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Potential for using an undersized mod-con boiler
First time poster here and could use some help. Sorry in advance for the long post. I'm building a custom log cabin at about 9000 feet of elevation in the Colorado mountains and I've gotten to the point where I need to start making some heating decisions. One thing I'm sure of is that we won't be doing forced air, since there's not an easy way to run ductwork given this construction style and I don't really want to do that anyway.
The cabin is a 40x40 square with a drive under garage. I ran pex for radiant heating before the slab was poured in the basement. On the main floor I think I'm more or less locked in to Warmboard or similar, since my structural engineer says that using lightweight concrete would be dangerously close to the design load limit of the joists and support beams.
It's hard to get a bead on the energy efficiency of this place due to the non-standard construction. The roof is insulated with rigid foam with an R-56 rating, and the windows are all high efficiency but the walls are ... well ... logs. Really really big logs, ranging from 18" to 30" in diameter, with rockwool insulation and cement based chinking to fill the gaps between logs. So getting a specific R-value is a head scratcher.
Here's my current dilemma. I'm at the point where I need to make decisions on the heating system, and I just found out a good friend of mine has a mod-con boiler and accessories sitting in a closet gathering dust. It's a two year old Lochinvar Knight already set up for high elevation along with a big box with the expansion tank and lots of the associated bits and bobs. He bought all this stuff planning to put it in his house then realized it wasn't going to work for a variety of reasons, but he wasn't able to return it. He's currently out of work and could really use some cash.
The only problem is that my best guess is that I'll need something in the 85-110k BTU range given the elevation and this boiler is only 55k BTU. I'm sure it would provide handle the basement / garage radiant and DHW via indirect with no problem. That's important in the short term, since I'm getting the well pump and pressure tank installed in the next few weeks and will need heat down there to keep things from freezing.
So what do y'all think? Buy the 55k unit from my buddy, knowing that I might need to buy another if it can't keep up? I can always add a second unit for the main level if needed, though that would obviously complicate the plumbing quite a bit. Or get the bigger unit from the start?
Thanks in advance for any insight.