Firebox requirements for hearth mount "insert"

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krazyderek

New Member
Sep 29, 2010
6
Dartmouth, NS
I got a used hearth mount "insert" which basically sits completely outside of the firebox on the hearth.

I'm doing the install myself, and the inspector gave me some directions for removing the damper to make some clearance for the liner that will attach to the insert. While i was removing the damper some of the fire box bricks were a bit lose, once i got the damper out i reached in and i could literally just pick up a quarter of the firebox and took it apart the mortar between the bricks had just turned to sand.

Is a firebox required for a hearth mount fireplace insert? The insert is a bit old so i'm not sure if i can get specs on it beyond what's on the back plate. The only part of the insert that enters the fireplace is the vent off the back of the insert. I'm not opposed to rebuilding the lose parts of the firebox, but i also don't want to create more work for myself if there's no real point since the insert is there for good and it will never be an open fireplace again.
 

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What's behind the area where the bricks & mortar came out?
Is it the outside of the masonry?
If so, you won't hafta do anything but block off the area
around the liner where it passes thru the damper, & insulate under the cap.
If it's an interior chimney, & there are some combustible building materials
behind it (studs, plywood, lath, sheetrock), you hafta ensure that you maintain
the clearances specified in NFPA-211...
 
there's a small 1/2" space and then it's right on exterior brick. I already have the damper out, so you're saying i should rebuild the firebox? and try and cap it around the liner once i'm done?
 
krazyderek said:
there's a small 1/2" space and then it's right on exterior brick. I already have the damper out, so you're saying i should rebuild the firebox? and try and cap it around the liner once i'm done?

No. If there's nothing behind the fireplace that casn burn, you should be good to go.
Insulate around the liner in BOTH the damper area & under the cap.
That will create a column of dead air which is the same as insulation...
Use rockwool or Kaowool of unfaced fiberglass insulation.
Folks will argue against that last material, but if it was dangerous,
it wouldn't be used for wood stove gasketing...
 
My fireplace is in the same shape, I pulled the old insert out and who ever installed the old insert already took the back of the firebox out. My fireplace is an exterior one as well. I sent an email to Osburn and they told me it should be fine but they cannot determine if it is safe for insurance or liability purposes and to have a fireplace technician inspect it, the last half of that sentence is CYA IMO. I then called the only two masons around here and both said it will be fine since the insert will not get hot enough to damage the concrete block and brick and because it is an exterior fireplace.
 
thanks guys, i talked to my inspector and he agree'd that i would have to insulate behind the insert if i didn't put the brick's back to reflect the heat.

I bought some insulation called JUSI (universal shielding insulation) made by selkirk canada, tested to CAN/ULC -s604 610 629 UL103 type HT and UL127, but the installation instructions talk about installing the insulation between the existing (or insulation sheild) and the insulated chimney length.

Problem is my clay liner only has room for a 5" flex liner, so is this JUSI insulation good enough to touch an uninsulated liner? I've only checked one hardware store so far, just wanted a bit more info on the insulation before i put it in there.
 
That firebox looks exactly like one I had when I lived off of Topsail Rd in Dartmouth. The bricks were all quite loose. Never used it. When my neighbor tore his fireplace out, we found charred timbers because it had been so poorly built. But I digress

The rules will say that you can install an insert into a code compliant fireplace. That said, on an outside wall, all you are dealing with is heat loss, so not a huge deal. You might as well put them back for all the effort it will take.

You don't happen to live in Woodlawn do you?
 
yup, two streets behind tacoma sobey's

Don't suppose you know a good place to get insulation that's safe to touch a non insulated flex liner around dartmouth?
 
The only insulation thats safe to touch an uninsulated liner would be liner insulation :)

Other than that you can get Roxul AFB from Home Depot. It is rock wool. It has some binders in it that will stink when they get heated the first few times, but is otherwise used a lot to block airflow around liners. Not my preference, but it works.

Have a real good look above where the damper was to see if there is any wood exposed. Like I said, the hearth design looks identical, down to the brick color and pattern, as the one we tore down. It essentially didn't have a smoke shelf, and was just a bricked in box with a damper on it. Soot and ashes would fall into the area behind the firebrick and burn there.

I assume this is a single storey house. With a 5" liner you are going to loose lots of draft vice a six inch. 5" has 2/3 the flow of a 6". Given that you are down in a bit of a valley there, you may have smoke spillage issues.
 
5" is the max i can get in my old clay liner unfortunetly :( it's one of those 8" x 5 3/4" rectangle ones.

I'll have another look to make sure there's no wood, but it's like you described, a brick box with a damper on the firebox, and empty space behind the firebox.
 
5" isn't your max, it's just your max round liner. You can get a 6" liner and have it ovalized to fit - essentially squished thru rollers to give it a oval shape. Or you can order an oval liner to start with.

I had a 5" liner on my insert for a year and had smoke spillage problems all year. The extra inch makes a big difference.
 
i was looking at the homesaver ovalflex liners, but they're 3 times as much as what i got and for a used woodstove that i'm only planning on burning in the evenings of the coldest months

the small oval flex liner is rated for 40.22 sqft so it would do a much better job then a standard 5" round, but what are the actual dimensions of the small ovalflex? i can't find the info anywhere on efireplacestore.com even if i did want to buy one
 
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