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I recently moved into a new house that has a heat pump (air-to-air) combined with a natural gas fired furnace.

My old thermostat failed, so I replaced it with a new EcoBee thermostat. During the setup it asked what the minimum outdoor temperature to still run the compressor should be. I am new to heat pumps, so I am not sure what the correct answer is.

I live in the middle of Iowa (Cold snow filled winters). Do I need to be concerned with snow on the outdoor unit? At what point is it more efficient to use the natural gas instead of the heat pump?

  • Snow is not of concern, it is the refrigerant must have enough of a temperature difference between the refrigerant temperate and the outside air to provide heat; although I am unsure of how to set this correctly. HVAC techs have alot of "curves", factors, and good-ol'-boy rules they deal with to properly set things such as this. – Damon Nov 11 '16 at 20:59
  • The balance point, is the point at which the heat pump provides the exact amount of heat required by the home. The balance point is usually between 15 - 40°F outdoor temperature, but it depends on the demand and the system capacity. You might find some helpful data, in the manufacturer's documentation for the heat pump. However, without doing a load calculation on the home, you'll still be guessing. – Tester101 Nov 12 '16 at 13:59
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    Can you provide the model number for the heat pump? As @Tester101 said, there is a balance point where the CoP for a heat pump is lower than 1, at which point the furnace is more efficient. Also, you need to determine if the manufacturer specifies a minimum operating temperature. You would select the higher of the two. – Hari Ganti Dec 21 '16 at 4:12
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For a natural gas furnace, cut off the heat pump at a fairly high outdoor air temperature (OAT). The cost of heating with natural gas in the US is similar to the cost of running a heat pump that has a coefficient of performance (COP) around 3, which many heat pumps only achieve at an outdoor air temperature warmer than 40-50 degrees F. Look up the cost of natural gas vs electricity per million BTU and the performance information on your particular heat pump to verify this. The Elk Public Utility District in Tennessee has a cost comparison of heating sources at the bottom of the following webpage:

http://www.erpud.com/comparison.htm

To find the cut-off temperature for the the heat pump vs the natural gas furnace: (1) Divide the natural gas MMBTU fuel price by the efficiency of the furnace, giving the furnace heating cost per MMBTU. (2) Divide the cost of electricity per MMBTU by the furnace heating cost per MMBTU, giving the COP at which the heat pump is break-even with the furnace. (3) On the manufacturer extended performance table for your heat pump, look up the OAT associated with the break-even COP. That is the OAT cut-off temperature.

Also accumulated snow on the outdoor unit is a problem, especially if the unit is always off when the OAT is below 45F. Snow can block airflow and eventually turn to ice. Severe ice buildup must be avoided since it can melt and refreeze, physically damaging the outdoor coils. If you get deep snow in your area, do consider building snow protection for the outdoor unit. Either that or disable it for the deep winter months and put a cover over it.

FYI, for someone with electric backup heat and not natural gas, the cut-off for many modern R410a heat pumps occurs around an OAT of 5-15 degrees F.

  • I think this is a great answer and would just add that there are two measures of efficiency - energy and cost. For my air->air heat pump the energy efficiency crossover is around a COP of 2 at -10C/14F (SCOP is in excess of 4, gas boiler is around 90% efficient; gas power plant delivers ~45% of the energy in the gas after wire losses in the form of electricity at the pump power socket). For cost, the crossover will be at a higher temperature and @chris has answered that comprehensively. – nsandersen Jan 7 '17 at 17:31

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