Comment by coryrc
14 hours ago
They don't work well with heat pumps. Heat pumps lose efficiency as the differential increases, so if you try to store heat in a tank, you quickly drop capacity and efficiency.
Versus resistance, which is exactly as efficient at 0°C and 1000°C, and why those storage heaters used to make sense.
(And storage is directly proportional to temperature differential above interior ambient)
Every air-to-water heat pump install will have a hot water tank. So I'm not sure why "don't work well" is the term used.
It is true that heat pumps coefficient of performance drops as the output temperature increases. So you need a proportionally larger hot water tank to store the same amount of energy. So it is fair to say there are tradeoffs. But hot water storage is still a necessary part of most heat pump installs - because peak output of heat pumps tends to be below the heat demand of showers.
Isn't it better to heat the water up to a point (let's say 40 or 50 degrees) and to heat it the rest of the way with resistance heating?
>Versus resistance, which is exactly as efficient at 0°C and 1000°C
It isn't. The difference is smaller than for a heatpump tho obviously.
Older heat pumps had max temperature limits and did often have resistance heaters to get that last push above 60C. Modern household heat pumps will reach 75C while staying above 100% efficient and can skip the resistance heater.
This is partly due to a change in the refrigerant used.
> It isn't. The difference is smaller than for a heatpump tho obviously.
Where does the energy go then?
Home hot water heating in the UK with heat pumps is about 250-300% efficient (slightly lower than the efficiency of home heating but still much better than resistive).
No one is storing 1000C water at home.
It is true that the temperature deltas affects efficiency. You can use the thermocline to draw from the cooler lower portion of the storage tank to push this further. Or less technically, just a bigger tank, though this has some tradeoffs.
In warmer countries they are set up differently can act as free air conditioning by extracting heat from indoor air at the same time as heating water.