Safe drywall temps

[unhdsm]

This is my first post at hearth.com

I fired-up my new Hearthstone Shelbourne woodstove this afternoon after the break-in burns.
I have the minimum clearances for a corner installation with the rear heat shield and double wall chimney pipe-- 6 inches from the back corners to the wall.
My concern is that the drywall still feels pretty hot. How hot can it safely get? I am afraid I will have to get some wall protection.

Thanks for the anticipated feedback.

 

[jtp10181]

I know mantle clearances are tested to 117 F above ambient temp.

This seems to be the universally accepted safe "combustibles" temperature when testing for clearances.

So that would be 187F if ambient temp is 70F

 

[CarbonNeutral]

Can you hold your hand on it?

 

[burleymike]

As long as you have met the clearances specified by the stove and pipe manufacturers it should be fine. The gypsum itself will not burn it is the paper you have to worry about along with the wood studs in the wall. If it were getting too hot the paint would bubble first then the paper would start to smoke.

When we bought this house the previous owners had an early 1980's blazeking in the laundry room. He furred two layers of 1/2" sheetrock from the walls and had the stove 12" from that. He then glued 1/2" veneer brick over the two layers of sheetrock. He also did not see a point in buying the support box for the pipe he made his own! It is amazing that the place did not catch on fire from that or any of the other stupid stuff he did.

 

[Slow1]

Welcome!

Well, if it is installed to code/manufacture specifications then you have nothing to worry about - really! The specs are very safe and I've heard it said many times (and I believe it) that nobody has had a stove that is installed to code set the wall on fire. Now, other things left around the stove like chainsaws being warmed on the hearth (fueled up and ready to go in the morning) have gone up in smoke and flames, but that is a different story...

Now - if you want to post more details on your install such as exact measurements and perhaps a picture (hey we all love to look at nice installs you know - and all stoves make for nice installs around here), then folks will be glad to help give you second (third, 4th, 5th...) opinions on how well the code/manual was interpreted during your install. Also, if you have an IR thermometer (what, you don't have one? you must be new here... ha!) then you can get a temperature measurement off that wall and post that. "It feels hot to me" is very subjective - but in general if you can hold your hand on it most folks tend to believe it is a safe temperature in any case, but if you can't what exactly does that mean since we all have different tolerances for heat (and it may depend on how cold our hands are at the moment we try).

 

[unhdsm]

Can you hold your hand on it?

I can hold my hand to it, but I did not really have the stove going as hot as it will get, and it was going for about three hours.
I plan to run it all winter, and much hotter.
I don't have a IR thermometer, but I would call the temp "very warm to hot". Probably 130-140F.

I guess I am questioning how they determine the the distance that the stove can be from drywall. It must be allowed to get pretty hot.

 

[jtp10181]

The way they test it is that they take a black wall (to absorb max heat) and it is covered in thermocouples hooked up to a computer to monitor temps.

They move the stove closer and closer until they exceed the accepted temp limit.

I have seen the rigs for testing clearances on gas fireplaces to walls and mantles. I imagine they test wood stove in the same manner.

 

[Dakotas Dad]

Can you hold your hand on it?

I can hold my hand to it, but I did not really have the stove going as hot as it will get, and it was going for about three hours.
I plan to run it all winter, and much hotter.
I don't have a IR thermometer, but I would call the temp "very warm to hot". Probably 130-140F.

I guess I am questioning how they determine the the distance that the stove can be from drywall. It must be allowed to get pretty hot.

If you can hold your hand on it, it's way cooler than 140 °F . That's second degree burn in moments territory. A good hot all you can do is dance around shower is only about 106-8 °F .

 

[unhdsm]

Here are two pictures.
The only concerning section is the drywall adjacent to the middle of the stove (height-wise), right in front of the heat shield.

 

[BrotherBart]

The UL testing standard is that it does not get over 90 degrees above ambient (room) temperature. But keeping it less than that ain't ever a bad thing because the studs behind that drywall get to a lower combustion temperature every time they get hot over time. Look up pyrolysis.

 

[begreen]

It looks like there's plenty of room on the hearth to move the stove forward a few inches. There is no rule saying you can't exceed minimum clearances. I always try to do that for greater peace of mind.

 

[fdegree]

Just to add another question the the discussion. I have seen walls turn a brownish color after extended exposure to high heat.

At what temperature will the wall start to discolor?

 

[jtp10181]

The UL testing standard is that it does not get over 90 degrees above ambient (room) temperature. But keeping it less than that ain't ever a bad thing because the studs behind that drywall get to a lower combustion temperature every time they get hot over time. Look up pyrolysis.

I always thought the UL testing temp limits were designed to prevent pyrolysis, otherwise, what would be the point?

 

[westside]

I'm also concerned with the wall temps of our new corner install , 138 degrees measured with an ir gun at the left rear corner of stove . According to englander the nc30 requires 15 inch clearance at the corners and presently has 16 1/2 inchs . These temps are with the stove cruiseing around 500 to 550 . Just worried that as winter sets in and start buring 24/7 the wall temps will rise even higher. What is a safe wall temp ? Or am i better off to just go ahead and install some wall protection now for peace of mind ? What wall temp are others seeing measured at the rear corners of there stoves ?

 

[westside]

The 138 degree hot spot is basicly a 6- 8 inch circular area just above the electrical receptacle where the wainscot and drywall meet, straght accross from the rear corner of the stove after that the temps start droping off.

 

[vvvv]

Here are two pictures.
The only concerning section is the drywall adjacent to the middle of the stove (height-wise), right in front of the heat shield.

try taping a piece of al foil over the hotspot, even with no gap, feel the wall temp behind the foil after time

 

[vvvv]

The UL testing standard is that it does not get over 90 degrees above ambient (room) temperature. But keeping it less than that ain't ever a bad thing because the studs behind that drywall get to a lower combustion temperature every time they get hot over time. Look up pyrolysis.

its called "pyrophoric carbon" & there seems to be some ongoing debate...me cant find good link

 

[firefighterjake]

As long as you do the install per the manufacturer's specs you should be fine . . . but as BeGreen mentioned there is nothing to keep you from exceeding those recommendations if you feel more comfortable . . . which is what I did . . . I still have the stove close to the wall . . . but exceeded the minimum clearances just to be extra safe.

 

[woodgeek]

As Craig likes to point out: hydronic/steam heating lines are in direct wood contact in millions of homes, and sit at 180°F or higher for long periods of time. I don't think there is any pyrolysis issue below 210°F, a good 140°F above ambient. The +90°F rule is conservative not b/c of pyrolosis, but to provide some safety margin in the case of a significant, unattended overfire.

 

[Battenkiller]

its called "pyrophoric carbon" & there seems to be some ongoing debate...me cant find good link

Nice, Pook! Here's a link I just found that covers the debate pretty well.

http://www.doctorfire.com/low_temp_wood1.pdf

 

[woodgeek]

its called "pyrophoric carbon" & there seems to be some ongoing debate...me cant find good link

Nice, Pook! Here's a link I just found that covers the debate pretty well.

http://www.doctorfire.com/low_temp_wood1.pdf

Wow. 77°C, huh? The logic of that paper seemed really tortured to me. I am still going to be skeptical of a couple events in a century of data across millions of homes....

Remember all those 'spontaneous human combustion' stories in the 1980's? Are we now going to say body temp is too high? Of course, it turned out later all the cases involved morbidly obese smokers wearing synthetic fabrics around bedtime.

 

[Pagey]

Well, 77C is about 170F, so that's pretty warm for a wall, I would think. Or were they saying that the wood exposed to 77C/170F over time turned to charcoal?

 

[westside]

Thanks for the link BK . After reading that i believe i'll play it safe and install some type of heat shield just for peace of mind.

 

[CarbonNeutral]

Don't buy that paper in the slightest - has to be something else going on, or we would see this ALL THE TIME. If by some amazing coincidence, with species of wood, whatever, this did occur, the chance of it happening regularly is still clearly hovering just above 0.

 

[Battenkiller]

After reading that i believe i'll play it safe and install some type of heat shield just for peace of mind.

Better safe than... you know.



BTW, don't go trusting your hand to act as a replacement for an IR gun. The way that skin perceives temperature is pretty complex. In a simplified explanation, it senses the rate of heat transfer, not the actual temperature itself.

Water can't be tolerated much beyond 115-120ºF for very long because water holds a lot of heat and it directly touches every pore in your skin. Aluminum foil OTOH can be held in the hand right out of a hot oven because there is very little heat contained in it because it has so little mass. Touch your cast iron stove at the same temp and you'll get zapped pretty good.

Insulating materials have there own properties that can deceive you. Wet wood is a fair conductor of heat, so it will feel hot at 150º, but dry wood won't feel hot at all at that temp because it is a superior insulator. I have had my firewood hit 145º at the recommended 3' side clearance for my stove. I could hold the back of my hand against it all day long.

Charcoal is an even better insulator. To prove this point to myself, I went down to the stove a few hours ago to check the surface temp of my coal bed. I got a reading of 225º. I put the sensitive back of my hand against and it just felt warm. I dusted off the fly ash and stirred the coals up a tiny bit and got a reading of 375º. Unbelievably, I was able to place the back of my hand on the coal bed without getting burned. This is the real secret of Indonesian fire walkers (that and a couple liters of coconut hooch).

In my physics text I once saw a photo of a cube of ultra-high insulating space age ceramic. It had a measured surface temp of 2500ºF, but the tech was holding it by the corners with his bare fingers.


Get an IR thermometer. They're dirt cheap now, and they work. It'll inform you a hundred times more than those stupid moisture meters everybody's using to give themselves a nervous breakdown about their seasoned wood.