Heat Pump Experts

[SuperD]

I have a 3 ton (36,000 BTU) Frigidaire heat pump with auxiliary electric backup on my home central system furnace. It is 10F (-12C) outside and the wind is ripping outside at up to 30 MPH. The heat pump is 10 years old and has a HSPF of 8.0. and goes through the defrost mode once every two hours. The heat pump is running continuous and can maintain the house temperature of 68F, with the occasional back-up electric heat kicking in. I am actually amazed that the heat pump can keep up.
My question is, is the heat pump at this low of a temperature cheaper to keep running verses the expensive back up auxiliary heating coils on the furnace? If it makes any difference the electricity here is $0.16 per KW.
Thanks in advance for your expertise.

 

[justinjoyal]

I dont have the specs because I would need the exact model number but there’s a good chance it is, probably near balance point though.

 

[yeldogt]

You should be able to get the manual w/all the data .. my guess is its still cheaper. Do you have any other fuel available ?

 

[TRWham]

The COP (coefficient of performance) for electric heat is 1.0. The COP for a heat pump is greater than 1 even at very low ambient temps, so the heat pump wins. It will lose capacity as the temperature drops, but as it does it also consumes less total energy. The only reason to run back-up heat is if the heat pump cannot hold your desired indoor temp.

 

[Milton Shaw]

The heat pump tstat keeps the heat pump only on until the temp drops more than 2 degrees below set temp, then it turns the resistance heat (backup) on. Most HP had a COP of 2-3 so they are half or a third of cost of operation of resistance heat. This thermostat 2 degree is why you don't turn thermostat back on heat pump to save energy. It uses high dollar backup heat to heat back up.

 

[yeldogt]

Previously -- prior to low temp units -- it was the rapid drop off in output that caused the high switchover temp to backup. They were still producing heat at lower cost vs resistance ...but it was not enough to heat the space. Also with the resistance upstream of the evaporator -- they could not work together. Its one or the other

 

[3rdgendslmech]

So I just had a new heat pump system put in last year. 16 seer Bryant/Carrier. When temps get below 20-25F the aux heat grids will come on 3-4 times an hour. It may seem like a lot but even with this last cold spell it's still saving 50 dollars a month heating and cooling cost

 

[SuperD]

So as long as it makes enough heat to maintain the preset temperature I am saving money based on the benefit of the COP over electricity back-up heat coils in the furnace?
Right now the thermostat and the back-up AUX heat is programmed perfect, if the wind howls and the temperature drops below -13, the heat pump keeps going and once in a while the back-up heat comes on. The heat pump has run continuous for the past 48 hours and is keeping up with the exception if the temperature drops below -13 C outside. Is it hard on the compressor and system to run continuous?

 

[yeldogt]

The coils will come on when the unit goes into defrost to minimize the cold air from the cycle.

When my Carrier infinity 5 speed was installed I noticed that you could add a downstream coil that it would control .... so some systems can keep working even with derating

 

[850xpeps]

My heat pump will maintain my house at -21. And thanks for the thread because I was gonna ask the same question. This is my first winter with this system. It has 4 stages of heat. First is heatpump only and second adds some resistance heat and so on as needed. Even stage 2 I think only calls for 5kw of electric. The heatpump is a 20 seer 3tpn variable speed Lennox. Time will tell if it was worth it.
But I was leery of the super long run time.

 

[aunsafe2015]

So as long as it makes enough heat to maintain the preset temperature I am saving money based on the benefit of the COP over electricity back-up heat coils in the furnace?
Right now the thermostat and the back-up AUX heat is programmed perfect, if the wind howls and the temperature drops below -13, the heat pump keeps going and once in a while the back-up heat comes on. The heat pump has run continuous for the past 48 hours and is keeping up with the exception if the temperature drops below -13 C outside. Is it hard on the compressor and system to run continuous?

Long run times are generally considered fine, good even. Certainly better than short cycling.

 

[SuperD]

Long run times are generally considered fine, good even. Certainly better than short cycling.

Makes sense, thank for the answer!

 

[firebirdparts]

So as long as it makes enough heat to maintain the preset temperature I am saving money based on the benefit of the COP over electricity back-up heat coils in the furnace?

No, you are saving money any time it has a COP greater than one. It has nothing to do with whether or not it can keep up.

 

[nsula_country]

Heat pump is cheaper, even running 100% duty cycle vs resistive. We have 2 heat pumps on 2200 sq/ft. Much cheaper to operate than resistive on last property of 1000 sq/ft, same POCO.

Heat pump build at our shop in signature.

CT

 

[SuperD]

No, you are saving money any time it has a COP greater than one. It has nothing to do with whether or not it can keep up.

Thank you for the comments, I learned a lot more about heat pumps. I also found the spec chart for mine, it is a 3 ton or 36, 000 btu unit. I also see that at -12*C (10*F) it is still putting out about 22,000 btu and running at at COP of 2.44. That means it would cost me 2.44 times as much to run baseboard electric resistance heat rather than the heat pump. I am also amazed that I can maintain the heat at 68*F ( I have learned to wear a sweater in the winter) for a two story home that is 1350 SF. on the main floor. The Nordyne heat pump is 9 years old and still chugging along.

 

[bazar01]

I have a 20 yr old 3-ton Rheem heat pump with TXV that has been running great.
I disabled the 10kW electric heat strips and it still puts out heat even in high 20s outdoor temps here in SW GA. It runs a little longer but it still turns off when indoor set point temp of 70F is satisfied.