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The boiler room door---To close or not to close?

Roddy Member Posts: 63
Approximately 40 year old Weil McClain boiler in good shape and maintained well. Original installation had a non-electric, bi-metal flue damper (now illegal to install in USA but still available in Europe) in the piping leading to the chimney which remained as is until one week ago. I'd notice during last heating season I was getting temporary levels of detection on my Carbon Monoxide Detectors, but nothing sustained that caused any of them to alarm/beep. Still, I was concerned. Started doing some investigating and determined the flu-damper was not functioning (opening quickly upon boiler firing and not opening completely) as it should. Consulted with a local heating guy with a lot of boiler knowledge who'd done good work for me in the past. After cleaning the damper and doing some maintenance it still did not function as well as it should. He suggested we get rid of the bi-metal damper altogether and just have an open flue pipe leading horizontally to the chimney. I agreed to getting rid of the damper as I didn't want to risk anything. I asked about installing an electric flue damper and he said for my boiler, he'd suggest not doing that as it would be difficult to interface and might be problematic. On new boiler models they're built in, and he likes them, but for my set up, so he suggested not adding one. Now to my question: With Chicago's NiCor gas pricing again going up 100% since last year's increases, I'm concerned about increased natural gas usage with no damper. My boiler is in a basement utility room approx. 10' x 18' that also has a natural gas hot water heater in it. The utility room opens up to the rest of a finished basement. The house is 100 years old and has lots of doors, including on the utility room entrance. Does it make any sense to keep the door closed, or does it matter? I'm thinking any cold air coming down the now non-dampered chimney would be somewhat limited to the utility room with the door closed. But...what do I know? Any thoughts or suggestions?


  • jhewings
    jhewings Member Posts: 139
    I think flue dampers usually keep warm air from going up. If your boiler room door has been usually open, if you close it, do you have enough combustion air?
  • KC_Jones
    KC_Jones Member Posts: 5,737
    edited October 2022
    @The Steam Whisperer I believe has some data for old boilers he has added flue dampers to and his approximate fuel savings. Perhaps he can chime in with his experience.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • pedmec
    pedmec Member Posts: 973
    Adding a flue damper only increases the savings on average by 2%. That's how the boiler manufacturers were able to increase there boiler design efficiency from 80% to 82% without a change in the boiler back when the DOE increased the minimum rating. Save your money. Depending on how well your weil-mclain functions you might get a little more, maybe percentage point, but no return on investment.
  • bburd
    bburd Member Posts: 912
    Cold air does not typically come down chimneys, unless the house is under severe negative air pressure. Rather, warm air goes up. And I would leave that utility room door open, the last thing you want is inadequate combustion air which can cause carbon monoxide to be produced.

  • The Steam Whisperer
    The Steam Whisperer Member Posts: 1,215
    I have some testing data, but by combining partial load efficiency information and typical Chicago weather data a typical cast iron sectional atmospheric boiler that is sized to match the design day heat load of the building has a seasonal efficiency of about 70%. This matches the AFUE ratings of typical older boilers without dampers. I had AFUE numbers for boilers that were produced before the minimums were raised, and the efficiencies were about so>>

    Cast iron sectional atmospheric gas boiler

    No damper, standing pilot 70-72%

    With motorized damper, standing pilot 78-79%

    With motorized damper and electronic ignition 80%.

    Most testing I've seen has been on commercial boilers that are probably utilizing outdoor air for combustion, so a typical residential setting would probably be different since heated indoor air tends to be used for combustion. Savings for commercial have been in the 2% range. Probably a big factor in the effectiveness of a stack damper is how oversized is the boiler to the design day heat loss. Most steam boilers for buildings built between about 1910 and 1941are 2.5 to 3 times the design day heat loss, so the stack damper is probably going to be much more effective on these oversized boilers since the boilers spend so much time hot, but no firing.

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  • fentonc
    fentonc Member Posts: 236
    @The Steam Whisperer - What are you using for partial load efficiency numbers?
  • The Steam Whisperer
    The Steam Whisperer Member Posts: 1,215
    An efficiency curve published by the National Bureau of Standards. You can find the same curve in literature published on multiple boiler heating plants, such as that by Burnham and maybe tekmar.
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  • fentonc
    fentonc Member Posts: 236
    Not to get too off-topic, but I plotted an estimate of efficiency (based on average baseboard temp when a zone was active) vs boiler run-time from my home for last heating season. This is with a nominally 83% efficient cast-iron natural gas boiler (about 4x oversized) with an automatic flue damper.

  • The Steam Whisperer
    The Steam Whisperer Member Posts: 1,215
    That is very interesting. Your efficiency doesn't hit a steep drop off until 5% load. On a boiler without damper it appears that point is around 30% ( see attached). It could be just scaling but it appears the damper is greatly effecting the curve between 5% and 30%. This is the operating range of typical sized to radiation steam boiler for nearly the whole heating season. The typical oversized steam boiler is even worse.
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  • fentonc
    fentonc Member Posts: 236
    If i reformat my chart to look just like that chart, it actually seems to match pretty closely:

  • Steamhead
    Steamhead Member Posts: 16,832
    Another thing to remember is that with the boiler off and no damper, air continues to draft up the chimney. This air has to come from somewhere, and if there is no outdoor air intake, it's coming from inside the building. This heated air is being replaced by colder air coming from outside.

    So the damper not only increases the boiler's efficiency, but also the building's efficiency.

    It's not that hard to wire a damper in if you know what you're doing.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
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