Air to Water Heatpump scenario for my abode.
@DCContrarian As discussed in
Here are the facts as they currently are, hopefully this is all of what you need to do a spreadsheet:
Zip Code 44130, almost exactly 6 miles due south of Lake Erie, and about 7 1/2 miles south-west of downtown Cleveland as the eagle flies. Design for this area I believe is 7f, but Lake Erie does create a bit of a micro climate(lake effect snow/rain anyone?)
Per my latest electric bill 8.38 cents per KWH from First Energy and 6.499 cents from Nextera energy for a total of about 14.88 cents per kwh - I do have a solar system that offsets about 110% of my usage over a 12 month period. However the past 3 months have been pretty abysmal with snow and cloudy days. Current months electric bill is $142. Highest bill I've seen in over 3 years actually, normally I have a credit. This month in particular has been bad because it has been so cold the snow has refused to melt off the panels, despite me removing as much as I can.
$.515/Ccf - This rate is just for the supply. There is a $19.36 charge for the distribution which works out to $.129 per Ccf.
~40yr old Weil McClain CGM-5 140K btuh input, 115K btuh output supply large cast iron radiators. In the past based on boiler run time at design I would use slightly over 60K btuh to maintain ~70f. Below are snippets of my home automation's tracking of the boiler, and of my most recent utility bills.
Here you can see water supply and return temps and the corresponding outside temp, t-stat is set to 70F. Before it spikes immediately after shut down the supply temp is getting to about 130F, and return is 115F at a outside temp of 23F. You can also see the on/off cycles. At 23F I'm seeing a run time of 19-20 minutes, and an off time of ~1 hour. If I did my math right currently @ ~23F the house is requiring about 28,750 btuh. That is taking total run time of 61minutes between 5:45Pm and now 9:45Pm(4 hours). Since it is a nice 1 hour of run time for 4 hours I can divide 115,000 by 4 and arrive at a rough estimate of 29K btuh. The temperature did fall from 23.4 to 20.8F in that time. Tomorrow and Tuesday I'll be able to give good numbers when the temps fall into the single digits and negatives.
Comments
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Here is that same graphic with just the time frame I referenced visible.
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OK, here's my whack at an analysis:
From your prices of gas and electricity I get the break-even COP at 3.36, which is what the heat pump gives at 52F. That's basically a deal-breaker for the heat pump, there's almost no usage above 52F. Running all-gas is $738.21for the heating season, a heat pump would cost you $220.02 more. (That seems really cheap to me, I guess I'm used to east coast prices.)
I didn't bother doing any of the analysis for things like split fuel, it's just not going to be worthwhile. A bigger heat pump would reduce the amount of backup heat use but that's only 8% of the total and not going to change the picture.
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This also kills the idea of replacing my 40yr old way oversized boiler with a modern modcon. Run it till it dies.
There is still a lot to be gained from insulating the house. There is very little except for the areas I've remodeled. (kitchen, downstairs bath, basement)0 -
Also I noticed you used 23f and 28K btuh as design, that is not the actual design temperature just the numbers I have for right now. At design, which is about 7F, the last time I clocked the boiler I was around 60K btuh.
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I just pulled all of my gas bills for the previous year… It's even worse than your numbers. Minus the minimum charges of about $50 a month, I spent ~$470 to heat the house last year. Granted it was a VERY warm winter.
If I managed to go ALL electric and shut off the gas I would save about $1100. Both usage and minimum charges. I wonder how that would work out if when the boiler finally craps the bed I went all electric and replaced it with a A2W HP with back up electric? This isn't as unrealistic as it sounds, the boiler is the last gas appliance in the house. My own hesitation at ever getting rid of it is if I ever bought a tri fuel portable generator I wouldn't have NG anymore.
Anyways thank you for the spreadsheet.0 -
The way the model works is if I put in any point on the load curve there it will calculate the rest of the values. (It assumes a straight line).
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The idea of getting rid of gas and saving the monthly hookup fee is interesting. That's what I did in my own home, I have a heat pump, heat pump water heater, induction cooktop and heat pump dryer and I love them all. But I'm in a pretty mild climate.
What tough where you are is that it just gets so cold. Usually when sizing heating systems you look at the 99th percentile temperature, 99% of the time it's warmer than that. For you that's 7F. But I also like to look at the average annual low, what the coldest is it gets in an average year. In most places that might be 10-15 degrees colder than the 99th percentile temperature. Where you are it's -24F, or 31F colder, which is enormous.
At -24F your heating load is 65,000 BTU/hr, which is 19kW, you'd need a 100Amp service just for that. Providing that kind of heat in that kind of weather is something gas is really good at.
Now, some people will design for the 99% temperature, the assumption is when it gets colder than that it won't be for very long and the house has some heat capacity to keep it from cooling down much, even if the heating isn't keeping up with the heat loss.
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I have never seen -24 personally that I can recall, I'm almost 37. I have seen wind chills that low. Actually -30 wind chills. That might be the work of that microclimate I mentioned. That large body of water helps warm the area a bit. It often gets colder south of here often.
I have 200amp service. Upgraded when the solar was installed, shortly after that I installed a hpwh.
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Another question is the cast iron rads I have, would I be able to use them with an a2w hp? They hardly ever get above 120 swt. The hottest I've seen them get was about 160 during a polar vortex and it was -10 outside, with high winds.
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lot of options and answers to your questions in theses issues. Pictures too. Some of the systems documented in these issues have been running in cold climates for 3-4 years now.
And they are free downloads.
www.caleffi.com
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
When I put your zip code into the climate calculator, it gave me the choice of five weather stations (I think they're all airports):
Akron-Canton Regional
Cleveland Burke Lakefront
Cleveland Hopkins Intl
Mansfield Lahm Regional
Youngstown-Warren Regional
I don't know your area, I just picked Mansfield Lahm. I can rerun it with a different weather station if you want.
Of course that doesn't change the break-even COP, which is what really drives the decision-making.
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CLE aka hopkins would be the one. Its about 10-15 minutes from my house. In fact if the browns build their new stadium, which would literally be right next door to hopkins international, the traffic during games is going to be a nightmare for...
Fyi Mansfield is about 2 1/2 hours south of me. About halfway to Columbus doen I71. They actually get colder I think. Well outside the influence of Lake Erie.
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I have similar costs as the OP, what made the AWHP worth it (besides boiler near end of life) was disconnecting the gas feed. Meter fees for a year do add up. The ROI is still long but not bad if I consider delta cost between a modcon and AWHP. We also have TOU metering and I can push a fair bit of the usage outside of peak into low cost time which helps.
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The temp outside just fell below design. But the time period shown was at design for most of the time:
Calculated btus used over two hours 95800, or 47900btuh. The boiler is literally 2.5x the size it needs to be.
With the swt hardly breaking 140 at designed would an a2w hp be viable with the rads that I have? Never mind costs... Just thinking out loud. And if I did decide to go all electric, how much in terms of electricity would it cost assuming I would have to use resistance backup to provide say 60k btuh for the short number of days/hours a year I could expect to?
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OK, I updated it to use Cleveland-Hopkins as the weather station. Looks very similar to Boston. Sheet is at:
I didn't change anything else. Let me know if there are any assumptions that aren't right.
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"With the swt hardly breaking 140 at designed would an a2w hp be viable with the rads that I have? "
There's the rub. Both the output and the COP really fall off the table at cold temperatures when you raise the water temperature.
Let me ask this: Do you have AC? Would you like to have AC? As a rough back-of-the-envelope guess, based on your heating load and the weather station data I'd say your cooling load is about 1.5 tons. I'd think about two heat pumps, an air-to-water that does most of your heating, an air-to-air that provides cooling in the summer and more heat when the air-to-water can't get the water hot enough in the winter.
The other option would be adding more radiators or other emitters.
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What if I cut the energy usage of the house in half with envelope improvements? How does that alter the calculus?
I've also played with the idea of doing radiant floors for the ground floor. About 700sq ft of 3/4" thick oak floors with 3/4" subfloor. I've thought of doing that even with the gas boiler but would have to do something to protect both the floor high temps and the boiler from cool return temps.
Edit: No a/c, and no duct work. Window shakers. And I'm closer to 2 - 2.5tons of cooling. I have a 14k btu window unit for the downstairs, and two 8k btu window units upstairs.
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