Getting wildly different boiler size quotes from contractors...

Discussion in 'Boiler Forum' started by stan1415, Sep 5, 2009.

  1. stan1415

    stan1415 New Member

    Sep 5, 2009
    Hi all,

    I'm in the process of getting quotes for having my 32 year old Weil-Mclain boiler replaced. I am looking at getting a high efficiency gas fired mod/con boiler. At the same time I want to replace my 12 year old gas fired domestic hot water heater with an indirect hot water tank connected to the boiler.

    My ranch style house in the Chicago area was built in 1959. It is brick, with an attic and basement. I had approx 18 inches of insulation blown into the attic last year. I'm not sure what if any insulation is behind the plaster walls. The living area is heated by cast baseboard, and there is radiant floor heat in the basement floor. The windows are original single pane, with some large dual pane picture windows. Storm windows are installed on the double hung single pane windows. During the winter I also install that 3m plastic on the inside of the windows for an extra barrier.

    The living area is approx 2200 square feet and the basement is about another 1800 square feet. I don't normally run the basement heat.

    There are four heating zones, two in the living area and and two in the basement.

    So I have had three contractors come through and given me estimates. One specd a 105,000 BTU Burnham Alpine, another specified a 210,000 BTU Lochinvar Knight and the third specified a 200,000 BTU Utica model.

    The original Weil-Mclain boiler was rated for 175,000 BTU in and 140,000 BTU out. It has kept the house warm, which leads me to ask why would two of the contractors spec boilers that are specd for around 200,000 BTU's out? Neither of them measured the baseboard, just looked at the old boiler to determine the size of the replacement. Later, I questioned the guy who suggested the Lochinvar and he said that yeah a 150,000 BTU unit would work fine too... Doesn't inspire a lot of confidence.

    I'm not a HVAC expert by any means but I want to make sure that I have the right size unit installed in my home. From what I read a heat loss calculation would help with the sizing but the sales guys don't seem inclined to do one when you mention it. Again, I'm not an expert but it seems critical to get the size right to make sure that the boiler condenses correctly to get as close to the 95% efficiency mark. The guys that quoted the 200,000 BTU models say that you can't oversize (and therefore short-cycle) a modulating boiler because it will adjust how much fuel it burns according to load.

    The guy who specd the Burnham 105,000 determined how much btu's my baseboard can put out per foot per hour with 180 degree water (said it was around 75,000 BTU) and came up with the 105,000 BTU quote. He said that that would leave enough if I wanted ever to run the basement radiant floor. I looked at the specs on the Burnham Alpine 105,000 btu and it is rated for an output of 96,000 DOE and 83,000 IBR. I am not sure which number to use and it does seem like it is cutting it a little close... It seems like that boiler might be running close to full bore for a lot of the winter. Maybe that's right, I don't know.

    Also, correct me if I'm wrong but these boilers should prioritize domestic hot water demands over the hydronic heat demands right? So is it correct not to include extra BTU's in sizing when including an indirect domestic hot water setup?

    So here I am, scratching my head and hoping for some advice that would help me make the correct decision for my family's comfort and our financial well-being... :) I have one more company coming out to give me a quote, I'll see what he says...

    I'll be happy to gather any more info that anyone thinks would be necessary.

    Thanks for reading!
  2. flamefix

    flamefix New Member

    Sep 4, 2009
    Heating engineer in the UK licensed to fit service
    Exeter, England
    It is the same problem here. It takes time to do a heat loss calculation and when you are quoting you have to be sure one is likely to get the job before doing it. I have a job currently It will take me 2-4 hours, which is a lot of lost time if it was done for every job.

    That said to be sure the best way is to have a heatloss calculation carried out. I posted a UK based HL calculator in an alternate thread you may be able to use that yourself to give you some idea.
    In the Uk traditionally heat sizing was carried out with a Mears calculator and this oversized by 25%. If it is the same in the US then the 140,000 might be ok.

    So initially I would use a rule of thumb to proviso a boiler size and cost if the client was happy then I would spend the time measuring the rooms, window doors construction and then asses the heatloss calculation.

    If the boiler you are having is a fully modulating boiler, then you don't need to worry as much if it is oversized as it will modulate to suit the heat load.

    Here it is not uncommon to fit a 35kW combi gas boiler into a duplex apartment because the the higher KW is for hot water flow and demand not to heat the property which probably only required 8kwW.

    It's your money you are spending so find an installer who wants your work and will provide a HL calculation, save him some effort and measure your room height length width and window door type and sizes, insulation levels and floor /ceiling construction.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Most indirect WH are run on a priority zone...this means that when they require heat, the rest of the house doesn't get any. This generally isn't a problem as it usually doesn't take all that long to reheat the tank. Almost any boiler will have more capacity than a typical standalone WH (although there are some available with huge inputs - often for commercial situations).

    A boiler actually running 100% of the time to meet the needs is more efficient than a boiler that cycles on and off. It takes a little burn time to get everything up to temp and reach max efficiency, then when it turns off, the heat not used may go up the flue, or into normally unheated space.

    So, unless it is critical to run the WH and the house simultaneously (which might happen if you had huge hot water demands and the lack of heat in the dwelling would be noticed), you normally wouldn't oversize the boiler to account for the indirect's use.

    Too small of a boiler won't automatically cause the house to be cold. Depending on the insulation, once the boiler is cranking out max and running at full output, as the outside temp (demand) goes up, the house will cool. So, say it can keep the house at your desired temp at zero at full blast, and it gets to minus 1, the house will get 1 degree cooler and stay there. ANother example, say you have it sized to handle -20 degrees, and it gets to -25, the house temp would drop 5-degrees, but how fast would depend on the insulation. Normally, it doesn't stay all that cold for all that long, so depending on the insulation, it may never cool off that much before it warms back up again outside. It's usually coldest at night, and most people like it a little cooler then anyways. A different situation is if you use setback, having excess capacity will allow it to recover faster.

    My small townhouse calculated out to only need about 22K BTU at a -20 degree day. I had a 30K BTU water/air heat exchanger. It would easily keep the house up to temp, but if it was setback, it took forever on a cold day to recover, what with the small excess capacity (the boiler was larger, but it couldn't transfer more heat than the heat exchanger's ability - sort of like the amount of radiators in the house).
  5. Dana

    Dana In the trades

    Jan 14, 2009
    The old beast with 140k output was keeping you warm even in it's aged derated condition, you've added significant insulation- the 105K unit should be just fine. It's duty cycle and as-operated efficiency should be higher than the bigger models, and with fewer cycles it'll have less wear & tear also.

    If you have decent record keeping you should be able to divine the a very accurate true design-day heat load based on fuel use & degree-day data using the FSA calculator tool downloadable here:

    The part-load efficiency model used in the tool was derived from the testing outlined here:

    I strongly suspect that even the 105K unit is oversized for heating-only, and may still be oversized with the indirect HW load factored in. There are plenty of 60K boilers running houses that size & age these days. But if your radiation is only capable of delivering 75K @ 180F you won't be running in condensing-mode very often, and you may do as-well from an efficiency point of view with a less expensive "perfectly sized" sealed-combustion mid-efficiency cast-iron boiler rather than the mod-cons you've been looking at. Seriously- if you can't deliver design-day heat at temps under 130F it'll take a decade or two to pay off the difference between an 85% AFUE cast iron and an oversized mod con with an as-used efficiency of about 87-88%. You need return water temps from the radiation to be under 122F to break 90% even on raw combustion efficiency (not counting boiler jacket or cycling losses), and under 110F to hit the mid-90s.

    If the FSA tool indicates a true heat load under 35K (which it might- don't be surprised), then your baseboard radiation probably WILL be able to deliver design-day heat at low enough temp to run at high efficiency. Baseboards deliver ~1/4-1/3 their 180F rating at 115F, or ~1/5-1/4 of their 180F rating at 100F. The Burnham Alpine has a turn-down ratio of 1/5, so the 105K version should have reasonably high duty-cycle even when it's 30F out, but there is an 80K version as well, which will do better. Going with the smaller boiler increases the efficiency, but slows the recovery time on the hot water. But going with a bigger indirect would make up for that, with only a very modest increase in standby loss (far less than the efficiency losses you'd get from more the frequent cycling of an oversized boiler.)

    When in doubt, go smaller on boiler, bigger on hot water storage. I'd be surprised if an 80K boiler wouldn't cut it in your application.

    But DO consider replacing any windows that don't have storms with something more efficient, eh? Storm windows on windows that are in otherwise good repair are more efficient than typical bargain-basement double-pane replacements, but if you have some aluminum frame sliders/picture windows, etc so common in the late 50s putting in just about ANY new window will cut the heat loss there by 2/3.
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