Insert just a little too deep for the upper part of the fireplace?

[Kominar]

New to this forum, after soaking up lots of great advice on inserts, firewood, etc. - seems like a wonderful community of knowledge and experience!

So... after much deliberation (and almost going with Regency i2450), we're about to have the Pacific Energy Super LE insert installed sometime in January.
I looked at all the dimensions, measured our minimum fireplace opening dimensions, and all seemed fine... UNTIL I realized the back of the fireplace starts sloping in gently at about 8-9" height. Now I'm really worried about the fit!

Dimension D, which the Super LE requires to be minimum 18 3/8", is just sufficient at the bottom of our fireplace (18.5-19"), but by the time you're at 17" up, it is reduced by almost an inch and half! The shape of the insert (see the diagram) requires D = 18 3/8" up to that height, after which the depth becomes less important.
Is this going to be a major issue to fit the insert? - what extra work/expense would be required? (we're really not wanting any surprises at that point!)
The fireplace interior is some sort of heat-resistant brick, but I'm not sure how deep... and whether just a couple of inches of the brick could be removed.
Thanks for any advice!

(In comparison, when we had a WETT inspection for the i2450, this issue didn't come up... since it only requires 17" depth)

 

[Kominar]

Just adding a diagram to make sure my description made sense:

 

[rwhite]

Is this a masonry fireplace or prefab that it's going into. Looks like you are describing the smoke shelf. Most that I have seen, the smoke shelf is not structural and can be taken down a few bricks

 

[begreen]

It looks like this is under the smokeshelf, just the sloping back of the fireplace. There are a couple options. One is to grind or cut out a notch in the back to fit the insert. The other is to extend the surround with a flange. This can be made from aluminum L stock attached to the back of the surround. If all you need are a couple of inches I would go for notching the brick.
If this is an exterior chimney I also strongly recommend adding an insulated block-off plate in the damper area. It will keep the insert warmer = more heat into the house instead of up the chimney and outdoors.

 

[Kominar]

Thank you for your input.
Indeed, it does look like it's all due to the sloping of the smoke shelf, and nothing structural. I like the suggestion of grinding a notch as all I seem to need is max 1.5" of extra space from 9" to 17" up the height of the back wall.
The fireplace is masonry, with an exterior chimney. Built back in the 60s, and it's starting to look a bit aged - a few small cracks, some bricks in that spot don't feel all that solid anymore... so I'm curious how they'd survive the grinding.
I'll have to read up on the block-off plate, I'm learning so much here!

 

[rwhite]

I think you will find that knocking the top 3 courses down with a chisel will be less work and mess than grinding. If you need 1.5" that's a 1/2 a brick on top. That's a lot of grinding for a brick that will probably break anyway.

 

[rwhite]

Taking down the bricks also allows better access to clean behind the smoke shelf. My last place was built in the 40s and I probably took 10 gal of creosote out from behind the shelf when I installed an insert.

 

[Kominar]

That makes sense, especially given the condition of that back wall.
I guess my assumption was that the firebox wouldn't have to be in the best shape for the insert, since it will no longer contain the fire directly. So would you recommend repairing it beyond what is needed to create those extra 1.5" of space in the upper part?

Also, wouldn't the insert installation (which will be done by a local supplier) involve taking out the damper anyway? (and adding a block-off plate, per begreen's recommendation). If that's the case, is there anything to consider in how that smoke shelf should be rebuilt, since my understanding is that it won't really serve any future purpose?
(I'm hoping I can do that myself before the installation, to save $$, but it will be my first time doing any masonry!)

 

[rwhite]

The damper will most likely have to be removed. The smoke shelf will serve no purpose once an insert is in.

 

[Solarguy3500]

Also, wouldn't the insert installation (which will be done by a local supplier) involve taking out the damper anyway? (and adding a block-off plate, per begreen's recommendation).

Don't assume the installer will install a block off plate or insulate the smoke shelf area. After reading about this on this forum, I asked a couple local dealers if they would do that as part of their installation and they said no.

At that point, I decided to do the installation myself because I realized I would end up with a much better install and much better performance from my insert, and save a ton of money at the same time. Definitely ask your installer what their installation includes and what it doesn't.

 

[Kominar]

Thanks, Solarguy3500. Hmmm, you raise a really good point there. I'll check but I suspect their answer will be similar to what your installers told you (which is a rather annoying realization given the $$ they charge already).
I'm not sure if I'm confident enough in my abilities to do the whole job myself... or what prep to do (e.g. build the block-off plate etc.) before the install so that it improves the performance but doesn't get in the way of their tasks. I guess I'd better read more on this.

 

[begreen]

There are several good threads on adding a block off plate. It can be a nice performance boost for inserts installed in exterior wall fireplaces.

Make a damper sealing block-off plate

How to make a block-off plate A block-off plate is particularly helpful with inserts installed in fireplace on exterior walls with exterior chimneys. Without one, the flue liner (which should be insulated) and the stove will try to heat the mass of masonry that is exposed to the outdoors...

www.hearth.com

 

[Kominar]

Quick update & follow-up questions:
Finally got around to expanding the firebox - removing some bricks from the back wall (smoke shelf) to create enough room for the depth dimension of the insert (Super LE). It was quite a process to get all that masonry out.
Here's what it looks like now.

As you can see, not much room behind it - the red bricks (presumably double) of the exterior chimney are right behind it. So I'm wondering how to proceed and whether anything else can/should be done. There's a bit of moisture on the red bricks now, but looks like it's just the condensation from the interior air against the cold bricks.

Some of the options I can think of:
a) leave as is
b) cover with refractory cement - but wouldn't it crack soon anyway, due to the big temp difference between the insert and chimney bricks?
c) fill in with firebricks - but they'd need to be cut thin, or else the firebox wouldn't be big enough again
d) put in some insulation? - if so, can it go all the way to the insert? (zero clearance?)

Maybe I'm overthinking this. A mason weighed in that option A would be just fine. And that the chimney bricks would have no problem dealing with the heat from the insert.

 

[begreen]

Put rockwool (roxul) or ceramic wool (kaowool) above the blockoff plate and behind the insert. Yes it can touch the insert and the much hotter flue liner.

 

[Kominar]

Thanks for the suggestion - that sounds like an easy solution!

 

[Solarguy3500]

Looks good! You're making progress.

This is going to be one of the next steps in my install project. Taking down some of the firebrick in the smoke shelf area to gain a bit more depth for my insert. My project is on hold now until we get a break in the weather, maybe until spring at this point.

As begreen mentioned, you should definitely insulate behind the insert and above the block off plate. What the mason told you is technically correct in that the insert would function fine and the brick would be fine on the exterior of the chimney without the fire brick, but the issue is that you would be losing so much of your heat as it would be heating the masonry mass of the brick chimney and transferring to the exterior. The insulation and block off plate will let you keep a great deal more heat in the room. The rock wool insulation comes in a few different forms. The regular unfaced batts are around 3" - 3 1/4" thick or so and are fairly flexible. If you don't have 3" behind the insert, Roxul makes a foul faced sheet of it that is 2" thick and is a bit more rigid. I am going to be using this for my insert project. Grainger and Zoro carry the foil faced stuff. If you use the unfaced batts, you may want to paint it with stove paint or somehow cover it with metal to contain the dust and fibers that can be released from it. The foil faced option would keep the fibers contained behind the foil layer. If you go that route, you'll want to make sure you tape up the seams between pieces with foil tape like the kind that is used for liner insulation.

I have a fairly large firebox so I'm actually going to make a series of metal heat shields that I will install on the back and sides of the fireplace over the roxul to contain any fibers and I figure it will also help reflect more heat back into the room.

Keep up the good work! You're getting there.

 

[Kominar]

As begreen mentioned, you should definitely insulate behind the insert and above the block off plate. What the mason told you is technically correct in that the insert would function fine and the brick would be fine on the exterior of the chimney without the fire brick, but the issue is that you would be losing so much of your heat as it would be heating the masonry mass of the brick chimney and transferring to the exterior. The insulation and block off plate will let you keep a great deal more heat in the room. The rock wool insulation comes in a few different forms. The regular unfaced batts are around 3" - 3 1/4" thick or so and are fairly flexible. If you don't have 3" behind the insert, Roxul makes a foul faced sheet of it that is 2" thick and is a bit more rigid. I am going to be using this for my insert project. Grainger and Zoro carry the foil faced stuff. If you use the unfaced batts, you may want to paint it with stove paint or somehow cover it with metal to contain the dust and fibers that can be released from it. The foil faced option would keep the fibers contained behind the foil layer. If you go that route, you'll want to make sure you tape up the seams between pieces with foil tape like the kind that is used for liner insulation.

Thank you, Solarguy, this is super helpful.
Good to hear I can put the insulation behind the insert. After the insert goes in, I will still have about 3-4" of room there now that the bricks are gone, so that's good.
I did ask the local company that has sold me the insert and will install it whether they do anything to keep the heat better contained. They told me they stuff some insulation above the insert - well, probably above the damper frame and in the space around the flue liner, so it doesn't escape up the chimney as much - but that's about it, nothing fancier than that.
I'm assuming the metal in the block off plate not only holds the insulation in place, but also reflects the heat back?

 

[kennyp2339]

I'm assuming the metal in the block off plate not only holds the insulation in place, but also reflects the heat back

Insualtion w/out a barrier will still allow air movement, the metal plate acts has an air stop and also holds the insulation in place, it doesnt need to be 100% sealed but 90% (figuring all the exposed edges added together) will make a big difference with air seepage.

 

[Kominar]

After much delay, the insert is finally in and keeping us warm as I learn to optimize its performance (reading many threads on this forum - thanks again for so much advice!)
I thought I'd add the story ending here:

I stuffed that firebox cavity that had been created by taking out the refractory brick (and exposing cold, exterior-chimney bricks) with ~4" of rock wool, then covered the entire back wall with a rectangle of 18-gauge galvanized steel that's almost touching the back of the insert. I also added a two-piece, makeshift version of a block-off plate above the insert. The pieces are not fastened to the firebox walls, just sitting at a slight angle on the rims where the firebox bricks meet the damper frame, with half-circle cutouts in each piece for the flex liner. Not 100% sealed, maybe not even 90%, but fairly snug at the walls without any drilling, and holding clumps of rock wool on top of them. And finally more rock wool stuffed in and above the damper frame around the flex liner. So between these two imperfect barriers the heat lost to the chimney should be fairly minimal.

The staff from the company who installed the insert was quite accommodating with me while I stopped their work to put in these improvements. But the insert was also many weeks overdue (this is another, much less satisfying story!), so I felt entitled to some patience on their part...

 

[NoGoodAtScreenNames]

A forum rule here is that if you mention building your own block off plate you must show a pic. Just saying... wouldn’t want you to get banned or anything.

 

[kennyp2339]

If you sell the house, you should put a tag on the fireplace that says modified since the refractory brick has been removed.

 

[Caw]

A forum rule here is that if you mention building your own block off plate you must show a pic. Just saying... wouldn’t want you to get banned or anything.

Pics or it didn't happen! That is the rule of the internet!

 

[Kominar]

Oh man... didn't take any pics... and there's NO WAY I can now!
Good point about the modification tag (although we have no plans of selling/moving for a couple of decades).

A great early March day here in NB to be working on my wood burning skills - 0F, with -24F windchill...

 

[NoGoodAtScreenNames]

The second forum rule is that when posting a picture of your crappy looking DIY block off plate you google a better looking one and pass it off as your own. Same goes for fire wood sheds and dogs laying down in front of the fire.

 

[Caw]

"This is all natural baby. Soak it in"
- Gordon