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mrd 04-21-13 06:25 AM

mackerel, that is a cool spreadsheet. I see there is an ASHP HSPF adjustment table at the bottom. The adjusted HSPF value has quite an effect on the cost of ASHP operation relative to other forms of heat. So it's important the value is accurate!

I don't know precisely how that table was generated, but logic dictates a COP curve versus outdoor temperature must be assumed. The actual curve differs depending on equipment. I have seen curves in design manuals for a few different minisplit systems and they differ quite a bit. A system advertised for 'low ambient' is going to have a curve that falls off much slower at colder temps, thus the table values would underestimate the performance. To what degree the estimate can be off, I'm not certain.

S-F 04-21-13 06:32 AM

It's pretty difficult to properly calculate the comparison between line gas and a mini split because of the varying efficiency of the ASHPs with the weather. I'm relying on utility bills to inform me. In January my electric bill was almost $400! The gas bill was $16, for heating DHW. In February (nearly the same temperatures) my gas bill was $56 and my electric bill, not using the mini splits, was around $45, as usual.

mrd 04-21-13 06:57 AM

There is a heat pump technology called 'enhanced vapour injection' which appears to alter the capacity (and perhaps efficiency) curve across various outdoor air temperatures.

See page 3 in this doc:

Or see figure 5 in this doc: It shows better COP at lower temps. Heat pumps with such technology would perform -a little- better than that spreadsheet's adjusted HSPF values would indicate, in colder regions. Figure 4a confused me for a moment!

mrd 04-21-13 07:04 AM

S-F, HSPF is a seasonal COP, it's supposed to be indicative of an average COP over the season, accounting for variable performance depending on outdoor air temperatures. Of course, every region has its own outdoor temp profile, hence that spreadsheet's adjusted HSPF values. I'm just questioning how accurate those adjustments are.

AC_Hacker 04-21-13 12:10 PM


Originally Posted by AC_Hacker (Post 29498)
Is this really true?

What is the price of electricity in your area, and what is the price of gas in your area?

Did you try the BuildItSolar Fuel Calculator? You can plug in your local values...

I'd be very interested to know your results.



OK, since S-F won't talk to me, I did my best to look up the prices from 3500 miles away...

FROM THIS LINK it looks like electricity is:

7.314/kWh = $0.07314/kWh

FROM THIS LINK it looks like gas price per therm is:


I don't know what S-F's Heat Pump HSPF might be, but my older heat pump has a HSPF of:

7.7 (really, not so good)

COP = HSPF/3.412, so...

COP = 7.7/3.412 = 2.26 (that's pretty low for today's mini-splits).

I'll assume that S-F's gas heat is 95% efficient (I'd be surprised if it was higher).

...and using BUILD-IT-SOLAR's FUEL_COMPARISON_CALCULATOR, I am getting the result that to provide 100,000 BTUs of heat would require:

NAT GAS: $1.40/100,000 BTUs

HEAT PUMP: $0.95/100,000 BTUs

So, it looks like the heat pumps would be a lot cheaper to run... like about 68% as expensive.

But, if we look at the environmental damage caused by CO2:

NAT GAS: 12.63 pounds of CO2 per 100,000 BTUs of heat

HEAT PUMP: 18.14 pounds of CO2 per 100,000 BTUs of heat

That would be the coal fired plants doing their work....

Looking at it this way, it looks like the heat pumps would actually increase the probability that Xringer's grand children would suffer from Asthma.

I guess that more and more insulation really is the best alternative...


AC_Hacker 04-21-13 05:13 PM

Unintended duplicate post.

(please remove)

S-F 04-22-13 04:57 PM

Sorry AC_Hacker, I don't mean to ignore you. I'm simply very busy these days and don't have as much time as I'd like to hang out on the forum, or really even make dinner.

On an entirely different note, a friend of mine told me yesterday that he recently got a retrofit house down to 20 CFM@50. I'd never heard of anything like that even with new construction!

AC_Hacker 04-22-13 06:53 PM


Originally Posted by S-F (Post 29551)
... a friend of mine told me yesterday that he recently got a retrofit house down to 20 CFM@50. I'd never heard of anything like that even with new construction!

Wow, really is impressive.

Does he know how to spell HRV?


S-F 10-26-15 03:39 PM

Hello all. Long time with no update because things have been progressing slowly around these parts. Here's the news: I replaced the big picture windows here with crazy triple pane Pella jobs. They were the only windows in the house that weren't replacements so they were the first to go. They are also by FAR the largest windows in the house. All the rest are tiny, tiny. Not even code minimum. That's fine by me because even the best window performs worse than the worst wall. Next summer the real DER will take place and all of the others will get replaced also. One window faces south so it's got a higher SHGC and in turn a higher U. It's U Value is .21. The other picture window faces West and would roast me out of the living room on summer afternoons so I got the extra low E coating. It's got a U of .15. The casements flanking it have a U of .17. Not bad for the money. Marvin wanted close to 6 grand for windows with a higher U. These came in right around $3,400. Pella has a big presence here and I have a sales rep I've been working with. Since a friend told me last fall about how much better Pella windows have gotten I've been using them exclusively.

Daox 10-28-15 07:29 AM

Looks good to me.

Nice to see you hanging around again too! :)

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