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So I’m getting a new boiler and have a deep fear of it being sized too large. House is 16.67×37, 4 stories, ceiling heights 8/10/8/8, all new windows, doors, 6 ins (R30) foam insulation on 480/620 square feet of roof, plus foam insulation on roof exterior (remaining portion of roof interior will have exposed beams with pressboard insulation between), possibly even insulation ceiling of basement below garden floor, plus both front and back walls insulated with r14 foam.

So my question is, can I get away with a 105,000 BTU boiler? My plumber says hells no, especially because I will be running indirect water tank off the boiler, but online heatloss calculators indicate my worst case scenario heatloss to be around 87,000 BTUs, which means the 155 BTU boiler advocated by the plumber is almost 2x too big. Plus, the place where I am buying my radiators told me sizes that were also almost 2x what I found using heatloss calculators. I’m not so worried about rads because it’s a mod con boiler, and oversized radiators mean lower water temps, which is good for efficiency…but panel radiators are expensive and I would like to size them accurately. Care to weigh in, Master Plumber or anyone else?!