Making a block-off plate - I have some questions...

[hstraf]

Hello,

I am about to start making a "block-off plate" in preparation to install a new insert into an open hearth. I need some help to understand a few things.

Here is the article I am using as a guide:

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

1) Is there any special metal I should use? My local home builder supply store sells a large square of sheet metal, but it is not stainless steel. Is this ok? (They didn't know what kind of metal it is... they said it's "metal flashing".)

2) The guide says that I should "Silicone the joint between plate and wall and furnace cement between pipe and hole…". What kind of silicone should I use between the plate and wall? Is there special fireplace silicone? I imagine the metal plate will get very hot, and the store has about 200 types of silicone tubes...

I already have the furnace cement for the pipe and hole, but I'm confused about the plate and wall silicone type I need.

3) Should I put insulation or anything above the block-off plate? Otherwise, what is to stop the heat from radiating through the metal and up the chimney? I was thinking of maybe making two block-off plates, with a 4" space between them filled with insulation. Is this necessary? Is it just overkill? Maybe I should tack on some reflective tinfoil or something on the insert-facing side of the plate to reflect the heat back down?

Thanks very much for any help!

 

[precaud]

1) Is there any special metal I should use? My local home builder supply store sells a large square of sheet metal, but it is not stainless steel. Is this ok? (They didn't know what kind of metal it is... they said it's "metal flashing".)

I've used light-guage sheet metal (mild steel) and aluminum before, both work fine.

2) The guide says that I should "Silicone the joint between plate and wall and furnace cement between pipe and hole…". What kind of silicone should I use between the plate and wall? Is there special fireplace silicone? I imagine the metal plate will get very hot, and the store has about 200 types of silicone tubes...

Regular ole household silicon will work just fine. No special considerations here.

3) Should I put insulation or anything above the block-off plate? Otherwise, what is to stop the heat from radiating through the metal and up the chimney? I was thinking of maybe making two block-off plates, with a 4" space between them filled with insulation. Is this necessary? Is it just overkill?

Two plates sounds like overkill to me. I used two layers of high-temp fiberglass on mine, leaving a couple inches of space around the pipe hole to be safe. I like the idea of having some insulation there, but I've nothing to prove that it makes a difference or not. Having the blockoff plate is the important thing!

 

[stoveguy2esw]

check with your local hearth store, they should have higher temp silicone (rtv) i'd look for 500F and above , it is available (although harder to find) at 1k or higher, but in my opinion that is overkill

 

[elkimmeg]

Hello,

I am about to start making a "block-off plate" in preparation to install a new insert into an open hearth. I need some help to understand a few things.

Here is the article I am using as a guide:

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

1) Is there any special metal I should use? My local home builder supply store sells a large square of sheet metal, but it is not stainless steel. Is this ok? (They didn't know what kind of metal it is... they said it's "metal flashing".)
Common galvanized 26 gage sheet metal will work 28 ga ok 24 gage ok

2) The guide says that I should "Silicone the joint between plate and wall and furnace cement between pipe and hole…". What kind of silicone should I use between the plate and wall? Is there special fireplace silicone? I imagine the metal plate will get very hot, and the store has about 200 types of silicone tubes...

If you may be able to do it with one tube of caulk any tube that has fire block standard 136 will satisfy all temp ranges If using two differnet types then 500 degree RTV can be used around the peremeter and gasket / furnace / refractory cement to fill the space between the liner and plate the highest temp area

I already have the furnace cement for the pipe and hole, but I'm confused about the plate and wall silicone type I need.

3) Should I put insulation or anything above the block-off plate? Otherwise, what is to stop the heat from radiating through the metal and up the chimney? I was thinking of maybe making two block-off plates, with a 4" space between them filled with insulation. Is this necessary? Is it just overkill? Maybe I should tack on some reflective tinfoil or something on the insert-facing side of the plate to reflect the heat back down?

Its not a bad idea to insulate the area above the plate However due to the heat in the liner common fiberglass should not be used Ceramic or Karowool is best for that application

However if the chimney cap is sealed and the block off space sealed then is that not a dead air space? A dead air space is insulation in itself or if you insulated your liner and have an extra piece that insulation would be ok

Thanks very much for any help!

A helpfull hint the plate can be more than one piece over lap the joints and caulk seal them

 

[precaud]

If you may be able to do it with one tube of caulk any tube that has fire block standard 136 will satisfy all temp ranges If using two differnet types then 500 degree RTV can be used around the peremeter and gasket / furnace / refractory cement to fill the space between the liner and plate the highest temp area

Elk, based on your recommendation a week or so ago, I just got a tube of the 3M FB136 product. The main ingredient in this stuff is Sodium Silicate, the same stuff that fireplace cement is made of. And it's labelled as a "sealant" not a cement. So I'm wondering if this is really a good choice for this application.

 

[elkimmeg]

good question the 136 standard is the ability to withstand 1200 degrees Boss product is tested to 3000 degrees and the DAP 136 product tested to 2000 degrees the 1200 degrees should be enough to satisfy the task and heat range To me it flows out of the caulking tube much easier than refactory cement and seems more plyiable bothe products have siilicates I do not know of any silicone that will withstand 1200 degrees I have seen RTV ignite applied to the space in the flue liner

 

[hstraf]

Can I use just the same cement stuff that is used between the flue/block-off plate ... for the block-off plate and the chimney wall?

 

[elkimmeg]

Yes if it is rated 1200 degrees or above which the 136 is

 

[hstraf]

Yes if it is rated 1200 degrees or above which the 136 is

Yes, I believe it is. My concern is more about "sticky power". I understand that the stuff used between two metal pieces (pipe and block-off plate) might be different then the block-off plate and the cement/brick wall of the chimney.

But it will stick to both with no problem?

Why does the block-off creation guide recommend silicon AND the cement, then? Why not just the cement stuff?

(Sorry.. I'm just very confused -- it happens easily.)

 

[DavidV]

You are not the onlyu one confused by all that. I am running my furnace right now so I can pull the insert and put in my blockoff plate.

 

[BrotherBart]

You are not the onlyu one confused by all that. I am running my furnace right now so I can pull the insert and put in my blockoff plate.

Seems to be the day for it. I had to screw mine up taking it out to replace the stove so I am making a new one today. BTW: I used furnace cement to glue the last one in and it held real well. Too well when you have to take the darn thing back out. The advantage to furnace cement over silicone is that if you do have to remove it you can take a little hammer and chip the cement off to get a clean surface to start again. Silicone would be a bit harder to get off.