Block Off Plate or Not

arcticranger

New Member
Nov 4, 2017
19
new york
I've got an old Elmira Stoveworks woodstove fully inside my fireplace and probably losing most of its heat up the chimney. It can't be pulled further into the room due to the way it connect to the steel chimney liner. I've been reading about blockoff plates and wonder if that would be as effective as a new woodstove standing fully in the room. Only 2" of my fireplace is inside the house, the rest is contained in an exterior chimney.

I'm confused by the descriptions of blockoff plate installs since the only damper I am aware of is a manual one on the stove itself. Is the blockoff plate just a metal barrier sealing the open space around the chimney liner or s it doing something more?
 

kennyp2339

Minister of Fire
Feb 16, 2014
5,041
07462
Is the blockoff plate just a metal barrier sealing the open space around the chimney liner or s it doing something more?
Yes, some put roxal insulation above for more insulation, block off plates were developed for inserts within a fireplace, but many people use them when they want to seal the fireplace to keep the warm air from going up the chimney
 

arcticranger

New Member
Nov 4, 2017
19
new york
oK great, I thought I was missing some deep science lol. Is installing block off plates a generally well known skill of woodstove installers? I think mine will be an odd shape. It's an old Heatilator fireplace.
 

jetsam

Minister of Fire
Dec 12, 2015
4,720
Long Island, NY
youtu.be
oK great, I thought I was missing some deep science lol. Is installing block off plates a generally well known skill of woodstove installers? I think mine will be an odd shape. It's an old Heatilator fireplace.
Everyone's is an odd shape. Several people have used cardboard to template out the shape and then used that to mark their sheet metal. Hearth has lots of threads on this with good pictures in some cases.

Installers may tell you that you don't need a blockoff plate because they are sealing the chimney with a flashing on the top. There are lots of threads on this subject here on hearth, and we seem to mostly disagree with that.

The blockoff plate is not a standard size and shape, so they don't have one they can grab out of the truck and slap up there in 5 minutes, so you will most likely hear about how you don't need one.

If your fireplace is exterior, you may also want to look into taking the stove out next year, insulating the fireplace, and putting the stove back in. A few people have done Roxul + cement board and reported good results, but you do need some free space in the fireplace, since you are losing almost 4 inches on the back and both sides.
 

arcticranger

New Member
Nov 4, 2017
19
new york
Good info all. I had already read a few threads here describing block off plate installs but wasn't sure it applied to my simple stove.

Do you always have to take the stove out to install a the block off? Do any people just try to work around it?

Also (sorry for all the q's) is there a rule as to where you put the plate?
 
Last edited:

Hogwildz

Minister of Fire
Typically you shouldn't have to remove the stove to sweep, if you have a liner from the stove outlet to the top of the chimney. Usually a matter of removing the baffle, or a couple tubes and baffle board, then sweep right down into the stove.

I installed the block off plate in mine just above the lintel. Easier to drill and tapcon the plate into masonry, than thick steel. If you make the plate so that it fits snug enough, you can wedge it in there and then silicone caulk the perimeter where it meets the lintel & masonry to help keep it in place. I used tapcons an then sealed with the silicone. If you have a gap around where the liner passes through the plate, you can stuff some stove rope gasket in there.

Don't sweat the questions, that's what this site is all about. Ask away.
 
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Ben Stark

New Member
Oct 31, 2017
37
Upstate NY
I’d like to add my experience with the block off plate as this is my first hearing season using one. I have an older Osburn 1600 insert installed into a 1960’s era Hearilator.

The insert was installed with an insulated SS liner due to being on exterior wall. I’ve used it for a few years to heat 1/2 my house, and take the chill off the rest of the place. I decided, after lots of reading on this forum, to add a block off plate because there was basically a gaping hole above the stove. The installer placed a blanket of ceramic insulation on top of the insert jacket and wedges some up into the hole around the liner.

Recently I added 1 inch blanket of unfaced Roxul behind the insert, it was easy to slide into place and friction fit to the old masonry. It’s 3 inches away from stove jacket. I did the same above the stove, basically friction fit the Roxul into place above the stove. I used two pieces so I wouldn’t have to remove the stove and I cut a hole for the liner that fits reasonably well. A bread knife worked excellent to cut the rock wool.

It’s not exactly air right, but I do feel it’s made a nice difference in the heat output. In the near future I want to add the metal block off plate basically as Hogwildz has stated. Only mine would be retrofit in two pieces. I’ll slide it up into place beneath the rock wool, seal it and call it job done.

The Roxul caused a smell as it first heated up, but this seems to be a one time occurrence. I’m pleased with the results of just the insulation, which slow down the heat loss. I’m thinking the metal plate will “air seal” the opening.

I never thought to ask the installer to do this because I was new to inserts at the time it was put in. (Though not new to wood heat in general, having grown up with a wood stove as our sole source of heat.)

At the top of the masonary chimney the installer used a SS plate to block off the chase, and sealed it with Silicone. He did not use any other fastener. I added Tap Con screws to the plate, a fairly easy job with a decent cordless drill.
 

arcticranger

New Member
Nov 4, 2017
19
new york
If your fireplace is exterior, you may also want to look into taking the stove out next year, insulating the fireplace.
I could get Roxul behind it and on the sides now, there is enough room to get at least R15 in. But if the back and sides are covered and I've got my block off plate in place, is the heat that warms the room just coming from the top of the insert and what's reflected from the blockoff plate? Seems like you'd want as much radiant heat from top and sides as possible.
 

Hogwildz

Minister of Fire
A 2 pc block off plate makes the job easier in many situations. I actually used 3 pcs.
Most inserts have an outer casing on back, top & sides, with space for heated air to circulate and make it's way out into the room. You do not want to encase the insert in insulation, which will defeat the purpose of the insert. If you want o install insulation, install it outside the casing, along the old fireplace walls etc. Some here have done so, I have never felt the need to since no issues with heating as designed.

The insert will convect heat from top, & sides. The block off plate keep the heated air from rising up the cavity of the chimney and being absorbed by the masonry and wasted to outside. It also serves to keep cold/ cooler air from dropping down and into the house.
 

Brian26

Minister of Fire
Sep 20, 2013
502
Branford, CT
My simple solution to a block off plate. Stuffed as much roxul up there as I could and then installed 2 layers of extra liner insulation with the metal facing down. Literally took 5 mins to do and simple to remove if I need do any work.

20181021_123417(1).jpg
 

jetsam

Minister of Fire
Dec 12, 2015
4,720
Long Island, NY
youtu.be
I could get Roxul behind it and on the sides now, there is enough room to get at least R15 in. But if the back and sides are covered and I've got my block off plate in place, is the heat that warms the room just coming from the top of the insert and what's reflected from the blockoff plate? Seems like you'd want as much radiant heat from top and sides as possible.
The problem is that the stove pumps tremendous heat into the exterior masonry, which conducts it endlessly outside.

To find out if you care, wait until it's cold out and point your IR thermometer at that side of the house. If your foundation is 0°F and the fireplace brick is 90°F, you are losing a lot of heat and should consider insulating.

People with interior chimneys don't care because the masonry still gets heated, but the heat winds up in the house.

I liked this thread because mellow did it in stages and took lots of pictures.

insulated-fireplace-2-jpg.jpg
 

arcticranger

New Member
Nov 4, 2017
19
new york
I’d like to add my experience with the block off plate as this is my first hearing season using one. I have an older Osburn 1600 insert installed into a 1960’s era Hearilator.
My woodstove is also set into a 1960s Heatilator which was built into an exterior chimney. It's rusted out like most but the flue is in good shape and has a new SS liner and chimney cap. So I vent right into the liner and bypass whatever is left of the Heatilator. I still have the 4 ducts which I presume do nothing. I've just refaced it with mortar and porcelain tile.

Is that your situation? I'm curious if there are any dangers to just bypassing the damaged heatform metal.
 

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arcticranger

New Member
Nov 4, 2017
19
new york
To find out if you care, wait until it's cold out and point your IR thermometer at that side of the house. If your foundation is 0°F and the fireplace brick is 90°F, you are losing a lot of heat and should consider insulating.


View attachment 231450
I'm sure I would benefit from insulating. But the more I read the more I think a free standing woodstove mostly outside of the firepit is going to deliver more radiant heat to the room. Are there any pros are for having a woodstove buried deep in a firepit? I'd have to build my hearth out but that's very doable. Maybe this is a totally different thread tho.
 

jetsam

Minister of Fire
Dec 12, 2015
4,720
Long Island, NY
youtu.be
I'm sure I would benefit from insulating. But the more I read the more I think a free standing woodstove mostly outside of the firepit is going to deliver more radiant heat to the room. Are there any pros are for having a woodstove buried deep in a firepit? I'd have to build my hearth out but that's very doable. Maybe this is a totally different thread tho.
The only plus I can think of to burying a stove in a fireplace is that it might draft better due to less horizontal run.

If you moved it out, you would get more heat, but will the stove still draft okay? Depends on the stove. No idea about yours, but some of the stove pros might.
 

arcticranger

New Member
Nov 4, 2017
19
new york
My simple solution to a block off plate. Stuffed as much roxul up there as I could and then installed 2 layers of extra liner insulation with the metal facing down. Literally took 5 mins to do and simple to remove if I need do any work.

View attachment 231444
That looks like a quick and flexible solution. What's holding the liner insulation in place, just tension like the Roxul?

I wonder if people using metal plates think one or the other is superior.
 

arcticranger

New Member
Nov 4, 2017
19
new york
The only plus I can think of to burying a stove in a fireplace is that it might draft better due to less horizontal run.

If you moved it out, you would get more heat, but will the stove still draft okay? Depends on the stove. No idea about yours, but some of the stove pros might.
Mine cannot be moved out and there's no height for a horizontal run of any kind. I wonder if there is a safety factor in having the fire basically outside of the house and away from framing.
The problem is that the stove pumps tremendous heat into the exterior masonry, which conducts it endlessly outside.

To find out if you care, wait until it's cold out and point your IR thermometer at that side of the house. If your foundation is 0°F and the fireplace brick is 90°F, you are losing a lot of heat and should consider insulating.

People with interior chimneys don't care because the masonry still gets heated, but the heat winds up in the house.

I liked this thread because mellow did it in stages and took lots of pictures.

View attachment 231450
That thread link was informative thx. I still have my old Heatilator steel wall behind my stove which I can slip some R15 behind. That will keep the Roxul off the stove. I noticed in that link the guy kept the Roxul away from the sides of his stove, is that standard practice?
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
Mine cannot be moved out and there's no height for a horizontal run of any kind. I wonder if there is a safety factor in having the fire basically outside of the house and away from framing.


That thread link was informative thx. I still have my old Heatilator steel wall behind my stove which I can slip some R15 behind. That will keep the Roxul off the stove. I noticed in that link the guy kept the Roxul away from the sides of his stove, is that standard practice?
Yes mist manufacturers dont want insulation packed around the insert they are concerned about overheating. I dont know how valid that concern is but i am sure it would void the warranty.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
He's using chimney liner insulation as his block off plate, the Roxul is on top of that.
That will work if it stays in place. Which without some mechanical fastener it will not