Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Post purge setting

Roy_10 Member Posts: 8
I wasn’t expecting a positive pressure over the fire and was somewhat concerned. You are correct, the instructions say to set it up as you stated. Thanks for your help.


  • Roy_10
    Roy_10 Member Posts: 8
    Post purge setting

    How long should the burner post purge be set for?

    We recently installed a Burnham MPO147 boiler vented into a chimney. The installation, operation and service instructions have boiler sequence of operations that states there will be a pre purge for 10 sec. And a post purge for the last 10-sec. The primary control shipped with the Becket burner has a fixed pre set 15-sec. Pre purge. The post purge is adjustable 30 sec., 2 min., 4 min., or 8 min. and came set at 2 min. Should the post purge be set to 30 sec? Most of the literature on this topic refers to flue power vents requiring lengthy post purges, not much mention of venting into a chimney. Seems like once the flue gas is vented, purge should end to keep the heat in the boiler.
    FYI - The stack temp is running 350 – 400deg. Also couldn’t install the 2nd pass flue baffles because the over fire pressure became positive. Draft at breaching is .04 and almost .01 over fire without the baffles.
    Would a CO2 reading taken after the post purge is complete be sufficient to tell if a 30-sec. post purge is long enough, if reading is at or near 0?
  • The MPO

    is a positive pressure boiler over the fire . The instructions say to take over fire readings as a reference only . You want to set the boiler between 0 and -.03 in. WC at the built in test hole in the back . What you want to see over fire is a minimum of + .02 to + .04 , depending on if the baffles are in or not . I remember the numbers from yesterdays MPO 147 .

    I agree with you on the post purge timing . I think 30 seconds is plenty to keep the boiler nice and clean .
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,450

    found that a short post-purge does wonders for those tight 80's pin boilers(68's& JOT etc.)

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Ron Schroeder
    Ron Schroeder Member Posts: 998
    I would do a CO test

    after the 30 second PP as PP is not just about venting flue gases but leaving it running long enough to take excess heat out of the chamber as that can cause reflected heat on the nozzle which causes nozzle drip and oil hitting a hot surface will cause CO and soot.
  • Jim Davis_7
    Jim Davis_7 Member Posts: 67
    Oil Post purge

    Back in the 80's when I sold induced draft oil furnaces or retrofitted sidewall venters, I found that oil usually needs 5-8 minutes of post-purge to get rid of the gases, moisture and odors and prevent heat from causing after drip. Gas appliances with inducers require 3-5 minutes post-purge.
  • Roy_10
    Roy_10 Member Posts: 8
    Post purge setting

    If a power induced vent is NOT used, would a 5 – 8 min post purge still be required? When venting into a chimney wouldn’t a 30 sec burner post purge be enough to push out the initial heat to prevent after drip? The natural draft still continues up the chimney after the burner shuts down. After about 10 min wouldn’t the remaining gases vent anyway, without removing so much heat from the boiler?
This discussion has been closed.