Ceiling too hot

Hello!
We are loving our RMH and I'm so proud of my husband for his work on it all. We just have one problem so far: our ceiling is falling. I think it makes sense to insulate the top of the barrel but he is thinking more of putting some kind of guard on the ceiling itself. Wondering if we have other options or what y'all might suggest.

Oops. Ceiling pic got excluded. Posting here.

Hi Holly;
It sure looks like your ceiling does not like something, probably the heat.
I notice directly over the barrel the ceiling seems solid, only at the joints is it coming off.
Have you used a temperature gun on your ceiling while the stove is running?
I would not insulate the top, I agree with your husband about using a metal heat shield spaced an inch off from your ceiling.
Depending on how hot your ceiling is getting, you might be able to place a large piece of stone( pizza stone, granite, slate) and diffuse how much instant radiant heat is hitting your ceiling.

That looks like your ceiling panel is expanding from the heat, causing cracking at the seams.

Thanks for the feedback. I have not used the heat gun on the ceiling but I am certain heat is the factor. I figured it might just be melting the glue or something but the expansion makes sense too. Appreciate the recommendations for resolving the issue. Will discuss with my husband tomorrow.

The ceiling is reading 250 on our current fire which is far from our hottest one and I doubt this is the hottest moment. Hopefully that can give a baseline though.

Make sure there isn’t a leak in the roof or the chimney flashing. That looks like what a little water will do to drywall joints. I have similar issues where pipes sweat above the drywall, not enough water to stain but enough to loosen the mud.

Just out of curiosity, I looked up automatic (spontaneous) combustion. It's interesting the temperatures at which things will start to burn. Just from heat. https://www.tayloredge.com/reference/Science/ignition.html.

We've heated with wood for 60 years now. I have never seen what you have going on. I'm not at your place, all I can do is guess and make suggestions from a distance. Which really isn't a good plan for you. Getting it right is far too important. We all can guess all day long. But I see your situation as too much stove, too much heat. Bringing fire into your house is a dangerous business that really needs to be done right to minimize trouble. The heat source needs to be matched to the space. You can try to correct some of the problem with various mitigations, shields and reflectors and deflectors and stones and such. But you're still going to have too much heat at the source. I think I might think about that, before deciding on all the other strategies.

If you are getting 250F at the ceiling, I suspect you have a bigger ratio of instant radiation to mass storage than you need. Putting a slab of stone or similar on top of the barrel, and possibly cobbing or otherwise enclosing some of the barrel with mass (while still allowing opening for inspection) would reduce the immediate heat load in favor of long-term heating. I would couple that with a metal heat shield an inch down from the ceiling, as is common with wood stoves.


Looking at the photo again, I note that you have about 1 1/2 barrels plus the metal firebox shroud exposed, which is more like a shop setup (where you want fast heat and don't care about long-term storage) than a continuously occupied dwelling. I am sure you could add mass to some of that and improve your balance.

I would be very concerned about what is happening.  It isn't just drying the the moisture in the sheetrock.

Sheetrock is fire resistant because when it is fully dried it is about 25% water that is chemically bound.  In order for sheetrock to burn or transfer high heat all that water has to be driven off which takes a lot of heat energy to do.   That process is called calcification and it begins happening very slowly any time the temp of the sheetrock is above 212F but it begins happening fairly quickly at 275F.

Once you drive off all the water that water you will no longer have the energy consuming process of breaking those bonds and the sheetrock will probably get much hotter that it does now even with the same fire as you have now.  Once that happens the heat will go directly through the Sheetrock and begin low temperature pyrolysis of the wood behind it after months or years of low temp pyrolysis  the wood structure may be able to catch on fire at much lower temps that it normally would.

I personally would not burn another fire in that stove until you have some type of heat shield on the ceiling.

I would also check the temperature of the wall behind the barrel and put shielding there if the temp is above 150-200F

Ok, some clarification: the drywall is in good shape. The plaster is the only thing so far affected. We are working to find a solution and appreciate the suggestions and comments

250° is WAY TOO HOT for the ceiling. Don't light another fire until you get some shielding installed. I'd put the shielding on top of the barrel, not attached to the ceiling. Don't attach anything to the ceiling! A nail or screw will conduct heat directly to the wood behind the sheetrock. BAD IDEA! Install two layers of sheet metal to the top of the barrel  with each separated by an inch of air space -- BARREL > 1" AIR  SPACE > SHEET METAL > 1" AIR SPACE > SHEET METAL

SHEET METAL
AIR SPACE
SHEET METAL
AIR SPACE
BARREL

Go to a metal fabricator (HVAC duct installer?) for pieces of sheet metal slightly LARGER than the barrel top to reduce direct radiation from the barrel. They'll cut it to size, crease it for added strength, and hem it if you'd like to eliminate the sharp edges.

Don't light another fire until you get some shielding installed.

It looks like you could use some shielding on the back side of the barrel too. Again, not attached to the wall, but to the back side of the barrel offset by a 1" air gap. That same metal fabricator can also cut / bend (roll) a piece of metal to match the barrel contour. For offset material you can use 1/4" copper tubing cut in 1" lengths and appropriate length self-tapping screws through the sheet metal/tubing into the barrel. With this very similar setup my barrel top is 650-700° and the ceiling is about 120°.

You could install a fan in the cealing.

From experience you need to get advise from Fire experts.
As others have said 250degrees is far to hot and even radiation shields may not work satisfactorily.
I think its a case of having built something unsuitable for you situation.
Somebody had to do it!
You may find a small unit or the lifting of the ceiling to a more suitable position may work.

DuraVent - using their triple wall chimney - recommends a minimum 2" clearance to combustibles. Hearthstone. With proper shielding clearances to combustibles are given in detail. Again, NO FASTENERS ARE TO PENETRATE THE SHEETROCK TO THE WOOD. (How is your sheetrock attached to the ceiling/wall? With heat-conductive nails or screws.) Shielding is applied to the heating appliance, then allow proper/required clearance to combustibles.

Don't light another fire until you get some shielding installed.

Holly Michelle wrote:The ceiling is reading 250 on our current fire which is far from our hottest one and I doubt this is the hottest moment. Hopefully that can give a baseline though.

Have you checked the calibration of your temperature gun? I bought a new one from Harbor Fright that was on sale over the Holidays, and it reads significantly and verifiably off at ambient temperatures at least...

Coydon Wallham wrote:Have you checked the calibration of your temperature gun? I bought a new one from Harbor Fright that was on sale over the Holidays, and it reads significantly and verifiably off at ambient temperatures at least...

True, IR thermometers are not all created equal, but the fact remains, that ceiling is getting TOO HOT. Your current installation is unsafe!. Do not light another fire without proper shielding.

What would be a good target temp not to exceed?

I'm not familiar with batch box design details, is there a necessity for the radiant chamber/bell being so tall? If it were a J tube RMH I'd say just remove the manifold section and cut it down as far as possible (perhaps it could even be done in place), leaving at least 2" of clearance to the riser, then clamp the upper barrel back on. The surface area for immediate radiation would be reduced while the distance to the ceiling would be increased, which should greatly reduce/disperse the heat reaching up there.

[edit: rethinking that cutting advice, it would mean cutting off the lip of the manifold so the barrels would not clamp back together. Another way of securing them together would then be needed. Or, make a new manifold from another barrel, then swap it in. Would save on down time for the system if that is crucial.]

We could offer better advice if we knew things like the cross sectional area of the system and such details. A side picture will help, as would any taken during construction...

I'm not familiar with batch box design details, is there a necessity for the radiant chamber/bell being so tall? If it were a J tube RMH I'd say just remove the manifold section and cut it down as far as possible (perhaps it could even be done in place), leaving at least 2" of clearance to the riser, then clamp the upper barrel back on. The surface area for immediate radiation would be reduced while the distance to the ceiling would be increased, which should greatly reduce/disperse the heat reaching up there.

In my 8 inch system there needs to be a minimum of 12 inches between the top of the riser and the barrel top. From there, there are calculations for overall  bell volume and surface area. The RMH is a little bit different beast than the J-tube design.

We could offer better advice if we knew things like the cross sectional area of the system and such details. A side picture will help, as would any taken during construction...

Correct, it would be nice to know the system size, but in reality, the existing barrel height is good for either 6 or 8 inch systems. This installation simply needs to get some proper shielding. That would fix the local area (ceiling/wall) over-heat issue.

Holly Michelle wrote:What would be a good target temp not to exceed?

Holly, I'm not certain what the "never exceed" surface temperature might be (I'm sure it's available somewhere??). The spontaneous combustion temperature table referenced earlier in this thread is interesting and instructive. There you'll note that "wood" combusts at 572°F, where "powdered-pine" is listed at 450°F. Not sure where one would get powdered pine (sawdust I suppose), but those temperatures are probably given at "normal" moisture content for kiln dried lumber - typically 6-8% for interior building materials (like what your house is made of). Continual heating above the RMH will drive that moisture content down with a consequent reduction in the spontaneous combustion temperature.  Heat conducting fasteners (nails/screws) penetrating the gypsum board can easily reach temperatures above that needed for combustion. Pyrolysis occurs around those fasteners further reducing the combustion temperature. Disassembly of an overheated  surface would reveal some scorching around the fasteners. Probably not in your case — yet, but . . .

It generally takes a bit of time for the drying/pyrolysis to occur (hours/days/months/years?) and before you know it, your "safely installed" RMH is sitting in a pile ashes and your house becomes a statistic, evidence that these heaters are "Dangerous and should never be allowed!" — when in fact, the heating appliance was not the problem, but the installation was.

Back to your original question— I'm pretty happy with 120°. Anything much more than that could be a problem. Anyhow, that's about what I'd aim for.

BTW: A little anecdotal story. I've been using wood heat off-and-on for the better part of 50 years with some quality stoves, so I'm not new to the game. I'm living in a new home construction where we had a kachelofen style , masonry heater installed, very similar to this, at the time of the house construction. It was unsuited to my space and has since been removed and replaced with one of these Hearthstone heaters which has been working quite well for me. (that's another story) However, as I was getting accustomed to burning the masonry heater I thought it would be a good idea to pre-dry my firewood in the warm (about 300°F) post-fire heater so that it would be absolutely dry for the next fire making the next fire easier to light. Since there was no fire or coals remaining in the heater I had also closed the damper. A couple of hours later that wood had reached spontaneous combustion temperature (whatever that was) and began to burn — with a closed flue damper. Needless to say, that was an exciting smoky few minutes. I'm glad we were home and could quickly open the flue damper or the new house would have been ashes.

You're playing with fire. Get some shielding on that barrel!

Holly Michelle wrote:What would be a good target temp not to exceed?

A common number that shows up for the highest temp you ever want to see is 110F over ambient temperature.  So if you keep your house at 70F you don't want to see temps over 180F.

I personally would want to see substantially lower than that.  My personal test is that I can put my hand on the wall and hold it there indefinitely.  If I can't do that I need to choke off the fire or add more shielding.  I assume that is somewhere in the 120F range.

That is a beautiful rmh.

To me, the damage looks like that texture might have been loosened when the hole was cut in the ceiling for the stove pipe. Especially since it was on a seam.

The texture just didn't fall off until it got hot.

If this were my house I would get a sheet of stainless steel.

First, cut the sheet of steel to cover the damaged area.

Then I would use quarter-round molding to go around the steel.  The quarter round could be painted the same color as the ceiling.

If a sheet of stainless steel was not available then I would use a sheet of aluminum roofing as I have seen other folks use as trim in houses.

My RMH is close to the wall—too close for combustibles. That wall is load-bearing (low load, but second storie floor) and was initially built from typical fir studs. Those wood studs were  removed/replaced with steel studs, covered with cement board/faux stone—all non combustible—and open on the back side for air circulation. The surface of the wall runs about 150-160° with heat soak through to the back side of about 130°. If this were gypsum board with screwed through heat conductive fasteners, it would be far too hot for safety.

Thank you We have installed a fan and are working on getting the shielding added as well. My husband also suspects that no primer was used when the plaster was initially added to the drywall. I recognize that this does not excuse the heat and so I'll simply chalk it up to a blessing that we are addressing it sooner than later.

Holly, for immediate help, have you considered placing the largest pan you have, as full of water as practical, on top of the barrel? This would mitigate some of the heat radiating up. Unless you are approaching 70% relative humidity, I'd think the additional moisture in the air would help also. Most people I know that use wood for the main heat source have a moderate version of this set up for normal burning.

Have you considered using a stove-top fan?

You just place a heat powered stove fan on top.  And it kicks into action.

As the heat rises the fan begins to turn and starts dispersing it about your room.  
It prevents the bulk of heat from rising up directly above the stove.

Sarah Flanagan wrote:Have you considered using a stove-top fan?

You just place a heat powered stove fan on top.  And it kicks into action.

As the heat rises the fan begins to turn and starts dispersing it about your room.  
It prevents the bulk of heat from rising up directly above the stove.

here's a pic of the type of thing I'm talking about: https://images.app.goo.gl/RTLvk3cdm2BEoe2D9

Leslie Walper wrote:In my 8 inch system there needs to be a minimum of 12 inches between the top of the riser and the barrel top. From there, there are calculations for overall  bell volume and surface area. The RMH is a little bit different beast than the J-tube design.

I'd heard of the surface area calculations though have no grasp of it. Is the riser clearance needed for complete combustion, or just system efficiency?

The latest theories on J tubes I've heard is that they also can benefit from a larger radiant chamber above the riser, but a 2-3" gap is minimum. I think the utility of that spot for direct heating of small items is tremendous, I have to wonder how much system efficiency could be gained from a larger 'bell'.

Sarah Flanagan wrote:Have you considered using a stove-top fan? You just place a heat powered stove fan on top.  And it kicks into action.

I have one on top of my Hearthstone heater—a gift from a well-meaning friend. It's a cute little toy, but not really effective for moving the amount of heat we're talking about. It spins, it's entertaining, but …

Coydon Wallham wrote:have you considered placing the largest pan you have, as full of water as practical, on top of the barrel?

I like the concept (might help the immediate issue, maybe?), but boiling water in an open pan over your head? Maybe not. Sounds like a recipe for disaster.

Coydon Wallham wrote:The latest theories on J tubes I've heard is that they also can benefit from a larger radiant chamber above the riser, but a 2-3" gap is minimum. I think the utility of that spot for direct heating of small items is tremendous, I have to wonder how much system efficiency could be gained from a larger 'bell'.

Improved system efficiency? Maybe? But there are limits — xx m² surface area per CSA of the riser diameter. For example, a 6" system can support 5.3 m² (57 sq ft); an 8" system can support 9.4 m² (101 sq ft) of heat absorption area. If you don't want to create any additional bell floor footprint size you can add columns inside the bell to increase the functional surface area within the bell, adding heat retention capacity to the bell, which heat can then be liberated over a longer period of time increasing the efficiency of the overall system. If you exceed this 1:1.77 ratio there are negative effects on draft and reduced heater performance.

The effective cross-section of the side walls of imaginary box positioned on the riser should equal at least to the effective cross-section of the riser.
If the riser is square and has the diameter of 200 mm (8") then the distance should be at least 200 mm/4 = 50 mm (2"). By adding 50%, for less restricted flow it would be  75 mm (3").
As effective cross-section I mean the area of a circle (ellipse) inscribed in a polygon.

Regarding problems with overheating ceiling, I would not try to shield anything but rather address the core of the problem - the barrel is too hot and too close to the ceiling.
In my opinion it should be rebuilt - for the peace of mind and safety.

What might be done, temporarily, is to use 4-5 small stones (1.5 inches) situated evenly around the barrel top edge and lay a large (full-sheet size 18x26"), steel baking sheet on top of the stones. Do that twice giving two layers of baking sheets with air gaps between. I'd also turn the top sheet 90 degrees to the bottom one encouraging some convective air currents around the setup. This could also be a more permanent solution if they were screwed in place with appropriate spacers. If you can get aluminum baking pans that will also do since aluminum melts at around 1200°F. Still need to do something about that back wall.

Cristobal Cristo wrote:Regarding problems with overheating ceiling, I would not try to shield anything but rather address the core of the problem - the barrel is too hot and too close to the ceiling.
In my opinion it should be rebuilt - for the peace of mind and safety.

Agreed, but adequate shielding is also effective.

Leslie Walper wrote:I like the concept (might help the immediate issue, maybe?), but boiling water in an open pan over your head? Maybe not. Sounds like a recipe for disaster.

With the time and fire management it takes to boil a few litres in a covered container on my barrel 3" over the riser, I'd think an open pan would outlast a batch firing with more heat dispersal in the bell. But a stock kettle would do a better job to prevent spillage when moving and any possible boil over...

Leslie Walper wrote:

Coydon Wallham wrote:The latest theories on J tubes I've heard is that they also can benefit from a larger radiant chamber above the riser, but a 2-3" gap is minimum. I think the utility of that spot for direct heating of small items is tremendous, I have to wonder how much system efficiency could be gained from a larger 'bell'.

Improved system efficiency? Maybe? But there are limits — xx m² surface area per CSA of the riser diameter. For example, a 6" system can support 5.3 m² (57 sq ft); an 8" system can support 9.4 m² (101 sq ft) of heat absorption area. If you don't want to create any additional bell floor footprint size you can add columns inside the bell to increase the functional surface area within the bell, adding heat retention capacity to the bell, which heat can then be liberated over a longer period of time increasing the efficiency of the overall system. If you exceed this 1:1.77 ratio there are negative effects on draft and reduced heater performance.

This was a side note about comparing batch boxes and J-tubes, sorry to distract discussion from the system in question.

The Love Shack was way too hot at the ceiling with the Minnie Mouse Rocket Mass Heater. Paul just released this video that shows how they remedied that.  A cob hat was added to the top of the Minnie Mouse. The cob absorbed the heat that was overheating the ceiling and dispersed it over hours.  Great fix!

Holly, I read about all the various suggestions that were given to you.  In the end, did I miss your final FIX and how effective it was?

With the wildly varying suggestions, I was wondering what you came up with?