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In a rural home I want to install a new oil fired boiler that will also provide domestic hot water. The existing boiler which also provides domestic hot water is original and 40 years old and input is rated 145,000 btu. I’ve performed heat loss calculations for the home and arrived at a needed boiler output of 70,976 btu, but that does not include additional capacity for domestic hot water. One installer wants to simply replace existing boiler with one of equal capacity but he did not measure square footage of home much less determine heat loss of ceiling, doors, walls, windows, above grade foundation, and unfinished basement. In general the home is a single story, 1552 s.f. of living space, 1344 s.f. of unfinished basement and R-30 attic insulation, minus 20 degrees F would be lowest winter temperature (home is in northern Michigan). Questions: What factor is “typically” added to boiler capacity to provide for domestic hot water? I’m certain that just matching new boiler to old will result in much excess capacity but what problems am I likely to have if this is done?
Thank you
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The drawbacks of buying too much boiler are spending extra money and having a system that turns on and off frequently (everything lasts longer if you let it run for a while at a time instead of many starts per hour). The downside of too small a boiler is a cold house and people NOTICE that. So many boilers are oversized.
Using a REALLY ROUGH rule of thumb, I get 110,000 BTU/hour for space heating at -20F. But why guess? Even a fancy calculation can't capture all the details of your house's construction but a simple test can. Time the duty cycle of your existing system on a cold day (for instance 30F). That duty cycle (like 30%) times 145,000 BTU/hour is your heat load at 30F or with a 40F temp differential (70F-30F). Multiply by 90/40 (90= 70F-(-20F)) to get the heat load at -20F. To time the duty cycle of your current system you could use a stop watch while you sit by the boiler with a good book. Or wire a Hobbs hour meter or old analog electric clock into some 120-volt contacts on the boiler. 10 hours later look at the elapsed time.
Note that in calculating the new boiler size from the old (145,000 BTU/hour) you need to compare apples and apples. If 145,000 is the OUTPUT, great, your answer will be in output BTUs. If it is the INPUT BTU's you need to factor in the efficiency of the old and new boilers. The new boiler is probably about 5-10% more efficient. (Or even more so, if you pay more for that).
I assume that the boiler heats a stand-by tank of hot water that is at the ready when you want a shower. Don't factor much at all for hot water because even at -10F, you'd still have extra capacity to eventually get that tank warm again. And the DHW is probably prioritized (ought to be) to be heated first. You won't miss the BTUs it takes for a shower until it gets down really close to -20F if that is what you size it to. -David
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In a rural home I want to install a new oil fired boiler that will also provide domestic hot water. The existing boiler which also provides domestic hot water is original and 40 years old and input is rated 145,000 btu. I've performed heat loss calculations for the home and arrived at a needed boiler output of 70,976 btu, but that does not include additional capacity for domestic hot water. One installer wants to simply replace existing boiler with one of equal capacity but he did not measure square footage of home much less determine heat loss of ceiling, doors, walls, windows, above grade foundation, and unfinished basement. In general the home is a single story, 1552 s.f. of living space, 1344 s.f. of unfinished basement and R-30 attic insulation, minus 20 degrees F would be lowest winter temperature (home is in northern Michigan). Questions: What factor is "typically" added to boiler capacity to provide for domestic hot water? I'm certain that just matching new boiler to old will result in much excess capacity but what problems am I likely to have if this is done?
Thank you