still think this is super interesting despite being a very obscure Russian invention with minimal English info available. It is the system of free gas movement as opposed to forced gas combustion - so I understand that typical fires need air/draft, but this seems different.
If I enjoy rabbit holes, does that make me a rabbit?
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Izzza -
Yes, the 6” intake you referenced is exactly what you would need, that checks that box nicely. Nice to hear that’s a code requirement! I believe the double bell design you’re referring to is intended to promote re-burn of combustion gases which increases efficiency and decreases emissions (not sure it would reduce the amount of combustion air required for a given burn rate, but either way no matter since you have the intake to provide combustion air). In our area we refer to this process as “gasification”. We have a lot of outdoor wood boilers here, and by state law any new wood-fired boiler installations (indoors or out) MUST be gasification units. The fact that a normally aspirated masonry fireplace design accomplishes this to some degree (pardon the pun hahaha) is a testament to the knowledge and craftsmanship of the masons or individuals who created it, intriguing! We do have similar insurance challenges, but there's always a way.
As far as your ASHP design goes, there is much to consider, and it seems you really like to have a good understanding of the systems so you can make the best decision possible, I applaud that. You absolutely should demand excellence from your build team and settle for nothing less. I wish more people were engaged in the technical side of their builds, they have a much better appreciation for the end result and feel good about their investment. It also eliminates any unwanted “surprises” since you have an expectation for all the different aspects of your project.
You asked if connecting multiple indoor units to a single outdoor unit (multi-zone) would be problematic (side note: we do not use the term “condenser”, since when it’s heat mode the outdoor unit is the evaporator, in cooling mode it’s the condenser). The answer to that is no, it is absolutely not a problem and is one option that has its advantages. I’ll explain -
In an earlier post, Akos referenced the better “turn-down” ability of a single zone, or 1:1 system over the multi-zone system. This is absolutely correct and is accepted as a general truth in the industry. It means the system has the ability to run at lower percentage of its maximum output. 1:1 systems typically have much lower minimum outputs and will provide better efficiency. That’s the primary advantage and in most cases, it makes the 1:1 system more efficient than the multi-zone. There’s a LOT more to it but no need to discuss that at the moment. Why does the single zone have better turn-down and lower minimum output capacity? Well, minimum output is determined by how low the compressor is designed to run. In these systems, the oil that lubricates the compressor is mixed with the refrigerant inside the system (referred to as miscible). The oil travels throughout the system and is returned back to the compressor with the refrigerant where it provides the necessary lubrication for the compressor. In a multi-zone system, there is potential for much more refrigerant piping. If the velocity of the refrigerant in the system is too low, the refrigerant will not deliver the oil back to the compressor properly and it will result in decreased compressor life. The increased piping volume in a multi-zone system requires that the compressor spins comparatively faster to ensure proper oil delivery back to the compressor. That’s the primary reason a multi-zone system does not have as good of a “turn-down” as a single zone or 1:1 system. Does this mean that a multi-zone system is a poor choice? Absolutely not! It is an excellent solution for many applications. There are certain extremely low load applications like net-zero type builds where a multi-zone solution should never be used, but in your situation a multi-zone system would be a good solution.
Multi-zone advantages – in many new builds, there are porches, decks, patios and windows everywhere. Sometimes it’s difficult to find a location for even one outdoor unit, forget trying to find a place to put the 3, 4, or 5 units that would be required if all zones in an average new build were 1:1 systems. Note – do NOT let them wall mount the outdoor unit! You will hear the system, it is not nearly as quiet as a unit detached from the structure (mounted on a pad/stand). Any wall mount should use isolator pads, and at a minimum the right side of the bracket (compressor is housed on the right side of the unit) needs to be lagged into a stud, but it’s still going to transfer vibration/noise. Especially if the wall is framed and insulated with closed cell spray foam, the wall is locked together and resonates like a speaker and magnifies the noise. A concrete wall is ideal for a wall mount, but there are limitations and noise can still be a concern.
With Mitsubishi equipment (which is all we install), another advantage of the larger multi-zone systems (branch box style) is that there is no low temperature cut-out feature. It was stated earlier that “hyper heating” systems do not turn off at low temperatures and they continue to run, just at reduced output. This is both true and false. The Mitsubishi single zone hyper heating 1:1 systems, and what we call “ported” multi-zone hyper heating systems where the indoor units connect directly back to the outdoor unit (no branch box), there is a low-temp cutoff feature at -19 deg F ambient (not wind chill). The outdoor unit shuts down, and it will restart when the outdoor temp comes back up to -14 deg F. This is why you’ll see the “-13 deg F” operation attached to Mitsubishi’s hyper heating marketing material. The larger hyper heating multi zone systems (3, 3.5, and 4 ton) that use the branch box or branch controller do NOT have a low temp cut-out feature and will continue to run. This is important when using the heat pump as a primary/sole source of heat. In our area, we do see -20 deg F and I’m sure you do also. Not often, but it does occur. During those cold weather events, those single zone and ported multi zone systems will turn off by design and there will be no heat output until the outdoor temp rises to -14 deg F. This cutout feature has been part of these systems for many years, I believe it’s slated to be discontinued but it still is present in most of today’s Mitsubishi equipment as described. To clarify, in a branch box style system, the branch box is piped and wired to the outdoor unit, and the indoor units are piped and wired to the branch box. It communicates with the outdoor unit and indoor units and meters the appropriate amount of refrigerant to the respective zone. The branch box can also provide greater piping distances required in larger homes or structures.
If your highest priority is highest possible efficiency, separate single zone systems may be the best answer for you. Keep in mind that any 1:1 combination is unique, and each design has a different efficiency. Yes, typically a 1:1 is more efficient, but not always. For example, a 1.5 ton 1:1 hyper heating ductless ceiling cassette system has a lower heating efficiency rating than an air handler on multi-zone hyper heating system. In fact, that particular 1:1 doesn’t even meet the minimum efficiency standards required to be a NEEP approved CCASHP. The 1:1 air handler systems are also not always particularly efficient either, the selected systems need to be evaluated carefully. Separate outdoor units does give you some redundancy, that’s true, but a properly designed and installed multi-zone system is extremely reliable so I’ve always rejected that view as a primary reason to use all 1:1.
Hopefully this helps you with your decision or raises new questions. For 12+ years of specializing in ashp technology, I’ve used Mitsubishi equipment exclusively because of the reliability and performance and have had stellar results. They’ve been making these systems since the 70’s, this is nothing new for them. No matter what company and product you choose, be sure to get references and see if you can visit a similar installation done by the company in that area. Talk with the customer, get their experience. These are expensive systems that need to be installed with care and attention to detail. Poor installations are problematic and unfortunately far too common, and issues may not be evident for quite some time. Proper leak testing and evacuation practices are very important and often side-stepped. Also, best to very careful when considering a company that’s fairly new to the technology, try to stick with someone that has a long history and many customers.
All houses are different, all people are different. There is no one approach that is “best” for everyone, and beware of any company that tells you what’s right or wrong for you without extensive evaluation of the technical requirements and your personal preferences regarding performance, comfort, and aesthetics. Many of our whole-house solutions are a mix of ductless and ducted, some are all ductless, and some are all ducted either centrally or with multiple air handlers. I can’t tell you what would be an optimal solution for you. I do recommend zoning floors separately, an air handler for the second floor to consolidate all of those low loads into a single zone. If upstairs air handler is in vented attic space, it’s critical this system is properly air sealed and insulated. Plenums need to be lined, then wrapped, you should have minimum R8. A good company can help you design and give you options to integrate ductless into your design which can improve your end result.
Ciao for now rabbit!
Hah, I have met my match(es) - thanks for the great info, even if I’m going to need to reread some of this like 10 times to comprehend it. And yes I agree about people giving absolute advice without any context or info. Like the installers all thought a 5-ton system sounded insane, they’ve never seen anything like it and most houses only need 3 ton, or whatever… I’m not sure how they could just know this without knowing the exact load calculations. Yet as far as I can tell, it is still not enough and we would need the backup electric in the coldest weather based on the original unit (VRV-4 S) which doesn’t seem great for cold weather performance.
Hmm, Mitsu was the other brand our engineer recommended but I can’t handle the controller/interface. It looks archaic! I honestly would not even know how to work something like that. They need to upgrade that, considering they make such good equipment. So as bad as it sounds, I just prefer Daikin for the Daikin One+ thermostat and the app 😂 Apparently third party controllers don’t play nicely with these kinds of systems, not sure how much truth there is to that. Finding out we can’t use something like a Nest to control everything was devastating. We got one installer quote for Mitsu ASHP and it was like $20k more expensive than the installer who quoted Daikin. Can’t explain that one but let’s just say in a high cost of living region, the markups are questionable.
Glad to hear that the single outdoor unit with multiple air handlers inside might not necessarily be the end of the world. Like anything, it depends!
OK - I posted a new thread, maybe the conclusion will just be yeah the engineer’s design is fine - leave it alone. But I don’t like the cold temp performance so we’ll see.