Sanity check on furnace to cold climate heat pump
Good afternoon!
I’m an architect looking for a sanity check on this load calc and equipment selection for my own house before pulling the trigger on a furnace to cold climate heat pump conversion. This is the last gas appliance in the house, so trying to take advantage of incentives before they are gone. I understand the principles of this process but this is a bit in the weeds of mechanical selection.
Climate zone 5A with winter design temperature of 7.5F to 10F depending on the weather station. I have been using the BetterBuiltNW tool to model the house parameters and feel fairly confident in the numbers. Been able to verify the ballpark based on winter furnace runtimes and summer loads based on window AC capacity. Plans and load calc PDFs should be attached. I’m planning on dropping from a 60k BTU furnace with a 2.5 ton AC to just a 24k BTU cold climate heat pump. This downsizing seems to allow for proper existing duct sizes since the furnace was so large.
Equipment selection and amount of supplemental heat are the major questions. The Mitsubishi SUZ/SVZ combo seems to be a nice balance between reliability, capacity in winter, and decent efficiency. This combo meets all but 2.8% (159 hours) of heating load – though it does low load cycle approximately 25% of the time. The existing furnace circuit is a 20amp which could be used for minimal supplemental heat. The existing electrical panel is 150amp and I don’t want to oversize a heat strip that rarely if ever will get used.
There are options (Bryant, Lennox, etc.) suggested by contractors that are more efficient at warmer temps, but with capacity drop off before the winter design temp. I have been using the NEEP Heat Pump tool to ballpark these other units but seeing major capacity loss (18k BTU) at around the 5F mark.
Questions: Any glaring calculation issues or other concerns? Does this sound appropriately sized where we will not suffer extended capacity issues during winter? Would you run a 30 amp or larger heat strip circuit? Any other units or brands to consider – my understanding is the defrost and variable capacity of the Mitsubishi unit is one of the best.
Thank you!
Aaron
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Replies
This post discusses how to do in-depth modeling:
https://www.greenbuildingadvisor.com/question/how-to-model-and-predict-electricity-usage-for-a-heat-pump
Thanks for the reference article. I have been using the NEEP database for referencing heat pumps proposed. It gives a great overview between specific units. Ultimately, there are so many models and units available it is overwhelming. For electric panel capacity sake, I don't see any reason to not minimize the usage of any resistance heat.
Been contacting multiple HVAC contractors for quotes and getting the same sort of responses. They seem dismissive of ownership over unit selection that actually meets loads at the winter design temperature.
Wondering if there are other recommendations beyond the Mitsubishi units I should be exploring as well.
This is pretty similar:
https://www.greenbuildingadvisor.com/question/38marb
I would make sure to spec a unit with vapor injection. Lot of the domestic units you see loose capacity as they don't have a vapor injection compressor.
I would also do some search of the newer R32 or R454 units as these are the new standard (ie SUZ-AK24NLHZ) . Lg and Midea also make some nice cold climate units with multi position air handlers that would be a direct swap for your furnace.
Thanks Akos. I did read this one again and found it similar as well with some helpful information. It reminded me of a question you may know the answer to. My understanding is some manufacturers have timed defrost vs a more intelligent demand defrost. That is why I was a little wary of the Gree Flexx/Mr Cool Universal option. My understanding is the cold climate output BTU ratings do not take defrost cycles into account so having timed defrost could lose BTUs when defrost is not necessary.
This Mitsubishi model I have asked for quotes on is that R454 version. The LG one looks intriguing but not finding many companies in my area listed on LGs website (also trying to keep to locally owned companies).
Now digging into the LG units I'm really hoping a can get a competitive quote for the KUSXA241A/KNSLA241A combo. It not only requires less supplemental heat 1% (which is well within the load calc range), but also modulates more with less low load cycling. Thanks for the pointer.
Aaron “I don't see any reason to not minimize the usage of any resistance heat.”
There are a few problems with this thought.
1 You need to be careful. Going big to get more heating capacity is great but now you are likely hugely oversized for your cooing load. If you are in a humid climate this is very bad in that your run times will be very short you will not run long enough to remove much moisture. This forces you to set a lower temp on the thermostat and you end up being cold and clammy.
2 Oversized system must cycle on and off much more often this is bad for efficiency and shortens the equipment’s life. The first few minutes of every cycle is wasted energy while system build up pressure it needs to work.
3 Comfort the unit is cycling on and off because it has gotten too far away from its set point so you are less comfortable.
4 Start up distraction. When the unit starts or stops you notice the change and it becomes a distraction. A smaller unit will have longer runtimes and move less air when it is running.
5 Sooner or later every system will have a problem and go down when this happens having resistance heat make this almost a nonevent instead of a full-blown crisis.
6 Supply air temp. an oversized variable speed HP will never run at its max speed. When a HP is at max speed it makes its warmest air. So, an oversized unit will blow more air around the house at a colder temp making the home less comfortable.
I think undersized is better for the home owner but if to many do this it could become a big problem for the grid operators.
Walta
A lot of these problems go away or are diminished with a variable-speed compressor that is capable of modulating.
Because of concerns about the effect on the grid of resistance heating, California is now requiring that heat pumps be sized to meet the heating load without resistance between the 99th percentile temperature and the annual mean minimum.
I've been living with some inverter window units for cooling while I plan this out and have really come to understand the benefits of this technology. It is one thing to read about it, but much better to experience the operation.
The soft start, smooth ramping and constant cooling to match load is much better than the old whole house traditional AC system design.
Absolutely checking to make sure I'm not so oversized that the cooling performance isnt too terribly oversized. The NEEP database has been very helpful for this. The Mitsubishi does low load cycle some but just meets the design load. There is a better LG model that I'm trying to source quotes for.
It is getting hotter and more humid so some supplemental dehumidification may be needed no matter what. 2, 3, 4 seem to be handled by the quieter variable speed/capacity communicating units. 5 can be handled by a few space heaters, which is better for panel capacity calcs than one circuit dedicated to a full load resistance element, but I see your point. Much easier to flip to emergency heat than pull out some space heaters. Currently with the gas furnace there is not backup heat either. 6 totally makes sense, which is why I'm trying not to oversize or undersize.
I appreciate the input and you have given me some good things to consider.
Variable speed equipment has it limits and oversized equipment will bump into its min speed limit a lot more. I think you will find even the best variable speed units only change the speed of the compressor and while having ECM motors for the indoor blower they do not slow the indoor fan speed when compressor slows down. They move the same volume of air just at a lower temp.
Space heater are a very bad plan. Each year, about 1,700 fires, 80 deaths and 160 injuries are caused by portable space heaters as estimated by the U.S. Consumer Product Safety Commission (CPSC).
Also note by installing oversized equipment you are shifting your run time to have less when it is warmer and more when it is colder. When it is 40° you likely will have a high COP 6 maybe more. When it is -20 you will be lucky if the COP is over 2.
Walta
I wouldn't want to rely on space heaters on a regular basis, but they would be fine for a emergency scenario while awaiting a unit fix.
My thought process is that I can include about 10kbtu in supplemental resistance heat on the existing 20 amp furnace circuit, so I'd only need 2 additional plug in space heaters to keep the house survivable for a few days in the event the unit malfunctions. If I switch to a 30amp circuit then I would only need 1 space heater.
10F is the NEEP winter design temperature, but I think my load calc was based on 7.5F per Ashrae tables a year or two ago. I hope to never see -20F! Attached is the NEEP estimate for modulation for the Mitsubishi combo and the LG option.
I don't think these look oversized, but it does appear the LG is better matched and/or more capable.
I understand the OP has existing ducts, but one of the ways I would fight the max/min sizing issue is more than one compressor/inside unit. Not only do smaller units usually have lower minimum output, but you could arrange your control strategy to take advantage of them
Like an upstairs downstairs split would in summer allow you to perhaps run just the upstairs unit until it was very hot. Perhaps the reverse in heating season.
I had this same thought and it would be technically possible to split the floors due to dedicated returns. There are practical considerations that I think preclude this approach - first is budget as it would almost double, second this likely would push to a electric panel upgrade (again budget) and third space within the basement utility closet is tight.
This is the ideal solution IMO.