New build small ADU over garage - sizing & heating (underfloor and domestic hot)

Most likely adequate — to get enough domestic hot water your going to need at least 110K BTUh output, and you'll be happier with 199,000. The underfloor heat isn't a significant part of that!

And by the way. Don't be too optimistic about the PV. That, plus a battery bank, may be enough to keep the combi unit running, but it will be nowhere near enough to heat the place. You're still going to need the propane.

@Jamie Hall : "And by the way. Don't be too optimistic about the PV. That, plus a battery bank, may be enough to keep the combi unit running, but it will be nowhere near enough to heat the place. You're still going to need the propane."

I would agree with the above if net metering is not available.

If you have net metering available that's a much better deal than a battery pack. Just rough numbers, a house that size in zone 4 probably has an annual heating load of about 100 million BTU. Using purely resistive heat that would be about 35 million Watt hours. In most of North America a PV installation gets the equivalent of about 1000 hours of production if it is unobstructed, so that would require a 35kW installation, which is big but not unheard-of.

If you use a heat pump you can cut the size by half or more.

That's all dependent on having net metering. If you don't have net metering the most you can hope for is some load shaving.

"The other factor I tend to consider is… what happens when the lights go out on a cold winter night?"

Buying a solar array doesn't mean you have to join a cult. If you live in a place where the power going out is a risk, by all means have a backup generator. And power it with fossil fuels.

you could go as low as a 55,000 boiler with indirect

I have the Lochinvar Knight wall hung 55,000, great boiler, lots of control options

@littletwig: "I used 12F/95F for coldest and warmest days in the Manual J location design conditions for LoopCAD. Rooms like bathrooms 75F heating design temps, and other rooms 72F. The 1st floor (6" concrete) will change temperature very slowly; the 2nd floor with PEX track just below the flooring will respond to changes a little quicker perhaps?

"A heat pump for supplemental heating/cooling, with an LPG furnace for backup heating is also included."

With a 12F design temp your climate is probably like NY City (13F design). There, January is the coldest month, average low of 29F, high of 40F.

If you're designing for 72F indoors, that's 60F of difference at design temperature. On a typical January day, when it's 34F out, that's 38F of difference, so your heat will run about 60%. If you design your floors for a temperature of 85F on design day they'd only be 80F on an average day.

Particularly for bathrooms and other areas where you're likely to be barefoot, a better strategy is to design the floors to be at 85F on the average day, but never go above 85. On days that are colder than average use that furnace or heat pump to supplement and make up the difference.

When I hear "fully conditioned attic" I think spray foam. Which means it's almost a certainty that you don't have the full R49 that zone 4 requires.

In zone 4, a side wall needs R30, or R20 plus R5 of exterior foam insulation.

I wish I had a nickel for every time we had someone come here who started off saying "my house is super-well insulated" and after pressing them for details it turns out it's actually sub-standard.