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Sizing a boiler in Watertown, MA
Hello, I'm one of the many trying to take advantage of National Grid's incentives to convert my 30-plus-year-old, forced-hot-water oil boiler to gas. My posting may be similar to others posted here but I'd appreciate any advice. My house is 1510 square feet, 2-story built in 1928. Last year, I had dense pack insulation blown into the walls and I personally replaced my old storms with good Larson Gold series storms. I have cast iron radiators. I also heat my house primarily with a pellet stove and for the past few years have only used oil to heat my DHW. But I want to upgrade to gas if I decide to sell my house and also to get gas into my kitchen while I live here. I've started the process, met with four contractors, and have received bids back from two. I'm set on a high-efficency boiler and indirect water tank, not a combi.
Here are my concerns:
1) None of the contractors, even the National Grid Value contractors did a full heat loss analysis. I know, I know -- it's the most important thing. Some measured the gross space and some counted radiator fins, and one guy took a little more time.
2) I did my own calculation using a series of online calculators and taking into consideration wall insulation, cold partition length, etc. I also used "old-school" methods of just relying on square feet or radiator size. I know, I know -- I'm not an expert but all of my numbers, including fudge factors, never came close to 40,000 BTUs for my house.
3) Here's the real concern: the two bids I have and the 2 bids I expect based on my lengthy conversations with the guys, have suggested boilers ranging from 80K (National Grid's preferred Burnham Alpine) to 110K (Lochinvar Knight). Everyone told me not to worry about the apparently large size because the boilers modulate down. And each told me that the DHW tank is the one that is driving their higher calculations.
So isn't this against everything I've read here. Shouldn't I go with a small boiler (say 50,000 BTUs if my calculations are right) and have it prioritize energy towards the water heater when I'm taking a shower. You're not supposed to size the boiler to the big BTU number on the 40-gallon DHW tank, correct?
I should note it's just me and sometimes a girlfriend in the house. Two back-to-back showers sometimes. The dishwasher may (but probably isn't) running as I shower. It's one zone. Listen, I know each situation varies and I've only sketched out broad details, but it's a big investment and I think I'm being led down the wrong path. Who knew HVAC could be so complex? thank you.