Heating Water with a Heat Pump
This is something I’ve wondered about off and on for a while: If I’m heating the house and domestic hot water with heat pumps, is it more efficient to do it in a single stage — drawing heat directly from outdoors — or in two stages, where the house is heated with heat drawn from outdoors, and the hot water is heated with heat drawn from inside the house (ie a standalone heat pump water heater and a heat pump for heat).
It seems that in the cooling season it’s clearly better to draw the heat from inside the house, you basically get free air conditioning — although the counter-argument is that it takes less energy to heat water using 90F outside air than 75F inside air.
In the heating season the heat that is going into the water heater was brought into the house by the heat pump. The question is, if you need water at say 120F, and it’s 20F outside, are you better off pumping the heat from 20F to 120F, or pumping it first from 20F to 70F and then from 70F to 120F.
I’m thinking about air-to-water heat pumps that have a domestic hot water option, so let’s assume that the operating properties of the heat pump are going to be the same regardless of how it’s configured.
Thoughts?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
> The question is, if you need water at say 120F, and it’s 20F outside, are you better off pumping the heat from 20F to 120F, or pumping it first from 20F to 70F and then from 70F to 120F.
I would think that a second piece of equipment (standalone HPWH) is going to have more overhead to run it, which would mean that more of the overall heat going to the water is effectively from the overhead waste heat of the unit itself (ultimately), whereas unless the CoP of the outdoor unit goes below 1, it still wins.
The math for series heat pumps is:
SystemCOP=1 / ( 1/SplitCOP + 1/WaterHeaterCOP - 1/(SplitCOP*WaterHeaterCOP) ).
So using actual numbers. UEF on most hybrids is between 2.5 to 4. So lets say 3. A decent heat pumps should also hit a COP of 2.5 at 17F so the overall COP is 1.7.
Looking at Chiltrix extended curve, to make 130F water at 17F the COP is 1.8.
So overall, it doesn't look like it matters much. Something like the Sanco unit does get slightly better COP but not by much.
Worst thread title rename ever.
GBA folks, please stop renaming threads!