How to seal exhaust flue pipe?

cluelessandlost · February 4

The cement around my flue pipe connection is cracking, and my city inspection won't pass the permit on my new boiler installation until I get it sealed airtight.

I've been researching how to seal the flue pipe but getting mixed answers. I originally bought some furnace cement and some trowels, and planned to scrape off the old stuff and apply the cement to it. But then someone was saying furnace cement for flue pipes tend to crack because it contracts/expands a lot during regular operations. And looking further, all the sites that suggest using furnace cement have been for wood stoves, not steam boilers. Then I saw someone on YouTube use fire block sealant with a caulking gun for a flue pipe, though it was for a water heater.

Which is the best option? Any important things I need to take note of during the process is appreciated as well.

Grallert · February 4

Unless things have changes furnace cement is all you need. Chip off the old and neatly apply the fresh stuff. It's not there to keep flue gasses from escaping it's there to assist with keeping a measurable, consistent and controlled draft at the flue.

cluelessandlost · February 14

Thanks! I did exactly as you said (minus the "neat" part - there was only so much my tiny t-rex arms could do behind all the pipes), and today it passed inspection!

pecmsg · February 14

ain’t neat but it works. 👍

EdTheHeaterMan · February 14

Screenshot 2025-02-14 at 10.33.58 AM.png

I have always found that Structo-Lite is the best for sealing single wall vent pipe to a masonry chimney. It is fireproof. There is nothing that can burn in this product. It is strong enough to hold the vent in place. It does not shrink when it cures. It can be troweled and finished like mortar and will stay in place on a vertical surface. And it looks good when you are finished.

Learned about this stuff from an old timer chimney sweep in Philadelphia.

cluelessandlost · February 14

Good info to know! If this current application ends up flaking or needing to be redone, I'll have that bookmarked.

Cyclist77 · February 14

How to seal exhaust flue pipe?

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