Insulate HWH flue

[HeadnSouth]

I'm replacing a HWH and after I got it all hooked up except the flue I noticed it was very lose. I opened the ceiling to find the 3" flue not connected to the 6" flue. The previous plumber just turned the 3" 90 up into the 6" and left it.

I'd like to install a 6" double wall 90 and reduce down to 3" to make the connection but I may not have enough room for a DW 6" 90. The flue is between the floor and it's to far back to trim without opening up more ceiling on the other side of the wall. I can get a 6" single wall 90 in the spot but that's not up to code.

Is there any way to insulate the SW 90? I thought of surrounding the flue with 1/4" wonder board.
Thoughts?

[HayZee518]

reducing fitting

the bonnet on your heater directs the burnt gasses upward with some surrounding air into a chimney.
it's temperature isn't really hot, but there is some warmth.
propane gasses tend to go down towards the ground, while natural gas fumes go upward.
a regular 6 inch to 3 inch reducer may not be tight enough to do the job.
ALTERNATIVE:
Get a 6 inch blank cap that'll fit into or over the 6 inch duct pipe.
Cut a 3 inch diameter hole in the cap that a 3" will fit into.
Cut 3/4" long cuts in the end of the three inch and bend every other tab at a 90 degree bend.
Place the straight tabs into the cap and bend these tabs back over the inside of the cap to hold the 3 inch pipe.
There! you've made a short reducer!

[HeadnSouth]

Thanks HayZee that's a great idea. I'm assuming any pipe inside the ceiling should be double wall due to code restrictions.