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Boiler size for conversion - Metro Boston

jh582jh582 Member Posts: 4
Apologies for the long post, trying to be thorough here!

For reference, the current oil boiler is 170K btu, and in the work estimate I'm requesting that the installer provision an additional 6th heating zone for a room that has a through wall heat/ac unit (like a hotel) - which is 21x22 with cathedral ceilings. Also, I've added an indirect water heater (60 gallon) that runs off of the oil boiler and will be integrated with the new gas unit.

Plumber "A" installed the water heater, did a good job. He quoted me a high efficiency wall mounted Burnham K2FT boiler @210K btu. He didn't ask for any information regarding linear sqft of baseboard or do a heat loss calculation/assessment.

Plumber "B" estimated I need approx. 135K btus (Dunkirk) and advised that I should NOT have a big unit (ie 200K btu) as it will create short cycling and that's not good for boilers, particularly high efficiency machines. Also mentioned Burnham boilers aren't great and prefers Dunkirk (I have no understanding of the brand hierarchy here, but I feel like each plumber has their own strong opinions). He also requested the linear feet of baseboard in the home and will provide a final estimate shortly.

My own research has led me to believe that a heat loss assessment is the best way to ascertain how big of a boiler is needed, though it's fairly complex and I'm not sure I can populate it accurately. That said, I know not everything you read on the internet is true, and I'm there's something to be said for an experienced plumber knowing what's required to do the job right.

Finally, I've also been forewarned that while the high efficiency units are >95% efficient, they do require much more maintenance, possibly so much so that any savings are lost in the shuffle, including any MassSave incentives that might come from purchasing the more efficient unit.

Appreciate the inputs!


  • BrewbeerBrewbeer Member Posts: 609
    For a properly sized modulating condensing natural gas boiler, 95% efficiency is achievable if the water returning from the emitters to the boiler is less than 100F. While this *might* be possible during a portion of the heating season, it won't be the case for all the heating season.

    Take the time to do a proper room-by-room heat loss analysis and emitter survey. The heat loss is critical for determine the proper size of the boiler, and the emitter survey is to determine what supply and return water temps are needed. Making the necessary measurements and calculations is a substantial undertaking and will require several hours (or more) investment of time to do it properly. The analysis should also look at the zoning aspect of your system, small zone can cause a boiler to short-cycle, which will really reduce efficiency.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread:
    System Photo:
  • John Mills_5John Mills_5 Member Posts: 935
    edited March 2018
    The potential for EXPENSIVE repairs is definitely something to keep in mind on a mod-con. Luckily they tend to have a bit better warranties than cast iron. And remember that they need 130° or lower return water temp which may not be real often on a baseboard system. If you had gotten a heat loss calc and measured your baseboard, you could compare. The higher the BTU output of the baseboard compared to the heat loss of the house, the cooler you can get by on your boiler temp.
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