The unit basically runs all day during the day on hot days. The difference is, as you state, it runs long cycles. The 15 minutes it's off is the break in the cycle itself. The unit I have is a communicating unit that "learns" the house in terms of all the internal and external changes. So it knows what fan speed and compressor rate to run to get to the setpoint - it doesn't just ramp up to the highest rate after a curtailment event. So it's basically running at a very low rate all day long, curtailment happens, and it ramps slightly if there's a need to bring any zones back to setpoint - but the thermostat is aware of the curtailment event and, as I was told, responds different after the curtailment is lifted. I talked to both the installer and the manufacturer regarding whether or not curtailment would negatively impact the system (performance, cost, longevity) and the consensus was that unless the curtailment events happened every hour there wasn't any downside to cooling. Heating would be another story, but I don't have any curtailment contract for heating in my home.
And I believe the contract states curtailment events can only happen up to 4 times in a given day and cannot happen more than once an hour.
Ah, so basically the variable speed and feed-forward PID make it so the unit is running all the time, so it's not like the 15 minute curtailment will come right after it kicked on to satisfy cooling demand. Rather your house is continually cooling, stops for 15 minutes, then goes back to it. That makes a lot of sense!
I do find it interesting that these terms are still valuable to a power company. Obviously being able to shed the load for 15 minutes is fantastic, but I would think the system would then just use more energy (say for the next 45 minutes) to make up for it. But perhaps staggering the curtailment times, and paying the debt of more draw from the units that were curtailed by doing even more curtailments, can kick the can down the road enough for the few hours they need to smooth peak load?
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?
> 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.
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?
And I believe the contract states curtailment events can only happen up to 4 times in a given day and cannot happen more than once an hour.