What is a Block off plate?

It's a plate up in your fireplace opening that blocks the heat from going up your chimney. The liner would run through it if you have one.
If you don't have a liner the plate would be of no use really. You should have both.

yeah, it'd get sucked right up that chimney

RayBurner said:

Thanks Hotcoals.

So in your opinion without a block off that would cause heat to go up the flue as opposed to going into the house??

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For sure if you don't have at least a plate at the top of the chimney. But even one at the top would not be as good has one down low
Do you have a liner?
If you just have a slammer instal ,that is your problem.

RayBurner said:

I do have a liner but I know I don't have a plate.

How difficult is it to have one installed?

Also how much of a difference will it make in heating my house??

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Well if you can't do it yourself you will need to get estimates.
You must have something at the top then since you have a liner. That helps but one below would help even more imo.
Some say they have shoved some insulting materiel up in there..like a insulating blanket. I forget the name of it but hopefully someone who knows will jump in with that info. I have no idea if that is a good idea or not but I know it has been done.

This is what some use,or something like it.
Get more opinions though.

http://www.homedepot.com/p/Roxul-Sa...3-1-4-in-Batt-Insulation-RXSS31525/202531875#

RayBurner said:

Much appreciate your help!!

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No prob. At least it's a start!
In time more will post that have done it..be sure to check back. Cheers!

Most that make a metal plate use typical 26 gauge galvanized sheet that you can buy in the duct section of your local Home Depot or Lowes.
You fabricate it so there are a 1" or so flange on each end so to fasten it to the interior of the fireplace at the old damper are or below.
You can put some Roxul on top of the plate when you install it, or not. Up to you. Overkill does not hurt here.
And yes in most cases, it is a noticeable difference, and keeps heat in the home, rather than up the chimney and transferring to the outdoors through the masonry.
Also keep cold drafts from coming down the from same area.
Here is a link to give you a better understanding with photos/diagrams.

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

Thanks, that's great information. I had the opportunity to take a look at my installation for the first time (removed the trim piece so I could get to the fan on my insert) and I was surprised at how the liner snaked up the chimney with so much airspace around it.

I think I'll ask my chimney guy to fabricate and install a block off plate for me next time he's here...

I think my chimney guy mentioned vermiculite if I did an install, but I think that would require a block off plate, anyway. Vermiculite seems kinda messy if one had to remove the insert or liner, as i imagine it would all fall down. Wonder why he didn't mention Roxul; that sounds ideal.

lml, Hopefully the chimney guy will be receptive to doing the job. Many are resistant. It can be a pain in the arse job, but for me, was well worth it. Many other feel the same way.

OLK: your chimney guy would not make as much money off the job stuffing some Roxul up there, as he would dumping Vermiculite in there. And yes, if you ever need to pull the insert or do anything with the liner, your going to have a mess.