How Low Should I Operate my Heat Pump?

UPDATED: Break Even COP’s for MA

How do you know if you should install a ductless mini-split, or upgrade your air-conditioner to a heat-pump and create a dual-source application during replacement this Winter?  I have been asked this countless times over the past week!  I personally always prefer the option of multiple fuel sources, particularly since energy prices have been all over the place during the last decade.  Fortunately, using averaged energy prices, there is some simple math to figure out if an aggressive assessment should be made for a dual-source heat pump application, based on equations from ACCA's Manual H: "Heat Pump Systems: Principles and Applications".  First, you will need the average energy costs for the selected fuels, and then plug the information into the equations below.  This will provide your "break-even COP", or the point where operating the heat pump will cost the same as the other source.  You can then take the calculated Coefficient of Performance (COP) and see what temperature the heat pump will be operating at; the lower the better!


I completed a little research for you, so let us insert the recent average prices into the Natural Gas, Oil, and Propane equations to see if the investment in a heat pump will make sense during replacement, or become your primary source of heat – and to what temperature.

As you can see, if installing a 96% Natural Gas furnace, the Break-Even COP would be 3.9.  Based on the Heating Performance Data for Fujitsu’s most efficient 12K RLS2 Heat Pump, it would need to be above 60F outdoors for the cost to operate the heat pump to be cheaper than the Natural Gas Furnace (see data at bottom, assuming 70F Indoor Temperature).



Not all homes in MA are lucky to have access to a Natural Gas supply.  There are more than enough Oil Tanks out there to keep the hundreds of delivery companies busy during most New England winters.  As you can see in the equation for oil, a resulting break-even COP of 2.6 indicates a significant savings can be realized. Based on installing an extremely efficient 87% oil furnace and the same heat pump performance data, the heat pump would still be cheaper to operate as low as 20F.  Of course, you must worry about the output of the heat pump at that low ambient, and in order to feel comfortable you will need to calculate the Thermal Balance Point.



If you decide to install a Propane tank you could realize the same efficiencies as the natural gas furnaces out there, but the increased costs in fuel/delivery is currently higher than oil, particularly when including a recent oil price plunge.  As you can see, the break even COP for installing a heat pump add-on above a 96% Propane Furnace is only 1.45.  This is even lower than the Oil application, resulting in a break-even COP of -15F, proving the recommendation of a more thorough calculation into the Thermal Balance Point and investment costs for going to Dual Source.



          With the recent technological advancements in the HVAC industry in controls and conventionally ducted VRF's like the Carrier GreenSpeed, break even COP's and Thermal Balance Points can be driven even lower.  This makes Dual Source Heat pump applications more attracting to New England homeowners, despite recent electric rate hikes.  Some contractors are still installing electric supplemental heat, hopefully in stages, for low-ambient operation.  Although not any more efficient than the heat strips it replaced, the new heat pumps could save more than enough above the temperature of the defrost cycle to still be worth it.  I would still prefer dual-source, you know these energy companies will not be leaving any money on the table over the long run!


8 comments:

  1. if I set my unit (heat pump) at 70 and its 20 degrees outside am I making it work to hard? We are very comfortable inside but it seems to go into defrost mode frequently

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    1. Interesting. An the pump heats the entire space without supplemental heat? How big is the space?

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  2. Another great post. During cold days our 5 ton heat pump cannot keep.up . The zone is about 2500 Sq feet. The supplemental heat kicks on and that supplement is oil furnace with hydro.

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  3. Hi Anonymous - yes, the colder the temperature outside, the less heat the Heat Pump will provide and more frequent the defrost cycles. Depending on the type of equipment, and use of supplemental heat, this can drive up operating costs. Fortunately, residential ductless heat pumps do not incorporate such supplemental heat - but this means if you are uncomfortable you will need to turn on or switch to your other source!

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  4. Hi Dr. Levine! Thanks for the compliment. What you are referring to, if the heat pump can heat the entire space, is called the Thermal Balance Point. As the temperature outside gets colder, the heat loss of your home rises. This means you need more Btu/hr to feel comfortable. Unfortunately, as it gets colder outdoors, the air-source heat pumps capacity decreases. The point which you can provide the needed Btu/hr, at the lowest temperature and still maintain comfort is your Thermal Balance Point. You have quite a large space for a single air source heat pump, hence the need for your back-up or supplemental oil hydro. This will be needed most often when temperatures get below 32F as defrost cycles increase with conventional or ducted equipment.
    Hope this helped!

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  5. Nice blog. Like it. If you ever need service on your ac repair please visit us at rheemteamcomfort.com

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  6. Nice blog........A heat pump uses some amount of external power to accomplish the work of transferring energy from the heat source to the heat sink.Trane Air Conditioning Repair

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  7. Be careful about fire hazard. The natural gas models can be a real pain if something leaks. Remember that the natural gas has a chemical added to it to make it smell bad. That is why I like the electric heaters more. If you have a solar power system for your home, the heating is entirely self contained.

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