Replies

1. Expert Member
   Dana Dorsett | Sep 13, 2019 01:45pm | #1

   The relative operating costs are relatively easy to estimate. HSPF 11 = 11,000 BTU/kwh. So per million BTU (MMBTU) it's takes 1,000,000/11,ooo= 91 kwh/MMBTU

   If your electricity is 9 cents/kwh it would be $0.09 x 91= $8.19/MMBTU

   If your electricity is 22 cents/kwh it would be $0.22 x 91= $20.02/MMBTU

   If you're running a 95% efficiniency gas furnace it's delivering 95,000 BTU out per therm, so it takes 1,000,000/ 95,000= 10.53 therms/MMBTU. If you're paying 62 cents/therm the cost is then

   $0.62 x 10.53= $6.53/MMBTU (plus the electricity to run the air handler & controls)

   If you're paying $1.47/therm it's $1.47 x 10.53=$15.48 /MMBTU (plus electricity).

   The HSPF efficiency is a somewhat squishy number, that can vary quite a bit with actual climate (beyond mere HDD), since the coefficient of performance varies with outdoor temperature, and it's also partly a function of sizing- even more so with multi-stage or modulating heat pumps than with 1-speed units. The model used to calculate the number is partly based on a presumptive duty cycle, so gross oversizing can result in a large error, as can gross undersizing and relying on use of resistance heating strip to cover the shortfalls during colder weather. With the Carrier Infinity it can also vary by whether resistance element re-heat is enabled or disabled during defrost. It's more efficient to skip the reheat, but cool air coming out of the registers during defrost can induce comfort issues that some people are willing to pay to avoid. I believe the default is reheat-enabled (lower efficiency), but my understanding is that reheat power during defrost is not accounted for in the HSPF test.

   To estimate the annual total heating MMBTU requires a minimum the design heat load of the house and the local HDD climate averages. Better than just a heat load would be a simulation, which takes into account things like passive solar gain, etc, not just outdoor temperature.

1. 
   Jon_R | Sep 13, 2019 11:26pm | #3

   > The relative operating costs are relatively easy to estimate....squishy...vary quite a bit

   In other words, impossible to accurately estimate with the data that is readily available.

   You should get closer using this HSPF adjustment (or is it outdated?):

   https://cleantechnica.com/2017/04/08/dragons-guide-to-a-100-renewable-home-part-4-heat-pump/

   Hmm, a rather disappointing heating season COP of 2.2 for a good heat pump here in Michigan.

1. Expert Member
   Dana Dorsett | Sep 16, 2019 06:53pm | #6

   The guy who wrote the article clearly didn't know how to run a Manual-J, or even read the spec for the Mistsubish MXZ-4C36NA, and even though he talks about cold climate mini-splits he lets the installer spec the non cold-climate version!!

   He writes:

   " Our Manual J calc said we needed 31.4kBTU of heat on the coldest anticipated day so our installer selected a 36kBTU heat pump. That would have been fine with a gas furnace because it produces 36kBTU regardless of outside temperature. On the other hand, our heat pump reduces to 68% of its optimal heating capacity when it’s 28°F outside. 0.68 * 36kBTU = 24.48kBTU, not nearly enough to keep up with our 31.4kBTU of heat required."
   -----------

   The max capacity of the 4C36NA2 is ~24K @ +17F, but maybe the earlier (non (-2")version undershot that:

   https://ashp.neep.org/#!/product/29041

   http://meus1.mylinkdrive.com/files/MXZ-4C36NA_Submittal.pdf

   But the COLD CLIMATE VERSION, the MXZ -4C36NAHZ is good for 45,000 BTU/hr @ +5F.

   https://ashp.neep.org/#!/product/26175

   http://meus1.mylinkdrive.com/files/MXZ-4C36NAHZ_Submittal.pdf

   But 45K would arguably be oversized for his more likely loads, since he was using (non-ACCA approved) LoacCalc.net as his Manual-J tool, and the high minimum-modulated output of that beast (really, only a 2:1 turn down?) makes it a pretty lousy choice for a totally ductless 4 zone solution.

   The 3C30NAHZ probably would have been a better choice that the non-cold-climate 3-tonner that was installed, despite the crummier turn down ratio.

   https://ashp.neep.org/#!/product/26173

2. 
   irene3 | Sep 13, 2019 06:47pm | #2

   FWIW, the ducted heat pump we had installed last year (Mitsubishi, but relatives of ours paid similar for a Carrier IIRC) cost nearly $15K total ($13.5K before tax). It has certainly saved us money, but that's by comparison to a very elderly furnace that at its best was 80% efficient. And it would have saved us a good bit more if we'd been able to go off gas entirely, due to the base charge every month (which isn't really worth it now that it's only the water heater that runs on gas, but the water heater is relatively new and we don't plan to replace it for a while). I would hope they would get cheaper as heat pumps become more mainstream in the US. Some utility companies have rebates for heat pumps (ours only does if you're replacing an oil furnace, and that one might have expired, can't remember).

3. 
   oldbungalow | Sep 14, 2019 03:03pm | #4

   thanks for the responses! Any thoughts on the hardware/install cost differences, heat pump vs conventional? Is it generally true that an installed Heat Pump is more expensive than a combined A/C + Gas Furnace combo?

4. 
   AndyKosick | Sep 14, 2019 05:22pm | #5

   Another factor to remember is that if you are able to ditch gas altogether you don't pay a service charge. Around here it's $141 per year whether you use any gas or not. This is a major factor right now in tipping the cost balance for all electric homes.