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Oversized boiler "fix"

Abracadabra Member Posts: 1,948
Have a Weil-McLain LGB-14 boiler (1690MBH input, 4313sq.ft. steam).  The LGBs are nice because they come from factory standard with a hi-lo gas train.  The LGB-14 has 2 identical gas trains, each firing half the sections of the boiler.

I've installed a vaporstat and a DPDT relay to kick the burners to lo fire when it hits 12oz pressure and kick them back to high at 4 oz.  Each pole of the relay controls the hi-lo fire of one gas train. Problem is that the boiler is so oversized, that even at low fire it continues to raise pressure to about 1.25psi. I was thinking to install a 2nd vaporstat to kill one gas train and leave the other one running until the pressure drops to 5 oz. 

Does anyone see a problem with firing only half of the boiler?  I'm also thinking that it may make a difference if I fire the side with the equalizer on it or the opposite side? The water feeder/LWCO and equalizer are of course on the same side of the boiler. I'm thinking that that would be the correct side to fire, since it's closer to the LWCO.

Any comments before I call up Weil-McLain for some guidance? 


  • Dave in QCA
    Dave in QCA Member Posts: 1,776
    Not on Steam

    You idea would work on hot water, but it WILL NOT work on steam. 

    I do have a suggestion though.  First, clock the meter to see what your firing rates are in BTU on both hi and lo fire.  Compare these to the rated input rate.  The honeywell valve that is used to provide the hi - lo  function is a regulating valve and both the hi rate and the lo rate are easily adjustable. 

    I understand from comments from one of the pros that the LGB maintains a very high effeciency even at low fire.  I would check and see just how low the low fire is running.  You may be able to turn it lower.  May be able to downwardly adjust the hi fire as well.

    Do you have the install manual or know where to find it on the weil mclain website?  If not, I'll find it later and put the link in here.
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
  • Abracadabra
    Abracadabra Member Posts: 1,948
    can't lower the lo fire

    I tried to lower the low fire.  There's a limit installed on the lo fire adjustment screw.  I can't run it any lower.  I did clock the meter on this one last week.  Didn't bring my notes home with me, but it did match what it was supposed to.

    I believe the WM limits the lo limit so that the stack temp doesn't drop too low. I do have the install manual.

    The manual talks about lo-hi-lo firing (which I understand), but also talks about "staged firing".  I'm wondering if what I'm trying to accomplish, by firing half the boiler is considered "staged firing".  Almost like having 2 seperate boilers, but they happen to be a single unit.
  • crash2009
    crash2009 Member Posts: 1,484
    edited January 2012
    Oversized boiler fix

    Don't know if you want to go this far.  An oversized dropheader, overventing the mains, and a resovoir tank can all help. 

    Not sure if this is recomendable but it is the way mine was when I got it.  The size of the gas line was reduced in diameter for a length, so as to reduce the input.
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