# Can ductwork get hot enough to ignite wood?



## Red Squirrel (Jun 29, 2009)

My guess is no, but I'm just asking to make sure. I'm framing a section parallel to the joists where I need to put some cross sections so I can put the top plate on, but those cross sections get in the way of the duct work. I chizzeled out so it's not as tight of a fit but a few spots the ductwork is sitting right on the wood. Any issues at all with this? I can't see how it would get hot enough but better safe than sorry.

If it is a concern, is there something thin I can slip between the ductwork and the wood? Small chunk of drywall maybe? Parchment paper?


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## sublime2 (Mar 21, 2012)

No way.


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## yuri (Nov 29, 2008)

the code says you should have 1" clearance and also to allow for expansion and contraction or you may get a noise later.


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## FClef (May 5, 2013)

Ductwork under normal conditions will not ignite or even char wood. There is no need to insulate the wood from the heat that may be generated.


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## Red Squirrel (Jun 29, 2009)

Cool that's pretty much what I thought. Especially considering it barely hurts to touch the main plenum with my hand.


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## ddawg16 (Aug 15, 2011)

yuri said:


> the code says you should have 1" clearance and also to allow for expansion and contraction or you may get a noise later.


The 1" you refer to is most likely related to B-vent's.....The ignition temp for wood is 572 deg F....if the vents in your HVAC gets that hot....you have other problems...


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## hvac instructor (Jun 8, 2012)

The 1" also refers to supply duct. if you look at the supply run, you see it hangs down 1" from floor joists as the return is hung tight to the floor joist. This is so if the furnace catches fire, if it gets into the ducts it buys a little time before the floor joists catch fire.


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## ddawg16 (Aug 15, 2011)

hvac instructor said:


> The 1" also refers to supply duct. if you look at the supply run, you see it hangs down 1" from floor joists as the return is hung tight to the floor joist. This is so if the furnace catches fire, if it gets into the ducts it buys a little time before the floor joists catch fire.


Ok....that makes sense.

But on a more proactive note...I prefer that the furnace does not catch fire.

Out of curiosity....how many incidents have there been where the furnace caught fire?


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## hvac instructor (Jun 8, 2012)

I had one. it was a roll out and burnt every wire and controls. I rewired the whole thing. usually when the flue is blocked the flame will roll out.
Now on rooftop units I've seen about a dozen. I just replaced the whole unit then.


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## countingcoup (Jul 29, 2013)

yuri said:


> the code says you should have 1" clearance and also to allow for expansion and contraction or you may get a noise later.


Code is just for the venting. 1" clearance on supply duct is common but not code. The 1" hang down on supply duct is to reduce noise. That is why almost all duct you see is hung 1".


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## itaintmesy (Jul 30, 2013)

countingcoup said:


> Code is just for the venting. 1" clearance on supply duct is common but not code. The 1" hang down on supply duct is to reduce noise. That is why almost all duct you see is hung 1".


Only for venting? Then you have not pulled many permits or attended a code class.


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## yuri (Nov 29, 2008)

Exactly. Must have 1" clearance on top of the furnace bonnet and technically over 90% of the expansion is linear so the 1" hang down is not for that anyway. Furnaces can catch fire but it is rare. I have seen limit controls fail to open or got bypassed from incorrect wiring and then the AC coil melted. Also had a oil flooded oil furnace ignite and get so hot the heat exchanger went red hot and the venting white hot. Had to get the fire dept down to watch it till it burned out the excess oil.


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## HVACTECH96 (Oct 16, 2012)

You can go tight to the joist once you get 8-10 feet off plenum.


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## MTN REMODEL LLC (Sep 11, 2010)

ddawg16 said:


> Ok....that makes sense.
> 
> But on a more proactive note...I prefer that the furnace does not catch fire.
> 
> *Out of curiosity....how many incidents have there been where the furnace caught fire*?


2 out of 2 we ever heard about.... Statistically speaking that is 100%


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