Heat Pump Questions
I own a twenty year old 2 zone HVAC house (upstairs and downstairs) in northern zone 5, Southern NH. One of the condensers died this summer and I’m planning on replacing it with a modern heat pump system. I know it’s better to have the furnace replaced at the same time, but my propane furnace was replaced in 2015, it’s direct vented, 95% efficient, and it’s running great. So I don’t want to replace it if possible. A couple HVAC vendors have recommended the Bosch 2.0 Inverter ducted series heat pump and the GE Connect heat pump. Both are variable speed compressors, with the GE being able to heat at lower temperatures (-22 F vs. -4 F) and the Bosch having a better COP. Any recommendations here on reliable brands or another heat pump system that might be better?
I understand it’s important to not oversize the cooling capacity, but if the heat pumps can be modulated during the cooling months, is performing an aggressive manual J calculation and getting a really accurate load calculation less important?
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Try this as a first pass: https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler. It’s for the whole house, so won’t answer what the zone’s heat loss is, but can keep things reasonable. If you have a backup furnace, I’d size it exactly for the cooling load and let the furnace handle the coldest days, even though the furnace is likely significantly less efficient. How did the AC work prior to failure? Low run times on hot days and low humidity?
Unfortunately we moved in to the house last September and it failed before the summer started this year so we didn't really get a good chance to feel it in action.
Most fuel suppliers will provide historic usage data to the new owner if they ask nicely.
Walta
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Whatever you decide, consider adding a whole house dehumidifier.
That's better in many ways, especially rainy or humid days that don't require lots of cooling. It also ensures that getting the exact sizing and balance of latent/sensible cooling doesn't matter (because you aren't trying to dehumidify with your air-cooling device).
Also, although this hasn't been discussed as much, I think those could be very useful for high thermal mass houses in places with high diurnal swings (hot afternoons, cold nights) where the diurnal swings mean that air cooling doesn't run long, but dehumidification is still required.
Hmm that's an interesting thought. Any articles on here you would recommend that I can research that topic? It would be nice to see how it integrates into the system.
This might be a decent starting point: https://www.greenbuildingadvisor.com/article/ventilating-dehumidifiers-for-improved-indoor-air-quality
One thing that has been an experience for us is that our household's comfortable temperature range is much more similar when humidity is low. ie when humidity is high, I want it like 5° cooler, when humidity is managed we are all comfortable at the same air temp.
Good point. Some AC/Heat pumps use electric resistance to reheat while cooling to accomplish similar dehumidification.
Right on. Do they have a humidistat set point that is controllable separately from the air temperature?
The GE Connect is made by Gree and is a rebadged Gree Flexx. The Mr Cool Universal is the same unit as well and made by Gree.
Thanks for the info maybe I'll look at reviews for those then
I would look at the minimum heating capacity on both units you are considering. That is how low can it modulate down to. You are right that oversizing matters less if it can run at low power, but if it can go down to 20% and one can go down to 40%, that ability to go down to 20% is a huge advantage.
Have you been able to find extended data for them, with COP and capacity at low and high, in different outdoor temperatures? If you posted links to that data some of us might enjoy looking at it, and might have useful comments about comparing them.
https://imgur.com/a/S36cJYo I tried to highlight the relevant apples to apples comparisons.
Bosch 2.0: 28,500 BTU cooling, COP: 3.6 / 21,100 BTU heating @ -4 F, COP: 2.5
GE Connect: 25,500 BTU cooling, COP: 2.6 / 23,400 BTU heating @ -5 F, COP: 2.6
At similar indoor/outdoor conditions, the Bosch has a better COP for cooling. Similar COP for heating but GE has slightly more capacity. From what I'm reading online is the Bosch has capacity down to - 4 F but seems to realistically be usage only down to ~ +27 F. I can't find much anecdotal reporting on the GE Connect.
I am not a fan of the dual fuel systems mostly because one would need a will of iron to not turn on the propane furnace that will blow 130° air around your house instead of the 88° air the heat pump will be making. Yes, your expiation can and will adjust so you will find living with a heat pump is comfortable but it takes time. If all someone needs to do is flip a switch and get the really hot air not many have that level of self-control.
If you happen to be that person and give that you already own the furnace and the quality of the HPs you are considering the furnace run time will only be a few hours a year assuming the furnace is set not to run as part of the defrost cycle, getting the installer to set it that way may take some convincing.
Walta
FYI We have a dual fuel system with a Mitsu hyper heat pump. Our thermostat has in-duct temperature sensors, and I've seen 115F air temps from the heat pump.