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Replacement boiler - which technology to use?

Bluethumb's picture

My 100+ year old wood frame/stucco house has a 28-year old gas boiler with a power vent connected to a 100+ year old brick chimney.  When I replace the boiler I want to disconnect from the chimney because I don't want the expense of lining the chimney, and the chimney leaks both through flashing and corbelled brick joints into the attic.  I would prefer to demolish the chimney when I redo the roof.

Some specifics about the heating system: The boiler is in the basement, and connects to 12 old tube-style cast iron radiators using a pump for circulation.  There is about 3' between the top of the foundation and the underside of 1st floor joists.  I have options for locating the flue at code-required distances from windows, equipment, etc.  The existing boiler has 150Kbtu input, but nothing listed for output.  I ran a heat loss calculator for the house which analyzed walls, windows, floors, ceiling, etc and came up with a load  of 75Kbtu.  (1500sf of heated space, 2 stories, heated basement, unheated attic, some insulation, storm windows, no fireplace, Chicago area)

My options for a replacement boiler seem to be a single stage power/direct vent exhausted by a b-vent  through a basement side wall, or a condensing boiler exhausted through PVC.  I was leaning toward the higher efficiency condensing boiler, but from what I've read my radiator system runs too hot to condense the latent heat out of the exhaust, and therefore much of the efficiency is lost.  However, each of the 3 mechanical contractors I've had bid a new system were extremely hesitant to do a side-wall vent, and recommended lining the chimney if I wanted to go with the lower-cost single stage boiler.  Since that option is not preferred they recommend a condensing unit with modulation responsive to outdoor temperature.

I see diagrams illustrating closed combustion and side wall venting for single stage equipment.  Why are these contractors steering away from them? Other than the additional profit to be made selling a condensing unit?  Or is the condensing unit really the best way to go with this house?

Do be aware that the (post #213997, reply #1 of 4)

Do be aware that the condensing ability of a furnace is related more to the input temperature than the output temperature.  A properly designed condensing unit feeds the hot gasses one direction and the fluid to be heated (water or air) the other direction, so that the hot gas gives a "boost" to the already-heated fluid, then the somewhat cooler gas "preheats" the incoming fluid (and condenses in the process).

So the effectiveness of the condensing action is dependent on how cool your water is coming back from the radiators.

And note that the above "counterflow" design also applies to your better non-condensing units.  It's just that they don't allow the flue gasses to get cool enough to condense (much), since that's destructive of a conventional flue (and a bit more expensive to arrange, to boot).

It may be that something like a variable speed water pump (coupled with a variable burner) will improve the effectiveness of the setup.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

My opinion is that a (post #213997, reply #2 of 4)

My opinion is that a condensing boiler with outdoor reset, and a variable speed Delta-T  ( temperature) is the way to go.

The heat loss calculations, and thus the system size, are for that 1 day ( the design day)  when all the heat available is required to keep the house warm.  The rest of the heating season, the system is running a partial load.

The outdoor reset on the boiler will control the water temperature based on the outdoor temperature, keeping the water temperature as low as possible to meet the heat load . The system is more efficient at lower temperatures, and allows the boiler to operate in condensing mode.  At the design day, when all heat is required, the water temperature may not be low enough to allow condensing, so the boiler will be operating for that period with the efficiency of a standard boiler.

The circulating pump should be sized so that there is a 20 deg drop in temperature between boiler outlet and return.  A standard pump and balancing valves will do that with some setup time, testing and adjustment.  Easier to use a variable speed pump and adjust.  Easiest of all is a variable speed ECM pump set up  for Delta -T, and it will adjust itself.  ECM circulators are readily avaliable at reasonable price for residential use.

Condensing Boiler products (post #213997, reply #3 of 4)

Have you any recommendations for manufacturer/model?  Or products to steer clear of?

Find a vendor first. (post #213997, reply #4 of 4)

Find a vendor that you want to do business with and see what brands they service and suggest. BTW, I will never again use HeatMasters as a vendor after various competence issues. Good luck.