How about if we start with this? What data, and how do you collect it? The site links to [1] which looks like it has some great info. But any potential adoptee will still need to translate that into some actionable steps to gather the data needed to know how to size the heat pump system
Note that if you have a boiler not designed to operate with condensing flue gases, this test will eventually damage the boiler [over the course of months, not hours]. If you have a plastic exhaust pipe on your current boiler, you're totally fine to do this.
More writeup here: https://news.ycombinator.com/item?id=39144329
In my area, the most common HVAC system is going to be a gas-fired forced-air furnace along with a central air conditioning system. What do you do there?
In some ways, that style of central heat pump is less talked about because it’s usually a straightforward replacement.
Ducts may need to be enlarged in heating-dominated climates though.
That's enough to allow you to size/resize a gas/oil furnace, as well as see what sort of costs you'll run.
To compare directly to a heat pump is difficult, as you also need to pull in your electricity cost, gas/oil cost, and if you have demand pricing, what sort of temp you keep your house at.
I will say though, going from something like a 9 SEER AC/heatpump to 17 is huge. 17 -> 20? Not so much.
The lower you can make the return water temperature, the more efficient your condensing boiler will run. The way to do that is to lower the leaving water temperature (and accept/relish the corresponding longer run times).
With my outdoor reset properly tuned, my Nest thermostat will run 22+ hours of every day and sometimes 24 out of 24. The outdoor reset control is the primary temperature control on the house and the thermostat serves more as a limiter than as a control.
This has the effect of using the lowest possible water temperatures (most efficient), which ends up being the most comfortable for the occupants because there's very little overheat/shutoff/overcool/turn-on cycling.
I found the minimum temperature to maintain 71°F at various outside air temperatures and then looked to see whether the ASHP could provide that flow temp (sometimes called "leaving water temp"). (This is also what the article's author did.)
In a case like that, I'm inherently proving that the pipes are sufficiently sized as well. Now, the downside is that it requires time and effort on the homeowner's part, requires a working old boiler, requires a winter season of variable temps, so it's not a practical way for a contractor to bid the job, but for HN readers, you can do it that way. (Note also that if your old boiler is not a condensing boiler, you will be damaging it by causing it to condense acidic water from the exhaust. I was OK with that, because I knew it was going to be replaced the following year.)
If I found that the house struggled to maintain temp, that method doesn't tell me whether increasing the pipe size would help (though having a larger than desired delta-T coupled with a low flow rate is a strong hint).
For a contractor bidding/design situation, there is a universal hydronics formula which is BTU/hr = 500 * delta-T (in °F) * gallons/minute. Pick the delta-T (often 20°F) and the heat load, and solve for gallons/minute. Look up pipe sizing tables for maximum velocity, then using the pipe size and total developed length, figure out the pump you'll need. Then check the emitter sizing to ensure that it can emit the heat required (to "cause" the delta T) at the lower flow temperatures characteristic of an efficient heat pump system.
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.
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, which matches the author's experience:
> this speaks to the third lesson in my story. The boiler market is focused on cheap and quick installation.