Is gas AUX heat more cost effective in dual fuel application?

Hi All,

Good morning!

Need some advice and guidance here regarding if I should set my Ecobee 3 Lite to use AUX heat instead using my heat pump. Below are some details on my HVAC systems:

  1. 5-ton Tarpy rebranded Heat Pump with 18 SEER rating and 10.5 HSPF. Not sure about the COP rating for the heat pump. I would assume it’s ~3.4 when outside temperature above 45F.
  2. 1-stage 80% AFUE gas furnace
  3. In San Diego, CA, the winter utility rates are ~$0.40 per KWh for electricity and ~$2.5 per Therm for natural gas.
  4. Ecobee 3 Lite is configured to use gas as AUX heat

Even though I have been told using heat pump is more efficient, however, if one factors in the Therm-to-KWh cost conversion ( 1 Therm = 29.3 KWh), and the efficiency rating of each (0.8 for gas furnace and 3.4 for heat pump), the cost of running gas heat per KWH is: $2.5/0.8/29.3 = 0.107/KWh. Whereas, the cost of running the heat pump is 0.40/3.4 = 0.118/KWh

It would seem that using gas for heat is still more inexpensive than using the heat pump.

Is my analysis above correct or did I miss out other significant factors?

Appreciate your feedback!


I can’t comment on your specific calculations, but I have heard that the typical recommendation for switching to gas is around 40°F. Of course that’s not considering your actual prices and efficiencies, but I would at least double check your numbers.

This has everything to do with your local utility rates. The MN ASHP Collaborative published a chart for this exact purpose (assuming economics are your top priority and not reducing fossil fuel use - the choice is yours here):

When calculating costs, make sure not to include any fixed monthly charges that you pay to your utility providers, and only consider the incremental cost of additional energy consumption.

I inquired about the heat pump used for the chart. It was a modern variety which sounds like yours with high SEER and HSPF.

For me, my economic switchpoint would have been around -10C or 14F which isn’t far from my winter design temp of -16C or 3F, so in my case I decided to eliminate gas and go all-electric.


If you can, look up your specific model on the NEEP ASHP list. They provide COP values for most heat pumps at 47°F, 17°F, and 5°F. They also provide HSPF values for Region 4 vs Region 5 if you’re in a cold climate and that matters.

A good seasonal COP estimate can be found with 10.5 HSPF / 3.412 Btu/W-h = 3.08, but I’d try to find what it is at your desired switchover temperature.

This generic chart is probably accurate enough if you can’t find equipment specific metrics.

FWIW, you can set your fuel equations equal and solve for COP. Looks like gas will be cheaper for any COP less than 3.74 at your rates - if economics are your only concern.

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Thanks for the info. Wow, it looks like the electricity rate in San Diego, CA is much higher here. With my systems, it would seem the gas is still a bit cheaper to run than the heat pump. However, last winter, the gas rate has also sky rocketed too :frowning:

Thanks for the info. Yes, based on the chart data, it would suggest running heat pump at 45F and above would be more cost effective than running the gas furnace.