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Downsizing boiler

HeatJockey Member Posts: 37
I have an apt. house customer that we are converting from oil to gas. The boiler didn't "feel" right, so I checked the actual installed load. The boiler ( Smith 28-6) is actually over twice the installed radiation.  I have 3 options:

a)  install a new boiler, if the customer is willing to spend the money

b)  install the burner sized for the oversized boiler and short cycle

c)  install the proper sized burner (for the load)  The boiler will make steam, but will the flue temps be too low?

I don't think removing sections from the boiler is cost effective.   

 The gas piping run changes from 2" to 3" because of the different loads.

Sharing any experience you might have had would be appreciated.  Thank You


  • Charlie from wmass
    Charlie from wmass Member Posts: 4,167
    Contact Smith

    They will know how far you can down fire. Some boilers they allow 50% down fire. You could also use a two stage or modulating burner. This would give you a fast recovery yet let you down fire to stop short cycling. Do not forget to use a vaporstat.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,426
    Oversized boiler

    With such an oversized boiler, maximumum main venting would reduce the short-cycling. A 0-3 psi gauge would tell you when you had reached the open-pipe equivalent. Making sure the radiator vents were slower Hoffman 40's would help.--NBC
  • HeatJockey
    HeatJockey Member Posts: 37
    Thank you all for your advice

    I spoke to Smith and they did say that the boiler can be downfired up to 50%.  I now have a plan.
  • Dave in QCA
    Dave in QCA Member Posts: 1,764
    50% should be good

    I installed a WM 680 last December.  It is firing on low fire at about 390 BTU, or pretty close to 50%.  The reason for the oversizing has to do with future plans to restore the old garage and chaufferer's quarters back to the steam system.  But, for the time being, firing about about 50-60% the boiler is perfomering exceptionally well!  Running about 30% less than the 1975 Pennco that it replaced.
    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.
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