Sizing a Heat Pump For Negative-Degree Temperatures
Should a heat pump be sized based on its reduced output at low temperatures? Building a house where the design temp in winter is -5F. Will be a ranch home with a walkout basement and currently looking at a separate ducted heat pump for each level like Mitsubishi SUZ+SVZ. Likely going to add electric resistance backup heat to the air handlers as it certainly gets colder than that sometimes.
These units have about 80% capacity at -15F, not sure what it is at -5F. Is the correct option to size them with the understanding that at -5F they will have reduced capacity, maybe something like 90% and then worry about resistance heat after that? Or would I be better off ignoring that and expect to need resistance heat use at anything below 5F?
Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The big risk with oversized equipment is in the cooling mode. When cooling the oversized equipment does not run long enough to remove moisture from the air. The residents end up turning down the thermostat in an effort to feel comfortable in the high humidity. The classic complaint is the house feels “cold and clammy”.
Have you considered the units with vapor injection every manufacture has its own name for the same technology (Hype Heat)
My system has the resistance heat locked out when the outdoor temp is over 8°f I have 11000 hours heat pump run time and 300 hours of resistance heat. My resistance heat does run on each defrost cycle. Some of the hours were during construction before the HP was on line and when the HP was down waiting for a part.
Having the resistance heat as backup means when the HP goes down it is an annoyance and not the crisis it would be without it.
Walta
Thank you for the response. Yes, my plan is to use the Mitsubishi HyperHeat or the Fujitsu XLTH with backup electric heat in the air handlers. It is difficult without a precise energy model for me to understand whether sizing my heat pumps based on their reduced capacity so they can meet load at -5F is worth the potential issues like you mention, so I was curious if there is standard practice around this.
Standard practice is to oversize the system, but you lose efficiency. Proper practice is to take into account the quality of the building enclosure. A Passive House or a well-done Pretty Good House will be able to coast through the extra-cold periods, while an old, drafty house won't be able to keep up even if the system is designed for an average new house.
How good is your building enclosure?
R15 under slab, R23 fiberglass BIBS in wall + R8 Comfortboard 80 walls, R60 fiberglass attic, average U value .17 windows. I mostly followed your PGH book, although I am a little short on the walls for Zone 5. Thank you so much for that btw, it was a great resource.
You are saying if the enclosure is good enough you would undersize/ignore the lost output capacity at -5 and allow resistance heat to take care of it when it arises to increase efficiency the rest of the year?
There are some "rule of a thumb" numbers of "capacity over outside temp."but for a hyperheat (-15F !) I am not so sure - better check the datasheets. You will have some higher capacity for sure.
Everything else is a question of what HP you pick - what the 80% means relative to the heat load of the house. A big one will need no backup (but it might still be wise to install it)
This a great resource for cold climate heat pumps. They also have simple sizing tool where you can enter your design temp and it shows you how much energy the backup heat will provide. Click on "Advanced Data-Sizing for Heating" in the corner to enter.
https://ashp.neep.org/#!/product/33489/7/25000///0
There is a good info from Mitubishi, their engineer documents have detailed output info at cold temperatures.
https://mylinkdrive.com/USA/M_Series/R410A_Systems-2/Outdoor_Equipment/R410A_Outdoor/SUZ_KA24NAHZ?product&categoryName=R410A_Outdoor
Most of hyper heat units will run at above COP of 1 even in very cold climate so you don't want to disable the heat pump when cold, what you want to do is supplement it with the strip heat in case it is needed (this can be configured in the setup).
In this case, I would size the strip heat to provide the extra capacity needed above what the heat pump can produce in case of those polar vortex days. This tends to be the small or medium strip heat in most case.
You can also do a quick check that the strip heat alone is enough to keep your house from freezing in case there are issues with the heat pump, no point for sizing the strip heat to carry the whole house by itself.
Overall I would size the heat pump to carry your place at design conditions. If the numbers are very close, under sizing slightly is fine, running the strip heat for a couple of hours a day doesn't change your overall energy usage much.
Wow, I have used that neep website before but hadn't noticed that button. That is a very insightful view to see how much backup heat would be needed in a given climate. The other data on short cycling and such is also great to see. Thank you.
I'd probably err on the side of oversizing to avoid resistance heat, but I wouldn't take my opinion too seriously. The resistance heat won't add too many kWh since it runs infrequently, so oversizing probably won't save you money.
But we all need to move to electrified heat. When every house is using resistance heat on the coldest day of the year, I have no idea how a mostly renewable grid is going to handle the peak demand. It's a future problem, and it probably won't be solved by a few individuals, but I still hate to make it worse.