Why Is It Wrong to reduce output of furnace by eliminating burners?

Discussion in 'HVAC Heating & Cooling' started by molo, Jan 22, 2012.

  1. molo

    molo Member

    Messages:
    845
    Location:
    cold new york
    I've heard there are reasons not to reduce the output of a furnace by eliminating burners (going from a 100btu down to a 35btu). What are the reasons not to do this?

    Thanks,
    Bill
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Well, for starters there is the "johnny cochran" issue: not legal to modify an appliance like that. You violate it's GAMA/AGA/UL listing, and hence lose the permit it was installed on.
    Aside from that, now you don't have the air flow balanced against the heat produced. That can't work out well. \

    And most important, you affect the operation of safety circuits like hi temp limit switches, etc. Those were all designed around a certain burner/fan operating scenario, and now...who knows.

    If you don't need 100,000 BTU, a whole new furnace is not that expensive, and if you want uber energy efficiency, you get a new furnace which is DESIGNED to operate the burners in stages, and modulate the air flow accordingly. The unit costs more, but in a very cold climate can save a lot of energy. If you are upstate like in Buffalo or something, this would be worth doing.

    Messing with a furnace on your own is a bad idea.
  3. DonL

    DonL Jack of all trades Master of one

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    It would be easier to change the orifice, and hope it don't blow.

    Bad Idea.
  4. BobL43

    BobL43 DIY Senior Member

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    las time I changed an orifice (jet) was when I experimented running my 58 Ford pickup on 100% Ethanol in the late 70's
  5. DonL

    DonL Jack of all trades Master of one

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    I hope you added some Lube with that Ethanol, Sounds like it ran fast for a short period of time. lol
  6. ballvalve

    ballvalve General Engineering Contractor

    Messages:
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    Location:
    northfork, california
    Many gas valves have a hidden screw [under a cap screw] that reduces gas flow to the orifices. That could safely downsize the output slightly.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
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    Location:
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    Most gas burners (at least should be) adjusted for proper air/fuel mix to minimize carbon and CO production. Just changing the gas (fuel) would disrupt that balance, and in itself would be unsafe unless you rebalanced the air/fuel mix. As already mentioned, just arbritraily going in and changing things is likely to make for an inefficient system and not increase your comfort or safety. The heat exchange output temp, drafting in the flue, blower speed, and other things would likely be compromised. The reduced heat output might also lead to condensation and self-destruction of the interior since it would not get as hot. Really not a good idea unless it was designed for it (some are, within limited range).
  8. Dana

    Dana In the trades

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    The massive excess combustion air will reduce the steady state combustion efficiency, but you'd also suffer flue condensation issues unless you narrowed it down with a liner. At that large a reduction you'd end up with laminar flow on fire-side of the heat exchangers (unless it has mechanically drafted venting), reducing the heat transfer efficiency.

    Derating a low-mass hot air-furnace has negligible effect on efficiency, even when done in a range that doesn't cause other problems. Derating a high-mass boiler can sometimes lead to fuel savings, even it's steady-state efficiency is slightly reduced, but even then the "comfort & efficiency" money is usually better spent elsewhere.

    What are the reasons you're contemplating doing this?
  9. BobL43

    BobL43 DIY Senior Member

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    Location:
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    Actually, it ran pretty well once it warmed up, which took a while, even in nice weather. I did not run it long enough to damage any valves. Used about 15 gallons of Ethanol in total during my experiments.
  10. DonL

    DonL Jack of all trades Master of one

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    Man, That is a lot of drinking. lol
  11. BobL43

    BobL43 DIY Senior Member

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    Location:
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    Brrrp, you got me pegged.
  12. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
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    Location:
    Minneapolis
    Serious business. A "furnace" should never be downsized, as the tin heat exchanger will not last long voiding warranty etc. A cast iron "boiler" could be de-rated, but the added stress to cold sections makes them fail prematurely, (so say the manufacturers).

    As pointed out early in this thread, all gas-fired appliances are UL/CSA tested and approved in a certain configuration and output. Changing orifices and plugging burners is not good practice and should never be attempted without the aid of an experienced HVAC technician with a current combustion analyser.
  13. DonL

    DonL Jack of all trades Master of one

    Messages:
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    Location:
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    I agree, It is better to be safe than sorry.

    They do make systems that cut back to save energy.

    It you only need 100btu, You maybe be able to get that from a candle, Use a smaller candle and get 35btu.
  14. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,689
    Location:
    Cave Creek, Arizona
    Usually, each burner has a "path" above it through the heat exchanger. if you remove burners you create an uneven heat pattern and that could cause metal stress and create cracks in the metal.
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