To Block Off Plate or Not To Block Off Plate

Another quandry

I have read about block off plates here on the site as I was referred to earlier and I have been calling around our area asking for information in regards to a block off plate at the damper area.

Two reasons have come up. One is that it keeps heat down in the insert area instead of going up the chimney and being lost heating the bricks there. My chimney is internal so the heat loss isn't as dramatic as it would be in an external chimney.

The other reason is a safety concern in that if there were a fire the block off plate would keep the flames from going up into the chimney and possibly going through any cracks in the masonry and catching attic materials on fire and thus a house fire..

If an insert has a ss liner, as mine does and it extends all the way up my chimney and out the top, how would the fire get out of it into the chimney proper?? My understanding is that a chimney fire, if it were to happen, in an insert with a liner would be contained in the liner and wouldn't get into the masonry part of the chimney at all.

I am getting conflicting info. in regards to if one is necessary with a full liner. I do have one at the top of my chimney which is where most people say one is a necessity.

It just seems to me that one isn't as necessary with an internal chimney and a fully lined insert.

So I'm back to the forum for more personal input from those of you who have had experience with a lower block off plate as well as having not had one previously. What have you found in having it both ways?

My installer came yesterday to inspect chimney and fireplace and said all looks ok for install of the Jotul 550. I asked him about the lower plate and he said it wasn't necessary due to how my set up is.... He is a full time fireman as well as an installer.....I'm befuddled so I figure just what I need is more info!

Okay - I get it. I am a knitter and not a builder so this all gets me pretty overwhelmed. I read the 'how to' article but I have to ask more questions because I can't comprehend something unless I actually physically do it. ( The only way I can explain this is that when I read a pattern to knit something it makes absolutely no sense to me. When I simply begin knitting one stitch at a time the directions begin to make sense and after I have completed whatever I am knitting I can then read the pattern and it makes sense to me.....it's a brain thing...)

Anyway, since I am not going to be making this myself I need to clarify where I measure. My installer has never done one of these things either so I need to be accurate. I don't think he minds the extra work as he will charge me for it which I don't mind paying because he is very reasonable and doesn't try to rip people off. Just a good person in general.

Do I measure below my damper area even though he will have to remove more of the damper apparatus in order for my ss liner to have a straighter shot to the top of the unit? (First installer didn't remove enough so when liner was pulled through he formed it into a bit of an oval shape so he could pull it through....not a good idea.)

When I have that measurement how do I determine where to have them cut a hole for the liner? It seems like it would be easier to have an extra piece with an indentation that can be added and fit snuggly around the ss liner if I get it cut in a collar type of cut.

What brand/type of insulation is rcmd. to be placed on top?

Sealants...I know many will not take the extreme heat of a wood burning flue and will simply get brittle and crumble out after a few fires. I have had that experience 3 times now with my current insert. It was originally sealed with a sealant that flaked off. Another person came out and said it had been the wrong type of sealant meant for gas fireplaces and said he had appropriate sealant for wood burners. He applied it.....It did the same thing. A few months ago I had a 3rd person come out (All independent chimney people. All with the right certifications/qualifications req. by the state) and he said last sealant not the right stuff and he applied sealant again that was recommended for these high heats.....It met a similar fate....

So you can see I am a bit leary of sealants. If any of you have successfully used a sealant and it is still intact please be specific with the brand name! People have rcmd chimney cement but I have no clue what that is. Can I purchase the sealant myself or is it something a licensed worker can only get??

I do agree with all of you in taking the extra measure so my hesitancy isn't in cutting costs. It's about trying to reduce my feelings of being overwhelmed with this project.


Just re read the 'how to' pages and I headed out the door to a neighbor's house who is a builder and I am sure he will be able to make sense of it. I do see that that despite what I have done to the damper area the measurement is down by the lintel area so what has been done above it doesn't matter...Am I right with that assumption?

Hopefully he can do this because my brain is close to panic!

Swestall - I think I am okay now. I took the article from this web site on how to build a block off plate to a neighbor who is also a builder and I also took the photos of iceman's install. He had no problem understanding what I need and will come over tomorrow to take the needed measurements for me. I will take the drawings to a sheet metal shop nearby and have them make the plate. My neighbor can't make it because he has no way to bend the metal for the flanges cleanly.

I'm calling my installer tomorrow to let him know I'm going with this plate. I can't imagine he will have a problem with it since I'm supplying all that is needed. It does seem pretty simple to install especially since the area will all be cleaned out.

What type of insulation should I get to put on top of it? How many inches deep?

What gauge sheet metal should I request?

How important is the cement around the liner and the plate? ( As you may have read I have had miserable luck with any of that stuff holding up to flue temps.) The plate he is designing will fit very snugly around the ss liner so the amount of air getting through it seems pretty minimal to me.

All of you thanks for your input!! It is greatly appreciated

Sounds easy enough! What gauge metal should the plate be made of?

swestall said:

You could use the same wool blanket type insulation and metal tape to secure it to the plate. If you use a bead of stove cement (get the kind that comes in the caulking gun tube) all around the plate and pull it down a bit for a good seal.
Run a bead around the place where the liner goes through also.

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Picture may follow install providing I can get my teenage son or husband to help me figure out how to do the picture thing on this site. I don't dare try to figure it out yet as all this install stuff has my brain on overload and any new info. is sure to begin popping my old circuits. I break out in a cold sweat just thinking about reading any more directions about anything right now!

Install won't happen for a few weeks as my installer is going on vacation at the end of this week.

Hogwildz said:

perplexed said:

Another quandry

I have read about block off plates here on the site as I was referred to earlier and I have been calling around our area asking for information in regards to a block off plate at the damper area.

Two reasons have come up. One is that it keeps heat down in the insert area instead of going up the chimney and being lost heating the bricks there. My chimney is internal so the heat loss isn't as dramatic as it would be in an external chimney.
IN your case not so bad.

The other reason is a safety concern in that if there were a fire the block off plate would keep the flames from going up into the chimney and possibly going through any cracks in the masonry and catching attic materials on fire and thus a house fire..
I think your understanding that backwards. If you have a chimney fire, and say the liner takes heat long enough to fail. The block off plate will also keep the flames from drafting back into the house through the damper area. Its a safety measure that is a no brainer for me. You can justify needing one, or not needing one. End result, with it, its another degree of safety and can hurt at all. Any extra safety measures is always a plus in my book.


If an insert has a ss liner, as mine does and it extends all the way up my chimney and out the top, how would the fire get out of it into the chimney proper?? My understanding is that a chimney fire, if it were to happen, in an insert with a liner would be contained in the liner and wouldn't get into the masonry part of the chimney at all.

I am getting conflicting info. in regards to if one is necessary with a full liner. I do have one at the top of my chimney which is where most people say one is a necessity.

It just seems to me that one isn't as necessary with an internal chimney and a fully lined insert.

So I'm back to the forum for more personal input from those of you who have had experience with a lower block off plate as well as having not had one previously. What have you found in having it both ways?

My installer came yesterday to inspect chimney and fireplace and said all looks ok for install of the Jotul 550. I asked him about the lower plate and he said it wasn't necessary due to how my set up is.... He is a full time fireman as well as an installer.....I'm befuddled so I figure just what I need is more info!
Ask the installer/fireman why would he advise against an added safety measure. He won't tell you that a block off plate is a PITA to install, and he won't make enough off it for the hassle. Ask him if the liner does take over 2100 degrees from a chimney fire for longer than testing standards, and the liner does for whatever reason fail............... what will stop the fire from back drafting back into the house at bottom without block off plate?
Bottom line, is if you can sleep ok at night, with you wife/kids/ yourself, and not worry, then don't sweat it and leave it off.
Personally, if a fireman or anyone for that matter came to install my insert, and told me I didn't need an extra added benefit of protection, I don't think I would be as comfortable with them installing my insert.
Per the manual if the liner is run all the way up, a block off plate is not required. (Only a direct connect requires per manual. But a go back to an extra added safety measure. Up to you.
Good luck with the install, and send pics

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Install date is set for Feb. 11 - weather permitting. I looked at the 'how to' info. for doing the pictures and it looks easy enough. I will have my 11 year old take the pictures because he really likes doing that kind of stuff and then he can transfer them onto the computer - I have no idea how to do that yet but it looks like I have a perfect opportunity to learn!! If I need help putting them onto the forum site I'll see if my 18 year old will help. He's pretty computer savy but not always willing to help his mother out...He's balanced precariously at that adolescent jump off point into independence which, translates into, somedays he acts human and on others I wonder who it is standing in front of me...One way or another, pictures will be forth coming after all of this prep. is finally finalized!!

Thanks for the help!