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Post purge setting

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Roy_10
Roy_10 Member Posts: 8
in THE MAIN WALL
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?
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