To be clear I own about 20 HVAC units. I really feel that lots of the push to go to heat pumps is greenwashing. Of course it depends on where you live, but they can be just as expensive and dirty to run as a gas furnace in the right (wrong) conditions.
I'm trying to imagine what conditions in the US would lead a heat-pump to be as expensive and dirty to run as a gas furnace, but I'm having trouble coming up with anything where both conditions are met. Certainly when natural gas is cheap and in areas that electricity is very expensive the economics are closer to being equal, but 'dirty'?
Can you give an example?
Well I think we all agree that in certain conditions a heat pump just turns into a massive space heater right? The heat strips come on and then it is consuming more than any other appliance. The heat strips on my unit are a double 50 amp breaker, the largest in my box.
So if you live in climate zone 4 or below I think heat pumps are fantastic. The heat strips barely run and heat pumps do their job pretty well.
Now where I live we are mostly on coal *checks notes* nope mainly natural gas I was way off! I think my argument is less persuasive with using gas rather than coal. But I still think the below is true.
How is it more efficient to (wastefully) convert natural gas to electricity, then ship it to me, to convert it back into heat? Would it not be easier to ship me the raw material (gas) and then have me convert it directly to heat in my furnace?
Note: I am not at all a geothermal/electrical engineer, or whatever smart people run our power grid. Convince me otherwise but I don't see how a giant ass space heater is better than a furnace.
Very rough calculations:
1 cubic foot of natural gas contains ~171mJ of energy, also equivalent to roughly 47.5kWh (this may not be completely accurate, and it probably varies regionally, but the exact number is functionally irrelevant - the rest of the calculations can be adjusted for whatever is input).
A 2015 estimate put the average performance of a natural gas powered electricity plant at about 56% (
https://www.forbes.com/sites/judeclemente/2016/04/10/u-s-natural-gas-electricity-efficiency-continues-to-improve/?sh=739c44fd35a4)
This means (using the number above), every cubic foot of CNG burned at a plant produces about 26.6kWh of electricity.
Your gas furnace is probably about 92% efficient, and assuming you have no leaks, for every cubic foot of natural gas you buy, your furnace can turn that into the equivalent of ~43.7kWh of heat.
A lower end central heat pump in a moderate climate has a COP of about 2.5 on a good day. Many heat pumps are edging toward 3, and some even closer to 5 (less in colder climates). Sticking with 2.5-3, that essentially means that for every kWh of electricity that's consumed by the pump, it ideally is producing 2.5-3kWh of equivalent heat.
Ignoring minor transmission losses supposing your electricity is coming from a natural gas fired plant, every cubic foot of natural gas that is consumed by the plant is converted into the equivalent of 66.5kWh (COP 2.5) - 79.8kWh (COP 3) of heat.
Or in other words, it's more efficient for the natural gas plant to burn the stuff if you have an air-source heat pump than if your gas furnace does so. On a good day, almost twice as efficient.
Now maybe you're thinking "but what about the days where it's too cold and the COP drops to 1 (aka the heat strips are on)"?
I like round numbers so suppose there are 180 days out of the year you're using heat from a system with a COP of 3 under reasonable conditions. I'd bet that there will be less than 20 total days (factoring in nightly lows) where the COP is at 1 because you're using the heat strips. And then suppose you're at about 75% average efficiency otherwise (I'm making this up, but you need to account for edge-temps, time the system takes to defrost, etc.).
So your average COP for the entire heating season is going to be about 2.1 in this scenario. That means for all the gas the power plant burns in a heating season, you'll be seeing an average of 55.9 kWh of heat per day from your heat pump AND backup strips. That's a considerably more efficient use of natural gas.
Better yet, natural gas plants are getting more efficient, renewables are a thing, hydro and nuclear say "Hi!" and coal is on the outs. So reality is that in 2022, you're going to be even better off efficiency wise with a heat pump.
Now clearly the underlying issue is that natural gas is still fundamentally less expensive, but that's for now and I'd bet that in the next decade or two as fossil fuels decline, residential natural gas will be not nearly as cheap compared to electricity.
ETA, this guy is one of my favorites to have on in the background. He lives in the Chicago area, and I based the above though exercise off this video of his. It's worth watching all his videos on heat pumps:
https://www.youtube.com/watch?v=MFEHFsO-XSI&ab_channel=TechnologyConnections