Cost of Heat Pump vs. Gas Furnace
Hi,
I am in the process of deciding what to do to add cooling to my 1915 uninsulated 1600 sq ft house (~ 800 ft/floor.) I figured I’d try to get an idea of what my gas and electric prices are on average and then compare the cost to heat with gas vs ASHP. Am I doing this right?
My average cost for electricity is 21.5c/kwh (after all fees and taxes)
My average cost for NG is $2.00/therm (after all fees and taxes – price gets lower per therm with more gas usage due to large “delivery” fee from utility that becomes a lower percentage of the total bill as it gets larger so I just used bills from the larger use heating months for this estimate – they ranged from $1.81-$3.06/therm and I omitted the $3.06 as an outlier)
Last year I used 450 ccf for heating with a 96.5% eff. forced air gas furnace. (I would use probably 3x that much if I didn’t use a wood stove every day in my living room heating and I also use a setback for several hours per day to go down to 58F at night and during weekdays vs 65F when I’m home.)
If I assume a heat pump would avg. 3.0 COP over the heating season, it would have cost me $946 to use it to heat our house assuming it was just plopped into the place my furnace is now and loses the same energy to ducts and leaky house, etc. and The gas cost me $900 over that time.
1) It seems this is not a fair comparison though in that the heat pump would have to stay at 65F 24 hrs per day or maybe use a slight setback that I would have to time so that the system recovers by the time I want it warm again? vs using a setback to 58F with the furnace. So, I’d be losing the efficiency of doing a setback with the heat pump. Can I account for that somehow? I guess I’d be using 7F more of heating for several hours per week with the heat pump in other words. Would that add an enormous amount to the heat pump cost for heating the way I do now with maybe a slight setback? Or is there a better way to think about how to use a heat pump in my case?
2) In practice is it pretty accurate to just apply a simple avg. COP if I had just kept my house at 65F using the furnace as a comparison vs using a setback?
3) I had thought I would either add a heat pump to my furnace either as a hybrid or as a separate unit and just use the furnace when it is really cold since I already have the furnace set up, but if it wouldn’t cost me a ton to just ditch the furnace, that would be preferable.
4) I also really really want to add central dehumidification/cooling to my house, as I said before, so I could just go with 1 mini split per floor or maybe 1 on the second floor to achieve that instead of adding some kind of air hybrid setup and also use it for some shoulder season heating while relying on the gas furnace for heating during cold weather and for bringing the house up from a setback.
Any thoughts?
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Replies
You want to use the marginal energy costs, so everything that’s $/unit. Anything $/day or $/month is irrelevant unless you plan to ditch gas entirely and leave the gas grid. For example, my $/therm is $1.40 and my monthly fee is $16. So when I ditched the furnace for a heat pump I used the $1.4/therm but when I eventually ditch the gas water heater (the final gas appliance), I’d use both the $1.4/therm and the $16/month.
The efficiency of a heat pump is also not constant, so while an average COP of 3 makes sense for the furnace or heat pump install decision, since you have the furnace already it’s more temperature dependent. Some hours it may be cheaper to run the furnace, others not. If the gas monthly fee is high enough, it might not be worth keeping the furnace financially even if 10% of the year it’s cheaper to run.
A central heat pump set up is the easiest and probably cheapest since you already have the ducts. It’ll be more effective than a single mini split per floor as well. Much easier to maintain and eventually replace.
Setbacks generally aren’t heat pumps strong suit, but a slighter setback could be fine. Their efficiency increases as indoor air temp decreases so you’ll be more efficient and have higher capacity at 58. At the end of the day, you’re talking about $50/year with the amount of wood you burn.
Thanks for the input. My cooling load is way less than my heating load, so I thought getting a small heat pump added to my furnace setup somehow would be wise. I suppose it's also possible to get two outdoor condenser units, one that can run in the summer most efficiently for the smaller load and a second that can run in addition to the first in winter for heating? Or is it best to just get one large outdoor unit for an air handler heat pump setup?
If you get a variable-speed unit for your heating load, and you are careful not to oversize it, it will work fine in the summer even if it's a bit oversized. If you don't quite meet the humidity control objectives in the summer, you can add a dehumidifier.
I need to have a good manual J done and see what the difference is between my heating and cooling loads. I'll look into how much heat pump air handlers can reduct output. The dehumidifier seems like a good idea. I assume that can be added into the main air handler setup?
My math says
Gas a 2.00 per therm = 99967.1 BTUs /2 $1= 49988 BTUs x.965 1$=48238.42 BTUs
Electric 21. kWh=3412 x4.65= $1 15869 BTUs x COP of 3 1$= 47607 BTUs
Making the gas 1.2% cheaper
It is impossible to beat the comfort of the hot air a furnace will provide.
Given your obscene utility rates air sealing and insulating seems to be the smart move and consider adding solar panels if you get enough sun.
Thanks Walter. Yeah, my utilities are high. The gas especially has gone up a lot. My electric has been pretty steady for awhile now as I recall, and it is 100% renewable, which is nice, but the bulk of the cost is the "delivery." Our local utility has a lot of bloat I think...
So, you'd work on air sealing and insulation, keep a gas furnace and add an air conditioner? Or add a separate heat pump for cooling?
I was starting to think about solar. Are you saying to stay on the grid or go off grid with solar? I don't know if the savings are that big if you stay on grid here. You have to still pay a hefty delivery charge.
What’s the delivery charges? $/kwh or monthly fee?
My bill from 8/20/21 - 10/20/21 was
816 kWh used
Delivery charges: $117.58 ($0.144/kWh)
Supply charges: $66.53 ($0.0815/kWh
Taxes: $10.48
Overall, $0.238/kWh
The total delivery cost goes up as you use more but there is also a basic $19.50 just to be connected.
Air sealing is always the first step. Tape a fan into a window blow air out of the house. Then use smoke from incents stick to find the leaks and caulk them. Only after the leaks are plugged should you insulate.
With your wood stove be sure to be sure to monitor the draft of the stove.
Solar with net metering is a generally a great deal. Solar off grid by choice seems silly to me. (Batteries are expensive and don’t last very long)
Walta
Ah yeah, the box fan and incense is a good idea. I read that under baseboards is a big area. I also know I need to do a better job around my kitchen range hood damper and bathroom fan exhausts. The holes where my pulleys hold the rope in my old windows can be bad too. I covered a few with rope caulk. I should probably do all of the ones that I don't open.
Check out the BEopt software, it can make the comparisons you are looking for:
https://www.nrel.gov/buildings/beopt.html
thanks, Erik! I'll check it out.