Quote:
Originally Posted by jeff5may
...Instead of 120 degF air coming out of registers, you will have 110 degF air, which feels "not so hot". Efficiency may rise a smidgen, but the overall "warmth" provided may be seen as diminished by the average user...
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Getting back to your "smidgen" comment...
The formula for max theoretical heat pump efficiency looks like this:
...where all temps are in Kelvin.
So I did a little spreadsheet with a typical date set. I'm more interested in GSHP than ASHP and forced air heating (which in my humble opinion is obsolete), so the data set is typical for GSHP scenarios, but the basic principle still applies.
Then I graphed
Max COP (values are %) vs
T warm F (AKA: condenser temp) and here is what it looks like:
The first thing to understand is that the efficiency is theoretical, and will never be reached, but it is the theoretical efficiency that lies behind the real world efficiency.
Next to observe is that the efficiency gain from decreasing condenser temp from 120 to 110 is not as "smidgen-ish" as one might guess. Even more important is that further reductions of condenser temperatures enjoy an accelerating efficiency curve.
So how dd we take advantage of the accellerating efficiency curve?
Reduce heating requirements:
- Reduce infiltration
- Increase insulation
- Reduce heated area of the house
- Reduce thermal bridging
- Increase solar window gain
- Reduce window heat loss (high performance windows, reduced window area)
- High performance doors
- Locate all heat ducting inside the heated envelope
- Locate water heater inside the heated envelope
When that is done, lower temperature heating becomes possible.
- Increase area of heating coil or area and efficiency of radiant surface
Then you are moving out of the smidgen area and moving into a more exciting area of increasing effficiency.
Best,
-AC