Heatpump COP vs outdoor temperature curves?

arcasOctober 21, 2011

Hi. Earlier this year I replaced my 20 year old gas furnaces with a dual-fuel setup. Currently the HP-gas switchover temperature is set for 40 degrees but finding the proper switchover point really needs to take current energy prices into account as well as knowing the heatpump's COP curve.

My setup:

Carrier 25HBC324A003 heat pumps

Carrier 58STA070 80% gas furnaces

CHPHP3017ATAT coils

Does anybody have HP temperature-efficiency data for this setup?

I'm in central KY. Electric rates are around 8.5 cents/KWH and gas prices are currently $5.64/Mcf.

I'm guessing that at these prices, gas will be cheaper at most temperatures:

1 Mcf natural gas = 1,012,000 BTU * 80% AFUE = 809,600 BTU usable

809,600 BTU = 237 KWh of 100% resistive heat = $20.14 at my current rate.

20.14 / 5.64 = 3.57

So the HP only makes financial sense at temperatures where the COP is > 3.57. Any idea what outdoor temperatures that corresponds to?

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With electricity that cheap, and only having an 80% efficiency furnace, I would think you could go down to 30 with the heatpump.

If you search carefully, tigerdunes has posted a nice fuel calculator that helps comparison.

    Bookmark   October 22, 2011 at 7:18PM
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There are two temperatures to deal with. The first is the uppermost that gas will be used. The second is the lowest the HP will come on. These temperatures should overlap by X degrees for both comfort and cost.

So you might try 38* for the upper and 28* for the lower. This means that if the HP cannot keep you warm, the gas takes over. Below 28* it would be gas all the time.

    Bookmark   October 24, 2011 at 4:43PM
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Hi. Thanks for the responses. Yeah, I was aware of the two cutoff temperatures from programming my thermostat. In fact, that's what started me down this path: if I'm looking to minimize heating costs, then I need to configure the thermostats to choose the fuel source that offers the lowest cost-per-BTU at a given outdoor ambient temperature.

Over the weekend, I was able to find a document from Carrier that, among other things, contained measurements from various HP-coil-furnace combinations at the standard 47F and 17F temperature points. While my particular furnace wasn't listed, I can glean a ballpark estimate for my configuration: At 47F, my HP's COP is probably somewhere between 3.6 and 3.7. At 17F, the COP drops to somewhere around 2.5.

So given the energy prices and the quick COP calculations made in the first post, my HPs appear to be cheaper than gas at 47F. As the outdoor temperature decreases, the COP drops. At some temperature, the COP drops below 3.57 and my 80% gas furnaces become cheaper per BTU. It's not immediately clear how COP behaves with temperature but assuming it drops linearly, the break-even temperature would appear to be between 43F and 45F.

So from a purely economic point of view, it seems the thermostats should be configured so that only gas heat is used below 45F regardless of whether the HPs can provide usable heat down to 30F or 25F...

    Bookmark   October 25, 2011 at 9:58AM
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I made an effort to see if I could find an AHRI matching number for your system. I could not. You might check with your installing dealer to see if they have or can research the number.

I will say this and you probably do not want to hear it.

Your HP is a low end base series 13 SEER model. Based on the various configurations I looked at on AHRI, I would expect your COP is around 2.5 at mid 40s. That's a big haircut from 3.5 COP.

You might want to take a look at the fuel comparison calculator on warm air.com. Keep in mind it will give you a close BTU cost comparison. Use as a guide only.


    Bookmark   October 25, 2011 at 12:30PM
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"So from a purely economic point of view, it seems the thermostats should be configured so that only gas heat is used below 45F regardless of whether the HPs can provide usable heat down to 30F or 25F.."

Obviously each situation is different. When you use a heat pump, it usually helps to upgrade the efficiency some because you take double advantage of it. I would suspect that most people using a newer heat pump are Seer 15 or better.

    Bookmark   October 25, 2011 at 7:23PM
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