Trying To Estimate Heat Pump kWh Usage

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.

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…

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

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.

@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.

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.

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.

18K for the ducts really jumps out at me too.

@Jamie Hall : "The COP of a heat pump varies with source air (or water) temperature. Therefore any calculation of power usage by the heat pump must take into account that variation. The relationship of COP to source temperature varies for different types of heat pump, so you need to be looking at specific makes and models for the calculations to make any sense at all."

If you look at the spreadsheet he linked to, that's exactly what he's doing. There's a row for each degree with estimates for heating load at that temperature, annual hours at that temperature, and heat pump COP at that temperature.

The COP is going to depend upon both temperature and percent of capacity, variable speed heat pumps have higher COP at lower utilization, at least until they start short-cycling. So it also depends upon how the system is sized to the house.

Do your ducts run through an unconditioned attic or crawlspace? Because if they're inside the building envelope there shouldn't be any loss at all.

Great job with the spreadsheet. Keep it accessible as rates change! Hoping you get the $.04/kwh shipping and handling rates we have in the mid Atlantic soon!

Great work!

@JustinS : "The ducts are all in unconditioned space, either the attic or the basement. Isn't that pretty standard?"

I probably shouldn't have said "unconditioned," that's kind of an obsolete word.

For most of the 20th century there was a belief that attics and basements were kinda-sorta inside the house and kinda-sorta outside, and didn't need to be insulated, and that heat would only go where you send it.

It's only in the past 25 years or so that building codes have come to realize that insulating a house is not like putting on a sweater vest, where keeping part of your body warm keeps the rest warm, and more like waterproofing a ship, where you have to cover the whole thing. So now the codes require that all six sides of a house — four walls, floor and roof — be enclosed to control the flow of heat, air, water and moisture. Those four barriers are collectively known as the "building envelope." And every part of the house needs to be decisively either inside or outside the building envelope, and parts that are outside should be treated as outdoors and typically ventilated to the outdoors.

So conditioned vs unconditioned isn't really the way that modern buildings should be thought of, space should be thought of as inside or outside of the building envelope.

By 2013 this was well-established in the code. Massachusetts has been very active in adopting the latest codes, and while code enforcement falls to the cities and towns, I suspect in Suffolk County it's pretty rigorous. So your house was probably built this way — as evidenced by the 2x6 walls and the R38 in the attic.

With all that preamble, it's still legal to put HVAC equipment outside the building envelope. It really shouldn't be, particularly in new construction, and it's not a best practice. But it is done. In older houses it was commonplace, particularly in buildings that were originally built without central heat.

So your attic and crawlspace may or may not be within the building envelope. The quick check is, is there a layer of insulation between the space and the occupied parts of the house?

If the ducts are within the building envelope, leakage is not a concern, it all goes into warming the house. If they are outside, it is a concern, some of the heat flowing through those ducts is going straight outside. How much depends on how well they were installed, how tightly sealed and how well insulated. And it's always possible to go back and improve the installation.

I would insulate the basement. When you're building this house it's actually cheaper, typically there's less area to the perimeter walls than the floor of the first floor. Insulating the basement also means you don't have to worry about your pipes freezing.

In new construction, for the second floor it's easiest just to make the second floor a foot taller and put the ductwork in a dropped ceiling.