As to “Why is this a thing now?” Well, …
Our next door neighbor when I was a kid worked for the local power company. He had a heat pump installed in the 60s. Which was mumble years ago. 
He touted the system to the neighbors but I don’t think anyone else got one. Local power was mostly hydro (and cheap) but initial cost, esp. back then, was a big deal. I can see pushback from natural gas and fuel oil businesses.
True, but underground temperatures do go down as you move north. Not nearly as much as air temperatures of course, but the annual average air temperature tends to track pretty closely with the ground temperature. What matters more is that being farther north requires more heating capacity, i.e. more wells/loops/trenches. However, Montana can get quite hot in the summer as well, so that may be a benefit for a ground source heat pump because you’re not wasting as much of your excess winter capacity with low summer loads. Generally, a very unbalanced winter/summer season (think Florida, Arizona, Maine, northern Ontario) requires a bigger well field to prevent “poisoning” the ground by dumping too much heat into it in the summer or sucking out too much heat in the winter. Any system where that happens, requiring the well field to “rest” or “recharge” is undersized. At least in a temperate climate an undersized well field will still equalize over the seasons.
Well, that depends on how far below the frost line. Just below the frost line, it’ll be just above 32 F.
Yeah, you’d want to trench about 8’ deep here. Luckily the soil is very cohesive (clay) so as long as no one needed to get in the trench you wouldn’t have to lay back much if at all. Still, per @jjakucyk you would probably need a lot of trenching.
For Arizona and Florida that is not an issue because the winter lows are still high enough we don’t need no stinkin’ wells. It’s those poor sods in Iceberglandia who do.
Ground-sourced heat pumps are much more efficient than air-sourced. In Arizona, rejecting A/C waste heat to the ground at 60-70* is going to save a boatload of money compared to rejecting the heat to air at 115°.
Right, and they don’t get clogged with dust and sand either.
That is something that should be taken into account, but might not matter given the cost difference. If I spend an extra $10k for ground source, and save $100 per year over conventional, I’ve probably made the wrong decision.
My example is when I got my new gas furnace. Stepping up to a more efficient model didn’t make any sense. It was something like the more efficient model costing 150% of the less efficient model, but saving 20% in cost. Problem was, at $200 or so per year to heat my house, it was going to take twice the lifetime of the furnace to see the savings (or something, I don’t remember the exact numbers).
Opposite was true when I got my gas hot water heater. After utility rebates, the more efficient model cost exactly the same as the less efficient model, so no reason to go with the less efficient one.
One thing to remember is that 70 degrees is not twice as hot as 35 degrees.
Even 35 degree air has lots of heat energy in it compared to the ~-460 of absolute zero. So, you are not doubling the temperature, you are only raising it by about 7% (if I did my math right.)
To the OP, I had always thought that heat pumps were standard, as the house I grew up in, that was built in 1978, had a heat pump.
It did not have gas, so that may have been a factor, as most houses I have seen do have gas, and do not have a heat pump.
So, FWIW, my experience is this–
We live in NYC. We have an older house (built in 1901), attached on both sides. When we moved in (25 years ago), we replaced the older oil-fired boiler with a natural gas-fired boiler. The house has steam radiators (not hot water, actual steam).
We’ve had very good results with the gas-fired steam heat (radiators are noisy, but that’s kind of homey). The house gets nice and warm and stays that way all winter (we did replace the drafty old windows, which made a HUGE difference).
But for A/C, we were working with window units, taking them out in the fall and putting them back in the spring. Those worked fine, too, but pretty much doubled our electric bills all summer long.
So two years agoo, we got an unexpected infusion of cash, and decided to install a mini-split A/C system for our summer needs, hoping to save some on the electric bills, and also to have a more comfortable, controlled summer environment (and stop moving heavy window units in and out of the windows and in and out of storage twice a year).
The HVAC company told us about a rebate program from the state–with that rebate, the total installation cost for A/C plus heat pump was about $5,000 LESS than A/C alone. So that was a no-brainer. We got the heat pump unit.
The winter was not terribly cold, but typical for NYC, and it turned out that the heat pump did not really provide the same level of comfortable heat in the house. It was OK, but required sweaters and socks and heavy blankets. And still just a little bit chilly all the time.
And worse than that, at least here in NY, at least for now (last winter), electricity to run the heat pump was MUCH more expensive than firing the boiler with natural gas. (and in NY, most electricity is generated by burning natural gas anyway, so there aren’t any real environmental benefits at this time). Like our electric bill was triple what it used to be. Much, much more than we wanted to spend.
So for the winters, we’ve just gone back to using our gas-fired steam system. In the summer, we use (and love) the mini-split which is ending up being much less expensive (again, about half) than the old window units were.
So we got a heat pump basically for free, but we’re not using it. If the balance between natural gas and electricity prices changes, we might need to re-evaluate. Now, though, we’re comfortable and happy and saving money (will we ever save enough to pay for the installation cost? Probably not. But the payoff in comfort and convenience is huge. And we hope to be able to contribute to environmental benefits, too, sometime).
We recently had a ductless system installed to heat/cool an addition. Works great down to 20F or so, but really sucks the juice! WAY more than we expected from the literature.
@Joeu what I would do in a situation like that is run the boiler when it’s below 40 degrees outside, but use the heat pump otherwise. Either that, or run the boiler at a low level, say enough to keep the house at 60 degrees or so, and use the heat pump to boost the rest of the way. The advantage of radiators is that they’re placed where there’s the most heat loss (exterior walls by windows), so they take the chill away.
That’s a problem with your old house which is probably poorly insulated, or not insulated at all. The heat pump has blow more air, and it’s probably up at the ceiling being a mini-split, so it’s trying to push the hot air down. With lousy insulation there’s going to be more temperature stratification between the ceiling and floors. By running the radiators at a lower level it lets you control the temperatures better per room with each heat pump unit without relying on them to take on the full load.
That’s not actually true. As long as the heat pump provides more heat than the conversion loss of the gas power plant and the electrical distribution network then the heat pump comes out ahead on resources used. I think the COP only needs to be around 3 to make it the most efficient way to get heat from any sort of fuel, including just burning it directly to heat your home.
Like if it takes 100,000 BTUs to heat your home, that’s 125,000 BTUs of gas for direct heating (steam boilers are generally only 80% efficient, and I think at best can maybe get up to 85% because they can’t condense the gas vapor like a hot water boiler or furnace). A heat pump with a COP of 3 can provide that same 100,000 BTUs with the equivalent of 33,333 BTUs of electricity, or nearly 10kW. If a gas turbine power plant plus transmission loss is 50% efficient then that power plant is still only using 66,667 BTUs worth of gas to make the electricity to power the heat pump.
I would add that if you buy a gas furnace how, it will be burning gas for its entire life. But if you buy a heat pump now powered by gas-fired plants, in 10 years it may be that most of the electricity now comes from wind or another clean source. It’s the same advantage that EVs have in that they automatically benefit as the grid gets cleaner.
Assuming this wasn’t a ground-source unit, heat pumps only have a tiny cost overhead compared to plain A/C units. At least this is what I’ve seen shopping for mini-splits, not to mention the basic physics of it (a heat pump really just an A/C running in reverse, and doesn’t need much more than a couple of extra valves and the like).
Thanks! This sounds like a good recommendation and we’ll try it next winter. And thanks, too for the info about efficiency and sustainability. My understanding has increased!