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View Full Version : At what temp should a heat pump be set at?



mreuter
02-02-2008, 08:33 PM
First, some background-a couple of years ago I had a heat pump installed and used my natural gas furnace as a back up, I am a carpenter by trade and not that up-to-date on HVAC. After it was installed, I never got the feeling that it was installed or working properly, installer came back a few times and adjusted some things, but I could never tell the difference but what did I know. This winter came around and I still didn't feel right about how it was working, (my energy bills confirmed this) called the installer and he wouldn't answer or return my calls, after a couple of months I called a different heating contractor, they came out and installed a dual fuel kit so that the heat pump and the back up didn't run at the same time and it seems to be working fine now.

Here is my question: I live in NW Iowa, we set the outdoor thermostat at 20 degrees, indoor thermostat is set at 70. Today the outdoor temp was 22 and the heat pump was running, I put a probe from a digital thermometer into a floor register and it stayed steady at 79 degrees, for comparison, I turned on my gas back-up and the floor register temp was at 98 degrees. At what point does the heat pump not save me money because it has to run longer? Should I raise my outdoor thermostat?
Thanks for any replies.

Martin Holladay
02-03-2008, 06:27 AM
MReuter,
You wrote, "I put a probe from a digital thermometer into a floor register and it stayed steady at 79 degrees. For comparison, I turned on my gas back-up and the floor register temp was at 98 degrees." Totally normal. Heat pumps will send air out of the registers at a much cooler temperature than a gas furnace. It doesn't mean the equipment isn't working. Don't judge the performance of the equipment by the temperature of the air coming out of the register. (Although it's good you measured.) The question is, does it satisfy the thermostat setting?

mreuter
02-03-2008, 08:20 AM
I realize that, I guess what I meant was, if my indoor temp drops one or two degrees and the heat pump comes on pumping out 79 degree air, whereas the back-up would pump out 98 degree air, would the heat pump run twice as long as the back-up to raise the temp two degrees? At that point, am I not saving money, or could a heat pump run 5-10 times longer than a gas furnace and still be cheaper to use?

BillHartmann
02-03-2008, 11:24 AM
I realize that, I guess what I meant was, if my indoor temp drops one or two degrees and the heat pump comes on pumping out 79 degree air, whereas the back-up would pump out 98 degree air, would the heat pump run twice as long as the back-up to raise the temp two degrees? At that point, am I not saving money, or could a heat pump run 5-10 times longer than a gas furnace and still be cheaper to use?

How long it runs, by itself, does not tells anything.

You also need to know the rate of fuel usage for the furance & heat pump and also know the incremental cost of the gas and electricity.

Kevin Ambrose
02-06-2008, 11:18 AM
On a duel fuel set up, I would set the heat pump so it would not ever go into defrost cycle, usually around 39deg outside air temp.

gregoryj
02-06-2008, 04:42 PM
On a duel fuel set up, I would set the heat pump so it would not ever go into defrost cycle, usually around 39deg outside air temp.

Is that because of the electric resistance heat required to defrost? What would have to be the price differential between gas and electric to make running the pump below 39F cost effective? In some areas of the midwest electric rates are very competitive with gas. If the OP set the heatpump to not go into defrost cycle then he would be using his gas backup essentially 100% of the time for at least 2 months straight in his NW Iowa location.

Edited to add: I looked up our electric rate and it is 4.6 cents per KWH. If his rates are similar would it make sense to run the heat pump in the defrost cycle?

Martin Holladay
02-07-2008, 04:46 AM
Gregory,
When an air-source heat pump switches to electric resistance heat, that isn't a "defrost" cycle. It means that the outdoor air temperature has dropped so low that the heat pump is unable to collect enough heat from the outdoor air to maintain the indoor thermostat setpoint; so it switches over to (expensive) resistance heat to keep the house warm.

One therm of natural gas usually costs between 80 cents and $1.20, depending on where you live. If you burn a therm of natural gas in an 80% efficient furnace, it produces 80,000 Btu, equivalent to 23.44 kwh. Where you live, that's about $1.08 worth of electricity. If you can buy natural gas for cheaper than $1.08 per therm, then heating with natural gas is cheaper than heating with electric resistance heat.

gregoryj
02-07-2008, 11:44 AM
Gregory,
When an air-source heat pump switches to electric resistance heat, that isn't a "defrost" cycle. It means that the outdoor air temperature has dropped so low that the heat pump is unable to collect enough heat from the outdoor air to maintain the indoor thermostat setpoint; so it switches over to (expensive) resistance heat to keep the house warm.

One therm of natural gas usually costs between 80 cents and $1.20, depending on where you live. If you burn a therm of natural gas in an 80% efficient furnace, it produces 80,000 Btu, equivalent to 23.44 kwh. Where you live, that's about $1.08 worth of electricity. If you can buy natural gas for cheaper than $1.08 per therm, then heating with natural gas is cheaper than heating with electric resistance heat.

Hi Martin,
I understood about the resistance heating on an all electric unit when it gets too cold for the heat pump but I don't think that was what Kevin was talking about.

Kevin was referring to the defrost cycle that must run in the outdoor unit when the temp drops too low. My limited understanding of this defrost cycle is that it is an electric resistance strip (on the outdoor coils) that runs periodically at cold temps to prevent ice buildup on the outdoor coils.

Kevins post implied that he felt that the cost to run the defrost cycle (in order to use the heat pump below 39F would be more than the cost to kick on the gas heat. I didn't know how to figure the cost to run the defrost cycle because I don't know how often it runs or how much juice it draws. I understand that ultimately at a low enough temp the heat pump is not effective but I thought the whole point of the defrost cycle was to be able to continue to use the heat pump mode in a cost effective manner for temps below 39F at least to a certain point.

Kevin Ambrose
02-08-2008, 03:18 PM
It's been 15yrs since I looked at a heatpump, so not sure about modern ones. Defrost consists of running the heatpump in A/C mode, and turning the strips, or whatever the back heat is, on to keep the air temp up at the register. So your expending lots of energy during that cycle. I'm sure given an efficiency curve, gas cost, electric cost etc. I could find a temp, possibly lower, that would optimize efficiency. I was merely using a temp that theoretically should keep the HP out of defrost, where the efficiency curve starts downward.