Cracked 8” thimble
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Demo and rebuild is not an option. A liner is the only feasible solution. I understand your concern. I don’t know how many chimneys were build with previous code…thousands? Millions? Are those homes in grave danger for an inch difference in clearance?
The codes are changed for a variety of reasons but I don’t have a book on why they were changed. Safety is the driving force in most building codes. I suppose tests were done and it was determined adding another inch would be better. But realistically, in a raging chimney fire will one inch really make any difference?
In Maryland, every new home is required to have a sprinkler system. I think a lot of people would agree that is a bit overkill.
Is my 1986 chimney unsafe because it has one less inch of clearance?
That code has not changed the 1" of clearance for external chimneys and 2" for internal has been required since the first codes concerning chimneys was made in the late 70s early 80s. And yes that clearance will make a huge difference. Testing has proven that. Now that doesn't by any means say that your house is going to burn burn down but it isn't as safe as it could be. Plus it will work far better with a properly sized and insulated liner.
And yes the vast majority of masonry chimneys are still built without the required clearances
I could check mine by cutting out the interior drywall but if it was built in 1986 it should have the 2” if this code was enforced in 1986.
Home construction in my rural county in 1986 was not booming to the point that inspectors would allow things like this to be missed but I suppose it’s possible. I know the fire marshal and could get data on chimney fires since 1980 with the causes and investigation results. It would be interesting to see if there were code violations.
Most inspectors don't catch it now so I don't see why they would have then. Out of the hundreds of chimneys I inspection and work on in a year I see 2 or 3 that have the required clearance
The vast majority of structure fires related to chimneys involve the wall passthrough area. They are almost never done right. But there are some caused by heat transfer through the masonry. Usually that happens after many years of pyrolysis lowering the kindling point of the adjacent combustibles.
When I had it cranking yesterday I used my pizza oven infrared thermometer and shot the interior wall where the chimney passes through as well as an exposed section on an unfinished garage wall. All temps were within a few degrees of ambient air temp. I’m not sure this is a scientific method. I just have this thermometer and used it this way as a test.
Ok what happens when there is a 2100 degree chimney fire inside?
I honestly think chances of a fire spreading from your chimney is fairly low. But it's not as safe as it could be. And it's isn't going to work anywhere near as well as it should.
brenndatomu
Minister of Fire
Normal operation is one thing, but what would those wall temps be during a chimney fire? That's where the required clearances come into play...it keeps any adjacent wood framing from pyrolysis during normal operation, and then if a chimney fire ever happens, it can keep the fire to just the chimney, not advancing into the structure through improper clearances.
I am never away from my stove long enough for it to get out of control. It is constantly monitored and I glance at the pipe temp often. If my pipe temp hold at 300° how hot is the flue gas? I had a chimney fire in 1995 in my first home (built in 1994). I had stuffed my Englander stove with wood and went to bed. I woke up hearing the roar. I went outside and saw the flames coming out of the chimney. I called 911 but was able to extinguish the fire before the FD got there. That was a scary night for me, my wife, and four children. That stove was used to heat the whole house and we continued to use it afterwards but from that day on I was extra vigilant and never took risks.
I see your point, a car with a seat belt is safe but why not upgrade to one with airbags and make your car even safer?
At 300 on the surface of single wall pipe internally it will be roughly 600. But will drop drastically when it expands into the 8x12 uninsulated liner. That really increases the risk of creosote buildup
That depends on moisture content and temps inside the chimney. It can be as short as a few weeks.
Isn’t the point of a hot start up to eliminate minor build ups from the previous burn?
No. That is to minimize buildup from the current burn. Once there is buildup in the chimney the only way to eliminate it is by cleaning. If you are removing it with the hot burn that's called a chimney fire
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