What is this red stuff?

I saw this red “caulk” on a furnace and water heater flue where they connect to the stack.

What is it? And is it okay for this use?

Looks like high temp sealant to me. I think the stuff is rated for over 500 F temps.

So it’s a non-issue? Correct?

I wouldn’t make it an issue

Thanks Chuck!

You mean besides ugly?


Hope it is the high temp sealant and not RedGard waterproofing sealing material.

It is rated well over 500 degrees.

I would be concerned they need patching at all though.
The straight B vent appears to be just that, as in Horizontal.(maybe even angled down)

Just what i was thinking

They look like site made connections and when the caulk fails (and it will) leaks will start.

Thanks guys.

Here’s what I said in the report:

  • Observed the flue connection from the furnace and water heater, where they connect to the main stack, appear to be sealed with a red material similar to caulking. This appears to be what is known as a “high-tempertature-selant.” While this is likely not an issue, it is recommend to remove the material and replacement with proper flue sealant/tapes. Improvement.

Yep sir

Is there a tape rated for exhaust flue temps and intended for use on flues?

I usually write up tape on flue joints, I wouldn’t have written anything for this.


Hey, at least they didn’t utilize duct tape. I’ve seen many vents with dried-up duct tape falling off.

A famous Canadain Man uses of stuff he has created

Something looks wrong. Two vent pipes the same size going into the main stack the same size indicates possible undersizing of the vent. This needs to be written up for HVAC company to do a sizing check on it.

Not the same size… 3 inch water heater on top, 4 inch furnace on bottom. Very normal sizes…