Vent duct heat and safety in basement

[MCB]

Hi all, 

I recently renovated a shed for a home pottery studio, and am installing a new kiln that is going in my unfinished basement this week. To vent the kiln I need to run the flexible duct about 30 feet across a room to reach a window that I am outfitting the outlet to go through. The best way for me to get the duct across the room is going to be by hanging it from the ceiling. Because the basement has low ceilings I would like the duct to hang down as little as possible, but I am wary of it being close to the exposed wood studs on the ceiling. The specs of the L&L vent-a-kiln say that it does not exceed 150 degrees, but I can't find any recommendations on safety precautions on what distance it should be from possibly flammable surfaces. In your experience, is the heat from the duct anything to be worried about?

This forum has been a fantastic resource for me as I built my studio. Thank you!  

 

[Bill Kielb]

If it’s a vent a kiln then you have a hood that is going over the kiln. It will draw room air so it’s not going to get very warm, beyond room temperature. Single wall duct and standard hangers. Distance should not exceed 10 ft without consulting the factory so definitely call them. Negative pressure is less likely to leak than positive. If you have a 5”265 cfm, then they Will likely suggest 6”round or better across the basement. If you have a 500 cfm then they will likely suggest an 8” round duct to traverse if you want to maintain the rated cfm .

All of the extension ought to be hard pipe BTW, 30’ of flex would be doable but generally a bad idea..

suction is easier as all leaks are inward, pressure would generally require your best joints not to leak a little. Either will likely require a way to bring in outdoor air when running especially if you have combustion appliances.

Best to consult the factory for your setup. Link to vent-a-kiln https://ventakiln.com/overhead-kiln-ventilation-systems/

Edited November 27, 2022 by Bill Kielb

[Hulk]

Hi MCB,

Is it the L&L "Vent-Sure" downdraft system that you're looking to install?
If so, good question on guidelines for routing the ductwork on or near flammables.

The L&L FAQs do have an entry on roof/ceiling penetrations:

"Do I need double wall duct when going through the roof?
You do not normally need double wall ducting when going through the roof since the pipe or duct does not reach high temperature. It is always advisable to check your local building codes for their requirements."

My guess would be to place somewhat between the duct and the flammable(s), a piece of tileboard perhaps.
Please check back for response from this section's Admin, who is very familiar with L&L equipment.

For the longer runs, smooth (rigid) pipe could be a better choice than flex pipe - less air resistance, easier to clean, and should last well.
 

The L&L documentation also indicates:

Length can be as great as 60 feet horizontal or vertical with up to four 90-degree bends.

Be sure to allow for adequate make up air - intake to equal what is being pulled from the space, hence an open window, vent, or somewhat to the outside.

[neilestrick]

I've installed many vent ducts against studs, walls, etc. It doesn't get hot enough to be a fire hazard.

[newclay]

150 degf is what fire departments leaving a fire hunt for. It is a bit below  the igniition temp for old, heat expoosed wood after years, which is why fire depts hunt for it.

 

[Bill Kielb]

6 hours ago, newclay said:

150 degf is what fire departments leaving a fire hunt for. It is a bit below  the igniition temp for old, heat expoosed wood after years, which is why fire depts hunt for it.

 

It’s an excellent point and possible under very special conditions and temperature rise (UL sets this as possible). But again if it’s a vent-a-kiln as the op specified, the duct will be near room temperature air. If it’s a downdraft that @Hulk mentioned then properly adjusted it will supply 100-130 degree air as the minor amount of kiln air must be diluted with room air in the mixing manifold at the kiln. Standard suspended single wall ductwork generally applies. Double wall and special clearance requirements generally apply to ductwork venting gas fired appliances.
Ensuring this is operating as the manufacture specifies is probably most important to get right and following the manufactures suggested venting requirements when extending.

[neilestrick]

25 minutes ago, Bill Kielb said:

if it’s a vent-a-kiln as the op specified

I think they're talking about the Vent-Sure since they specified L&L.

@MCB If you're concerned, you could always mount the ductwork with a hanger that is slightly offset from the wall.

[MCB]

Thank you everyone for your responses. I was referring to the l&l vent-sure downdraft system. Just to be safe I will hang it slightly down from the ceiling so it isn't touching anything, but good to hear it doesn't get too hot in people's experience.  I appreciate the recommendation for rigid pipe, and I am planning to have a window open with a small fan going. Thanks! 

[Cobraone]

Any wood stove or fireplace store sells fiberboard a heat resistant board that could be cut and attached to the wood your concerned about. Your L&L manual describes wood over a period of time of low heat that looses moisture can flash at lower temperatures, I would buy a ir temp gun ( harbor freight sells them cheap} and find out the exact temp the ductwork is running at,  it’s hard to be over concerned when dealing with fire safety, 

I hope this helps!

best regards!