Attaching fireplace blockoff plate

I have two areas of concern, attachment and noise. I expect the plate to distort during each heating and cooling cycle. My current plan is to install angle brackets to all four sides of the enclosure and set the plate on top of them. Should the plate be attached to the brackets? I'm afraid that if the plate is attached to the brackets, the movement from distortion will eventually loosen the anchors holding the brackets to the brick. If the plate rests on the brackets without being attached, I think it might make loud noises bumping against them as it distorts.

I can make the plate from 12, 16, or 20 gauge sheet metal, which I have on hand, or use lighter or heavier metal if I was persuaded that it would give better results.

All thoughts, observations, suggestions, and advice are welcome.

Thanks, Stan

The reason I want to install a blockoff plate is so that I can fill the space between the chimney liner and the masonry with vermiculite to the lowest possible point. This will provide my house with added protection from a chimney fire, as well as holding the heat which radiates from the firebox at a level where the masonry that absorbs it can transfer it to the living space rather than to the attic or the outdoors above the roofline.

I have a plan for closing the blockoff plate snugly, not tightly, around the chimney liner, so the only advice I think I need is about how to deal with possible noise and attachment issues from the cyclical warping of the metal of the blockoff plate.

Stan

Enlightened fire departments deal with chimney fires by blocking the top of the chimney with something non-combustible. Less enlightened departments run water down the chimney, thereby adding tens of thousands of dollars of unnecessary damage to the situation. But it would be a monumentally incompetent department that would pull the burning appliance out into the middle of the room so that they could get at the chimney from the bottom. Thanks for the dire warning, though.

Stan

"blocking the top of the chimney with something non-combustible" only works when the fire in the chimney is small and there is not much buildup of combustible materials in the flue and the flue is intact and airtight.

Some chimney fires are so deeply embedded that there is no way to put it out without removing the burning material. They will need to get to the seat of the fire after the stainless steel liner burns through. To do that requires removing whatever heating appliance the "monumentally incompetent" homeowner put there.

I'm glad that you now say you are putting in the stainless liner, it will help to keep the soot and creosote under control, and it will help the flue draw better. We've had to deal with some really nasty cases where somebody just put the insert in without a liner.

Check your smoke detector. You never know when you may need it.

All the best, Stan