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Post Purge

Does a 2 min post purge affect my efficiency much? It must cool down the water temp some in the boiler. No? I currently do not have a post purge in my direct vent boiler and I am seeing some varnishing on the nozzle.

Thanks- Tom


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    Why 2 minutes?

    What type of equipment do you have? Burning/varnishing of the nozzle is more about proper burner set up, nozzle size, air settings and draft, not about post purge.

    Yes 2 minutes is way too long without some special circumstances.

    There was an error rendering this rich post.

  • Steamhead
    Steamhead Member Posts: 16,889
    Direct-vent boilers

    often specify these long post-purges to eliminate odors. Check with the manufacturer before changing it.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Ron Jr._3
    Ron Jr._3 Member Posts: 603
    Weil Mclain

    has the Beckett burners for their packaged Gold boilers set for 2 minutes . We always cut that back to 15 seconds . I'd have to guess there is some cooling action going on if the fan runs for 2 minutes , but I never checked .
  • Burntoacrisp
    Burntoacrisp Member Posts: 20
    edited September 2012
    Post Purge

    So post purge does nothing for the nozzle or actual function of boiler. It's just "purge" of fumes after shutdown in a direct vent. I do not get any smell of fumes even though I have no post purge currently. I do have about a 4-5 ft vertical rise in my direct vent pipe. I think that the small draft incurred takes care of any fumes after shutdown. 
  • Jim Davis_3
    Jim Davis_3 Member Posts: 578

    Not having proper post-purge is the biggest mechanical defect on all equipment today that has some type of mechanical venting, whether it is gas or oil.  It is not provided because of a fictitious efficiency reduction on the AFUE.  

    I first started using post-purge on gas and oil furnaces that were retrofitted into homes that had heat pumps and no flues, so they were power vented.  Usually within days water was running out of the seams of the flue pipes because there was no post-purge.  Depending on the length of the run, 3-5 minutes was necessary to clear 100% of the moisture from the flue pipe and heat exchanger on gas appliances.  This seems long but that is what trial and error testing confirmed.

    Oil appliances need a longer time, usually a minimum of 5 minutes and many times 8 minutes.  Are were losing some heat.  Yes!  But a sooted up oil furnace is going to lose a lot more.  On oil it is not just the gasses, moisture and smell that is a problem.  I have found that if there is not adequate post-purge, the heat of the combustion chamber radiates back into the burner tube.  This heats up the oil in the drawer assembly and can cause a delayed after-drip.  It also causes glazing and blackening of the nozzle.

    About the only equipment I have found that can use less than 2 minutes would be a packaged unit.

    The only reason 80% furnaces started condensing in flues is because of lack of post-purge and not the supposedly oversized chimney which is a bunch of foohpah.
  • Burntoacrisp
    Burntoacrisp Member Posts: 20
    Nozzle Dribble

    I am getting some dribble and darkening of the nozzle. But, when its it pulling outside air that is -25 to -30 it must really cool the boiler down. That's when I don't like outside air and post purge.
  • Jack
    Jack Member Posts: 1,047

    You have discovered one of the primary reasons to have post purge. At shut down the pump stops the oil flow. The remaining oil in the burner is heated, expands and is forced out, peeing or dribbling into the chamber. No good! Put your post purge back on. Every system has a personality. You want a post purge sufficient to clear odors and reduce the heat enough to eliminate the "dribble".
  • Burntoacrisp
    Burntoacrisp Member Posts: 20
    Post Purge Dribble

    I have replaced my R7184P with a new R7284P. The 7184 purge circuit must have been damaged somehow as it was not purging. I set the purge so it would drop my flue gas temp to 200 and then shut down. Boiler manual claims this sufficient for post purge. I will check it again as we get out of the shoulder season and adjust the purge cycle downward if needed. Thanks again to all for the help.
  • billtwocase
    billtwocase Member Posts: 2,385
    direct or power vent

    post purge should be at least 4-5 mins .
  • Burntoacrisp
    Burntoacrisp Member Posts: 20
    4-5min Post Purge

    Boiler manufacturer states in manual that post purge should run until flue pipe temp drops to 200 degrees. 2 minutes does it for me. :)
This discussion has been closed.