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Efficiency rating and sizing of a new gas boiler

Belmont Member Posts: 1

1. Is there a formula or standardized process when sizing for a natural gas boiler?

    For example, a  house with 1850 square feet (that's the current area being

    heated) with 2 heating zones (2 thermostats - one on the first floor and one on the

   2nd floor). There is additional space in the house that is not heated (i.e. unfinished

   attic and basement).

2. Is there a direct relationship between the number of therms used and the efficency

    rating of the boiler? For example, if 100 therms are used in one month with

   a boiler that has a 70% efficency rating would a boiler with a 95% rating use

  only 75 therms for the same period?  (In this example I'm assuming all other

  factors are the same (i.e. outside temperature, same thermostat settings, same

  thermal conversion factor).



  • Tim McElwain
    Tim McElwain Member Posts: 4,506
    The best way to determine

    is to do an actual heat loss calculation with any one of many procedures for doing the heat loss. It takes into consideration amount of insulation, type of windows, type construction etc. Something Like ASHRAE Manual J or Slant Fin has a program available on the Internet

    To answer question #2 there is obviously a direct relation ship to the amount of fuel burned based on a degree day analysis. It then depends on many other factors. Example I could install a very high efficiency boiler on a very inefficient system or building. That would make the boiler in actuality less efficient due to poor system design.

    Most legitimate contractors will be able to do an effective analysis ( heat loss) and give you a design that matches your particular needs.

    Boilers are advertised with AFUE ratings as to efficiency. Those are sometimes suspect when you do an actual combustion analysis or thermal efficiency calculation.

    The decision as to size may also take into consideration the existing system and boiler. Be careful however as in the past many boilers were over sized

    You also want to determine the rating of your emitters (baseboard, radiators or convectors).
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