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Condensing Boiler

I just went to contract on a house in Long Island NY. House was built circa 1890. Heat is currently an oil boiler in basement single zone hot water. There is separate gas fired water heater manufactured in 1979.
There are radiators (old cast iron) on the first floor and baseboard on the second floor.

I want to insulate of course and replace the boiler with a condensing model coupled to an indirect water heater. The boiler will be direct vented bypassing the chimney.Chimney to be retained for future use(it's already lined).

Is the mix of rads and baseboard a drawback for a condensing boiler?

Re: Condensing Boiler


It sounds like you have already talked to a boiler installer when you describe the direct venting---yes, baseboard and radiators can be used together, even with condensing boilers,....but read on.

I completely agree that you should install an indirect hot water heater with either the existing or new boiler, this will eliminate a wasteful fuel-burning water heater with an indirect that will efficiently use the boiler water to heat the domestic HW & eliminate an unnecessary flue connection.

You're also on the right track by first concentrating on insulation, weatherstripping and getting tight windows---it only costs a few hundred $$$ to have cellulose insulation blown in---they do it from the outside, removing a shingle or piece of siding here & there & doing it usually within one day---an excellent move---R19 for all exterior walls & R40 for the attic.

However, to buy a condensing boiler may not be a good option---a condensing boiler will cost you double what you would pay for a good 3-pass boiler, or even a standard cast-iron boiler that will heat in the 83% AFUE to 87% AFUE (3-pass)range;($4k or $5k as opposed to $8k to 10K for a condensing boiler).

Condensing boilers are mostly designed to be used with radiant floor heat or radiant slab heat where many hundreds of feet of plastic (PEX) tubing are stapled under the flooring & circulate hot water almost constantly in the 100 degree to 120 degree temp range----however, the radiators and baseboard currently in place in the house are designed to operate at 180 degree water temps---at 180 degree temps, baseboard outputs approx. 560 btu/hr per foot of baseboard, not 80 btu/hr per ft. at the lower temp.

Radiator output can be calculated by assigning 170 btu/hr for each sq.ft. of rad. area, then multiplying by the # of rad sections: thus, a 24 section rad 1/2' wide & 1.5' high would = .5 X 1.5 = .75 sq.ft. X 24 = 18 sq.ft. X 170 = 3060 btu/hr output at 180 degree hw temp, again it would be much lower at 100 degree water temp.

A rough estimate of the amount of heat you need for the building can be obtained by multiplying the total sq.footage (including utility rooms, boiler rooms,basements, etc., by 40 btu/sq.ft. (this assumes living in a cold climate, a reasonably tight building with some insulation in walls & attic, tight windows); thus, a 2000 sq.ft. house X 40 btu/sq.ft. = 80,000 btu/hr heat loss & the amt. of btu/hr to heat the house (also size of boiler)---this is a rough guess; the installer must do a HEAT LOSS CALCULATION to determine an accurate heat loss for the building.

You can calculate if too many feet of baseboard & rads were originally put in by calculating the total heat output in BTUs/hr of the total rads/BB in the house & comparing that figure with the rough heat loss calculation (total sq.ft. X 40).

In addition, the amount of baseboard and rads that exist in the house now is SIZED (designed) for 180 degree hot water, not 110 to 120 degree hot water--trying to run the rads & BB at such lowered water temps wouldn't heat the house very well, except perhaps in late fall or early spring seasons---yes, there would be some savings, especially if the amount of radiators originally installed was oversized (as it often is) but such a condensing boiler would otherwise operate in the non-condensing (high temp) mode most of the time, negating any savings expected; and buying a very expensive condensing boiler that has a shorter lifespan than cast iron models doesn't seem practical.

I would recommend getting several other estimates & opinions from local installers as to the best type of boiler to be installed---don't ignore fuel oil dealers, who are also licensed to install gas boilers & often charge less.

Also, get a feature called outdoor reset, which would go far to make any newly installed boiler act as a condensing one---ODR monitors the outside temp & lowers the boiler water temp down to as low as 140 degrees to save fuel.

Re: Condensing Boiler

Thanks Nashua Tech,
I understand about the condensing boiler not running very efficiently in the high temp mode but isn't that offset by the condensing mode the rest of the year?
I guess only a heat loss calc will answer that question.
It's still in the planning stages right now.

I was informed today that the town where the house is does not allow direct venting at all. I am trying to get firm answer on that one.

Re: Condensing Boiler


Yes, as noted, there is plenty of time once you get into the new house to get at least 4 to 6 estimates from local heating contractors/installers---only those who are familiar with local conditions and town regs can properly advise you which way to go---you will get a wide diversity of choice of heating equipment as well as type of equipment, and price ranges will vary widely---that's why it's important to get as many different quotes as you can.

Eventually, a consensus will dovetail toward a more obvious, best choice.

I would tend toward a 3-pass cast iron boiler with an outdoor reset as a good choice.

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