# Is gas AUX heat more cost effective in dual fuel application? **URL:** \ **Category:** Feedback & Discussion\ **Created:** [December 9, 2023, 6:03pm UTC](https://community.beestat.io/t/is-gas-aux-heat-more-cost-effective-in-dual-fuel-application/1017 "2023-12-09T18:03:23Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Matt](https://community.beestat.io/letter_avatar_proxy/v4/letter/m/a6a055/32.png) [@Matt](https://community.beestat.io/u/Matt)\ **Post date:** [December 9, 2023, 6:03pm UTC](https://community.beestat.io/t/is-gas-aux-heat-more-cost-effective-in-dual-fuel-application/1017/1 "2023-12-09T18:03:23Z")
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! Regards, -matt ---
**Author:** ![ziebelje](https://community.beestat.io/user_avatar/community.beestat.io/ziebelje/32/633_2.png) [@ziebelje](https://community.beestat.io/u/ziebelje)\ **Post date:** [December 10, 2023, 1:33pm UTC](https://community.beestat.io/t/is-gas-aux-heat-more-cost-effective-in-dual-fuel-application/1017/2 "2023-12-10T13:33:04Z")
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. ---
**Author:** ![tkzzz](https://community.beestat.io/letter_avatar_proxy/v4/letter/t/94ad74/32.png) [@tkzzz](https://community.beestat.io/u/tkzzz)\ **Post date:** [December 11, 2023, 3:47pm UTC](https://community.beestat.io/t/is-gas-aux-heat-more-cost-effective-in-dual-fuel-application/1017/3 "2023-12-11T15:47:06Z")
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): [https://static1.squarespace.com/static/5eebc734f52c736dceefa2ec/t/5fd3e084ee621d2d63fadaf1/1607721094613/MN+ASHP+SetPoint\_Chart.pdf](https://static1.squarespace.com/static/5eebc734f52c736dceefa2ec/t/5fd3e084ee621d2d63fadaf1/1607721094613/MN+ASHP+SetPoint_Chart.pdf) 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. ---
**Author:** ![tdykhuis](https://community.beestat.io/letter_avatar_proxy/v4/letter/t/50afbb/32.png) [@tdykhuis](https://community.beestat.io/u/tdykhuis)\ **Post date:** [December 11, 2023, 9:06pm UTC](https://community.beestat.io/t/is-gas-aux-heat-more-cost-effective-in-dual-fuel-application/1017/4 "2023-12-11T21:06:37Z")
If you can, look up your specific model on the [NEEP ASHP list](https://ashp.neep.org/#!/). 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](https://learnmetrics.com/heat-pump-efficiency-vs-temperature-graph/) is probably accurate enough if you can’t find equipment specific metrics. ![IMG_0671](https://community.beestat.io/uploads/default/original/1X/233f686f062b708f909744e99e9b8e948f2f3261.jpeg) 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. ---
**Author:** ![Matt](https://community.beestat.io/letter_avatar_proxy/v4/letter/m/a6a055/32.png) [@Matt](https://community.beestat.io/u/Matt)\ **Post date:** [December 12, 2023, 2:19am UTC](https://community.beestat.io/t/is-gas-aux-heat-more-cost-effective-in-dual-fuel-application/1017/5 "2023-12-12T02:19:45Z")
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 ☹ ---
**Author:** ![Matt](https://community.beestat.io/letter_avatar_proxy/v4/letter/m/a6a055/32.png) [@Matt](https://community.beestat.io/u/Matt)\ **Post date:** [December 12, 2023, 2:49am UTC](https://community.beestat.io/t/is-gas-aux-heat-more-cost-effective-in-dual-fuel-application/1017/6 "2023-12-12T02:49:48Z")
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.