Sealing block off plate question

[KennyK]

The new stove is coming shortly (Woodstock Ideal Steal), which will sit on my hearth about 14 inches in front of my fireplace firebox opening (completely outside of the firebox), rear vented through insulated stainless steel liner in my masonry chimney. When the company installed the liner for my current stove they put on a block off plate and didn't do the best job - the plate is a little short on one side, and not sealed off on any part rather just wedged into place, and one side has a gap of almost an inch, but it does have plenty of insulation behind it. It also seems like pretty flimsy material (perhaps aluminum?). I have some 22 gauge steel that and plan on making a better block off plate, I have four questions about sealing it off:

1) Do I actually need to seal it off with silicone and/or screws or if it's tightly wedged into place and fully covering the opening of the chimney, is that fine?

2) The only high temp silicone I can find locally is Imperial High Temp Red (http://www.imperialgroup.ca/stove_maintenanceproducts.cfm?c=330). It says it's rated up to 500F and 600F for shorter periods of time. If I do use sealant, is this the correct sealant I should/can use? The block off plate I am making will be in two pieces, so even if I can just wedge it into place, I'll want to seal those two pieces together, so I assume I'd use silicone sealant for that.

3) If sealing, is silicone sealant alone enough or should I also screw the plate into place? Or, could I screw it into place without sealant (or as mentioned prior just wedge it in tightly)? If I should screw it into place, I believe I will need to screw into a very seriously heavy duty lintel and possibly some brick. If so, what kind of screws should I use? Do I need to pre-drill?

4) If there is a little gap between the block off plate and the liner of a 1/4 inch or so, should I put a bead of silicone on that? Again, there is a lot of insulation behind the block off plate around the liner.

Thanks!

 

[begreen]

Check the wiki examples for damper sealing. You can use regular silicone around the perimeter. Don't put it around the liner.

 

[KennyK]

Check the wiki examples for damper sealing. You can use regular silicone around the perimeter. Don't put it around the liner.

Thanks Begreen! I've searched this site many times looking for answers, but must admit, I hadn't used the wiki until now - looks like there's a ton of helpful information there! This thread is helping me out a bunch for the block off plate: https://www.hearth.com/talk/threads...ock-off-plate-for-a-masonry-fireplace.147679/

Thanks again for another amazing resource here!

 

[Hogwildz]

Silicone around the perimeter as advised, leave a small gap around the liner, and fill with stove door rope gasket. Furnace cement will harden and fall off.

 

[KennyK]

Silicone around the perimeter as advised, leave a small gap around the liner, and fill with stove door rope gasket. Furnace cement will harden and fall off.

Can you elaborate a bit on how to apply the rope gasket? Do I wedge it in the gap and then cover the outside with furnace cement? How big of a gap are you talking about?

 

[Hogwildz]

Can you elaborate a bit on how to apply the rope gasket? Do I wedge it in the gap and then cover the outside with furnace cement? How big of a gap are you talking about?

Around 3/8" to 1/2". doesn't have to be perfect. Get the next size larger rope gasket and wedge it on along the gap. Done. Nothing else needed. Being pinched in the gap will hold it in place.

 

[KennyK]

Around 3/8" to 1/2". doesn't have to be perfect. Get the next size larger rope gasket and wedge it on along the gap. Done. Nothing else needed. Being pinched in the gap will hold it in place.

That's easy! Two questions (for you and/or others):

1) Is the gap and the rope gasket to allow for expansion and movement of the liner? Is there a different reason to use this over some sort of other seal? Just curious.

2) Do you think there's a disadvantage to making a block off plate in two pieces with semi-circle cut outs that I then connect? I've seen some models like this, and I ask because I already have something like this I could reuse and quickly adjust for this purpose, but if it will make a significant difference, I could always go out and by another piece of steel.

Thanks!

 

[begreen]

Two piece is ok. At the end of this article there are several linked examples of block-off plate installations
https://www.hearth.com/talk/wiki/why-damper-seal-is-needed/

 

[Hogwildz]

That's easy! Two questions (for you and/or others):

1) Is the gap and the rope gasket to allow for expansion and movement of the liner? Is there a different reason to use this over some sort of other seal? Just curious.

2) Do you think there's a disadvantage to making a block off plate in two pieces with semi-circle cut outs that I then connect? I've seen some models like this, and I ask because I already have something like this I could reuse and quickly adjust for this purpose, but if it will make a significant difference, I could always go out and by another piece of steel.

Thanks!

Actually, 2 pcs makes it easier to install & uninstall if needed in the future. Mine is actually 3 pcs due to the liner coming through that area on a slight angle before straightening out to mate to the stove outlet. Rather than cut an oval and struggling while installing after the insert was in place, I cut a larger hole, and a 3rd pc over where the overlap would have been in front of the liner. I originally tried furnace cement, which is messy a shitzgy, and only ended up cracking and falling off over the next few seasons. The rope gasket is easy to install, no mess and easy to remove if needed. Makes a good seal with no mess or worry of failure & falling off. Silicone around the liner will smell as it gets hot, and may singe off in time if the flue temp gets high enough.

 

[KennyK]

Okay, two more related questions:

Should I screw the two pieces of the block off plate together when it's in place (that's how the shoddy one I have now is), or can I seal the two parts together with silicone or another adhesive?

The firebox in my fireplace is completely lined with a thick metal (cast iron?) plate all the way up to the lintel. There is no way for me to screw or nail the block off plate into just brick, rather, I'd have to go into the thick metal plate and/or the thick metal lintel. I anticipate this being quite a challenge. Can I just forego screws or nails and wedge it into place? If I do this should I put some silicone or other adhesive between the block off plate and metal in firebox/lintel?

 

[DuaeGuttae]

Okay, two more related questions:

Should I screw the two pieces of the block off plate together when it's in place (that's how the shoddy one I have now is), or can I seal the two parts together with silicone or another adhesive?

The firebox in my fireplace is completely lined with a thick metal (cast iron?) plate all the way up to the lintel. There is no way for me to screw or nail the block off plate into just brick, rather, I'd have to go into the thick metal plate and/or the thick metal lintel. I anticipate this being quite a challenge. Can I just forego screws or nails and wedge it into place? If I do this should I put some silicone or other adhesive between the block off plate and metal in firebox/lintel?

I’m no block-off plate expert, but I did a lot of reading when I planned to install one before we ended up relocating. I think I recall that some, perhaps @weatherguy is one, have used rare earth or welder’s magnets to secure the plate to metal inside the fireplace.

 

[begreen]

Why is there no way to screw into the brick? Brick is soft. Use a masonry bit to make a hole for a rawl plug, then you can screw into the rawl plug anchor. Or use tapcon screws.

Use silicone on the overlap of the sheetmetal panels, then screw or pop rivet the two pieces together where they overlap.

Review the threads listed at the end of the wiki article I posted for examples.

 

[KennyK]

Why is there no way to screw into the brick? Brick is soft. Use a masonry bit to make a hole for a rawl plug, then you can screw into the rawl plug anchor. Or use tapcon screws.

Use silicone on the overlap of the sheetmetal panels, then screw or pop rivet the two pieces together where they overlap.

Review the threads listed at the end of the wiki article I posted for examples.

I don't have any exposed brick in my fireplace. My whole fireplace firebox is lined with a thick metal (perhaps cast iron) lining. This goes all the way up to the lintel, which in my chimney is a big piece of steel that is on all sides of the interior of the chimney, so the metal on top of the brick in the firebox goes right up to the metal of the lintel. I would need to screw or drill into the metal before getting to any brick and I anticipate this being very difficult (though I don't have experience with this, so maybe it's not as hard as I think). Any thoughts?

 

[begreen]

Metal could be even easier to drill into. Use a fresh 1/8" metal bit and #8 SS sheet metal screws.

 

[KennyK]

Metal could be even easier to drill into. Use a fresh 1/8" metal bit and #8 SS sheet metal screws.

Thanks. Will that go into thick cast iron and the thick steel of the lintel? This is stuff that was built in the 1890s and they used pretty heavy duty materials back then! Do I pre drill with the bit and then screw in the SS sheet metal screws?

 

[begreen]

It might go easier than you think. Use good tungsten tipped drill bits. Put a drop of oil on the tip of the bit before drilling. First drill 3/16" clearance holes in your b.o. plate, then mark the holes on the fireplace metal. Remove the b.o. plate and drill the pilot holes. Then put the silicone on the edges and screw the b.o plate into place. 1/2" long screws should be fine.

Or if you prefer you could just use 1/8" pop rivets. Drill and rivet in place with no clearance hole drilling. You would need longer maybe 1/2" pop rivets depending on the metal thickness. On the lintel if it is a real pita you could silicone that edge and clamp in place overnight to let the adhesive set up.

 

[bholler]

Thanks. Will that go into thick cast iron and the thick steel of the lintel? This is stuff that was built in the 1890s and they used pretty heavy duty materials back then! Do I pre drill with the bit and then screw in the SS sheet metal screws?

We do it all the time usually we just rivet them fast in metal fireplaces but if we think it will need pulled down for any reason we use screws you just need to predrill with the right sized bit. And i seriously doubt it is cast iron. I have never seen a fireplace lined with cast. But many many boxes made from plate steel.

 

[Hogwildz]

Make sure you use the right drill bit size for the screws you use, is you are using s.s. screws. The s.s. screws threads strip very easily, and the screws themselves break very easily. Go easy screwing them in.

You can seal the joint between the two pcs, or just screw them together. Either is fine.

 

[KennyK]

Block off plate coming along, and not a moment too soon... new stove install scheduled for tomorrow. Here's a few in progress pics, getting ready to drill.... Right now it fits snug around the chimney liner, not tight but less than 1/4 inch space between metal of plate and liner. Should I make that a little bigger so I can wedge gasket in there?

 

[Hogwildz]

Block off plate coming along, and not a moment too soon... new stove install scheduled for tomorrow. Here's a few in progress pics, getting ready to drill.... Right now it fits snug around the chimney liner, not tight but less than 1/4 inch space between metal of plate and liner. Should I make that a little bigger so I can wedge gasket in there?

View attachment 226421 View attachment 226422

As long as you can stuff some rope gasket in there, any size gap is fine. 3/8" and 1/2" gasket is fairly common, so I would cut the gap to accommodate one of those.

when you caulk the perimeter of the plates to the walls, that will help hold it in place. You don't have to go nuts with the screws around the perimeter.