Need To Know: Best Sealant to Use on a Blockoff Plate

[bren582]

I am doing some maintenance on my masonry fireplace before the new insert arrives and I noticed that some of the high heat mortar I used to seal the block off plates (Top damper, side and back) cracked pretty bad, some falling off entirely. The damper plate was the most affected as its not a perfect fit. I'm thinking if the stove runs in the 700Deg range the damper plate maybe gets to 300 -400 degrees?

Is it OK to use 600Degree silicone (Rutland 600Deg RTV) this close to the insert? Would it give off odor when heated to several hundred degrees?

Or should I go with something like Rutland "Seal-It-Right" which according to the description is good to 800degF, does not release odors once cured and remains somewhat flexible, at least compared to the mortar or cement products?

Thanks,,

 

[begreen]

Yes, RTV silicone or similar high temp silicone is fine.

 

[bren582]

Thanks BeGreen.

I've been doing some research and even called the manufacturer that markets Mill-pak and the gentleman I spoke to was very helpful and willing to share his knowledge on the subject. He indicated as you have that high temp RTV is ok for the lower temp areas but also added that RTV is really only good to 400DegF. Even the products claiming to be good to 600 are only good at those temps for a brief exposure. anything beyond 400Deg for an extended period is not a good place to use silicone of any type.. Seems some of the Silicone formulations can off gas nasty stuff life formaldyhide when heated beyond 400 deg for long periods. I think I've seen posts from BrotherBart maybe? stating he doesn't believe all the hoopla about Silicone's being marketed as high heat versus regular.. Silicone RTV is all the same he says.. I am now inclined to believe that..

So I will purchase a tube of mill-pak for the real hot areas like around the top of the stove inclusing the stove collar and connectors and proceed cautiously with the use of silicone in around the insert.

 

[BrotherBart]

Thanks BeGreen.

I've been doing some research and even called the manufacturer that markets Mill-pak and the gentleman I spoke to was very helpful and willing to share his knowledge on the subject. He indicated as you have that high temp RTV is ok for the lower temp areas but also added that RTV is really only good to 400DegF. Even the products claiming to be good to 600 are only good at those temps for a brief exposure. anything beyond 400Deg for an extended period is not a good place to use silicone of any type.. Seems some of the Silicone formulations can off gas nasty stuff life formaldyhide when heated beyond 400 deg for long periods. I think I've seen posts from BrotherBart maybe? stating he doesn't believe all the hoopla about Silicone's being marketed as high heat versus regular.. Silicone RTV is all the same he says.. I am now inclined to believe that..

So I will purchase a tube of mill-pak for the real hot areas like around the top of the stove inclusing the stove collar and connectors and proceed cautiously with the use of silicone in around the insert.

Vindication at last after four years. Thank you sir.

My block off plate over the top of the 30-NC is $1.99 on sale plain old silicone caulk from Ace Hardware and it ain't looking like it has noticed getting hot in three seasons.

100% silicone vaporizes at 800 degrees. And the masonry around the edge of that block off plate is a heat sink. If that seal between the block off plate and the brick gets to 800 degrees it will be about the time the house falls into the basement.

 

[Hogwildz]

I used HH silicone around the perimiter of the block off plate and furnace cement around where the liner goes through.
The furnace cement cracked and fell off. The silicone on the perimeter is like the day it was put on.
I took BB's advise and stuffed stove door rope gasket around where the liner goes through and haven't had to touch it since.
Thanks BB

 

[semipro]

I suspect the block-off plates don't get as warm as some might think at least where they contact the masonry. When my stove top was 600 degrees per electronic IR thermometer my steel block off plate that sits about 16" above it was only around 250 degrees at the edges. I suspect the cool air from the chimney on the other side might keep the temp lower as well as the contact with the masonry.

 

[woodgeek]

I packed rockwool tightly in the small space b/w the liner and the plate. I'd use silicone around the outer edge (if I was doing it again).

 

[dreezon]

Is it absolutely necessary to seal the block-off plate? I was under the impression that the block-off plate itself is somewhat optional... more for reducing heat loss up the chimney than for safety? And what if you're putting a layer of 1.5-inch ceramic insulation on top? Wouldn't that make sealing less necessary.