They're not efficient when the temperature drops below a certain point. This is predictable and based on system design criteria. It doesn't mean it's not effective, it's just that it wouldn't be a reliable source of heating year round. It may still make a worthwhile addition to another system, or combined with several other upgrades, may become an acceptable single solution.
A solution that is inefficient two week a year but efficient the rest can still be cheaper than some hybrid setup.
If it is expected it simply won't work a few weeks a year, sure, that's clearly not effective.
To know whether a heat pump (air-to-water) can replace that boiler effectively and maintain comfort, I had to find out whether the house would be comfortable with water temps of 135°F or so. Is there an amount of “that’s just looking at the building construction” to make that analysis? I think maybe technically yes, but practically no.
As it was, to get an answer, I abused my old boiler by turning the water temps down (causing condensation and slow damage [planning to replace it anyway]) and seeing what happened on cold days.
Then, because the answer is almost always going to be "yeah, it's going to be really close...", I felt well-advised to prove it via experimentation rather than commit to changing the heating plant to a system that could not provide 150°F flow temperature.
If your heater can go low (mine bottoms out at 50 unfortunately), by far the easiest is to just test.
Alternately, to get a realistic measure, you'll need to set your boundary conditions about what the heat flow out of the room will be, which is a bit simpler to setup with U-values, area, delta-T, and heat capacity of materials. You'll also need to do this to every other room in the building simultaneously. This is a Manual J, or heat balance method or the radiant time series method load calculation that will balance out with the amount of heat leaving your radiator without knowing its specific shape.
I am not familiar with the nuances, but this appears to be an equation with two variables - water temperature and radiator surface area. Maximising surface area should slow you to use lower water temperature. And the lower the water temperature, the more efficient the heat pump will be?
As a homeowner who bought a 1920s house because I like the character of a 1920s house, would I like to do that? Absolutely not!
A two-part answer:
From the Mechanical Engineering/Thermodynamic angle:
> Ultimately, I proved to myself that a heat pump could work down to an outside air temp of about 18°F [which is slightly above our 99th percentile design temp] with flow temps of 135°F, so an air source heat pump could work with slightly reduced comfort on about 2% of days or could work all the time with supplementation with a 9kW [30K BTU/hr] electric boiler.
From the commercial angle:
> What killed the project is no heat pump installer was interested in doing the work (as reflected by outright declining to bid, while bidding a 4-hour gas boiler swap, or by bidding so high that they might as well not have bid, while also cheerfully bidding a 4-hour gas boiler swap). So my house still burns gas for heat.
On the engineering front: I think the answer is often going to "<hissing inhale> It's going to be close; we should probably test it..."
Calculating the heat loss of a house is really not trivial. Often you'll have no idea of the materials used or the quality of installation, and not really a way of finding out unless your up for some destructive investigation.
You will have to invest a lot more electrical energy and you might not be pumping heat as much as just converting electrical energy to heat, but let's not spread the myth that a heat pump will leave you freezing once outside temperatures drops below a certain threshold.
I'm looking at the datasheet for my Fujitsu system and it is specified to work down to -25°C.
Does it lose efficiency when it's cold outside? Sure. But guess what, it's still more effective than resistive heating! I am starting from a SCOP of 4.89.
They're devices. They're engineered. They have specifications. The specifications have implications as to how they will function in a given environment.
I'm not here to sell heat pumps or to make you feel good about your past purchases. Are you?
I am thinking in Nome, Alaska where such heat pump could prove its versatility as an anti-freezing component of living quarters or even maintenance shed.
That is not a myth - I have a heatpump in my house and it will leave me freezing on the coldest days. The system is sized so that it cannot keep my house warm at -5C (the system can deliver heat, but the house will cool down). Worse, it did get below -25C here for a couple days which is as cold as any heat pump will work - I don't know of any house that is insulated so well as to be warm when the outside temperature is below -25C for a few days without some heat - but mine isn't one.
A correctly sized heat pump can keep your house warm to -25C, but if the installer doesn't give you a correctly sized system it will not. Most installers don't know how to size heat pumps.
If we standardised ground source heat pumps as a must have all multi dwelling developments like apartment blocks and rows of terraced houses who could share one well et cetera, It would not be a noticeable cost at all