Heat Pump Break-Even COP's

  How do you know if  should upgrade your air-conditioner to a heat-pump and create a dual-source application during replacement this Summer?  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, based on average 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 in 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 furnace.  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 did a little research for you, so lets insert the average annual prices into the Natural Gas, Oil, and Propane equations to see if the investment in a heat pump will make sense during replacement.

As you can see, if installing a 96% Natural Gas furnace, the Break-Even COP would be 6.8.  Based on the Heating Performance Data for Goodman's most efficient DSZC18 Heat Pump, it would need to be above 65F for the cost to operate the heat pump to be cheaper than the Natural Gas Furnace (see data at bottom).  Even if I installed an 80% furnace, the break even COP is still at 5.7, much too high to realize a savings.

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 1.17 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 -10F.  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 can realize the same efficiencies as the natural gas furnaces out there, but the increased costs in fuel/delivery could be even higher in than oil, even after including recent oil surges.  As you can see, the break even COP for installing a heat pump add-on above a 96% Propane Furnace is only 1.07.  This is even lower than the Oil application, 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.  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!


10 comments:

  1. Good Blog - Dixey from CCH

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  2. Very interesting calculations. I live in Chevy Chase, MD just outside Washington, DC. I heat with a very old gas steam boiler (Bryant vintage 1949-1956) and have two separate zones for AC - basement air handler and attic air handler. What would the calculations look like here for a Heat Pump replacement or HP hybird gas system? I imagine that electricity and gas prices are similar to those found in New England.

    Scott

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  3. Hi Christopher,
    I only have access to oil heat which drives me crazy considering the speculative nature of the market/prices. I decided to put in a PV solar system and Greenspeed heat pump to provide low cost electricity and an alternative heat source. My cost of electricity is close to $.09kWh. I am trying to run some numbers to determine the cost comparison at low temps to determine when i should switch back to oil. According to carrier, the system can run to very low temps but it is hard to track down the COP for any useful comparison. Can you provide any perspective on using the greenspeed during a cold winter?
    Thanks.

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  4. Hi jinx1832,
    Thanks for reading my blog! In order to comment on any individual residence, I would have to do a Cost-Benefit analysis on your heat pump installation. ACCA Manual H outlines how to complete this math, but what it boils down to is how low can you go and still provide the heat needed to match your heat loss calculation. Personally, based on your price of oil vs. electricity, I would recommend doing just this. Once your btu output gets consistently below the "defrost knee", and needing supplemental heat, I would consider firing that burner. The fact that you have a high COP is good for efficiency, but it may not meet your needs regarding load!
    Hopefully this helps...good luck!

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  5. This comment has been removed by a blog administrator.

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    Replies
    1. I don't understand where the 0.9 in your equations comes from.

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    2. I also live in ma, but my natural gas costs are much higher:
      $0.776/therm for supply and $0.553/therm for distribution for a total of $1.35/therm. My supplier is NSTAR but I surprised that gas prices vary so significantly. My COP, even without your 0.9 factor, is a much lower 3.0. Making my heat pump much more economically viable than your calculation.

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  6. How to determining the coldest operating temperature for your heat pump

    BALANCE POINT (heat supplied = heat loss)
    Capacity of the heat pump is limiting.
    At balance point, heat pump runs constantly (or nearly so)
    The larger the capacity of the heat pump, the better insulated the house, the lower the balance point.
    Because heat pumps are generally sized based on cooling requirements, there is frequently excess heating capacity, and therefore the balance point is frequently not the limiting factor, at least in natural gas settings.

    ECONOMIC BREAK-EVEN
    cost of heat pump operation = cost of backup heat source

    Heat pump required COP for economic break-even
    $/kwh*kwh/btu*COP = $/therm*therm/btu*furnace eff (natural gas furnace backup)
    COP=($/kwh)/($/therm)*btu/therm*kwh/btu* furnance eff
    Nstar gas = supply +delivery = 0.776 +0.573 = $1.35/therm
    (1 therm = 10^5 btu nat gas)
    Nstar elec = supply +delivery =0.0 7 +0.104 =$0.17/kwh
    (1kwh = 3412 btu) (furnace efficiency = 80%)
    COP = 3.0 corresponds to a temp of 7oF

    More expensive energy back-up systems (fuel oil, propane) would have lower COP and be able to economically operate at lower temperatures. Of course a heat pump is always more economical than
    running an electrical backup heating system, so other constraints dominate.

    OPERATIONAL
    High humidity coupled with cold temp, forces the heat pump to ice up and require frequent defrosting.
    This may make it desirable to adjust the temperature setting upward.

    COMFORT (the colder the outside air, the less warm the inside supply)
    The heat pump can heat the house but the air supplied feels cool, although it is warmer than interior air.
    Adjusting the temperature to user preference is not a true constraint.

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    Replies
    1. There is an error in the previous post. The calculated COP of 3 corresponds to temperature of 27oF not 7oF. This value is found by looking at the manufacturers specifications and may require some interpolation.

      It is interesting to note that I found a heat pump to be able to operate economically in Massachusetts at much lower values then the original poster.

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