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Modcons, boilers with a lower min fire rate than 8kbtu/hr? Future?

SuperJ Member Posts: 605
It seems like the min fire rate on modcon's has stuck at or around 8000 btu for a couple years.
I would love to see a 4000-5000btu/hr min, it means substantially less cycling if you have a micro-zoned system, and if you still need a buffer it can be much smaller.

Has any manufacturer come out with one, or are they even working on it?

It seems like the high turndown ratios of 10:1 or higher are reserved for 80kbtu or higher boilers. Most of the 50kbtu boilers seem to be a throttled version of the manufacture's 80kbtu and have similar low fire rates.

Is there some sort of technically reason that makes combustion control difficult at low fire rates.


  • hot_rod
    hot_rod Member Posts: 19,260
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • clammy
    clammy Member Posts: 2,910
    I don’t think we will see min fire ever that low look at the supposedly best in world self proclaimed ( no pun ) the big v ahead of everyone w lowest mod only in the 15 to 20 mbtu range ever wonder why they have such a low issue rate w min fire cause there min fire are high by my standards the real issue rear there head on low fire and if u don’t mod that low there never a issue It seems the lower u go is where the issue are .in a closer look for a real low turndown the whole boiler itself would have to be re designed w every part smaller including the whole gas burner assembly and the max fire my feeling is ain’t never gonna see it just ain’t a market for a 50 mbtu w 10 to 1 plus where it would installed surely would be for some one w a very small home and limited money so it would be limited to who would purchase on a budget no no they would go cast plus all the junk sounds great but if the emitter cannnot function at low water temps it’s all bs no money saved . I know this no one has money for panel rads and low temp just hack in some hot air and a ac coil done . Only a person like myself in the business would go the whole dealo and spend serval grand on panel rads and size them for low temp did it in my own house last month gas bill w the the record cold wave was a whopping 114 buck but x 1000 and u got my system My supposedly crappy munchkin goin on 13 years old running max out at 140 @ 5 keeping my house at 70 .it s a super hard sale usually w big number no body like big numbers I believe is the lowest on modulation as far as what I see is on http stuff and maybe lochivar with a say best 15 year run span boilers and heating have caught the bug of the car industry replace every few years boiler and heating system are cheaper then anew car plus everybody moves so often who know what line of bs to believe in must have been the last guy sorry for the early am rant to much coffeee lol peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • DC123
    DC123 Member Posts: 69
    Viessmann goes down to 1.9kw in Germany, who h is under 7000 btu...
  • SuperJ
    SuperJ Member Posts: 605
    DC123 said:

    Viessmann goes down to 1.9kw in Germany, who h is under 7000 btu...

    That's a good start. I would like to see a small modulating 7000-10000btu air to water heat pump, that could replace the boiler under light loads, and let the boiler take over for the heavy lifting as the load goes up.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited February 2019
    Consider the output of the stove top burners on a modern gas range and suddenly 8K BTU turndown doesn't seem too bad for a boiler.

    Left Front Burner 15,000 BTU
    Left Rear Burner 9,500 BTU
    Right Front Burner 17,000 BTU
    Right Rear Burner 5,000 BTU
    Center Rear Burner 9,500 BTU

    I do agree that new tight homes with micro zoning (t-stat in each bedroom) will present a problem even with 8K BTU turndown.
  • Gordy
    Gordy Member Posts: 9,537
    HTP versa hydro with a 10:1 tdr on the heating side, and 80 gallons of mass. Is an answer depending on situation.