I have a combined radiant heating system/domestic hot water system. This unit is a middle efficiency unit rated at 87% The boiler is connected to a B-vent chimney with single wall vent connector. The boiler is also equipped with an electric damper to reduce air going up the chimney when not operating.
The B-vent was replaced 15 years ago and is in good shape. The vent connector is so corroded that last night it collapsed. It was replaced at the same time.
I have shutdown the unit until I can get it vented properly again.
A: Why is my vent connector pipe corroding so badly?
B: How can I prevent this from happening again?
My suspicion is that the boiler is oversized (90,000 BTU) and is short cycling. Even on cold winter days it rarely runs more than 2-3 minutes at a time with 10-20 minute off times. This results in hot humid air in the vent and chimney which is kept there when the electric damper closes. Water condenses in the exhaust system, and never really dries out.
My thoughts:
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Disable the damper in the open position.
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Punch a hole in the damper so that it doesn't shut all the air off in the off position.
Best Answer
I would start with a couple of small 1/2” holes if you disable the damper totally you loose efficiency quickly where several smaller holes will allow a draft and the combustion moisture to dry 15 years is a fair amount of time but I have had systems last much longer in some cases when doing annual inspections I have recoated single wall with zinc rich paint it could have the last piece dipped in a batch but excess moisture is probably the cause so start small and see if that works without totally disabling the energy saving feature of your damper.