Based on my experiences, working as a Bellingham, WA home inspector, I would say that one of the most misunderstood components to be found in the home is the B-vent. The B-vent is commonly seen at gas furnaces, gas water heaters or gas fireplaces...it is that metal chimney like device you see coming out roofs. Since the vent is double-walled, it seems that everyone thinks it can be in direct contact with wood, cardboard, plastic flex duct or attic insulation.
The fact of the matter is, a B-vent requires 1", minimum, clearance from combustibles. It says that right on the B-vent in most cases, if the label has not worn off. As often as not, the B-vent has attic insulation packed around it. Sometimes the insulation around a vent was cleared away at one point, but it has been disturbed and ended up around the vent anyway. The photo below is an example of that....close, but no cigar and the situation will probably get worse since nothing is really holding back the insulation.
The best solution I have seen to remedy this problem is a metal insulation shield. The shield goes around the vent. Insulation can be tight around the shield, but the shield provides the proper clearance from the vent to combustibles. In case you are wondering, insulation may not be all that combustible, but it is still considered to be a combustible when taking it into consideration when installing a B-vent.
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