At What Temperature Does Wood Combust ?

[Guswah]

I'm building a Russian banya (sauna) in a standalone building. In a traditional banya, a woodstove projects through a passage in the wall and into a separate room where wood is fed into the stove thus separating it from the "hot" room. I framed the wall that separates the two rooms with fir lumber and left an opening with about four inches clearance all around the stove. It's a little late, but now I'm worried that the opening might have been bigger and that heat from the woodstove in the opening could eventually start the wall on fire.

Currently the entire wood-framed wall is fiberglass insulated and clad with fireproof drywall. Over that, I've installed a layer of quarter-inch cement backing board. The opening itself is likewise covered with those materials, but I've also cemented in a course of refractory bricks around the opening's perimeter. The space now remaining is about an inch and a half all around the woodstove.

I wish I knew what the shell temperature of the woodstove will be once it is operational, but I do not. It's a Fisher stove -- the old type without double walls, chosen specifically for this purpose. After all, we want that thing to radiate heat quickly. My question is what to do with the remaining inch and a half of clearance to best isolate the opening from the stove's outer surface where it passes between both rooms. Another course of brick? Fiberglass insulation? Nothing at all? What will provide the best protection from the stove's surface?

I'm grateful for any discussion anybody cares to offer!

 

[brenndatomu]

The stove walls could see 8-900*...even more...what stove is it? Most stoves require at least 18" to any combustible...some even up to 3'...I don't think there is anything you can do with your current setup to keep the framing from burning eventually...other than tear it out and go steel framing, or all masonry.

 

[Guswah]

The stove is a Fisher. Those are now obsolete, but when then were in production there were three models called "Papa Bear", "Momma Bear", and "Baby Bear". This one's a Momma, chosen because it's low and long.

 

[Wood Doctor]

I have a stove thermometer on my Federal Airtight 288 that measures flue gas temperature in the secondary combustion chamber about 1" below the top surface of the stove. I have sometimes reached 1400 F, but the average peak secondary exhaust temperature on a cold day with a good draft is about 1300 F. Peak stove skin temperature, therefore, must be close to 1000 F but could be a tad higher I suppose.

The Fisher Momma Bear is a good wood stove. It's not really obsolete as you say. They just aren't made anymore.

 

[Guswah]

It goes without saying that ANY wood stove has a clearance spec of eight or more inches. I have no illusion about that. But what I have done is protect the wood-framed wall as I described in the hope that the usual clearances are not necessary. In a perfect world the stove door would protrude and the stove walls are completely sealed so that sauna occupants have complete privacy and ambient heat stays within the chamber. To accomplish that, there must be materials that will prevent conduction from the band around the stove's girth and up through the firebrick I've installed and end up heating the framed wall. Those treatments are not prescribed in any building code because they are complete overkill if you are adhering to the large clearances that are typically specified for wood stoves. By the way, as a rural, remote, and detached building, I'm not required to follow any building standards.

 

[Wood Doctor]

Today I watched my Federal Airtight 288 surprisingly kick out some sparks right through the small air vent valve opening on the left side, I have brick all the way around the stove and the nearest combustible is about two feet away from it. These sparks jumped out about a foot and right through an unbelievably small opening. It happens.

No problems here, but I often wonder why anyone would want to compromise recommended clearances. Stoves get hot and fires can throw sparks in every direction. Simple as that.

 

[ChoppyChoppy]

I've witnessed ignition several times over the past 18 years working in an OSB mill. Dried wafers with core moisture around 6% and surface around 8% and these will vary a little in the drying process. Temp of the press runs 410 to 415 range. That's heated oil in the platens. If there's anything to stop the line that's when we have to be very careful if the press is loaded and can't unload. If it looks like we'll be down over 30 minutes we can dump the hot oil and cool the press down to like 385 and have no problem. Course we use wax and resin and that may help ignition a little.

It's a sight to see 14- 8'x24' mats of unpressed pine wood ignite. Only takes seconds to be engulfed. Fire fills the area with smoke fast. Spray with water and have to be careful you don't get scalded by the hot water falling back on you. If you can't knock it down in seconds it's a run (feet don't let me down this time) situation and let the sprinkler heads do their job. Once the smoke gets down to your head level it's time to get out fast. So I'll say 410 for 35 min. or a little more with the right moisture content can get a good fire going. Once it starts then the smoke burns too.
RUN.:yoyo:

By chance do you work for JM Huber in Crystal Hill? My Dad works for them at the OSB mill in Maine. Back in the 90s we almost moved to VA for a job they offered him.

 

[Guswah]

This is a Fisher stove. There are no side air vents. The only place for sparks to fly out is the door, and there are no clearance issues there. Regardless, my post was not about sparks. It was about heat convection from a 5" x 20" area on the stove's topside where it passes through an opening in a wood-frame wall. I'm not sure how many times I need to describe the scenario.

 

[Guswah]

I've just found this at a website called Hearth: "The benchmark temperature, which determines safe clearances, is typically 115 degrees F over the ambient room temperature. In other words, if the ambient room temperature is 70F, the benchmark wall temperature cannot exceed 185F." I think that is what I was looking for because it allows me to monitor the susceptible area, and if there is a problem, I can probably overcome it with a fan or explore other remedies.