How to make the most of WaterHeater:HeatPump heat output?

I have a model which contains a WaterHeater:HeatPump which can provide 2,000 kWh per year from the DX coil. However this leaves the dwelling with unmet hours in the winter so it needs some extra heat input.

The way I see it, my options are to use an electric element in the WaterHeater:HeatPump itself, or to have a secondary tank which acts to supplement the output of the heat pump when required. My problem is that both of these approaches mean that the heat pump only provides around 1,000 kWh over the year, since even when heat is available from the heat pump, there is a good chance that the tank has already been heated by the element.

Is there any way around this, or is this just something I'll have to accept?

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Jamie Bull

asked 2015-01-13 09:45:37 -0500, updated 2015-01-13 10:10:25 -0500

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You could put the secondary tank on the supply side of a second plant loop. Then connect the second plant loop to the first using fluid to fluid heat exchanger. Try to arrange the controls so the fluid HX only requests flow when the primary tank cannot meet its setpoint. Locate the HX just downstream of the primary tank and use component setpoint based operation.

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Archmage

answered 2015-01-14 07:16:30 -0500

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If you put a second tank in series on the same branch after the heat pump water heater tank, and make it small, it should only come on if the heat pump water heater tank is not up to temperature. This is similar to the instantaneous water heater that is used in the EnergyPlus example file SolarCollectorFlatPlateWater. That file also has a tempering valve in case the solar tank is too hot, but that would not be necessary for this application.