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boiler sizing

Did the calculations for my square feet of radiation and got 235 square feet. Have had (3) contractors come by to give quotes, all are coming up with more than 235 square feet for the boilers they are providing. I'd prefer the correct size, however the next size down (for all 3 guys) is less than 235 square feet. (usually approx 200 square feet).

Am I better off with a slightly too large boiler or a slightly too small?

Please advise...


  • RodRod Posts: 2,067
    Sizing Boilers

    Hi- Does your 235 square feet of radiation figure include the "pickup factor"?  Just thinking that maybe the contractors' figures do and if the 235 square feet figure doesn't, that might account for why their speced boiler sizes are larger.

    Just a thought.

    - Rod
  • Steve NicholsSteve Nichols Member Posts: 124
    how much different

    are the calcs between you and contractors?  Are you world's apart, 10% different, etc  Are all the contractors getting about the same values?  Essentially, if you have rads, know the type, height, depth and # sections, the numbers should be pretty nonvarying.  If you have that under control....

    I'd say check over your EDR values. Your contractors might be quoting a specific boiler type because that's what they prefer to use. There are a range of EDR's that steam boilers can support.  Going outside of one particular brand can give you more options.  I'm sure some of the more seasoned pros on here can chime in and tell you what's what.

    A short answer is you want to at least "match" the connected load.  Slightly above is acceptable.  I'm 30% oversized (one more section than necessary) and my boiler short cycles if I have a multi degree setback.

    Hope this helps.
    striving for peaceful coexistence with an oversized boiler....
  • JStarJStar Member Posts: 2,668

    Use a wet-base boiler with a power burner. Fire the boiler at the EXACT rate that you need. Steam happy.
    - Joe Starosielec
    [email protected]
  • BobCBobC Member Posts: 4,441
    edited September 2013
    How low?


    On your advice i called Carlin tech support a couple of months ago about my loud EZ-Gas on my G8-3 steam boiler. They suggested I dial the gas pressure down to 3.2 WC from the normal 3.5 where the regulator was set to see if that quieted the beast. Doing that did help some and as a result I ordered a new orifice from Carlin, knowing the firing rate was really too high for my systems needs.

    My current orifice was supplied with the EZ-Gas specifically for the G8-3 (7/32") for 105,000 BTU's, it clocked out to 112,000 BTU's. My EDR is only 210 vs the 283 the G8 is rated for, 30% oversized. I was going to drill the new orifice to a #10 (0.193) and figured that was about as low as I would dare go, being concerned not to downfire too much. Should I drill the orifice smaller (easy enough to drill it out). I'll drill it out myself on my drill press before the tech installs it because I'll get a better hole than he will with a hand drill. I'll have him put the regulator back to 3.5 WC at the same time.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • DsissonDsisson Member Posts: 47
    my calcs

    My calcs are the total of the standing radiation, with no factor for losses in the system. The installer hasn't given me his calcs, however the boiler SF includes a 1.33 increase for losses.

    What is a wet base boiler with power burner? Can you give me a brand name and model (or multiple?) The current quote is for a weil mclain PEG-35

    I would say that they are 50% oversizing the boiler for the current load, however I plan on adding a main and a couple of radiators (addition to the house), so maybe it's only 10% oversized once the new loads are added.

    I like the idea of something that's adjustable - produce less heat until the addition is complete and new radiators are online.
  • BobCBobC Member Posts: 4,441
    Wet based boiler

    A wet based boiler surrounds the flame with a cast iron water jacket rather than having a cast iron pot on top of  a gas burner. The wet based boilers are inheritly more efficient because they capture more heat from the flame.

    I am using a Smith G8-3 boiler that came without the usual oil gun, in it's place I have a Carlin EZ-Gas that looks remarkably like an oil gun, That series of boiler is meant to feed anything from 105,000 to 245,000 BTU input; if you need more you can go with the larger Slant Fin or Smith boilers butb you would need the larger gas guns made by Carlin or Midco.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • JStarJStar Member Posts: 2,668

    BobC. We fire the 8S3 as low as 65,000 with non-condensing flue gases.

    Dsisson. Check out the photos below for examples of the Smith G8 / Carlin gas combo. You get better efficiency with this arrangement, and a lot more flexibility.
    - Joe Starosielec
    [email protected]
  • DsissonDsisson Member Posts: 47

    so, I'm convinced there are no steam experts here in Providence RI. The guy who suggested the weil mclain seems to be the best so far, but my questions have revealed that he's not an expert.

    I can't get in touch with gronski. they don't answer the phone.

    really about ready to have some knucklehead install a boiler and hope for the best, because I need to get it done before the snow flies. all the advice I've gotten on here is awesome, but I can't implement it without someone who knows what they are doing. I swear, by now, I could probably do this myself.
  • Choosing an installer

    Whoever you choose, just make it clear in writing that the piping will be done as shown in the instruction manual for the boiler, with no deviation. Don't listen to any statements on his part to the contrary, such as, "I've been installing boilers for 20 years-I know what to do!"

    Doublecheck your edr/radiator measurements. Did you tell us what the existing boiler ratings were?

    The flexibility of a wet base boiler would enable you to have more exact control over the output, but they are a bit more noisy.--NBC
  • Dave in QCADave in QCA Member Posts: 1,660
    W-M Chart

    First, I agree with all of the comments as to the benefits of using the Smith boiler with a power burner. If you can find an installer qualified to install this, it would be the best way to go, I agree.

    However, with your original post regarding sizing, and the contractors proposing grossly oversized boilers, here are the sizes from Weil-McLain.

    EG30 = 196 sq ft

    EG35 = 258 sq ft

    EG40 = 325 sq ft

    With your current connected load of 235 sq ft, the EG35 would be a near perfect selection. However, you indicate that you may be adding on to the system. How much radiation are you going to add? The sq ft (EDR) rating of the boiler must be equal or slightly larger than the connected load.
    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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