In my quest to get rid of oil, I'm now evaluating ASHP. I live about 20 miles north of Boston. Our average winter temperature is about 40f, but it obviously gets much colder than that. I want to try and calculate my expected COP based on HDD data for the past couple of years. I also want to make sure that the designed system is able to heat our home for those really bitter days where it gets below 0f.

I know this will likely vary by manufacturer. Most of the heat pumps I'm looking at advertise an HSPF of 11 to 12, but I have to assume that's best case and at a specific temperature. They don't make it easy to find that data. Not only that, but I don't like the idea of relying on the company to tell me the truth, so if there are any good independent sources of this type of data that would be ideal.

I'm hoping to use the following table to get a true average COP:
Temp (f) % @ or Below
----------------------------
-15        0.00%
-10        0.00%
-5        0.00%
0        0.16%
5        0.16%
10        0.16%
15        0.79%
20        1.10%
25        3.30%
30        6.13%
35        10.85%
40        16.19%
45        21.07%
50        28.93%
55        32.70%
60        38.21%
65        50.94%

Do you have detailed heat loss data on your house? An energy audit, a blower door test, or other information? You should preform (or find someone to preform) these and do a heat loss study to determine the size of the units you'll need. Minisplits are great - I converted my 2700 Sf farmhouse to them after upgrading the insulation & we're very comfortable and happy with them.

I do have some heat loss data - entirely based on oil consumption. Dana1 was good enough to help me with that. Short version is that I have pretty high loss right now and an energy audit is being done next month to determine what's going on. Peak BTU/h as high as 120k for the coldest period this year so far. This is quite a bit more than my home should need (circa 1997 - 4500 sqft or so, well insulated). So I won't be able to calculate real ROI until I know what my future consumption will be and size accordingly, but I'm creating some scenarios to give me a good idea of what to expect just based of COP and comparing that to oi at my current usage.

Steady state COP at temperature is only half the story. The duty cycle, idling time, and spin-up efficiency hits matter quite a bit with air source heat pumps. Some of that goes away with modulalting systems, and ductless systems also gets rid of duct loss and duct design issues. But it's pretty easy to screw that up too- modulation ranges are not infinite.

The as-used COP of modulating heat pumps varies quite a bit by modulating levels. The as-used HSPF of 1 or 2 stage ducted heat pumps is affected by the oversizing factor, since that determines the duty cycle and efficiency losses from spin-up from cold start.

Assuming the heat pump is right sized for the actual loads, 20 miles north of Boston is in the zone IV heating hours zone (not to be confused with DOE climate zone 4), at which the HSPF is tested & rated.

http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-413-04/images/Figure5_lg.gif

A modulating ductless system can sometimes beat it's rated HSPF if right-sized, but will underperform (sometimes by quite a bit) if it's oversized to the point that it cycles rather than modulates at the mid-winter average temperature (an error that's really common when they take the approach of a ductless head in every room.)

Given how atypically high your fuel use heat load calculation came in, it's better to use a real room-by-room Manual-J for sizing the equipment. Also, the ~120K /hour is an instantaneaous rate, not "over the coldest period". Fuel use during the colder periods are part of the input data by which the load at 0F was calculated, but the stated load is at 0F, not over the period.

A reasonable third party tested graph of COP vs. temperature & modulation level for a popular older 1-ton mini-split can be found in Figure 5 , p10 ( p18 in PDF pagination) of this document:

https://www.nrel.gov/docs/fy11osti/52175.pdf

Note that at 35F at minimum modulation the COP is about 4.5, whereas at maximum speed it's only about 2.5. As temperatures below 10F it's consistently under 3 (at any speed), but is still hitting ~2 at -4F. The latest-greatest ductless systems are slightly better than that, but in your climate IF PROPERLY SIZED that old 12RLS2 can realistically hit near it's rated HSPF 12 (COP 3.5) for a seasonal average, but only ~2.5 if oversized by 2x. For a modern cold climate multi-split reasonably sized for the 99% load, assume a COP of 2.7-3.0.

Single speed ducted systems will be all over the place- assume a seasonal average of 2-2.5, if sized reasonably, only 1.5 if oversized, or even worse if the duct system isn't both well designed and well constructed.

There are a few modulating ducted ASHP systems out there (Carrier Greenspeed, etc.) , but the turn-down ratios are limited, only about 2.5:1, so getting the size right is essential for them to be modulating enough of the time to hit near their HSPF numbers, yet still keep up with the 99% heat load. They're pretty pricey too- usually more expensive than multi-split ductless/mini-duct solutions.

Dana1 - it's like you do this for a living or something.



I was looking at Greenspeed, but I'm a bit scared by the frequent reports of really bad noise problems caused by the scroll compressor. Instead, I'm considering one notch down - the 25vna8, which is 5 stage. I did find this: http://dms.hvacpartners.com/docs/1009/public/0e/25vna8-01pd.pdf

.

Based on that and using a weighted average COP with the temperature data for my area, I come out to about a 3.35 average COP. Yes, assuming everything else is perfect.

GBTalk you may have read already shares on many postings that WHO is installing can really vary some real world resulting-efficiency. -seeing above I.E.) that Dana1 directed attention to sizing, for one.

A savings that you can ask about might be WHAT is practical with a couple AHP's with sufficient back-up heaters. It may be practical to also consider, as over the past decade, it surely seems lower-efficiency with OTHER 'life-costs, @$ of about a fifteen-year review'.
 - (in your reviewing a high-speed/stage rating which will result in a very high first-speed/stage performance, meaning here that one may review a 'lower efficiency' )

Such in lower-efficiency lines you may have seen is that of a two-staging AHP with step-speed select- very efficient blower motors may beat first 'buy' substantially. Some seem best significantly /by maintenance dollars in some comparison over fifteen years as to of some higher-efficiency variable AHP set up.

But too, there is some 'bet' that a decent appliance home-owner's policy a few years after purchasing, would further cover 'higher-efficiency' repairs.

You may have considered mini-split head aesthetics already, to mention.

-and also with at least one AHP as a two-staging two-zoned / , with a second in 4500 square feet, with more information yet to consider, one may be completely reasonably ahead of in all R.O.I. , etc.; here, just in (presuming somewhat then by OP) including a full basement (?half- walk-out) three or four-zoned

If cleaned regularly, seems more-other savings is with some Daiken AHP's and any alike, now with 'heat-recovery' to a generator of hot water production to go into a HW tank, and during cooling modes to move household heat into a water heater tank as a system.