[RE-wrenches] Heat pump water heaters (was: On demand Electric with pre heated water)

[Kurt Albershardt]

On 2/9/09 3:36 PM, wlbryce at pineridgeproducts.com wrote:
> We are trying a HeatPump heater as backup for a solar hotwater system.
> Trying to get away from propane.
>
> http://www.airgenerate.com/products/specs.html
>    
If that 7,000 BTU/hr rating is correct, it equates to 2kW of resistance 
heat, which is typical of under the sink bathroom applications in 
restaurants and gas stations.  Might work as makeup heat if you have a 
large enough tank.  The Stiebel-Eltron unit we used draws 36 kW (!)

The COP of compressor-driven heat pumps drops as output temperature 
increases (which is why they make such a nice pairing with radiant floor 
heat.)  Even at the lower COP that a 120-140F hot side produces, it 
would be a huge win in the southwest deserts if it pulled that heat from 
the interior air.  Remember that in the winter, you are stealing heat 
from the house to heat that water.   If you put the evaporater outside 
like Solahart does, you are then pulling heat from the outside in 
winter, which is harder work.  Ideally, you'd want a vented box that 
could use either inside or outside air depending on the season.

Better yet, find a source of waste heat like refrigeration or 
dehydration.  We are working on a commercial project where we will 
recover heat from the kitchen refrigeration systems (12-14 compressors 
overall) to heat domestic water.  I'd love to see this applied on a 
residential scale.