COP vs HSPF
Which is more important. I am looking at two heat pumps:
1- HSPF 12.5 / COP 17F 2.46
2- HSPF 8.9 / COP 17F 2.7
I was very surprised that the lower HSPF HP actually had a better cold weather COP, which I believe would mean it will be cheaper to operate at that temp. Is HSPF just the ability for the heat pump to extract heat at lower temps then. In my case my utility supplier switches my HP over to my furnace at 10F and below.
Let's draw you a picture.
The HSPF is a bad metric. But we will need another point to draw a line.
If your contractor can't help you go to AHRI for the second Capacity point.
Your AC guy can get you all these points easily.
Try this. You can also get COP"s(BTU/WATT) and heating capacities on these pumps @ 47 degrees.
Example: For a 3 ton pump you might have:
@47 36,000 BTU/HR and a 3.4 COP
@17 22,000 BTU/HR and a 2.5 COP
Now get out your graph paper out.
X axis is outside temperature,lets use 0 to 70.
Y axis is Load/ Capacity in BTU/Hr, lets use 0 to 60,000
To Plot your two pumps connect the dots to make lines. In reality they will not be perfectly straight but this will get you close. Now you can see the Heating capacity curves of the two pumps.
Now, lets add a third line to our graph. Our Heat loss line.
Point 1. Lets say @ 65 outside your house has no Heat Loss. Put a point here (65,0).
Point 2. Lets say @ 0 outside your house has a heat loss of 50,000 BTU/HR, PUt a point at (0, 50,000)
By connecting these points you get a picture of what happens. The colder out it gets the more heat loss you have. Yet, the colder out it gets the less capacity you have. We call the point where the heat load crosses the capacity the "balance point". Below this point you need backup heat since your pump cannot supply enough. In your case your Heat Pump will turn off and run your gas backup.
Fifty Bucks says you will have to turn your heat pumps off at a much higher outside temperature than 10 degrees when you complete this graph. I'll bet it will be closer to 30-35 degrees when you hit your "balance point".
If you want to get crazy with your graph you can also figure out at what outside temperature it is cheaper to run your gas based on the price of Gas vs price of electric. To do this figure 100,000 BTU per CCF of Gas and multiply this times the efficiency of your Furnace to figure your delivered $/BTU with gas.Recall that COP is BTU/Watt. So if gas is $1.00 CCF and Electric is $.10 per Kwh (1000 watts) You have all you need to use your graph to determine your "Economic Balance Point". The point it is cheaper to run gas.
With low gas prices your graph may tell you turn on your gas on before you have to because it is less $/BTU.
I hope this helps you,
HSPF takes into consideration how often it will go into defrost. So the lower HSPF may have to use defrost a lot more, and cost you more.
Always helps to know the BTU output at those lower temps.
Thanks ACBD, that's good advice. Will do this tonight as I have all the numbers, I even have the balance point charts.
Originally Posted by AC Bad Dog