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!

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Comment by Bob Blanchette on June 18, 2012 at 8:17pm

That's just it, only "high end" heat pumps offer demand defrost. Unfortunately they account for less than 10% if installations.

Agreed, people DO need to stop fiddling with the stat, especially on heat pumps.

Comment by tedkidd on June 18, 2012 at 10:30am

Bob, don't blink or technology will move beyond you.

Many high end heat pumps do have demand defrost.  And these high end units (Greenspeed, for example) will stage backup either - upon what I call user interference (fiddling with tstat or using setback), or when it can't satisfy load on it's own.  

Ideally you do not "interfere" with the equipment, and it never experiences load shock such that backup is necessary.   The challenge is getting people to understand that attempting to game savings with setback is hard on this equipment and drives energy cost the wrong way. 

Comment by Christopher Morin on June 15, 2012 at 3:34pm
Hi tedkidd! Thanks for taking notice to my blog! I think you are 100% correct on all counts. I will correct my math as the number supplied to me for natural gas does not appear to include delivery charges. Even still, it does not appear very favorable for natural gas (today). Also, this break-even COP calculation is just the beginning to the cost-benefit analysis needed to correctly determine if a dual-source is cost effective. I think I can speak for most New England contractors in stating that comfort is one heck of a thing to sell when everyone is so wrapped up in price! Thanks again for the comments, I appreciate any and all feedback!
Comment by tedkidd on June 15, 2012 at 1:52pm

I'm not sure those numbers line up quite right.  Seems you may be including delivery cost on electricity, but not on gas.  

In our area gas is about $1 a therm, electricity 11 cents a kwh.  


I've sold a lot of natural gas hybrids.  I'm finding unprojectable and unexplainable savings associated with them. 

With Hybrid COP calculation is not likely to accurately reflect the efficiency of the heat pump.  COP is heavily dependent upon outdoor temperature, so unless you can really dial in what time at temperature the heat pump will run, you can't have a handle on cost.  

Furthermore, I think tremendous savings is achieved by fewer cycles and long run times these devices are able to obtain.  Output that can better match load means equipment that runs non-stop, replacing only the energy lost rather than the blast furnace/off old school approach.  Again, I think a lot of unexplained savings comes from gentle, efficient, long cycles.

Also, air flow is a function of BTU output.  By having the ability to run low and slow, pressure and thus leakage both in duct and in pressure imbalanced spaces is diminished.  So there are savings achieved here.

Heat loss diminishes as delta t decreases.  At lower stages the duct temperature may be lower, resulting in less loss through ducts.  

Load matching and lower output means more even temperatures, and better humidity control.  Don't undervalue the importance of occupant comfort, or you miss a big opportunity for savings.  When people are comfortable they don't adjust things and equipment can do it's thing.  Leave it on the highway with the cruise control on - that's how you get the most MPG for the least trouble. 

I think we need to stop looking at this like a simple math equation, it diminishes the tremendous importance of good design.  There are a lot of subtle issues that can have a big impact on consumption.  


Comment by Bob Blanchette on June 12, 2012 at 6:45pm

Our utility offers 1/2 power on all use over 600KWH during the winter. Even with 5 cent power it's still hard to justify a dual fuel with 40 cent per therm gas prices. The real gains come from going all electric and dropping the $27/mo service charge for the gas meter. But then you pay more for dryer/stove/water heater by making them electric.. You're right the utilities DO have it figured out, they are very competitive.. Of course with the cost of propane, even running strip heat is cheaper at the 5 cent rate. Oil isn't used in Oklahoma so no comparison there.

Ideally there should be lockouts on how many heat strips can come on based on outside temperature. None above 40 degrees, 5KW down to 30, 10KW below 30 or something similar. Of course "emergency heat" would override the lockouts.

The real crime with heat pumps is the lack of "demand defrost", heat pumps should only go into defrost when NEEDED, not every 45 minutes based on a timer.. A pressure switch can be an effective way to determine airflow loss through the frozen coil and initiate the defrost cycle.

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