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How do I actually downfire a gas boiler?

Lem Member Posts: 1
I have a steam heat system in my house with an oversize gas boiler.  My EDR is 471, and my boiler's EDR is 633. Needless to say this is causing a myriad of problems.  Having just purchased the home, I do not want to replace the boiler yet.  With all the talk of down-firing oversize boilers, I was wondering how this is actually accomplished. 


  • Rod
    Rod Posts: 2,067

     Hi- What's the boiler's make and model? What sort of problems are you experiencing?

    - Rod
  • kcopp
    kcopp Member Posts: 4,432
    edited March 2012
    With gas...

    most likely it can't be done. No boiler manufacturer will go along w/ it. W/ oil you can drop a nozzle size down..... you  can't safely do that w/ gas or plug off an orifice.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    Over-fired steam boiler

    Probably you are experiencing short-cycling of the boiler as it so quickly overwhelms the system with excess pressure. In such a case, massive main venting can alleviate a portion of the short-cycling. this would be accomplished by buying the venting tables in the shop here, or by installing a 0-3 psi gauge( gauge store.com), and adding main vents until the back-pressure of venting drops to 1ounce in the venting phase of firing.

    Why not check with the boiler mfg. for advice on the down-firing. Ask if there is a hi-lo option available, and if that option could fire low all the time.--NBC
  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,846
    It depends.

    In my case I was able to turn down the pressure regulator to lower the firing rate, but then, I was really correcting a situation where someone had previously turned it up too high. Then again, I probably over-corrected by a little, but I had a technician come out and check the combustion, and he gave it a clean bill of health.

    If you can determine, by clocking the burner or direct measurement of the manifold pressure, that you can reduce the pressure and still be within the range specified in the manual, try reducing it nearer the lower end of the range. Then have a qualified technician check it out. If this doesn't get you there, discuss having smaller orifices installed.

    While smaller orifices can give you a lower firing rate at the same manifold pressure, there are other things to consider. For example, the flue temperature needs to be high enough to prevent condensation and create an effective draft. You can't tell how much wiggle room you have unless someone checks all these parameters throughly.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
  • crash2009
    crash2009 Member Posts: 1,484
    Having an oversized boiler

     is not the end of the world.  Many of us are in the same boat as you.  In my case, my oversize ratio is about the same as yours 377 edr installed on a 510 edr boiler.  About 25% too big. 

    There are some relatively inexpensive/expensive projects that can reduce the pain of an oversized boiler. 

    1-insulate all steam carring pipes.

    2-adjust your thermostat for the optimum settings (anticipator or cycles per hour)for steam.

    3-maximise your venting.

    4-get a vaporstat and force the boiler to shut down well before it runs out of water.

    5-add an extra 1/2" of water so the boiler has a bit more in reserve.

    6-think about adding a condensate reserve tank.

    7-add a couple more radiators.

    8-if you are planning to re-do some of the near boiler piping, think about adding an oversized header.
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