I'm replacing my atmospheric venting natural gas water heater and I'd like to clean up the type B venting solution while I'm at it. I've had a high efficiency furnace put in so now only my gas water heater is venting through what I've found to be a flexible flue liner going into my brick chimney. The existing solution is the flexible flue liner is mortared inside of a clay thimble, with a sleeve protecting the liner in the mortared portion. The mortar is badly cracked and I pulled out pieces out by hand.

What would be the right solution to have a tidy connection to the flexible flue liner? Should I remove the wye, connect the water heater vent to the appropriate increaser and mortar the flue liner with refractory cement? Is there a better product to reduce the clay thimble to the size of the flue liner?

Water heater at a distance

Vent wye fitting

Closeup of clay thimble and flexible flue liner

Edit: Here is the venting after the repair: Flue liner passes through mortar sleeve and adapts to Type B gas vent

Mortar seals the mortar sleeve in place

  • After a lot of research - I think I've figured out a good solution. I posted it generically as an answer. I used a Rapid Set mortar mix (a fast setting, CSA cement based mortar) which worked well to set without cracking. Probably overkill on strength for the application.
    – Barrett
    Commented Jan 3, 2022 at 1:58

1 Answer 1


There is a flexible aluminum flue liner inside the chimney, and that passes through the clay thimble by way of a mortar sleeve. To repair this: remove the wye, adapt the flexible liner to type B gas vent, and use type B gas vent fittings for the remainder of the run to get to the single water heater appliance. The mortar sleeve is fixed inside the clay thimble with any mortar suitable for brickwork (e.g. Type N). Because it's a fairly wide joint, care must be taken to cure the mortar properly to resist cracking (like the previous installation has suffered) especially given the drying effect of the heat from the vent while the mortar is curing.

The mortar sleeve slides over the flexible liner: Mortar Sleeve in Flexible Liner

A special Type B vent adapter (made specifically to mate to flexible flue liners) can be attached to the liner, and Type B venting connections are made thereafter to get to the water heater. Adapting Liner to B Vent System

Pictures from example manufacturer's installation instructions: DuraFlex® Aluminum Chimney Relining System

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