Haha, thanks Dana. Great explanation as always. I really wish there were well-rated contractors who used HTP and Lochinvar routinely down in this neck of the woods. This seems to be largely Weil-McClain country, which doesn't get you very far.
Yeah, I was bit surprised that the company itself was recommending a significantly larger boiler, rather than, say, the Tx81 if they really felt the 51 was too small. I can't quite figure out their reasoning for why oversizing by 100k BTU would be a good idea. That being said, the contractor stated that he sizes by IBR, and it is true that the Tx51 IBR rating is only 45k, which doesn't leave a ton of error margin.
Ultimately, this is the art of the possible, since I can't find a contractor around here who routinely installs the KHN85 or HTP UFT80. The fire tube design does seem superior to the water tube in terms of potential maintenance issues, so that is tempting. Water content for the TFT154 per the manual was 3.9 gallons.
A more careful fuel use over the highest two winter months last year: 361.4 therms (our gas use in the summer is only about 5 therms, so I don't think the water heater or gas stove is making a huge difference). 1533 HDD (based on a base temp of 60F). So, 361.4*100k*80%/24hr/1533HDD = 785. Using an outside temp of 40F and a delta F of 28F to get to 68F, that yields 22000. So maybe TFT154 isn't too crazy? Alternately, since replacing the boiler isn't an emergency, I guess I could wait for a year or so until more manufacturers get with the program and make high turndown firetubes available in this area.
This is awkward, but...
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