Trying To Estimate Heat Pump kWh Usage
Considering installing some heat pumps for my mom's ~50 yr old house in Massachusetts and so I am trying to get an idea of how much her electric bill might increase.
Looked at her oil deliveries for the past year and it looks like they add up to around 900 gallons. She tells me that she has a 65% efficient boiler that was installed in early 90s and delivers heat via aluminum fin baseboard.
As I see it, that means that she only gets ~91K BTU per gallon of oil, right? How much more should I take off for any distribution losses? Any other losses?
For quick and dirty, I had used 100K BTU net per gallon ⇒ 90M BTU from the 900 gallons. By my calculations, 90M BTU converts to 26370 kWh (90M BTU / 3413 BTU/kWh). Considering a heat pump with a HSPF2 of 8.3 (COP = 2.43), it seems that the heat pump would use 10547 kWh to produce that heat, right?
That seems like a crazy amount of kWh to me?
Am I getting confused somehow?
Comments
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Your math looks right.
This post explains how to do a more detailed modeling:
https://www.greenbuildingadvisor.com/question/how-to-model-and-predict-electricity-usage-for-a-heat-pump
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The last time I filled my tank I paid $4.11 per gallon. That equates to $0.35 per kWh, which sounds about what you'd pay in Mass.
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This is a helpful site.
https://coalpail.com/fuel-comparison-calculator-home-heating
An intro to A2whps here
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Nothing wrong with your math that I can see. Her oil bill would decrease — but the electric bill will increase by almost the same amount. In most of New England switching to a heat pump won't save any money — it just depends on whether you want to pay the oil company or the electric company.
Then there's the capital cost of the heat pump; they don't come free. If saving money is the objective here, the first step is to make sure the house is at least reasonably insulated. Then the least expensive next step would be to swap the existing boiler out for a modern one, which will be able to hit 80 to 85 percent efficiency. The most expensive option will be the heat pump…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
We're mostly considering the heat pump because she'd like to get AC other than her existing window units.
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Well then… other than the capital cost of the heat pump, it will be close to a wash on heating and probably not much different on the AC
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Good to know that my math looks right but it's still confusing. I also have heat pumps in my house (installed late 2023) and they configured with a balance low limit of 30F and high limit of 33F. They have an HSPF of 10 and I've only seen my electric usage go up by about 1200-1500 kWh a year since then. Maybe I shouldn't be looking at her total annual heat load but rather just a range similar to mine?
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Not much different on AC? Different from what? Shouldn't it be more efficient than the window units that she has now?
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Maybe. Depends on how good the window units are now, and what the relative spaces being conditioned are.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
I’m at 1.2 kWh per heating degree day for a heat loss of about 20kbtu at 15F. So very reasonable that a higher heat loss home in a cold climate would tack on 10,000+ additional kWh. Which is good actually - the northeast has high delivery rates because people don’t use much up there. As usage increases, those costs will come down.
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Always the optimist, @Hot_water_fan ! While it may be true that usage of the grid assets — mains, substations, distribution — may be less in some ways in the northeast, maintenance of those assets, particularly the secondary (typically 23 KV) distribution, never mind drops, is an ongoing nightmare for the utilities. That doesn't come cheap, and the cost isn't going down as people and PUCs become more demanding and less patient with outages.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
@Jamie Hall I’m watching the new electric heating rates in MA. Could be what the future looks like! More kWh moved across the same assets seems like a win win.
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Curious how I can know what the outside design temperature should be that I use? I tried looking for what it might have been in 1977 but no luck, I think.
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Here's sticker that's on her boiler - seems to me that it's saying that burns 1.45 gph of oil… not sure what the 175k and the 152k BTU ratings are. 152k is 86% of 175% so I thought maybe that this yields the efficiency but 1.45 gallons of oil isn't only 175k BTU. Can someone clarify? thanks!
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The challange with your energy costs is to get the system to hit nameplate efficiency in heating. This means getting right sized heat pumps, each on its own outdoor unit (no multi splits). Based on the oil use, the heat load is about 40000BTU (run the link from DcContrarian for more accurate result) which is well within the range of a heat pumps.
The challenge with older houses is you have a lot of rooms. The best way to get heat to all of them is with a ducted unit, which is doable if the basement ceiling is open.
The next best things is a two larger wallmounts. One on the main floor in the most open area for the bulk of the heat and 2nd one on the upper floor hallway for the rest of the heat and cooling. Again, these should be on their own outdoor units. With point heat source and older structure you'll still have the run the oil to even out the heat near the perimeter but the bulk of the heat will come from the heat pumps.
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Could you elaborate on how you're getting the heat load of 400K BTU?
As his link suggested, I went and got one of her oil deliveries (12/26/24 - 1/29/25) which was 195.9 gallons.
I had used an efficiency of 65% (that's what she told me but the nameplate earlier seems to indicate higher) yielding 17.8269M (195.9 gallons x 65% x 140000 BTU/gal) BTUs supplied. I got 1129.7 HDD from a local station at 65F for that period yielding 15780.2 BTUs/DD or 657.5 BTUs / DegreeHour.
I'm not sure what outside design temp to use (the directions use 20F). Using his same 45 delta T, I get just under 30K BTU/hr.
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You can find your current design temperature here:
It's virtually certain that in 1977 the installer just used rules of thumb to size the equipment. There's basically no penalty for oversizing an oil boiler.
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One too many zeros, should have been 40k. Heat loss is roughly heating season therms used x 42. Why 42, the math works roughly out to that in heating climates and because it is also the answer to life universe everything.
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This is near my calculation too. Energy cost wise at my 31c/Kwh rate oil has to be more than $4/gal for heat pumps to be cheaper
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So how do I interpret that nameplate that I posted earlier? Does she really have an 86% efficient boiler?
And if we're thinking that her heating load is ~40K - what do you all think of the 1st proposal we received. It's for a 4-ton ducted system downstairs (Bosch) and a ductless 4-ton (Fujitsu) upstairs with (4) handlers, basically one in each bedroom (7k, 12k, 15k and 24k for the loft area above the garage).
Kinda seems like a lot to me, especially the loft area.
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I did the detailed analysis for Boston using the process outlined in the post I linked to above — use NEEP to get hours for every temperature — and for an annual heating load of 90 million BTU's I get a design load of 38,100 BTU/hr at 13F.*
So now I'm shaking my head and wondering what kind of sorcery @Kaos is capable of.
*(I didn't do it for this question, I had the spreadsheet done for Boston for a different project and just had to plug in the BTU's.)
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For heat pumps, nameplate capacity is meaningless. What matters is capacity at your design temperature or even your average annual minimum temperature.
But eight tons does sound like a lot. The four head unit in particular can be problematic. Most variable speed compressors have a minimum modulation of about 25% of output, which would be 12,000 BTU/hr. So when it's running that's the least it can produce. If the load is small and the heads are calling for less they will be forced to take more than their calling for, sometimes even if they're off. This can lead to swings in temperature which affect comfort.
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Understand, just trying to make sure that I have correct heat load from her existing system per your directions. Using 65% right now, not sure if that's accurate.
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