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> Also just a guess: is the story with heating that your heat pump is sized based on your max heating load, and so losing up to 25% of its capacity on the colde

> Also just a guess: is the story with heating that your heat pump is sized based on your max heating load, and so losing up to 25% of its capacity on the coldest days means it would never be able to catch up? The unit I have has a HSPF2 rating of 10.5. It's very efficient at the transfer of the heat available. Heat pumps can extract more energy than the consume. New heat pumps can operate over 300% efficiency. The best furnaces are generally in the mid-90 percentages. I have a dual-fuel system and can set where the heat pump locks out. The system is rated to a COP of 1.75 at 5F. Meaning it's still over 100% efficient at that temperature. The unit will operate down to -22F but would be at, likely, less than 50% capacity of output. To save on wear and tear of the unit, however I have my setpoint lockout at 15. It would have to be significantly below 0 for days on end to have a problem. But even then the furnace would be providing primary heat. There are people in my area that heat their entire home with ductless heat pumps [0]. My unit is rated at 48k BTU. The furnace at 100k BTU. I only really "need" the furnace for emergency. But we do have sub-zero highs rather often and during those days the heat pump would be running continuously. I also have an 80 gallon hybrid heat pump water heater which has saved us thousands over the last few years. 98% of the year it only uses the heat pump. [0] https://www.energyvanguard.com/blog/can-a-heat-pump-work-in-minnesota/ https://www.energyvanguard.com/blog/can-a-heat-pump-work-in-...

mindslight 3y ago
It seems you took my comment as an attack on heat pumps, which I certainly didn't intend! Such is life in our needlessly polarized information environment. Thank you for the description of your system though! I've got thoughts of a heat pump in my future, and I will be well involved in the design if I don't outright DIY, so it's nice to hear about other people's setups. By "lose 25% of its capacity", I mean because with curtailment it could be off up to 25% of the time (assuming the same curtailment schedule as summer). The focus on "coldest days" is because that is when your house needs the most heat (regardless of source type). Based on the climate, I assume your heat pump was sized based on the minimum it needs to supply for heating loads on the "design day" (coldest day generally expected for your climate), meaning on the coldest days it requires most of its capacity to keep the house temperature steady. Whereas it's oversized for summer so it can still keep up even if it loses a bunch of capacity due to curtailment. I'm a bit surprised with a solid backup heat source that winter curtailment isn't more attractive to you (it would also help exercise your backup heat). Does it really not make financial sense? Or is it more of an idealistic thing of not wanting to burn oil/gas as much as possible?

mindslight 3y ago

It seems you took my comment as an attack on heat pumps, which I certainly didn't intend! Such is life in our needlessly polarized information environment. Thank you for the description of your system though! I've got thoughts of a heat pump in my future, and I will be well involved in the design if I don't outright DIY, so it's nice to hear about other people's setups. By "lose 25% of its capacity", I mean because with curtailment it could be off up to 25% of the time (assuming the same curtailment schedule as summer). The focus on "coldest days" is because that is when your house needs the most heat (regardless of source type). Based on the climate, I assume your heat pump was sized based on the minimum it needs to supply for heating loads on the "design day" (coldest day generally expected for your climate), meaning on the coldest days it requires most of its capacity to keep the house temperature steady. Whereas it's oversized for summer so it can still keep up even if it loses a bunch of capacity due to curtailment. I'm a bit surprised with a solid backup heat source that winter curtailment isn't more attractive to you (it would also help exercise your backup heat). Does it really not make financial sense? Or is it more of an idealistic thing of not wanting to burn oil/gas as much as possible?