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Re: Deciding on a new boiler

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Posted by dana on October 14, 2008 at 13:20:28:

In Reply to: Re: Deciding on a new boiler posted by rbeck on October 13, 2008 at 17:50:48:

: Even a Modulating gas boiler has the importance of a heat loss. You want to match the heat loss as close as possible on the high side so the low side goes lower on warmer days. Let's assume you have a heat loss of 60k. A 150k boiler will cover the heat loss. It turns down to 50k. But what about the 60% of the season you need less than 50k. Use an 80K that turns down to 30k and more fuel savings.
: For more info on heat loss see
: www.comfort-calc.net/home-page and FAQ on that sight.

There is a quick & dirty pretty-good easy to use freebie heat loss program downloadable from Slant-Fin:


Then to figure out what temperature to insert into the SlantFin tool as the design day temperature, for CT select the temp for the nearest city from this list:


Measure carefully, make some educated guesses about the construction if you can't tell for sure, then see what the tool delivers. I'd be surprised if ANY decently-insulated 1500' house in CT is over ~50KBUTH, and most are probably under 40BTUH (which is less than the high-fire output of even the smallest condensing boilers.)

Mind you, there is some overage built into the program- you won't actually be cold even if you pick a boiler that's undersized by 25% from the estimate delivered by the heat-loss analysis, unless you have some pretty significant but easy-to-rectify air infiltration issues. (But if you went with almost any wall-hung modulating Takagi, Bosch or Rinnai HW heater for a boiler, the max output will be in the 140-200KBTUH range anyway- it's pretty hard to go wrong with those on a house that size no matter what the insulation level.)

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