Single Air Handler vs. Multiple Ducted Minisplits
We’re doing a new construction project in Portland OR (Zone 4C), and I am deciding between using 2-3 ducted PEAD systems or a central SVZ-type air handler.
I’ve read John Harrod’s excellent articles (https://www.greenbuildingadvisor.com/article/planning-a-furnace-to-ducted-heat-pump-retrofit) which list a number of advantages of air handlers over a house full of ductless minisplits, such as centralized condensate removal, better access, easier maintenance, less equipment). But would the trade still be the same for 2-3 ducted minisplits vs one air handler?
The house will be about 3500 sq ft, long and narrow, single story slab on grade, and no attic – so ducting will mostly have to be routed through soffits.
I’m attaching a layout of the house and the Manual J load calcs. The ADU is a separate area and is not shown in the diagram.
The way I see it, we can have one ducted minisplit for the pink area, and one or two minisplits for the green, blue, and gray areas, which would need to be zoned.
Or we can have one ducted minisplit for the pink, and one zoned air handler for the other areas.
Or in the ideal case, have a single zoned air handler for everything.
How to decide on which way to go?
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I would think a lot of it would depend on where you are comfortable putting the air handler(s) and how the ducting/bulk heads lays out. Cost would be another important factor.
Why do the green, gray, and blue areas need to be zoned separately?
The green and gray would be on the same zone, as they are "common" area. The blue is the master bedroom suite, so we'd like to have it controlled separately from the common area.
I get the considerations of ducting and cost, and the way I see it, ducting would be pretty similar for ducted minisplits and single air handler. The only difference may be not needing to run a duct across that central clerestory area if using two separate systems (image below). But we can run a duct across there if a single system was otherwise feasible.
As for cost, my HVAC designer says the two approaches are "comparable". I'm thinking that if the performance of the two systems is comparable, and cost is comparable (give/take 10-30%), the advantages of the single unit outweigh the disadvantages?
Do you plan on keeping the master suite at a different temperature than the common areas?
If the cost is the same, I would vote for two systems. You have some redundancy in case of a failure and the systems will probably be operating closer to their minimum giving a higher COP. But it’s hard to say how much of a difference until the equipment is specd.
The more I think about it, the more I'm not sure I need the master suite to be zoned separately from the common area. 99% of the time it'll be just my wife and I in the house, so it seems like we should be able to adjust the thermostat as we want for daytime or nighttime.
I'm trying to avoid what we have in our current two story house, which has no insulation and tons of non-low-e glass - it can easily have a temperature differential of 5 deg or more between the living room on the first floor (where the thermostat is) and the bedroom upstairs. So if I set cooling to 77F at bedtime, and living room temperature reaches 77F, it'll stop cooling, even though the bedroom temperature will still be 80F.
I'm guessing that won't be as much of a concern in our new construction, with a single story and much better insulation. So maybe I don't need zoning for the bedroom?
What you experience on the second floor of an older home is common. This should not be an issue with your energy efficient single level home. Adjustable dampers at each register will help balance the air flow during commissioning to get even temperatures across different rooms.
That makes sense, thank you. That'll certainly reduce complexity in the design, which I am always a fan of.
Overall, a single centrally ducted system is usually the cheapest for a single story.
The issue is that without a basement or a crawlspace, there is nowhere to easily run a duct across the house without a lot of bends. Too many bends and imperfect install, chances are the guest area will never get proper conditioning.
There is a way to make space for ducting though if you really want to. The roof on either side of the popup doesn't need to be flat. You can slope it towards the clearstory on one side which will give you plenty of space to run ducting underneath it and still have a flat ceiling at the same height as the other side. Not sure if this design change is worth it at this point.
Since the design ship has mostly sailed, two units is really the best way to go.
One mid static or multi position air handler for the living+bedrooms, maybe a bit of zoning for the bedrooms plus a 2nd slim ducted unit for the guest area.
Since the guest area shouldn't need to run all much, it really won't add all that much extra maintenance.
It is easy to do clean modern design when you omit essential details such as mechanicals. Design clean and modern with all of those details properly designed and well hidden is a lot of work. All those clean details take a lot of time to build, so there has to be a be a balance in any project.
P.S. I don't remember seeing shading on your clerestory windows. Without any shading they can cause some serious overheating issues in the space plus a lot of glare. All that direct sunlight can make being in the space difficult. In my home, there is a two week window that on a sunny day at certain times it is hard to work in the kitchen because of the light from the clerestory.
Yes, unfortunately our architects designed the house without HVAC in mind, other than "we'll have a minisplit in every room". I wasn't smart enough at the time to call them out on that, plus I was too busy focusing on the layout. It wasn't until I started delving deeper into the details that I found how unattractive the "minisplit in every room" approach is - and that's when the architects told me that their design was just "notional", and it's up to my HVAC designer to figure out the mechanical. Funny, I never knew I needed my own HVAC designer.
But like you said, at this point the ship has largely sailed - the plans are going to permitting in a couple of weeks, and I don't want to delay and pay for additional efforts by the architects or structural to make significant changes. So I'm focusing on designing the best HVAC system for the current structure, and then find ways to add soffits/chases/bulkheads to run the ducts as needed.
I agree that the guest/office suite is probably best served by its own mid static, which avoids running ducts across the great room. That leaves the option of a single multi position air handler for the entire west wing, or (as the HVAC designer has) one zoned mid-static for the common area and primary bedroom, and a separate small ductless for the workshop.
It sounds like maybe I don't need the primary suite to be zoned separately from the common area? I'm wondering about the workshop - I plan to use it only occasionally, but I'd like it to be conditioned when I do. So does it make sense to give it its own ductless minisplit, or have a separate zone go there from the air handler? The workshop is only about 1.8K heating and 0.8K cooling.
And thank you for the catch about the clerestory window and shading. I've been thinking about it in the back of my mind, but never really focused on it (priorities!). But it just so happens that the pop-up is the only portion of the house without roof overhangs, so that window will get no protection from the sun. I can see how at some times of the day, depending on season, we could get direct sunlight exactly where we don't want it. I'll have to give this some thought.
Seems like heating load on Manual J may be a bit high for your location/design temperature (+29 F) for 3500 sq ft. I'm in zone 5 with -1F design temp with 2100+ sq ft and at 28k. If I multiple 28k by 3500/2100, that's about 45k at -1F and the Man J says 41k at +29F.
Sometimes you can make changes after permitting without problems, such as overhangs, windows, interior walls, closets, etc. (unless you're in some HOA that regulates exterior or really strict municipality?). Unfortunate that it was designed without HVAC, to me the excuse doesn't fly. I've given lectures to architects and they tend to be pretty energy aware. Does your municipality require a HERS rating? That could give you another data point. My energy consultant/HERS rater worked closely with everyone: HVAC, flooring, insulators, etc. OK'd bathroom fans, range hood, flooring glue/nails, wall assembly, roof assembly, etc. I planned three ducted units well in advance and designed spaces to accommodate them.
As to clerestory windows, I added an additional one once I spent time in the house during construction as I could see how a north facing great room with some western exposure would benefit from an east facing clerestory (about 15 sq ft), which pulls in light in the morning even on winter days. The light traverses the kitchen in the summer but its quite mild from the east (glad I made the choice and it replaced two casement windows). As the design is mountain mid-century, clerestory were part of the design, they offer some nice views, even moonlight.
I haven't looked closely at the Man J calcs yet, I'll have a look and see what his assumptions were. We do have a lot of glass, I'll have to see how he modeled it in terms of U and SHGC. He does have "tight" selected for "construction quality", so that's good. I'm also finding that he has different numbers in the short form that I posted earlier, and the more detailed form that I asked for later. The long form has 37K heating and 29K cooling, whereas the short form has 41K heating and 33K cooling. Not a big difference, I guess.
I'd love to "borrow" your HERS rater (just kidding) - I can't find someone like this to save my life. No one seems to care other than to get paid, which is so frustrating. I'm having a telecon today with the architects and the HVAC designer, I'm hoping to get them to work as a team in designing the equipment and how to incorporate it into the house design. Our architects are actually very energy aware too, that's why they kept singing the praises of ductless minisplits (admittedly the most energy efficient option) - but I am finding out that they are very inexperienced in the real world of construction.
Our clerestory windows are along the North and South walls. The North one will not have any direct sunlight, so that is safe. The south one is the one I'm concerned with - I love the idea of seeing moonlight and sky, but direct sunlight doesn't sound like fun.
As many say, be sure not to oversize and remember that heating design temps are a small fraction of the days in a year. And, check out City of Missoula info for a list of HERS raters: http://ci.missoula.mt.us/DocumentCenter/View/5100/2012-International-Energy-Conservation-Code-form?bidId=
I wrote a book chapter in 2007 on green procurement codes and Portland was quite advanced if I remember correctly. Thus, I'm surprised that a HERS rating or equivalent is not required on new construction, from a quick search seems like it is required for sale of an existing home.
As to the clerestory windows, I'd make sure the electrician runs an outlet/box (or two) so you can set up motorized blinds/screens (with floor level switch box/control box). Normally I'm against such things, but given the height and exposure, you may consider - possibly even recessing the blinds/screens as you have that opportunity with new construction. I imagine it's even possible to have a heat/optical sensor to trigger or a timer so you don't have to worry about it. Moonlight and stars, maybe better than a skylight ;-)
Attaching a couple photos, the west facing clerestory windows could have benefited from a screen, east facing is not a problem.
How are you going to use the workshop? Dust can be a major factor. It may be best just to use electric radiant heat.
Good question - it should not be dusty or dirty, it's mostly for small household projects or tinkering with simple car repairs. I'm hoping to build a large detached workshop for all the messy/serious/dirty projects.
Than with the low heat load, I would just condition it 24/7 and include it in the common area/master bedroom zone
I don't think you need too much zoning, but I would do the simple zoning at least I mentioned earlier (overprovision the bedroom area, install in-line zone damper+thermostat without any connection to the air handler). It would only add a couple of bucks to the build but give you some additional control over your bedroom.
Multi position or mid static only really changes the blower size. The choice is mostly about where you have space. Multi position is simplest to mount it in a closet, mid static in the ceiling. HVAC installer might prefer a multi position unit as it is closest to a standard furnace and won't require any custom ducting adaptors.
If you are going to be doing anything that you don't want to smell in the house in the workshop, in your climate with mostly heating a ceiling panel heater is your best bet. An oversized unit can get the place up to temp in no time plus they have no issues with dust/dirt. Something like https://mightyenergy.net/products/ can also be mudded into the ceiling for a flush install.
Also figure out how you will integreate your venitllation setup(HRV or ERV). If you are in wild fire area, it would be a good idea to include an fresh air intake filter that can accommodate a filter stack (ie merv8+carbon+hepa).
Fully ducted is the cadillac setup but does require more ducting. In 99% of cases the installers prefer to go with a simple install where the unit is just plumbed to the return. My preffered in-between setup is a hybrid ducted setup. Run dedicated stale air pickups (bathroom, big one in kitchen area about 8' from stove, laundry room and a small one in the main bedroom) and connect the fresh air supply to the return of your air handler. This works the best to go with one of the auto-balance units (ie Panasonic Intellibalance, Zehnde Q series or Vanee AI series) as they won't be effected by flow changes in the air handler. This setup also lets you skip the bathroom exhaust fan, so one less hole in your building envelope plus a bit of cost saved.
PBP1, thanks for the suggestion for the electrical provisions at the clerestory windows for future shading. Will definitely consider that.
Akos, we had a call with the HVAC designer and the architects, and I think we have a good plan. We'll stick with the PEAD for the guest rooms and office, and then have a SVZ air handler for the west side of the house. It'll be in the utility room by the garage (just north of the laundry room).
We'll have one supply trunk line over the laundry room and mudroom, which will then split into three zones - one for the great room, one for the gym, and one for the primary suite. It's probably an overkill, but it gives us flexibility, and the HVAC designer recommended going that route.
A central return trunk line will pull air from the hallway area, and go to a cabinet filter at the air handler. A couple of transfer and jumper ducts will connect primary bedroom and the gym to the hallway. I'll post the proposed layout once the HVAC designer sends me the updated version.
I really like the idea of that infrared ceiling panel heater for the workshop, that looks perfect for my application. Thanks for that!
I think that is a solid plan.
Make sure they figure out how to get heat across the living room as well, you have some large windows right up against the ceiling which won't leave any room for bulkheads.
Also don't forget the ventilation setup. Tight houses always need mechanical ventilation, something that needs to be part of the design.
I'm attaching the draft sketch of the ducting plan - there will be several outlets into the living room.
As for ventilation, he is suggesting supplying fresh air for all three units, as well as having make up air for laundry and kitchen hood (shown by pink arrows in the sketch). I have to admit that I have not researched ventilation all that much (too many balls in the air all at once), but this sounded reasonable to me after doing some reading.
What do you think?
The left side of the living room is well conditioned, but there is nothing on the other side. Either figure out how to run ductis from the SVZ to there or connected it to the zone in the guest suite.
For running ducts along, I've had good luck with running 3x14 ducts against the ceiling, with with only side support for the drywall bulkhead. This makes it only a 4" thick bulkhead which is barely noticeable (see attached, here it is thicker as it couldn't be right against the ceiling).
You are in a mild climate, so energy or heat recovery might not be worth it. These units don't cost all that much though, so I would always include it. In either case, make sure there is a decent filter for the fresh air intake. I run a combination of Merv13+Hepa because of ragweed allergy at home.
I've heard people have issues with the Mitsubishi zoning setup, make sure your installer is familiar with it and gets is set up properly.
I was thinking that the supply from the West side of the great room would be sufficient to condition the entire room, but sounds like you disagree. Is there a rule of thumb for how far air would reach into a room? There is no easy way to get ducting across the great room (possible, but not easy), and if I added ducts from the guest suite heat pump to also condition the great room, they'd be on different zones (I guess I can also zone the guest room heat pump?).
Speaking of zoning, can you elaborate on the Mitsubishi zoning issues?
Edited: I asked the HVAC designer about the great room supply air, and he said the following: " My intention was to select grilles that would throw air across the room. I think with a slight upward slant, the air would provide good mixing within the space.
But, if you'd prefer to have a trunkline travel across the room within the TJI ceiling joists and have ceiling supplies, we could take that route too."
I'm attaching the sketch, red is "across the room" air, and blue would be "up into the ceiling with ceiling supply" air. Blue is obviously more ducting, is it worth it? Or would "across the room" work?
It is hard to say without somebody doing the design work on it. You can get long throw registers, it is a fine line between getting good distance being too loud or too much air velocity around people. Generally this works pretty well for cooling but harder for heating as you can get a fair bit of stratification along the floor along the far wall where there will be very little/no mixing.
Running along the ceiling that high up might also not work for heating as it is a again a fair bit of throw to reach the ground. A bit lower down, I find ceiling registers pointing down at 8' work well for both heat and cooling, just make sure they are adjustable to keep air flow from directly blowing at people.
Instead of running along the ceiling, can you build the roof popup wall slightly thicker and run the duct along the bottom of the clerestory window? You inset the window to compensate, this will also help with the shading without effecting the design at all. It would be the cleanest solution I can think of as it would be invisible if you use a linear diffuser.
Another option is to put a larger low mounted return on the other side, this would help with stratification and mixing. This would still be a fair bit of ducting, maybe it can be run behind some of the built-ins.
This is a good resource if you want to read more about air distribution:
https://www.priceindustries.com/content/uploads/assets/literature/engineering-guides/air-distribution-engineering-guide.pdf
P.S. When you see those clean modern houses in design magazines, the utilities are almost always run inside the dropped ceiling. Essentially the whole ceiling is installed lower down to make space for it, similar to a commercial construction. Since there are no references, you don't see this at all. You do have to make the structure slightly taller to compensate.
I’ll dig out my anemometer readings for cfm blower speed and fpm/fps throw from my two 14”x14” supplies at 9’ high that supply from a low static SEZ 15k, the throw is over 20’. Breaks up stratification with minimal whoosh in 24x30 great room 9’ to 13.5’ ceilings. Throw was heavily dependent on filter size and filter type. Great room is supplied from one side only with returns on that side too.
First plot shows feet per minute throw for "old box" (small sq ft for filter), "new box" (more sq ft for filter), and without a filter - data for the three fan settings: lo, med and hi.
Second plot shows FPM for "old box", one supply versus two supplies with and without filter.
For low static air handlers (Mitsubishi), the filter size and type (and presence of the filter) makes a big difference. At "lo", the new filter box had FPM over 300 FPM (w/filter) and, at "hi", over 400 FPM (w/filter).
If you get 350 FPM, then you may get over 20 feet of throw.
As to Figure 3 of the pdf in the link from above post (reproduced below), I believe the envelopes are for "terminal velocity" meaning that there is 100 FPM at 20+ feet out from the supply and nearly 150 FPM at 20 feet out (dark blue envelope). In that case, the velocity at the supply is going to be quite high (maybe 350 FPM or more?). I did not really feel (or measure) air movement 20 feet out from the supplies at under 300 FPM at the supply.
With a double deflection supply grille (pdf per link), you can tailor the size and direction of the supply. I also have one linear diffuser with two slots (not in great room), it has some adjustability too.
Thanks guys, this is helpful! Our HVAC designer says that with 200 cfm of air at the grilles, the throw should be about 23-33 feet (at 50 fpm), using these grilles - https://www.hartandcooley.com/products/l-series/linear-diffuser-for-ceiling-sidewall-or-floor-applications
Unless he's completely off base, which I have no reason to believe, this seems like a simple and cost effective approach (?).
Seems like it is reasonable.
A quick warning on my "2 supply" plots, for those data each supply has only half of what is shown on the x-axis. So, where it says 400 cfm, for two supplies, it's actually 200 cfm through each single 14" x 14" supply to give a max exit velocity (on center) of about 300 fpm, which gives 20+ft of throw (using the "hand test" at that distance and anemometer at supply, room 24x30 and I can feel the throw at 6' high at 24').
Will those be: "Note: G Grid blades are assembled parallel to short dimension. Order with desired diffuser for factory fitting." Some amount of adjustability is nice to have.
We got back the proposed HVAC design from the designer. I'm still looking it over, but I'm not sure I understand all of it (e.g. which ducts are flex, and which are rigid). Does this look complete, does it make sense, etc.
One thing I find odd is that >90% of cooling requirement for the primary suite is from glazing, and about the same in the gym - but maybe in a well insulated house that is right?
Would appreciate any feedback from those who know how to make sense of these attachments :)..
Even when it is very hot outside, the temperature difference between outdoors and indoors is very small. This means any well insulated house, only a very small amount of your cooling load comes from walls and roof. Almost all cooling load is solar gain.
Looks like a very thorough design. The big ducted heat pump seems a bit oversized in your milder climate. Most cold climate heat pumps put out much more than rated output in your weather, you can get away with a 1.5TON unit:
https://ashp.neep.org/#!/product/34578
If you compare it to the 2.5 ton
https://ashp.neep.org/#!/product/34582
The bigger unit can do almost an extra ton of heating over your load at your design temp. Looking at the turn down on both units, surprisingly there isn't all that much difference between the 1.5 TON and the 2.5 TON unit, so oversizing in this case is not a big deal.
I would make sure the zoning is sorted out as some people have reported issues with zoning on these:
https://www.greenbuildingadvisor.com/question/a-warning-and-a-question-about-mitsubishi-zuba-zoning
Good comments, we'll plan to revisit the sizing of the SVZ after we select the windows, and again after the blower door test.
I asked the designer about the zoning issue, he said the following - " We will be using the thermostat interface module with Honeywell dampers and control. You should not expect any of the AirZone issues because that’s not what were using. I’ll make sure the dampers/controls are called out. I also plan to put on a barometric dump-zone bypass in to relieve pressure in the rare case that the gym zone calls alone."
The Honeywell zoning is a pretty low tech setup, you get no modulation as all control of both zone dampers an the SVZ is simple on/off. It does work, you are giving up a the benefits of a modulating setup.
The Airzone setup is much better setup( it runs the SVZ in modulating mode, full control of fan speed and zone dampers are also fully modulating) my point was that it needs somebody knowledgeable to set it up and get it to work. If you can't find this, the Honeywell setup will work.
In either case a bypass damper is not the nicest solution, the better option is to set the closed position on the larger zones to allow some airflow so the extra capacity can be diverted to the rest of the house. More efficient plus better mixing of the house air.
Thanks for the tips Akos. I read the thread you linked about the gentleman having zoning issues, and in the end, it's not clear how he solved it or if he solved it well. I certainly am not interested or qualified to even think about programming my own HVAC :).
I'm wondering if I should leave the zoning discussion to the actual installer, since my HVAC designer is in NC and the house is in OR. So in the end, I need the installer to be comfortable with the design, since he'll need to be the one to stand behind it. Does that make sense?
The more I read about zoning, the more I start disliking it, especially for situations like ours where one of the zones is a small room (the gym). I'm thinking I should ask to have the design changed to just two zones for the SVZ - the common area (include the gym in that) and the primary bedroom suite.
As for bypass, I did read about the downsides of bypassing into the return air duct, that makes sense. But our designer is having the bypass dump into the common area, which as I understand is the same as just not fully closing the common area damper, right? I'm attaching that detail below.
Bypass to the living room is fine, that is the better setup. The small zone with barometric bypass or living zone damper closed stop adjust should not be an issue. Plus it is nice to be able to supercool the gym when needed.
I took a quick look through the manual of the Mitsubishi thermostat interface module you would use with the Honeywell zoning.
You can actually get an OK modulating setup provided it is set up properly.
The thermostat module can keep the fan speed modulating provided the jumpers are set correctly and the zoning controller is only driving the G input.
The module can also be configured to be two stage heat/cool with the first stage being modulating. Make sure this is set as the default is only single stage and the Honeywell units are all configured as two stage control. This is important as a number of times I've had to fix this where the HVAC installer couldn't be bothered to wire up and configure both stages even when both thermostat and furnace had it.
The engineer in me would still prefer the Airzone setup. So many more things to adjust and tweak!
Thanks Akos, those are some great tips, I'll be sure to revisit this when we get around to installation! And maybe I will keep the gym as a separate zone, the supercool options sounds fun (jumping from sauna into supercool gym!). Really appreciate all your advice and guidance.