Q on ducting 1" clearance to combustibles

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motoguy

Burning Hunk
Jan 7, 2015
134
Central MO
Managed to move the Kuuma from it's storage area in a spare room, to it's new location today. Didn't hook up the flue, but did a test fit. Happy with it.

Which brings me to ducting. The Kuuma is located in our walk out basement. The trunk line runs in the center of the house, the length of the house. Branches exit the trunk at 90 degrees to the registers. The trunk is insulated metal, branches are insulated flex ducting, located between joists for the upstairs flooring. Basement ceiling is unfinished.

Ducting from the Kuuma to my existing trunk won't be an issue. This flex ducting...may be. Specifically, the wiring that runs between the joists (perpendicular to joists). In several places the flex ducting is pressed against, or in some places between, electrical runs (Romex). Not an issue currently, but likely more so when replaced with rigid metal ducting.

So...this 1” to combustibles requirement...if I need to squeeze round metal ducting between wires, or against wires, can I just put a barrier there? Will 1" of concrete board (or ceramic, or firebrick, or whatever) wedged between the ducting and the Romex suffice? Is that acceptable, as it gives a 1" barrier from pipe to (I assume combustible) Romex? Does code require a 1" -air- barrier?

If I can do something like that, replacing all this flex tube won't be too big a deal. If I have to arrange a 1” air gap between duct and Romex...life might suck.

Ideas?

While not my home, this is the type of thing I'm talking about. Romex spanning joists, perpendicular to the joist direction.

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Maybe I'm way off here but I'm not aware of a clearance to combustibles requirement for HVAC forced-air ductwork? I'm pretty sure my ductwork (installed/inspected) comes into contact with all sorts of stuff as it runs throughout the house.

My only other thought is that you may want to skip the flex ducting where possible. You'll get better performance from rigid.
 
It's my understanding that requirements exist for solid fuel furnaces, which don't exist for gas/propane/electric, due to the ease of which their output is regulated. Unlike a wood furnace, which keeps building heat when the power goes out.
 
This clearance issue is brought up quite a bit and in the case of modern (caddy, tundra, kumma) add on wood furnace I apply the clearance figures to the run from the wood furnace to the plenum of the main furnace be it natural gas/LP or oil. Certainly applying the clearance to combustables to ALL forced air ducting in the house would make most add on wood furnace installs impossible. IMHO of course. Now if it's a all new install from scratch (no existing heating system) you might have a better chance of maintaining those clearance figures through out. Don't know what you'd do about those vertical runs from basement to 2nd floor that hug the studs in wall cavities.... and hundreds of other spots where your typical sheet metal ducting is in direct contact with floor joists, wall studs, plywood flooring, etc

In the case of my Drolet tundra tied into the existing plenum of a propane FA furnace, even with the fan off those ducts are not getting hot enough to cause me ANY concern. Now it might be a different situation with a different wood furnace, say one of the smoke dragons with a huge firebox and no provision to automatically shut down the inlet air in the event of power failure.
 
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motoguy said:
It's my understanding that requirements exist for solid fuel furnaces, which don't exist for gas/propane/electric, due to the ease of which their output is regulated. Unlike a wood furnace, which keeps building heat when the power goes out.
Click to expand...
Exactly.
The plenum and the first 6' of run is often times required to be 6" of clearance. The 1" is NFPA code I believe.
@3fordasho is right though, there are places on many houses where the 1" is impossible to maintain. If nothing else, at the register there will be contact. For me, I maintained as much clearance as I could, for as long as I could. That and I sprayed all the wood in my furnace room with a fire retardant. Also, installed a "emergency heat dump door" (made by Yukon) on the plenum (maybe overkill I know, buy hey, makes me sleep better)
I would say as long as you can meet Kuumas clearance requirements for the plenum and main trunk line at least, then I would replace the branches with metal duct, then get some 1/2" ceramic fiber insulation blanket material with the foil backing on it (think chimney liner insulation) then I'd put two wraps of that anywhere that has less than 1" clearance. That what I would do...which means nothing to anybody else really, it's your house you hafta do what you feel is correct, which may be to meet actual code requirements...?
Does it need to be inspected by anybody?
 
No issues. Wire is rated at 90C IIRC and this is much higher than any expected air temp in ducts.

Personally I'd be much more concerned with the flow loss (CFM) of having the branch circuits off the supply plenum running all that flex. There is good reason the building code limits the length of the flex crap.
 
1" applies to the supply plenum and the main trunk, not branches. For wood fired though, many areas require 6" to meet code so you'd best check with the local code people where you are located.
Given the fact that it is impossible to immediately "kill" a wood fired piece of equipment in an emergency situation, I'd advise more rather than less clearance.
 
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heaterman said:
For wood fired though, many areas require 6" to meet code so you'd best check with the local code people where you are located.
Click to expand...
Also, the manufacturer, in this case Kuuma, calls for 6" for supply plenum and supply duct for the first 6'. I think many local codes will refer back to what the manufacturer calls for, unless local code is more stringent.
 
Everything heaterman says, Also absolutely NO flex duct that is not metal.
 
Motoguy,

They make round to oval ductwork connections that could be installed to clear those wires in the center of your floor joists. This could provide you with the proper ctc (clearance to combustible) needed for your application. I was under the impression that no flexible ducting for wood furnace. What is the measurement of those floor joists? What are you doing with the ceiling?
 
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