Warm air duct insulation

[Socratic Monologue]

Hello all,

I have a US Stove 1557 (this is a warm air central furnace) which is ducted independently of our propane furnace. The wood furnace sits outside of the building envelope (in a large furnace room), and pushes heated air through two 8" round ducts which are each about 6 feet long, then into 8x14 rectangle duct which runs through the basement wall (poured concrete) and continues through an 8x14 trunk which feeds six branch ducts connected to floor vents throughout the house. Hope this is clear.

My question: the furnace room is warmer than I'd like it, and the house cooler than I'd like it. Now, the 8x14 trunk in the basement is insulated (foil-backed fiberglass), as are the branch ducts. This seems to keep the heat out of the basement pretty well (which is why I insulated them last year). The two 8 inch round ducts that lead immediately off the furnace are not insulated; I'd like to insulate them to keep more of the heat in the ducts and out of the furnace room. There is a hot fire going now (600F on magnetic temp above the door, which is as hot as I ever let it get intentionally), and these 8" ducts are about 160F (surface temp with IR gun) near the furnace, and the temp drops about 5F each foot of distance. (The main trunk in the basement is 130F about 12 feet from the furnace.) Can I insulate these round ducts that exit the furnace? If so, how close to the furnace can I insulate?

 

[RobC]

IR guns. Form reading on here I believe that you need to shoot gun off a black or dull & dark, surface to get an accurate measurement. I would check on that. Might be as easy as putting on some electrical tape.
For high temp insulation try searching Garn insulation. Some of those guys use a blanket type that may suit your needs.
Don't think spray on foam is the answer you will have a high temp problem, or worse, some is flammable and wouldn't be a good suggestion for furnace room.

 

[Socratic Monologue]

Yes, I only shoot the IR gun at a dull surface. The round duct is actually black stovepipe (installed by previous owner -- not what I would've used, I guess, but it works fine and looks, I don't know, more wood-stoveish). The 8x14 duct is galvanized steel, so I shoot the IR gun at patches of the duct mastic (dull gray), and get what I believe is an accurate reading.

Thanks for the info on what kind of insulation I might use. I was just thinking I would use the same foil-backed fiberglass that I put on the trunk in the basement. It is specifically marketed as HVAC duct insulation.

What I'm really concerned about is (this sounds silly when I type it) heat buildup inside the round ducts near the furnace. I know they are full of heat, but I thought I recall reading somewhere that warm air ducts from solid fuel furnaces shouldn't be insulated for the first so many feet from the unit. I forgot the details of this, and I wonder if it is even true.

 

[RobC]

I could buy into that from the standpoint of it being wood burning. "No combustibles within X" of appliance".
I know with boilers some suggest that if you run pex pipe to start off with a length of copper to get away from boiler a few feet.
There is a lot of extra heat near these appliances as you mention 600F at the door.
Good luck.