Where is the most efficient place to get the heat for hot water from? – GWC Mag

This is a question I’ve been kicking around for a while. It came back to top of mind because GBA had a review recently of the Sanco2 water heater
( see https://www.greenbuildingadvisor.com/article/sanco2-heat-pump-water-heater )

which provides domestic hot water from a heat pump located outside the building.

The question I’ve been kicking around is, if you’re pulling heat from outside for your hot water via a heat pump, is it inherently more efficient to do it in a single stage with an external compressor, or in two stages with a HPWH that takes heat from inside the house, and a heat pump heating system that replaces that heat from outdoors?

We kicked the question around a bit in January in this thread: https://www.greenbuildingadvisor.com/question/heating-domestic-hot-water-with-a-heat-pump-one-stage-or-two , but didn’t really come to a conclusion. And we touched on it a bit in this thread in March: https://www.greenbuildingadvisor.com/question/minimal-radiant-heating-options . 

So I thought I’d take another whack. I built a spreadsheet model with the following assumptions: 
1. Heat pump COP is linearly proportional to the temperature difference between the source of the heat and the destination.
2. All the heat pumps being used — the HPWH, the home heat, and the dedicated external water heater — have the same performance curve. There’s no reason to believe that any one is going to have inherently better performance.
3. In winter, the HPWH pulls heat from the building, which has to be replaced by the heating system.
4. In summer, the HPWH pulls heat from the building. If you’re not using a HPWH, and instead using a dedicated external water heater, the heat that would have been pulled by the HPWH instead needs to be pulled by the AC.

I’ve attached two graphs showing winter and summer performance. As you can see, the only time the dedicated external water heater gives higher COP is in the winter when temperatures are between about 40F and 70F.

I used the performance line from Chiltrix because I had already entered the data for an earlier question about heat pump performance vs temperature. If you look at their performance graph here: https://www.chiltrix.com/Efficiency-curve.jpg

you’ll see that the assumption that COP is linearly proportionate to temperature difference is a very good one.

I assumed the interior was at 70F and the water heater was at 110F.