I too was a bit disappointed that the data in question wasn't explained. The only thing the article has made me do, is become even more wary about heatpumps!
I don't get the impression that I should be more wary about heat pumps, but that if I have the data collected in advance I would be ready to switch when the time comes. Armed with that data, you could also do some advanced prep work so that you can purchase a smaller system that will still maintain high levels of comfort. And any prep work would likely improve the performance of the existing system you have.
Your last point is excellent, especially if you have a modulating/condensing boiler. (Do you have plastic exhaust piping and a small plastic line that drips water from time to time when the boiler is in operation? You do.)
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.
Adjust the boiler so that the output flow temperature is 55°C (or 130°F) or lower. See if your house remains comfortable on the coldest day you expect to have.
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.
I do appreciate the writeup, but I feel as though there has to be a better way :)
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?
Air to air heat pumps are a pretty straightforward retrofit there. (Including central units with reversing capability; heat pumps don’t have to be per-room minis.)
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.
1. Heating Degree Days
2. Net heat delivered (BTU delivered * efficiency)
3. Calculate BTU used per heating degree day
4. Calculate your heat load (BTU/hour)
5. Multiply by the ASHRAE sizing factor (1.4).
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.
