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Re: Deciding on a new boiler

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Posted by dana on October 14, 2008 at 10:38:23:

In Reply to: Deciding on a new boiler posted by Louise on October 13, 2008 at 15:51:58:

: Our home is 1500 square feet in CT, and we are deciding on gas boilers.
: Possibly Munchkin (condensing or not) or Buderus condensing.
: Looking for high efficiency, but not sure what is the reasonable option for people without a lot of money. What we had before was a too large Utica oil boiler. Is a heat loss calculation somewhat of a moot point with these high-efficiency boilers which cover a range of BTU's? We've had some reticence to do one from contractors who've come by. The house is new to us - we haven't even lived a winter here yet.
: Any advice is greatly appreciated!

A heat loss is always important, but unless your house is completely uninsulated you'll be looking at the SMALLEST of any of the condensing boilers. What may be more important for your case is choosing the one with the lowest modulated output, since that is the mode it will be operating in MOST of the time. Cycling losses are significant, and the condensing boiler with the fewest & shortest OFF cycles will inherently provide greater efficiency. Even a NON condensing wall hung boiler like some on-demand hot-water heaters that modulate to a much lower flame will outperform a condensing boiler that is 3x oversized, with the lowest fire being over 50% of your design-day load. Do a heat loss- if the boiler you have on offer won't modulate down to well under 1/2 of the design-day heat load it probably not worth the extra money- it won't perform for you.

This is likely to be the case if you're already well insulated. And if you're not well-insulated, using the price-difference buy more insulation & air sealing will almost always be the better value.

In my moderately insulated ~2000' house in central MA (with a ~6800 degree-day heating season) I can even buy auxilliary solar-thermal inputs with the difference in cash outlay that would have a greater impact on the annual gas bill than the marginal efficiency increase I'd get out of going with a tiny condensing boiler vs. a non-condensing modulated boiler like a Takagi T-K3 or KD20.

Do a heat loss analysis, no matter WHAT. (It's now required by building code in many place.) Don't buy too much boiler- it'll be more money up front, and more money every year for 20+ years or more. I modeled the tiny Peerless PI-T50 condensing boiler with the non-condensing Takagi KD20+ a buffer tank against my heat load (both with minimum outup of ~16.5kbtuh and while the condensing boiler outperform the hot-water heater by about 120therms/year, the annual maintenance on Peerless will be more than the ~$150-200 saved in fuel. With the difference in price I can (and plan to) add a single evacuated-tube solar collector boiler loop & controls to the buffer tank that would supply about 200 therms/year. (I'd need more tank or a summer time heat-dump loop were I to add more solar.)

I suspect your design day heat load is even lower than mine (or could be, with cost-effective insulation upgrades.)

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