Barometric Damper Slapping when system shuts off

masterclass44

Hi, I recently had a new oil furnace installed (Williamson low boy) and have had the tech come back many times to fix minor issues. However after his last visit, I’ve noticed the barometric damper slaps open every time the system shuts down after hearing the house. Is this normal? I didn’t notice it before but could hear it upstairs for the first time today

STEVEusaPA

Probably normal-ish. What were the combustion numbers & over fire and breech draft measurements?

masterclass44

sorry not sure on those. I have had to leave all my trust in the furnace tech as I am not familiar with anything hvac related unfortunately (but trying to learn!)

EBEBRATT-Ed

@masterclass44 the tech should have a print out of the combustion test

EdTheHeaterMan

The barometric draft control will always slap open at the end of a cycle.  There is a fine balance of expanding gasses in the combustion process. Liquid fuel expands rapidly to a vapor then a flame. This causes the pressure inside the combustion chamber to push back against the fire tube of the burner and push against the exhaust vent connector to the chimney. To achieve proper combustion, the barometric draft control is adjusted to maintain a balance of slightly negative pressure (compared to the atmospheric pressure in the boiler room) in the boiler or furnace. 

Consider what happens when the flame stops at the end of a cycle. All that expanding gas stops instantly, but the chimney draft continues to exit the boiler. The combustion chamber pressure rapidly drops at the same time. This results in a momentary rapid decrease in pressure in the exhaust vent connector where the draft control is.  The draft control acts as a relief valve and rapidly  opens to  equalize the low pressure in the vent connector. The only way to decrease any noise is to operate the burner fan for a few seconds after the oil valve stops the flame. A 15 second post-purge might help.   You may also want to consider using the next size larger barometric draft control like using a 7” control on a 6” flue pipe connector. 

masterclass44

Thanks Ed. The noise doesn’t bother me but more so just wanted to make sure that it is ok to slap open at the end of the cycle. It doesn’t slap any other time at start/while running