Help with HVAC design for new ducted heat pumps
We are trying to finish a long process to replace our gas furnace with heat pumps. I’m already grateful for the help I received in previous phases of our project.
I need further help to confirm that what we’re currently looking to do makes sense.
We are in zone 6, near Ottawa, ON, design temperature is -13F.
We got a room-by-room heat and cooling load calc done by and independent engineer. We are in the process of having him design the HVAC system.
Our idea is to have 3 zones with 3 1:1 compact ducted heat pumps.
We have to do with Carrier units as the only installer who accepted to work with us on the project using compact ducted units only sources Carrier and Mitsubishi and they tell us that they’re not getting enough clarity on Mitsubishi unit availability at this point.
We initially wanted to go with 2 x 18 000 Btu units + 1 12 000 Btu Carrier units (blue are on the plan), but our engineer told us he couldn’t make the 12 000 btu unit work because of static pressure constraint (0.2 WG for 353 max CFM).
Our plan B is to go with 3 40MBDQ18—3 units using the configuration in the attached plan. We decided to go with 3 rather than 2 zones because of a change in direction of the joists which resulted in a big loss of space in our laundry room with only 2 zones.
Total heating load is approximately 47 150 btu, divided this way
Heating load for blue area is between 10 000 and 14 000 btu (basement + first floor, part of dining room will most likely be shared)
Heating load for the red area is between 15 073 btu and 19 000 btu (basement + first floor, part of kitchen area will most likely be shared)
Heating load for yellow area is 18 109 btu (2nd floor).
Based on rough calculations, he is pretty sure the 18 000 btu unit will work as far as static pressure goes. Before asking him to redraw the system, I wanted to get your input.
I’m worried about the capacity of the system in the shoulder seasons. Will the 16000 btu minimum capacity cause problems (3 x 5500 btu at 47F per neep website)? Or is this an ok compromise given my situation?
I considered asking for the 2-ton unit for the second floor, but I believe the increased static pressure capacity at 0.64 WG doesn’t compensate for the increased airflow from 480 CFM to 780 CFM. Was that the right decision?
Thank you a for your help!
P.S. I added the previous duct draft. In the new one, the basement bedroom is supplied by the unit in laundry room. I also edited the title as I think the reference to the pre-existaing gas furnace was unhelpful.
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I have to ask why you are looking at the slim unit if you are replacing an existing furnace? In most cases the existing furnace is oversized, which generally means you can swap it out without much issues with a right sized heat pump with a multi position air handler. I can see wanting some zoning, unless you have some highly variable heat loads such as a sun room, two zones should be enough.
With modulating units you can always do a bit of partial zoning on a smaller area with an in-line zone damper driven by a local thermostat.
For some reason, it doesn't look like your sketch of the HVAC layout made it into the PDF, maybe you can try reposting it.
When you have multiple one to one units, I wouldn't worry too much about min capacity during the shoulder season. You can always turn one of the units off or run one in heat mode and one in dry mode if you have humidity issues.
2nd floor units is mostly needed for cooling. It will provide some heat when the bedroom doors are closed but most of the time, the main floor unit is what provides the heat load. I would not oversize it. You might even be able to save a couple of dollars and go for a non-hyper heat for the 2nd floor provided it will continue to operate at your design temp.
I've done ducting with 0.15" rated units and it is not a problem. Does require a bit of extra planning, oversized ducting and a big air filter but it can work great. I wouldn't try to run it up from the basement from the 2nd floor though, best if you can mount the unit in the ceiling of the hallway or in a closet.
Hi Akos, good question. I think I should have omitted that from the title. The short answer is it was terrible ducting.
We have issues with air flow to a number of rooms and all the second floor ducting is in the attic. There aren’t enough returns upstairs. I could go on for a while, but it’s just simpler to start from scratch.
We do have a sunroom, the area labeled as a playroom is also a sunroom. The sunroom faces rougly east and the front west. We constantly have variations in temperature between rear and front in the summer. Would that be resolved by having two well designed zones (1 (basement + 1st floor) and 1 for 2nd floor)? Or are we better with the 3 zones as designed?
I tried sending the pdf again with the same result. I see the colored areas on the original pdf, but it doesn't seem to transfer it when I upload the document. I will try it another way tomorrow.
I thought about your suggestion of moving the unit supplying the second floor. The layout of the house makes it reall hard to do without losing too much headroom so we revisited the project.
The line of thought was to go for two units:
a 40MBDQ18---3 unit that would cover laundy room + bedroom in the basement and the rooms above on the first floor. We would try to put the Kitchen on it as well, static pressure permitting to better balance the loads on the two units.
a 40MBAAQ24XA3 air handler for the remainder of the house.
We would get a worse coverage of the design load, but reduce the number of units and have a more powerful motor to supply the second floor.
Does that make sense?
It makes sense to put the 2nd floor onto a high static unit in the basement. This would definitely make ducting easier as they can be smaller.
In terms of actual sizing, your heat loss was somewhere between 35k and 45k. The 18K+24k you propose deliver around 80% of their rated max capacity at -5F, so you are looking at a total of right around 35k. Ballpark, but I would add a strip heater (5kW / 17000BTU/h is enough) to the one feeding the main floor for those polar vortex days.
Since the heat pumps sizing is driven by heating needs, the basement/main floor unit will be doing most of the job so it might be better to flip them around and use a 24k for the main floor.
There is an 18k high static air handler you can get for the 2nd floor:
https://ashp.neep.org/#!/product/62695/7/25000///0
Thanks for your input. I wanted to go with the 18K unit high static i your link, but I can't find it anywhere and neither can our installer (I've spent way too much time on the neep website). It would have been perfect.
I guess I have a knack for not being as clear as i want to be. The 24K unit will also do part of main floor as it was ovesized for just the second floor.
I was planning on having the units based on different sections of the house insteal of levels.
So first unit doing basement + part of first floor. Second unit doing second floor, some of first and even a room in the basement.
According to my calculation, the 18k unit is able to supply 100% required BTU at -13F for the area it would cover and the 24K unit about 55%.
I was thinking of putting a strip heater only on the 24k since it will do most of first floor. Since I'm sharing the first floor with the 2 units, would you put strip heater in both or just the 24k?
I think that should work. I would try to group the loads so that it matches. For example, you want rooms facing west on one unit as they will see more solar gain in the afternoons.
Since the 18k can cover your design heat load, I don't think it needs any help. I would put a strip heater into the 24k unit to make up for the shortfall in output.
Noted! One more question presented itself after talking to our engineer, he rightly pointed out the loss of zoning for the second floor with the single zone 24k unit option.
We currently have quite the stratification of temperature in the summer and the ducting doesn't keep up as it is in the attic. It's one of the main reasons for the change of systems + ducting.
He suggested we look into a Greenspeed unit instead of the 24k unit, like the 25VNA436A*030* as a compressor to have two zones, one for second floor and one for parts of first + basement.
https://ashp.neep.org/#!/product/32736/7/25000///0
I see that the unit has a fair amount of modulation, but I have read over time on GBA about the downsides of zoning with one unit. It will also be significantly more expensive, although cost is not a huge factor.
So my question is: Do you think it will provide a significant advantage over the single zone 24K unit?
Our goal would be to have the 2nd floor reasonably close to first and fairly even temepratures, give or take 1C ideally.
Generally 2nd floor cooling problems are cause by air leaks and lack of sufficiently big high mounted air return. By moving your ducting and air handler out of the attic, you are fixing a lot of your air leaks, and from what I remember there are some pretty large returns in the HVAC design. I would just make sure the return grill is near the top of the wall.
Most basements are pretty low cooling and heating loads. This means you can put a bit of simple zoning. You can put a zone damper on the trunk feeding the basement and connect it to its own thermostat. This thermostat doesn't have to be integrated with the ducted unit at all. You can adjust the zone damper closed position to provide a small amount flow for air circulation. Even if the damper is set to close all the way, these inverter based heat pumps can easily modulate around that bit of pressure change without issues and the basement airflow will be delivered to the rest of the house. This way if upstairs is hot, you can crank the unit without worrying about over cooling the basement.
The Greenspeed units are not quite up to heating in Zone 6. Their modulation is also not that great, I think the Neep info is wrong. Turn own on most is 1:3.