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Draft on cold boiler

General question. What sort of draft numbers should be expected on cold flue/boiler? Purpose of such a test is to determine if there are negative pressure issues in boiler room/building. Thank you.


  • Jim Davis_3Jim Davis_3 Posts: 578Member

    If you mechanical room is positive you will have a slight draft in the chimney, even with a cold boiler, but this is not the norm.  Most mechanical rooms are negative unless some type of mechanical fan is running during the off cycle.  The important question is what is it during the on cycle.  Without some type of mechancal combustion air fan the room is still going to be slightly negative.  The question becomes is whether or not the flue can overcome it.

    What would be considered negative enough to indicate excessive negative pressure in building?TY
    neg pressure

    What I really want to know is what is the best way to determine (using a combustion tester) if there is an unsafe negative pressure situation in a building.
  • Jim Davis_3Jim Davis_3 Posts: 578Member
    Negative pressure

    An unsafe negative pressure would be one that doesn't allow the boiler to start venting within a few minutes.  To determine if an appliance is venting with a combustion analyzer(which is the only way it can be verified) is to watch the O2 and CO readings.  If they are stable after 3 minutes (could still be going down slightly, but never up) you are venting.  If they start to rise after they bottomed out you have a venting problem.

    If the numbers go up how do does one know conclusively that there is a negative pressure issue in the building as opposed to some other CO causing issue
  • Jim Davis_3Jim Davis_3 Posts: 578Member
    venting problem

    You look at the draft reading after several minutes.  If it is -.02" or higher you do not have a negative pressure problem, but if it is lower you do.

    If the draft is -.02" or higher and the CO is rising that is an equipment venting problem.
  • VictoriaEnergyVictoriaEnergy Posts: 126Member
    Test Standard

    I believe the certification standard for Cat I appliances mandates successful establishment of draft from stone cold start-up with the mechanical room being depressurized to 10 Pascals, about .04"WC

    I've known this for a long time & can't remember where I learned it from or who told me.  May have been HVI. 
    Home Owners Please Note:

    You are receiving advice from some very skilled pros completely free of charge. One of the reasons I participate is to sharpen my own troubleshooting skills. So; did we get it right? I would be grateful if you extend this courtesy back by posting the final outcome of the issue you are inquiring about. Thanks
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