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Combustion air

STEAM DOCTOR Member Posts: 1,369
Good morning. I am going to look at a job soon. Customer says that he is burning 407 mbtuh. He says that the boiler room is enclosed. There is a 5" pipe running from outside into the boiler room. The 5" pipe is the only sorce of outside air. The customer would like add a fan to the 5" pipe to ensure adequate combustion air. Is there a formula for determining what size fan etc to use? Obviously too small is no good and too big is no good. Again, I haven't seen the actual job yet.

Thanks in advance for the advice.


  • Tim McElwain
    Tim McElwain Member Posts: 4,481
    You could use a Fan in a Can or Fan in a Drum

    from Field Controls their catalog will match you up to the correct size by the BTU of the equipment.
  • Jim Davis_3
    Jim Davis_3 Member Posts: 578
    combustion air

    If you check the National Fuel Gas Code they will say you need .35 cfm per thousand btus of input.  However if the boiler has a drafthood, you actually need .50 cfm per thousand btu of input.  If the boiler had a barometric and no drafhood, then you would need .33 cfm per thousand.  Now if the boiler was induced draft, noithing in the flue then only .25 cfm per thousand is needed. 

    Field sizes their fans using old numbers like .83 cfm per thousand btus which is too big.  I don't have my catalog with me on the road so I can't tell you right now which one I would recommend.  But using the numbers abovr you can figure the cfm.
  • Bob Harper
    Bob Harper Member Posts: 882

    The code does allow for "engineered" options. Putting a powered MUA unit in is one version. He can install an inline duct boosting fan run off a rheostat. Dial it in to the lowest speed that gets the job done as proven by combustion analysis. You can slave it off the primary on a call for heat. Note that a cold intake pipe will sweat and condense. If it is not stainless steel or aluminum, it will rot out. When not in use, it can have a modest backdraft damper that swings open when the fan energizes or you can wire in an automatic damper. If you duct it down into a large bucket that will tend to minimize cold air infiltration a little bit at standby.

  • RJ_4
    RJ_4 Member Posts: 484
    combustion air

    I have used fan in a can on several jobs, on 1 job i had to install 2 of them,  they have air flow safety switch that you can wire boiler control circut through,  good product, they were about a $1000.00  4 yrs ago for a 10 inch mod. but well worth it
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