Masonry outer chimney and fire safety

[electrathon]

I am curious about masonry chimneys and the danger during a fire. My questions all are revolving around worst case. If I have a existing chimney that has a clay liner it is code and allowed to vent a stove directly into it. In the event of a chimney fire the overheat of the chimney can crack the clay liner and potentially pass into the house and burn the house. It is commonly recommended here to remove the clay liner and drop in a stainless insulated liner. Out of control fire, liner failure and one layer of brick till the fire passes into the structure. If the liner was not removed the structure would effectively have a second layer of defense protecting the house from burning. It really seems from a fire safety direction the extra lining would really help in safety. I am not referring to performance here, I am talking safety. A lot of performance is sacrificed in in the wood stove world for safety (example is heat robbers on a chimney) so I am wondering the safety arguments compared to the performance arguments of the second liner.

 

[pen]

If there is room for an insulated liner without removing the tiles, I'm not sure I would see the point in removing them unless they were caked with difficult to remove glazed creosote?

I think the safety in a SS liner comes in several ways. 1. A good many masonry chimney systems are installed incorrectly and wouldn't handle a fire as intended even if the terra cotta were in good shape, so the SS and or insulated SS simply increases the safety. 2. The SS liner, especially if insulated keeps the flue warmer and reduces the chance of getting the buildup that would cause a chimney fire in the first place.

 

[Blazing]

I have my clay tiles which were in great shape as well as an insulated liner. I definitely see it as double the protection from fire. That said if I couldn't fit the insulation because of the tiles they would have been removed in favor of insulation.

 

[electrathon]

I am not advocating removing the tiles and then a non-insulated liner. My question revolves around the removing of the tile so that insulation will fit. If I pull out the tile and insulate, is the total safety a step lower than if I leave the tile and then use a non-insulated liner? Again, this is a safety over performance question. There are a lot of things that will improve performance that would be looked down on in the spirit of safety.

 

[VirginiaIron]

Imo: I think it is a safety issue FIRST. I would imagine there is concern about not removing all the creosote that remains in cracked tiles or deteriorated seams, however little. So, a non insulated liner, in theory, will accumulate more creosote. More creosote means a greater chance of fire in the liner that is tight to the, possibly, creosote contaminated clay liner. The outer insulation on my install was 116*F when the stove was near 600 and the connector pipe was 400+.

 

[yooper08]

I wouldn't say it's commonly recommended to remove the existing clay liner. That's only recommended in the case where an insulated liner wouldn't fit otherwise.

That said, insulation is used to protect the house from a non-code chimney, which includes not having a clay liner. The absolute worst case scenario without a clay liner is the stainless steel liner fails in a chimney fire, exposing the chimney itself to the chimney fire. The UL testing of liners is such that the liner can withstand the chimney fire, and with insulation, keep combustible materials safe.

If you have the option, it is much safer to knock out the tile and put an insulated liner down than to keep the tile with an non-insulated liner.

 

[bholler]

I am not advocating removing the tiles and then a non-insulated liner. My question revolves around the removing of the tile so that insulation will fit. If I pull out the tile and insulate, is the total safety a step lower than if I leave the tile and then use a non-insulated liner? Again, this is a safety over performance question. There are a lot of things that will improve performance that would be looked down on in the spirit of safety.

No insulation will without a doubt improve the safety. The danger of a chimney fire spreading to the house is almost never from fire actually making its way through the masonry. The danger is from heat transfer through that masonry. A clay tile has a negligible r value so it does next to nothing to stop that heat transfer. Now if there is room for an insulated liner inside that clay and the clay is not glazed to the point it is easier to remove than clean then there is no reason to remove it.

 

[bholler]

And the performance and safety are not completly seperate issues. A chimney that is insulated will draft better and stay warmer leading to less chance of creosote. Without creosote there can be no chimney fire.

 

[electrathon]

So why is it safer? The clay tile that is being removed is much more durable than the thin layer of insulation. You are removing a very durable very tough safety layer in order to put in a thin layer of insulation. If there is no tile, absolutely insulation is better. With tile it would be two separate layers of safety instead of one.

 

[yooper08]

The tile doesn't protect against heat transfer, therefore doesn't provide any safety net to the house. That's the real danger in a chimney fire, not the flames themselves. If combustibles surrounding the chimney structure get too hot, they will combust, flames or no flames.

 

[electrathon]

No insulation will without a doubt improve the safety. The danger of a chimney fire spreading to the house is almost never from fire actually making its way through the masonry. The danger is from heat transfer through that masonry. A clay tile has a negligible r value so it does next to nothing to stop that heat transfer. Now if there is room for an insulated liner inside that clay and the clay is not glazed to the point it is easier to remove than clean then there is no reason to remove it.

OK, that is what I thought. It is safer to leave it. The air gap around a non insulated liner to the clay liner would make a great safety layer. You must loose the first layer of stainless, then loose a second clay layer that by itself is a legal separation layer. Effectively two safety layers instead of one. Like double wall pipe compared to triple wall pipe. Also the trapped in air between the clay and the stainless would be a great insulation layer.

 

[electrathon]

And the performance and safety are not completely separate issues. A chimney that is insulated will draft better and stay warmer leading to less chance of creosote. Without creosote there can be no chimney fire.

Yes, this is the balance between efficiency and safety. Sort of like why people buy economy cars and trust their family's safety to it instead of buying a Suburban.

I do wonder if the thin trapped air layer between the pipe and the clay has a similar effect of total r value as the insulation does in the large open masonry cavity after the clay tiles are pulled. Very possibly even more.

 

[electrathon]

The tile doesn't protect against heat transfer, therefore doesn't provide any safety net to the house. That's the real danger in a chimney fire, not the flames themselves. If combustibles surrounding the chimney structure get too hot, they will combust, flames or no flames.

I question this statement. If they have no R value why are they installed in the first place? They are still used in new construction and a chimney meets code with them and does not if you build a chimney with no liner.

 

[bholler]

OK, that is what I thought. It is safer to leave it. The air gap around a non insulated liner to the clay liner would make a great safety layer. You must loose the first layer of stainless, then loose a second clay layer that by itself is a legal separation layer. Effectively two safety layers instead of one. Like double wall pipe compared to triple wall pipe. Also the trapped in air between the clay and the stainless would be a great insulation layer.

That is not at all what i said. That clay offers no safety adgantage at all and if it is left in place in lieu of insulation it is a big step down in safety. And in double and triple wall pipe those extra layers have absolutly nothing to do with protecting from burn through. They are there to block radiant heat and protect from heat transfer.

 

[yooper08]

I question this statement. If they have no R value why are they installed in the first place? They are still used in new construction and a chimney meets code with them and does not if you build a chimney with no liner.

I question your statement. Who says chimneys are built to code? Builders and inspectors that truly understand the code are few and far between, hence why the vast majority of installs you see here that are recommended to have insulated liners.

OK, that is what I thought. It is safer to leave it. The air gap around a non insulated liner to the clay liner would make a great safety layer. You must loose the first layer of stainless, then loose a second clay layer that by itself is a legal separation layer. Effectively two safety layers instead of one. Like double wall pipe compared to triple wall pipe. Also the trapped in air between the clay and the stainless would be a great insulation layer.

I think you misunderstood. bholler is advocating for insulated liner.

I question your logic of air gaps and material strength while completely ignoring the fact of r value and the more serious danger of heat transfer, which has been mentioned at least twice.

Trust us, insulate it. Seriously. The science and testing is on our side.

 

[bholler]

Yes, this is the balance between efficiency and safety. Sort of like why people buy economy cars and trust their family's safety to it instead of buying a Suburban.

I do wonder if the thin trapped air layer between the pipe and the clay has a similar effect of total r value as the insulation does in the large open masonry cavity after the clay tiles are pulled. Very possibly even more.

No that air gap does not have close to the same r value because it is free to move through the cavity and is trapped at the top. The reason air cooled pipe works is because that air is allowed to excape at the top carrying the heat with it. If it cant escape but is allowed to circulate in the cavity it offers little protection. And for the record many smaller cars are much safer than big suvs.

 

[bholler]

I question this statement. If they have no R value why are they installed in the first place? They are still used in new construction and a chimney meets code with them and does not if you build a chimney with no liner.

Yes a chimney can be built to code with clay. But it needs the proper clearances between the outside of the chimney and combustible materials because of that lack of r value. With an insulated liner you dont need that clearance because the insulation slows the heat transfer enough to make it unnecessary. And from years of experience in the feild i can tell you very few chimney are built with proper clearances even when inspected and passed.

 

[electrathon]

I question your statement. Who says chimneys are built to code? Builders and inspectors that truly understand the code are few and far between, hence why the vast majority of installs you see here that are recommended to have insulated liners.

I think you misunderstood. bholler is advocating for insulated liner.

I question your logic of air gaps and material strength while completely ignoring the fact of r value and the more serious danger of heat transfer, which has been mentioned at least twice.

Trust us, insulate it. Seriously. The science and testing is on our side.

Are you saying that modern clay lined chimneys do not meet current building code? It sounds to me like you are. I don't quite know how to respond to that. I know that it is recommended here that stainless is the thing to use, that does not mean that it is code required.

I know bholler is advocating for insulation. I brought up the question of safety of two layers of fire protection verses one layer.

 

[bholler]

Are you saying that modern clay lined chimneys do not meet current building code? It sounds to me like you are. I don't quite know how to respond to that. I know that it is recommended here that stainless is the thing to use, that does not mean that it is code required.

I know bholler is advocating for insulation. I brought up the question of safety of two layers of fire protection verses one layer.

He is not saying that clay lined chimneys cant meet code he is saying that most do not because they in many cases are not built with the required clearances

 

[bholler]

And again the danger from a chimney fire spreading to the house is not from burn through it is from heat transfer which is not reduced markedly by clay liners

 

[electrathon]

No that air gap does not have close to the same r value because it is free to move through the cavity and is trapped at the top. The reason air cooled pipe works is because that air is allowed to escape at the top carrying the heat with it. If it cant escape but is allowed to circulate in the cavity it offers little protection. And for the record many smaller cars are much safer than big suvs.

Insulation is the opposite of air cooling. The trapped air (lets call it a 1" cavity) becomes heated without any insulation inside. The warm trapped air would help keep creosote accumulation low. insulation is all about trapped air.
There is a reason why small car crash standards are judged against cars of similar sizes and not small car compared to big cars. I am very familiar with the issues of trucks being rated on a different scale (actually a lower scale because they have frames and frames are dangerous because they do not collapse the came as uni bodies do).

 

[bholler]

Insulation is the opposite of air cooling. The trapped air (lets call it a 1" cavity) becomes heated without any insulation inside. The warm trapped air would help keep creosote accumulation low. insulation is all about trapped air.
There is a reason why small car crash standards are judged against cars of similar sizes and not small car compared to big cars. I am very familiar with the issues of trucks being rated on a different scale (actually a lower scale because they have frames and frames are dangerous because they do not collapse the came as uni bodies do).

Yes that trapped air will increase performance but the problem is it is not really trapped it is allowed to circulate withon that space transfering the heat from the liner to the masonry. There is also no radiant barrier so the heat radiates off the liner directly into the masonry. This all leads to allot of heat transfer. You also have to consider that there is no way to be sure that liner does not touch the sides of the masonry anyway. Which gives you direct heat transfer. Your thinking is wrong and that has been proven by the tasting done in the industry.

 

[electrathon]

And again the danger from a chimney fire spreading to the house is not from burn through it is from heat transfer which is not reduced markedly by clay liners

OK, I can buy that. I do often see when people have had a fire they are told to have the chimney inspected for failure (I am assuming that means burn through) before it is used again. If it rarely happens maybe it is a little overstated. I was reading about fires and potential pipe failure is virtually always stated, it seemed if it was such a big issue a second safety layer would be a good idea.

 

[electrathon]

Yes that trapped air will increase performance but the problem is it is not really trapped it is allowed to circulate withon that space transfering the heat from the liner to the masonry. There is also no radiant barrier so the heat radiates off the liner directly into the masonry. This all leads to allot of heat transfer. You also have to consider that there is no way to be sure that liner does not touch the sides of the masonry anyway. Which gives you direct heat transfer. Your thinking is wrong and that has been proven by the tasting done in the industry.

Insulation that is squished has very little r value too. If the pipe presses against the side of the raceway there is minimal r value at that point, allowing for heat transfer.

 

[bholler]

OK, I can buy that. I do often see when people have had a fire they are told to have the chimney inspected for failure (I am assuming that means burn through) before it is used again. If it rarely happens maybe it is a little overstated. I was reading about fires and potential pipe failure is virtually always stated, it seemed if it was such a big issue a second safety layer would be a good idea.

Yes pipe failure is possible but it would then have to go through 4" of solid masonry. That does not happen. Now if the masonry was already comprimised it could happen but in that case the chimney should not have been lined anyway. That is why you need a full inspection before a liner. And the reason to inspect for pipe failure after a fire is so you are not letting the products of combustion out of that liner which could lead to a fire outside of that liner. Which i have seen and it can be very dangerous and hard to put out.