Insulating air duct from furnace

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Socratic Monologue

Burning Hunk
Dec 2, 2009
196
WI
I posted this question on The Boiler Room, but am still looking for more feedback. Hopefully someone here has some advice for me.

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?
 
I would think it would be ok to insulate them. Just be sure no combustibles are involved. Also use insulation to isolate the 8x14 trunk duct from the concrete if it isn't already.
 
Fiberglass has to get a lot hotter than that to burn. If you are concerned though, You could use Kaowool for the first couple/few feet, and fiberglass over that.
 
I believe that *officially* fiberglass bat insulation is NOT considered non combustible in the building codes. This is why you will see Mike Holmes freak out when he sees insulation withing 10 ft of a chimney flue for example.

In the real word - fiberglass is made from spun glass fibers. Silica class cant burn AFAIK, but has a meliting point somewhere north of 2000 F. If its paper faced the paper might catch fire at 454 :) but other than that you have nothing to worry about.

If it makes you feel any better my steam boiler and the piping system is all fiberglass insulated. the 215F temps dont bother the glass one bit...
 
jharkin said:
I believe that *officially* fiberglass bat insulation is NOT considered non combustible in the building codes. This is why you will see Mike Holmes freak out when he sees insulation withing 10 ft of a chimney flue for example.

In the real word - fiberglass is made from spun glass fibers. Silica class cant burn AFAIK, but has a meliting point somewhere north of 2000 F. If its paper faced the paper might catch fire at 454 :) but other than that you have nothing to worry about.

If it makes you feel any better my steam boiler and the piping system is all fiberglass insulated. the 215F temps dont bother the glass one bit...

Not sure if "burn" was the best term to use, however, when welding large castings, I have resorted to using fiberglass to slow the cooling rate, for items too large to fit in the ash can. The smell and burnt brown/black look of the glass after contact with too hot metal is easiest described as burned.
 
I knew what you meant Dune... directing my comments more at the OP. I guess there could be other fillers or coatings mixed in with the glass fibers that might burn off... didnt think of that. But I think we all agree he has nothing to worry about....
 
jharkin said:
I knew what you meant Dune... directing my comments more at the OP. I guess there could be other fillers or coatings mixed in with the glass fibers that might burn off... didnt think of that. But I think we all agree he has nothing to worry about....

I do agree he has nothing to worry about, however, lining the first few feet with kaowool AND fiberglass will boost the performance significantly, since that is the area of greatest heat loss.
 
Thanks, everyone! This makes me feel much more confident. Last season I was dissuaded on this forum from attempting to insulate the furnace itself (in the way that the Caddy is insulated) since there was a risk of excessive heat build-up especially if the blowers were to lose power. I wasn't sure if there was some similar concern with insulating the ducts, but if you all think it is safe, I think it is safe.

Just curious -- BeGreen, the 8x14 duct is in fact insulated from the concrete by fiberglass batt, but I did this to slow air infiltration between the basement and the furnace room (the furnace room gets quite cold when we are on vacation and the wood furnace is not in use). But your comment suggests there is some safety-based reason to insulate here. What is that reason?
 
Not a safety based reason, just that concrete is a heat conductor, and at an average temp on 53-56 degrees is stealing heat from your ductwork if in contact.
 
Dune said:
Not a safety based reason, just that concrete is a heat conductor, and at an average temp on 53-56 degrees is stealing heat from your ductwork if in contact.

Oh, I should've thought of that. I've got safety on the brain whenever it comes to burning wood in the house (hope I never lose that attitude!), so that's immediately what I thought of. Thanks for the clarification.
 
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