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Replacing draft vent into chimney
I had my boiler serviced a few years ago and the tech examined the exhaust and interior of the chimney. When he put it back together he used some chunks of the old concrete and slathered furnace cement all over it. It looked bad and there was a small hole in it, so Ive decided to redo it.
Take a look at the existing penetration after I've removed all of the cement. It is 4" pipe going into a 7" hole, so it would be a lot of void to fill.
I have test fit a 7"x6" reducer to a 6"x4" reducer with a 4" coupling. It seems to fit nicely in the hole and I plan on using furnace cement around the reducer and taping the rest of the joints with foil tape.
Is this an acceptable way to do this, or does the 4" pipe need to protrude into the chimney void by a certain amount?