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excessive draft and stack temps

clammyclammy Member Posts: 2,608
 Hey timmie wondering if i'm off base on this ,Looked at a job with a large natural draft gas fired boiler ,The boiler block was raised up about 4 to 6 inches just on the boiler blocks side while the rest ofthe underside of the boiler was left open . it was my belief that whan raiseing a boiler up on block that the  blocks  or a  poured pad should be larger then the physical size of the boiler as to not  increase the air space between the open burner tray and floor due to it increasing draft across the burners  increasing the excess air and increases the draft across the boiler  .The boiler has a built in draft divertor i did not chk the draft because i was there for something different  but the chimmey is about 35 to 40 ft high.The boiler seems to light off fine but it kinda of concerned me being the flue pipe had that i have been running a hi stack temp look that i have come see oh so much of espically with dirty oil boiler that where never properly cleaned oil co love those hi stack temp more oil burning,This is a larger peerless gas boiler so the bottom of the burner tray is open unlike say a utica which has a closed bottom tray on it.Thanks for ny input peace and good luck clammy
R.A. Calmbacher L.L.C. HVAC
NJ Master HVAC Lic.
Mahwah, NJ
Specializing in steam and hydronic heating


  • Tim McElwainTim McElwain Member Posts: 4,436
    Yuo are correct if the bottom of the boiler is

    open and gets its air for combustion from the room (atmospheric boiler) then a full pad must be placed under the boiler extending 2 1/2" out on all sides. Putting such a boiler up on blocks will drastically increase excess air and can cause high levels of Carbon Monoxide along with a high stack temperature and excessive draft due to the larger package of air and products of combustion that needs to be removed.

    It is amazing how many of these I see in the field.
  • clammyclammy Member Posts: 2,608
    Thanks Tim

    I knew this to be true but i needed some else smarted then i being you  to agree with me on it ,I also see it alot and i have always thought about it but most where on smaller boiler and lower btu outputs and didn t seem to have thetell tale signsi usuallysee with hi draft and excessive air .But this some slapped in the face due to the extremely tall chimmey and the extra draft you get from that  .Thanks Tim for giving me more ammo peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating

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