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!

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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).

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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.

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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.

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          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!

http://excessair.blogspot.com/2015/01/how-low-should-i-operate-my-heat-pump.html

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Comments

  • Peak energy use is a summer issue. TOU rates apply. Located in OKC. That's why I mentioned local rates structures vary A LOT.
  • Great points Bob!  Can I ask where you are located?  Here in MA, our utility providers have a tiered structure to charge more when using more...I guess it depends on when, or even if, you have a peak demand problem in your locale.  These demand issues are well documented on the West Coast and where I live in the Northeast.  Although we still do maintain a minimum monthly service charge...so using less or least is only beneficial to that point.

  • Don't forget to account for meter/delivery charges when comparing the costs of various fuels. Some charges change with use, others are "fixed". Some utilities tier their rates. Our utilities have a "negative tier" where they charge LESS per KWH/Therm as you use more per month.

    The NG company charges $176/yr in monthly meter charges plus $3.73 per DTH if you use less than 50DTH per year. If you use more than 50DTH you pay a flat $363/yr but no per DTH delivery charges. The NG itself runs about $5/DTH and varies month to month. Go dual fuel and get under 50DTH/yr, you will be effectively charged a higher "per therm" rate from the NG company.
    https://www.oklahomanaturalgas.com/en/CustomerCare/RateInformationT...

    The electric company has a similar deal, all use over 600KWH is 6 cents instead of the normal 11 cent winter rate. The electric company only does this in the winter, summer rates are higher once you go over 1400KWH.

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