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Replacement boiler sizing

Bob Bona_4
Bob Bona_4 Member Posts: 2,083
how much bb there is for the moment, that does not dictate what size boiler to get, heat loss of the house does. The only things the existing bb will tell you (once you have your heat loss in hand) is a)there is the right amount b)there is more than enough (opening the possibilty of running lower water temps to do the job..with the caveat that ALL the bb is proportionately over length) or c) you are short on bb.

The indirect load is looked at next. If you are doing a mod/con, and you are looking at a 40 gallon indirect (sounds a tad small), you want something that will ramp up to at least 75K for decent recovery. Of course, the indirect is prioritized, so in most cases, the boiler actually gets sized for the DHW! Mansions and extreme human habits excluded....


  • Jim P
    Jim P Member Posts: 29
    Replacement boiler sizing

    When doing a heat loss for a boiler replaceemt (FHW baseboard), what is the better way of doing the heat loss? For example do you measure the existing baseboard footage and mutiply by 580 btu/ft or do you perform a whole new heat loss (measuring exposed walls, room size, windows ect..). The job I just looked at had a 130,00 btu gas fired boiler. The house has 107ft of baseboard, and at 580 btu/ft that gives you 63,000 btu heat loss. I am adding a 40 gal. indirect fired water heater. I choose a 80,000 btu mod/con boiler for the job. What do you think?

  • Mark Hunt_4
    Mark Hunt_4 Member Posts: 68
    Hi Jim

    The length of baseboard does not tell you what the heat loss is, it only tells you how much heat you can put into the structure.

    By your numbers, the current boiler with 130k btu input is about twice as much boiler as the baseboard can actually handle.

    A heat loss will tell you how much baseboard is actually needed.

    That being said, if the home is over-radiated you can take advantage of designing the system to work with lower supply water temps and outdoor reset.

    HWBB does work with water temps under 120°. I've done it.

    Sounds like your customers are going to see a dramatic reduction in their fuel consumption!

    Mark H
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