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I know I'm missing something here, having a brain lock
Customer called with a spill switch that was tripped, they reset and I went out to take a look.
140,000 btu boiler and 40,000 btu water heater. 6" wye with a 4" connected for the water heater, boiler was installed about six years ago, I was told their was a new liner installed.
Put my draft gauge in the 6" boiler flue will making steam and had very little draft(just passed the 0 line). Boiler shuts off and damper closes and the draft gauge goes up to 2.5" wc.
My question is why would it rise after the damper shut and not during it running?