Building a Block Off Plate

Nice score!You stole that insert if you only paid $75 bones.The seller must have needed quick cash for some crack?

I am also looking for some good advice regarding the block off plate. The previous owners of my house had a woodstove venting through the damper. It was 'loosly' sealed up with insulboard. This stuff eventually started to fall apart. Years ago i replaced the stove but kept the insulboard in there. Last year, I pulled out the stove, and the insulboard, and made a cardboard template of the damper opening. I then transferred the template onto some heavy gauge aluminum sheet metal. I put the trimmed peices up there around the pipe to seal the opeining. I then sealed the edges with copper impregnated high temp silicone. I burned several cord through it over last winter, and everything so far looks brand new, and holding up. But I question whether the aluminum was OK or not.

https://www.hearth.com/econtent/index.php/articles/making_a_block_off_plate/

Rainer for $75? That is INSANE - congrats! Now I need to go repent for my serious envy! (since I spent about $1800 on mine just a few months ago!)

I read that. It just said sheet metal, didn't really specify what type of metal. Plus I forgot to mention that I sealed around the pipe and the block off plate by tightly stuffing some gasket and sealant around it.

I read it here somewhere, I can try to find it, but 24 gauge steel is what's recommended. I got mine for $1.59 at the hardware store

here's the link: https://www.hearth.com/econtent/index.php/forums/viewthread/4670/#49366

Could I use cement board to build the block off plate? We have a lot of it lying around here at work and it will just be thrown out. I would think this would give more insulation than tin or thin steel. Any thoughts?

It should be said that, at least in my install, that I stuffed ceramic fiber insulation around the damper and then put the plate underneath that. I don't know about cement board other than I think steel would be much easier to work with and in bending the ends like to article says it will simply "slide into place" Again, the steel cost should be minimal.

It does not pay to reinvent the wheel - 24 ga sheet metal is easier to work with and will do the job just fine - we have seen it last for decades. Insulation on top is overkill, but who knows - you might get another 1% in efficiency. This depends somewhat on other factors. If you are using front sealing plates (in an insert), you are trapping so much heat back in the fireplace anyway, that adding a percent or two is worthless.

Now if you really want to add, install the stove with just the block off plate and leave it open around it....

Exactly - another of the (many) possible false stats given by stove manufacturers is the efficiency of their fireplace inserts. From what I have heard, these do not include heat loss to the fireplace wall and then outside, etc. - In other words, the efficiencies are actually as if the stoves were free standing. In my opinion, certain fireplace installations could take 10-30% off those numbers! That's a lot....

Ah, someday maybe we'll have the Hearth.com labs with a calorimeter room and all (that is an insulated room which you pull the heat out of an measure......so it is extremely accurate)......I'm just learning about some of this stuff, but it is my understanding that these rooms are not used any more with stoves - the efficiency is calculated by formulas...in other words, by the stack temp and chemicals in it, plus the weight of wood burned.

Webmaster said:

Exactly - another of the (many) possible false stats given by stove manufacturers is the efficiency of their fireplace inserts. From what I have heard, these do not include heat loss to the fireplace wall and then outside, etc. - In other words, the efficiencies are actually as if the stoves were free standing. In my opinion, certain fireplace installations could take 10-30% off those numbers! That's a lot....

Ah, someday maybe we'll have the Hearth.com labs with a calorimeter room and all (that is an insulated room which you pull the heat out of an measure......so it is extremely accurate)......I'm just learning about some of this stuff, but it is my understanding that these rooms are not used any more with stoves - the efficiency is calculated by formulas...in other words, by the stack temp and chemicals in it, plus the weight of wood burned.

Click to expand...

for the record, my rainer insert is in a 2 sided fireplace without any face plates. I'll have to take a pic of the stove and post sometime soon but here's the fireplace from one of my first posts.

Cement board meet the intent of code being a rigid non combustible material

Unit 40 no problem using heavy gage Alumium

Gasket material is fine to fill the cutup hole and the plate

We had s discussion in the past about lining the fire box sides with insulation to prevent heat loss to the outside.

the discussion also asked if sheet metal reflective values were worth installing

I know insulation above the 24. or 26 gage plate will prevent heat loss

Thanks for the approval Elk. I've got the 3CB burning as I type this..

Yeah, in theory aluminum would soften and melt at certain pipe temps, but in reality there is not enough pipe surface touching the side of the stove pipe to ever make this happen. It is more expensive, though - so we never used it.

Cement board is not actually tested for use with stove pipes running though it, and as others here have reported, it contains everything from wood to poly foam pieces. I would not use it unless I had to.

There is really only a need for alternative materials when the preferred one cannot be attained.......on this board, we sometimes discuss everything to death.....it's easy to see how committees and government rarely get anything done, but one person with some tools...now there is productivity!

We didn't even discuss liners, etc......but if the stove is being installed with a full liner, than the removal and reinstall of the plate is not going to be needed.....that was one advantage of the sheet metal - relatively easy removal and replacement.

If a person was going to insulate on top, even a small amount (1 inch) or fiberglass (no paper or foil) or koawool, rock wood, micore, etc. would do the job. Whether this does much depends on the stove and how close it is to the plate. In my fireplace at home, I was always able to touch the plate when the stove was running - evidence that not much heat was headed in that direction.

Elk has a good idea with that fireplace wall insulation - we should develop and package a new product - something that you can cut with a utility knife, does not throw fibers out, and simply glues or otherwise fastens on the rear and sides of exterior fireplaces (the interior walls of them).

Get that stove paint and tile ready bro, we'll have her goin' this weekend!

Thanks for the advice, I'll skip the cement board. We have some steel around from all the heating and air conditioning going into the building so I'll grab some scrap from that.

I insulated my fireplace wall in the back as Elk mentioned. I figured it couldn't hurt since my chimney is on an exterior wall. I installed mineral wool and covered it with sheet metal and painted it black, which I imagines negates reflectivity of the sheet metal. It makes it look better though, and protects the mineral wool just fine...

For my block off plate I found using one piece to cover the entire area easier than using two pieces. The problem, however, is getting the hole lined up right. Because of this, I cut the hole large enough so that I wouldn't have any trouble getting the liner through the hole. Next, I used a smaller piece of sheet metal with a hole cut to almost exactly the diameter of the liner, and fastened this piece to the main block off plate. I put some caulking in between the main plate and the smaller piece, and then used sheet metal screws to hold it in place. This made installing the whole thing pretty easy. I'll attach some pictures below.

elkimmeg said:

We had s discussion in the past about lining the fire box sides with insulation to prevent heat loss to the outside.

Click to expand...

These pics show the block off plate before being fastened...

and after...

and these show insulating the back wall and covering with sheet metal...

When I made my block-off plate I got the metal cheep at RE-Store ( Habit for Humanity re-store), a air duct I could cut up. I used extra foil faced flue liner insulation on the the back of the plate. I over lapped the edges of the plate and foil taped the liner to the stove side of the plate. The foil and insulation made a air tight seal. A little hi heat silicone away from the liner finisher the job.