Chimney condemned; wood stove conversion question

Burdman44

Hello all, a professional chimney service recently condemned the chimney for my wood burning fireplace in my 1930s era house. There is a missing mortar joint between the terracotta tiles near the top of the 27ft chimney and a small amount of glazed creosote in said joint.

With money being tight, I'm wondering if I can safely use this chimney with a small wood stove in the original firebox if I cover the stove's exhaust pipe with a fine high heat screen to prevent any embers going up the chimney.

In my reading, I've found that wood stove exhaust temps range from 300F to 600F (mine would be very small so probably towards the lower end of that scale), creosote has a flash point 165F with an ignition source (spark or flame), and an auto-ignition point of 451F (spontaneous ignition point without external ignition source; ie without spark or flame).

I'm obviously no professional, but it seems to me that the screen would prevent the flash point danger, and there would be no way the exhaust/smoke temps would be anywhere near that 451F auto-ignition point by the time it got all the way to the top of the 3 story tall chimney.

The hearth, smoke chamber and damper are all in good shape and the area around the firebox is otherwise safe for burning. Also worth nothing that last year a different chimney service told me everything was fine, so I burned tons of firewood in the normal open fireplace all winter without issue. A little scary considering the damage to the chimney definitely did not happen over the course of one year, and this new chimney service tells me this chimney is a severe fire risk (yes, they are fully licensed and came highly recommended by everyone in town; 40+ years in business, etc).

All that said, what do you guys think? Safe to set up the small wood stove with the screen? I cannot afford the $700+ stainless steel liner that would otherwise be required to make the chimney safe, so it's really my only option. Thanks in advance.

Big Ed_4

I would get a second opinion ….

Jamie Hall

I would definitely get a second opinion on that chimney.

But no, just putting a screen over the exhaust from a wood stove isn't going to fly. That is NOT a safe option.

Burdman44

Thank you both for your responses. In an effort to avoid another inspection fee ($300 for the last one), I have asked 2 other chimney services who are business friends of my tree company about the bad mortar joint. Both told me that bad mortar joints are the #1 cause of chimney fires as they collect creosote. One ember gets in there and it could easily light up. Both said they didn't even need to see the pictures - if there's a bad joint, it's dangerous to use the chimney in all cases. I know chimney guys are often found to be using scare tactics to generate repair business for themselves, but both of these guys knew ahead of time I wouldn't be fixing the chimney, and therefore had no monetary incentive to give me bad information.

That said, the missing joint is several feet above the highest roof of the house and thus is not next to/near a wall or anything else flammable, so if it were to light up, I'm not sure it would be "dangerous" anyway.

Jamie, could you expand on why you think the screen wouldn't be safe? Also, is there any other cost effective option you're aware of in lieu of or in combination with the screen that would be safe?

Thanks again.

Bob Harper

No, no, and No! The chimney must be suitable for the class of service. Even a brand new masonry chimney built perfectly to code (never seen one) would need the ss liner in this case just for sizing. As soon as you realize you have combustibles touching the chimney, it would need an insulated ss liner.

Your flue tiles are worn, have staggered joints, missing mortar, spalling and flaking. etc.

Once creosote gets out of the flue into the interstitial space between the tile and the outer wythe wall you cannot remove it. You must tear the chimney down and rebuild.

Even regular chimney caps with a 3/4" diamond mesh clog up from wood burning and you want to put a fine screen right over stovepipe? BTW, you cannot stuff a little stovepipe up the chimney and hope it finds its way out.

FYI, a proper ss insulated liner must be installed to meet its 2,100F rating at zero clearance btw the outer chimney and combustibles.

Refer to NFPA 211 for full details. You can read it online for free at NFPA.org

Jamie Hall

I can see the point about the bad mortar joint. What isn't obvious is that if there is significant creosote accumulation in the rest of the chimney, if it ignites at the bad joint the fire will propagate down the chimney as well as up. One would like to think not, but… it will. And a chimney fire is no joke at all.

The problem with the screen is two fold, and paradoxical. First, while it would stop most embers, in the event of a really hot fire in the stove, for whatever reason (who put that piece of pine in there????), the gas passing through may be hot enough to ignite creosote further up the chimney. Second, paradoxically, the screen itself may accumulate enough creosote if it is not really heated up regularly and, itself, form a nice ignition source when a hot ember catches on it — and an ignition source with a nice built-in flame holder to stabilise the flame while it gets organized.

I really think, though, that you should contact a few stove or insert companies and enquire about the cost of a full length stainless steel liner connected directly to the stove or insert. It may be possible — that chimney looked pretty straight — and may not be prohibitively expensive. It's quite a common repair for older chimneys and, if the chimney itself is reasonably structurally sound, doesn't require demolition of the old chimney.

Burdman44

Thanks Bob and Jamie! Glad I asked people who know what they're talking about! Both of your responses make sense and I will definitely not be doing the screen and stove thing. Also thanks Bob for confirming the chimney itself is shot - good to know I'm not getting scammed into paying for things that aren't necessary. Guess I'll just save my pennies and see about a wood stove conversion with the full length insulated liner for next year. Not worth any risk whatsoever in this matchbox of a house, so thanks again for helping me avoid burning it down.

…now if only I could get that accountant who's never touched a chainsaw in his life to be similarly humble enough to let me decide how best to bring down the dangerous tree over his house instead of insisting I do it "his way", lol. Happy holidays gentlemen :-)