paint for heat shields

[Ashful]

A few of you may remember some FLIR thermal images I posted of my house, showing very clearly the location of each stove within. If not, here's the pertinent post, quoted below:

These are the two photos of most interest to me:

The stove which is much closer (3") to the stone wall reads only 38F on the outside of the wall, whereas the stove that's much further (12") from the stone wall reads 49F on the outside. I guess the big wall is a decent heat sink, on a night when it's 16F outside.

I'm wondering if it's worth playing with some shielding between the stove and wall. Not much room there on the new install, and I fear a user-designed shield placed that close to the rear of the stove may inadvertently cause damage to the stove, by trapping and reflecting more heat at the back than the designer ever intended. Sure would be nice to limit what heat I'm contributing to the great outdoors, though.

Well, I obtained some 22" x 24" heavy steel (looks to be 0.050"ish) panels for rear heat shields, and I have a sheet metal shop bending up more of the same material for lintel shields (safety first!). The material is something the sheet metal shop had laying in their bins from an old job, so it does need some cleaning up, prior to painting.

My current plan is to drop all four shields off at a local sandblasting shop, who've done a good bit of work for me on prior projects, and then prime/paint them black or dark gray after the sandblasting. Installed locations to be these two stone fireplaces, which everyone has probably already seen too many times:



Best thing to paint them with? I was thinking black metallic stove brite (would match the one re-painted Jotul), but I'd be interested in knowing if Stove Brite is the best thing to use, and if there might be a better color to blend in. You will see the bottom 1" of the lintel shield, all the way across the opening. The rear shield won't be visible when viewing straight-on, but you will see it from any angle other than straight-on.

 

[pen]

Read the directions carefully for whatever paint you use. I have yet to use a high heat paint that called for priming, just good and clean metal.

Also, when using spray high heat paint, I have found to get awful overspray / dust! Make sure you are in a location where it won't be a problem if you go that route.

If these shields are going to block the stone / brick work then

pen

 

[webby3650]

Stove Bright is the best product out there in my opinion! I'd go with that. They offer a primer but I have never used it or needed it. My chimneys are painted with their satin and it's holding up fine to the elements. It's very durable.

 

[ddahlgren]

I did mine in aluminum and left it bare. It makes a very good reflector as well.

 

[EatenByLimestone]

This sort of use is what radiant barriers were made for. I vote for shiny if possible. White to match the mantle if not.

Matt

 

[webby3650]

A bright color or left bare would reflect better I guess, but man that will be hideous!

 

[pen]

If I did this, it would be a shield basically wrapping around the back of the stove so as not to hide that nice stone work!

I'm hoping this is not meant to fill that whole space.

Are you having troubles heating the house and trying to squeeze every last BTU out here? Even though the images showed hot spots here, I wonder how much is actually lost or can be prevented considereing the alcove-ish installation.

pen

 

[Ashful]

Hey, guys! Thanks for the thoughts. Don't worry, pen... no way I'm blocking out all of that stone work! I made a 24" W x 22" H shield to go directly behind each stove, to catch the broad-side radiation off the back of the stove. If painted the same color as the stove, I'm thinking you may never even notice it's there.

I do like the idea of going white for the mantle shield, whereas I had originally just figured on painting that black, as well. In fact, that's far enough away, perhaps I can get away with painting it with the same exact Ben Moore oil paint used on the mantle?

 

[EatenByLimestone]

It's certainly worth trying the ben moore paint.

Matt

 

[mellow]

I vote for hand painting it to match the stone work

 

[Ashful]

I just realized I forgot to answer pen:

Are you having troubles heating the house and trying to squeeze every last BTU out here? Even though the images showed hot spots here, I wonder how much is actually lost or can be prevented considereing the alcove-ish installation.

Yes, this is the case. I found that pushing both stoves pretty hard last year, we still could not keep temps above 70 in the old part of the house (4 story section shown in first photo of the OP). Even the room adjacent to the stove room could barely hold 68F, without kicking on the furnace. I was burning a mix of dry poplar (poor) and fairly green hardwood (poorer), and we also had four old drafty windows from 1773 that were under construction, with no storms installed on those four windows. I recall going thru more than a full cord of poplar in 8 days, at one point, when temps really dipped last winter. We were still cold.

This year, I'm burning a mix of hardwood that was split Thanksgiving weekend of 2012, so it's at or over 1 year. I am currently missing storm windows on 7 old 1773 windows, but they are being delivered tomorrow, and will be installed within a month. I am also installing some temporary neoprene weatherstripping on the only two old wood doors that have none (will likely follow up with permanent metal weatherstripping, if it makes a positive difference).

I believe the rear heat shields will make a big difference in the amount of heat radiated directly into the exterior stone wall behind each fireplace, particularly in the old part of the house, as that stove is VERY close to the stone. Beyond that, I am still dealing with stoves pumping heat into an alcove, so my next step will be focusing on moving the air out of the alcove. Each fireplace has a block-off plate, but they're mounted up at the 8 or 9 foot level, whereas the lintels are only 4 or 5 feet above the floor. That amounts to a large cavity of hot air trapped up in the fireplace, although I am not sure that makes a big difference in how much heat radiates out to the room.

 

[jharkin]

FWIW, when I repainted my stove I used Rutland high temp. The nice thing is they offer it in a brush on can, so you dont have overspray issues. I brushed it on with throwaway foam brushes, it dries in minutes with no brush marks and it didn't even smoke when I got the stove up to 600. Smell was not too bad either.

(broken link removed)

 

[DenverDan]

I just realized I forgot to answer pen:


Yes, this is the case. I found that pushing both stoves pretty hard last year, we still could not keep temps above 70 in the old part of the house (4 story section shown in first photo of the OP). Even the room adjacent to the stove room could barely hold 68F, without kicking on the furnace. I was burning a mix of dry poplar (poor) and fairly green hardwood (poorer), and we also had four old drafty windows from 1773 that were under construction, with no storms installed on those four windows. I recall going thru more than a full cord of poplar in 8 days, at one point, when temps really dipped last winter. We were still cold.

This year, I'm burning a mix of hardwood that was split Thanksgiving weekend of 2012, so it's at or over 1 year. I am currently missing storm windows on 7 old 1773 windows, but they are being delivered tomorrow, and will be installed within a month. I am also installing some temporary neoprene weatherstripping on the only two old wood doors that have none (will likely follow up with permanent metal weatherstripping, if it makes a positive difference).

I believe the rear heat shields will make a big difference in the amount of heat radiated directly into the exterior stone wall behind each fireplace, particularly in the old part of the house, as that stove is VERY close to the stone. Beyond that, I am still dealing with stoves pumping heat into an alcove, so my next step will be focusing on moving the air out of the alcove. Each fireplace has a block-off plate, but they're mounted up at the 8 or 9 foot level, whereas the lintels are only 4 or 5 feet above the floor. That amounts to a large cavity of hot air trapped up in the fireplace, although I am not sure that makes a big difference in how much heat radiates out to the room.

I think addressing the air flow issue (convective heat) is your best next approach. I have a somewhat similar (albeit much smaller scale) setup in that I have a Jotul F100 fully tucked inside a brick fireplace. Initially (without a fan or block off plate) I had good radiant heat but I was not impressed with the stove's ability to heat even the living room in which it was located. I then added a tangential fan (cylindrical blower) under the stove with the air flow directed up the back of the stove. Room temperatures improved dramatically. But it was not until I added a block off plate at the level of the lintel that the stove really hit its stride. The block off plate directs the air flow (even with the fan off) out of the fireplace opening. Now with the stove cranking and the fan on, there is a steady flow of warm (even hot, at times) air out of the fireplace opening. Temperatures can hit the 70's in much of the (small) house.

So, as an experiment, I'd get an inexpensive 20 inch box fan (I know, big and ugly) and when the stove is up to temperature direct the air flow on one side of the stove/alcove and see if you don't get a flow of warm air out of the alcove. The moving air will help recover some of the radiant heat that is 'lost' to the stone work. If it helps, you could use the fans when 'big and ugly' are not an issue (nights). Or you could try a more discrete fan arrangement (smaller/hidden). But, just as with my situation, I don't think you'll really see the potential of your stoves until you have some sort of block off plate (block off ceiling?) close to, or at the level of the lintel. I think much of your heat output is being trapped and lost in the cavity between the lintel and your current block off plate. Given the size of your fireplaces it would be a big area to cover with a block off plate and may require some support (hidden above) for the sheet metal but I think the heat gain in your living area would really be worth it.

 

[ddahlgren]

A bright color or left bare would reflect better I guess, but man that will be hideous!

I find the beauty in extracting as many btus as possible and having a warm house with the smallest amount of wood. As an engineer I find beauty in function as much as form if that makes some sense. For me I still have some problems with the secondaries lighting off reliably and will start a new thread on it with what I think is wrong. Even with that i am cruising with a low fire with stack 500 on the probe and stove top 475 80% of secondaries buring and house 73 with outside 37. It is time to break out the box fan though because the room the stove is in is 85 so time to get some air moving.

 

[bryan]

A shiny piece of metal will be the most reflective I believe (think spacecraft shielding for solar radiation), but I heard a talk at a conference a few weeks about metal roofing and the speaker (a coatings expert) stated painted metal will emit energy better significantly better than untreated metal so I'm not certain which is best in this instance. I believe you'd rather just reflect the heat rather than have it absorbed and then re-emitted. In any case I would definitely leave the side facing away from the stove untreated if possible. You don't want it emitting into the masonry

 

[Bluezx636]

Not sure how much is lost from above with your setup but Roxul stuffed around your flue just above the stove might help you

 

[Papa-Yankee-Romeo-Oscar]

I'm going to agree with DenverDan on this. Do you have blowers on the stoves? I think getting the heat out of those stone holes and into the room air would do the most. Set a fan on one side blowing into the fireplace and see how that works. I think it will get the heat off of the stonework and into the room air before it can soak to the outside. A blower on the stoves will get more of the heat off of the stove and into the room before it can make it's way to the stone.

 

[Ashful]

Thanks, guys! I'm already working on the air circulation. We tried a box fan in a few different locations, but between the noise, high velocity air (felt like a constant draft), and the dust it would stir up (we have allergies), that was just overkill for us. I just got a smaller fan, and found a location for it, pushing cool air along the floor toward the stove, that seems to be really doing a good job to circulate the warm air to all the rooms on that side of the house.

The stove on one end of the house (new construction around old summer kitchen fireplace), actually does a good job at heating that end of the house, and it's located in an open space of about 1400 sq.ft. with 14-foot high ceilings and multiple ceiling fans. The troublesome one was in the old part of the house, where the heat loss is likely too great for ANY single stove to do the job 100%. When I installed that stove, I did consider the idea of putting a block-off plate at lintel height, but had to choose against it, for a few reasons. There are multiple wood beams and cabinets built into that fireplace, hiding up behind the lintel, and I did not want to cover them. Not only are they one of the most interesting / curious features of that room, but I want to be able to see if they're getting too hot. I currently use these cabinets to store my gloves, tools, Supercedars, lighter, etc.

 

[Papa-Yankee-Romeo-Oscar]

Is there anything at all up there blocking off the area around the pipe? Maybe a block off plate above the little cubbies so you still have access to them. As Blue mentioned maybe stuff some Roxul up there.

 

[Ashful]

Is there anything at all up there blocking off the area around the pipe? Maybe a block off plate above the little cubbies so you still have access to them. As Blue mentioned maybe stuff some Roxul up there.

Yep... there's very well-insulated block-off plates in both fireplaces:

Each fireplace has a block-off plate, but they're mounted up at the 8 or 9 foot level, whereas the lintels are only 4 or 5 feet above the floor. That amounts to a large cavity of hot air trapped up in the fireplace, although I am not sure that makes a big difference in how much heat radiates out to the room.

My heating troubles have much more to do with 26 windows, 3 exterior doors, and 4000 sq.ft. of un-insulated stone house, all built in 1773, than circulation or block-off plates! This part of the house was once heated by 5 wood stoves, as evidenced by all of the old thimbles and chimneys... now I'm trying to do it with one.

 

[mellow]

Would a TEG powered fan help?

 

[Ashful]

Been looking at those, actually... although I'd be much more thrilled with something more steam punk styled. A nice Stirling engine fan would fit the bill, if I could find one that looked nice, and was actually available for purchase today.

I'm a professional electron pusher, and play with those TEG's at work (although we use 'em in reverse, as TEC's), so I prefer to play with mechanics at home.

 

[Bluerubi]

A shiny piece of metal will be the most reflective I believe (think spacecraft shielding for solar radiation), but I heard a talk at a conference a few weeks about metal roofing and the speaker (a coatings expert) stated painted metal will emit energy better significantly better than untreated metal so I'm not certain which is best in this instance. I believe you'd rather just reflect the heat rather than have it absorbed and then re-emitted. In any case I would definitely leave the side facing away from the stove untreated if possible. You don't want it emitting into the masonry

Speaker is correct. Shiny metal doesn't emit much at all. Reflects a lot, but doesn't give off much IR radiation thanks to the very low emissivity. If you have a hot shiny metal surface and you want it to give off as much heat as possible, paint it flat black and blow a fan on it.

I think the best bet in this case would be to take high temperature aerogel blankets and attach it to all the stone work, followed by thin spacers and then polished aluminum panels. This gives you the benefit of very good reflection of IR into the room off the metal, very poor emittance from the metal to the aerogel blanket so low transfer of heat towards the stone, small gap between metal and blanket limits convection, and blanket itself slows conduction. Then add small low watt muffin fans below to the stove and blowing towards the back to increase airflow with essentially no sound (convective heat transfer off the stove will go way up due to higher film coefficient). Setup wouldn't be particularly attractive, but the heat loss through the masonry would drop off dramatically. Would be like a grown up version of an easy bake oven that has a monster light bulb sitting in the middle.

 

[Ashful]

A nice concept... but I'm looking to go from 0 to 85. No need for 100, if it's going to cost anything in the looks department.

 

[ddahlgren]

A shiny piece of metal will be the most reflective I believe (think spacecraft shielding for solar radiation), but I heard a talk at a conference a few weeks about metal roofing and the speaker (a coatings expert) stated painted metal will emit energy better significantly better than untreated metal so I'm not certain which is best in this instance. I believe you'd rather just reflect the heat rather than have it absorbed and then re-emitted. In any case I would definitely leave the side facing away from the stove untreated if possible. You don't want it emitting into the masonry

Being a shield the first goal is keep heat off the wall second extract as much heat as possible from the wood and radiant heat is not to be dismissed. The way I see it a brick hearth will absorb then radiate 360 degrees to possibly unheated space. Metal will reflect radiant the convect on the back side if there is an air gap to allow it.. I see no upside to pumping btus to a wall and unheated space.