What is the benefit of a block off plate? Insert, exterior chimney

[CONick]

I've found bits and pieces of this conversation on searches, but nothing concrete as to "why block off plates are installed at top of firebox?".

I recently had a Regency i2450 Insert installed in an exterior chimney (about 25' tall). The installation team sealed the top of the chimney really well (absolutely no draft coming down the chimney around liner) and after burning the stove almost non-stop for a month, it's performing phenomenally. 6", non-insulated liner. Colorado front range (just outside Denver). I asked the installer multiple times about a block off plate at the top of the firebox, and they repeatedly said "not necessary", and said, "You can jam some insulation up there if you want. We'll leave a roll behind". Which they did.

I finally received the faceplate for my stove, and before I install it I'm trying to decide if I should shove some insulation up the chimney above the stove (pack it around the liner), and/or install a block off plate at the top of the firebox?

But I can't find any information as to WHY one might do this. Can someone shed some light on the situation please? I'm trying to determine if this is necessary (or at least beneficial) for my situation.

 

[RomanW]

I don't pretend to know, but if I was to guess, I'd it's to reduce the amount of heat lost going up the chimney beside your liner, and keep most of it down by the stove so it can go into your room.

 

[CONick]

I hadn't really looked at the roll of insulation until now. Seems like a bad idea to use kraft-faced insulation around a liner!

 

[bholler]

I've found bits and pieces of this conversation on searches, but nothing concrete as to "why block off plates are installed at top of firebox?".

I recently had a Regency i2450 Insert installed in an exterior chimney (about 25' tall). The installation team sealed the top of the chimney really well (absolutely no draft coming down the chimney around liner) and after burning the stove almost non-stop for a month, it's performing phenomenally. 6", non-insulated liner. Colorado front range (just outside Denver). I asked the installer multiple times about a block off plate at the top of the firebox, and they repeatedly said "not necessary", and said, "You can jam some insulation up there if you want. We'll leave a roll behind". Which they did.

I finally received the faceplate for my stove, and before I install it I'm trying to decide if I should shove some insulation up the chimney above the stove (pack it around the liner), and/or install a block off plate at the top of the firebox?

But I can't find any information as to WHY one might do this. Can someone shed some light on the situation please? I'm trying to determine if this is necessary (or at least beneficial) for my situation.

View attachment 306324 View attachment 306325 View attachment 306326

It's to keep as much of the heat produced in the house. You are loosing quite a bit up the chimney then through the masonry.

And why would a chimney company have a roll of fiberglass on the truck much less craft faced stuff that concerns me allot about their quality of work

 

[bholler]

I hadn't really looked at the roll of insulation until now. Seems like a bad idea to use kraft-faced insulation around a liner!

Fiberglass is a bad idea. Craft faced is a horrible idea

 

[CONick]

I don't pretend to know, but if I was to guess, I'd it's to reduce the amount of heat lost going up the chimney beside your liner, and keep most of it down by the stove so it can go into your room.

I was thinking this as well. But then I thought to myself, "but isn't it a good thing to keep your liner warm, to help prevent creosote buildup"? I am also planning to install a faceplate around the stove (closes the opening), so that open area will be closed off. See the attached photo from their website.

Ideas I'm kicking around. Pros and cons.

 

[CONick]

Fiberglass is a bad idea. Craft faced is a horrible idea

Right?! What the heck.

 

[bholler]

Right?! What the heck.

Like I said it really makes me question their work. Did they check for clearances before installing an uninstalled liner? Did they clean it properly?

 

[RomanW]

I was thinking this as well. But then I thought to myself, "but isn't it a good thing to keep your liner warm, to help prevent creosote buildup"? I am also planning to install a faceplate around the stove (closes the opening), so that open area will be closed off. See the attached photo from their website.

Ideas I'm kicking around. Pros and cons.

View attachment 306327

Well that space will be heated up by the liner anyways. Since its blocked off up top and bottom, there no cold air intrusion and it will just keep that space warm

 

[CONick]

It's to keep as much of the heat produced in the house. You are loosing quite a bit up the chimney then through the masonry.

Thank you for the response. This makes sense, although I worry about my liner being too cold, if I'm using block off plate (I've got a 25', non-insulated liner). Is this something I even need to concern myself with? Or is the block off plate just always the superior solution?

 

[CONick]

Like I said it really makes me question their work. Did they check for clearances before installing an uninstalled liner? Did they clean it properly?

Yes and yes. They cleaned it before install, and they did mention that I had "a ton of adequate clearance around my liner".

 

[bholler]

Well that space will be heated up by the liner anyways. Since its blocked off up top and bottom, there no cold air intrusion and it will just keep that space warm

But you are loosing lots of heat through the masonry

 

[bholler]

Yes and yes. They cleaned it before install, and they did mention that I had "a ton of adequate clearance around my liner".

Not around your liner. Between the masonry structure and any combustible material. That is what matters for code compliance

 

[RomanW]

But you are loosing lots of heat through the masonry

Right, but it would still be warmer in there with a hot liner than the liner out in the wind by itself.

 

[bholler]

Right, but it would still be warmer in there with a hot liner than the liner out in the wind by itself.

Yes which means more BTUs go through the masonry because of a higher temperature differential. Meaning more heat loss and more heat transfer to adjacent combustibles

 

[bholler]

The block off plate has nothing to do with the liner really it's about directing the BTUs produced by the stove into the house

 

[CONick]

The block off plate has nothing to do with the liner really it's about directing the BTUs produced by the stove into the house

Ok, this is what I was thinking, but I wanted to check with my setup that a block off plate was still a good idea. I was worried that with a non-insulated liner, I might have to worry about my liner not getting hot enough to prevent creosote buildup.

Per your other comments on the thickness of masonry: The team also commented that I had very thick masonry, but I'm glad you called this out. I'll do some research and then see what I can check myself with measurements.

When I bought the stove, the salesman at the fireplace store (who was also part of the install team) said they "very rarely sell insulated liners, as they aren't needed in our climate, which is not super cold. It's not worth the extra cost, and is not needed". So I took his word for it. But I'll see what I can do to doublecheck clearances.

Our winter temps are probably very similar to what you see in central PA.

 

[begreen]

Well that space will be heated up by the liner anyways. Since its blocked off up top and bottom, there no cold air intrusion and it will just keep that space warm

To some extent true, but the heat loss is notable. In a tall chimney, it's significant. Look at some FLIR images to see how much.

 

[bholler]

When I bought the stove, the salesman at the fireplace store (who was also part of the install team) said they "very rarely sell insulated liners, as they aren't needed in our climate, which is not super cold. It's not worth the extra cost, and is not needed". So I took his word for it. But I'll see what I can do to doublecheck clearances.

That is the standard answer of shops that don't want to be bothered with the difficulty of doing it properly.

 

[RomanW]

To some extent true, but the heat loss is notable. In a tall chimney, it's significant. Look at some FLIR images to see how much.

I'm not debating that, the idea in question was why put a block off plate when you want to keep the flue warm. Which both bholler and myself answered. I was just responding to say even with it there, taht space will still be protected against the elements outside of the masonry unlike running class A outside where it's exposed. You'll obviously lose some heat, but there will be some protection for the liner in there, and help it retain it's heat better.

 

[bholler]

I'm not debating that, the idea in question was why put a block off plate when you want to keep the flue warm. Which both bholler and myself answered. I was just responding to say even with it there, taht space will still be protected against the elements outside of the masonry unlike running class A outside where it's exposed. You'll obviously lose some heat, but there will be some protection for the liner in there, and help it retain it's heat better.

A class a even exposed outside will generally perform better than an uninsulated liner in a masonry chimney. The masonry just conducts heat way to well

 

[Garbanzo62]

So if I understand this correctly, the block off plate is covering the opening where the old fireplace damper existed. So this is effectively preventing heat from the top of the stove from going up through that opening and heating up the masonry in the chimney. Do I have that correct?

 

[bholler]

So if I understand this correctly, the block off plate is covering the opening where the old fireplace damper existed. So this is effectively preventing heat from the top of the stove from going up through that opening and heating up the masonry in the chimney. Do I have that correct?

Yes you are right on

 

[begreen]

Why damper seal is needed, block off plate install

Should fireplace inserts and hearth stoves have a metal sealing plate installed at the Damper area? Here is the simple answer. It is almost impossible to correctly seal most insert panels to the fireplace front. A lot of air can get through them...

www.hearth.com

 

[CONick]

Thank you begreen! Good link 👆

I'd like to summarize to make sure I have all the key points right (please call me out if I have anything wrong!). Also, I have an uninsulated liner which I'm not going to change right now. I just had it installed, received probably poor advice, but I don't plan to change it out yet unless there is a fire hazard.

• Lower block off plates (where damper was) help keep more heat in the house where you want it.
• Exterior masonry chimneys transfer a fair amount of heat to the outdoors, so even if your chimney is blocked at the top, you are losing good heat up the chimney without a lower block off plate.
• A lower block off plate is beneficial with an exterior masonry chimney, whether your liner is insulated or not.
  • It would be better if you had an insulated liner, but if you don't, block off plate still better than not having one.

Do I have all those correct? Especially the last bullet? ("...beneficial whether your liner is insulated or not?")