Pulled the trigger on a new stove-- time to replace the flexliner? (photos)

[Whirled Peas]

After a million years of waffling around and spending my woodstove budget on other parts of the house, I finally got around to ordering a new stove (WS Absolute Steel-- thread to come). Now my issue is figuring out the details of remodeling my hearth and determining the health of my flue liner.

- I believe the flue liner is ~20 years old (to match the double door F400). It is uninsulated and runs ~24' from the cleanout to the chimney cap. For many of those years, it was cleaned from the top via metal brush and I've recently starting cleaning it from the bottom via a poly Sooteater. From what I can see when I clean it out, the liner appears to be in ok shape to my uneducated self. What kind of life should I be expecting from a liner? If it doesn't show signs of breaking down, is it still prudent to replace the liner for safety reasons based on age? Would installing a new, insulated liner give me a noticeable performance benefit over my existing set-up? I've attached a photo of the Tee to try to capture what looks like is maybe corrosion?

- The existing set up hasn't been up to snuff in regards to hearth floor fire protection and is definitely a project I'm relieved to be tackling. My plan here is to remove the square tiles and another ~20" of wood flooring and re-build with cement board and then brick tiles. Anyone see anything else I should be considering?

The AS will have a left-hand door and be rear vented.

 

[clancey]

I am no expert with all of this but I will give you my opinion---anyway----lol.. I would get a new insulated liner and I am so glad you are going to fix those rotten bricks as well...Safety First I say...Heck new stove---go all the way for safety or get one of those things that you can look up into the pipe with and see the condition of it...My I almost got killed by carbon dioxide so get a new liner one that you can clean better than getting up on a roof too...Mrs. clancey

 

[bholler]

After a million years of waffling around and spending my woodstove budget on other parts of the house, I finally got around to ordering a new stove (WS Absolute Steel-- thread to come). Now my issue is figuring out the details of remodeling my hearth and determining the health of my flue liner.

- I believe the flue liner is ~20 years old (to match the double door F400). It is uninsulated and runs ~24' from the cleanout to the chimney cap. For many of those years, it was cleaned from the top via metal brush and I've recently starting cleaning it from the bottom via a poly Sooteater. From what I can see when I clean it out, the liner appears to be in ok shape to my uneducated self. What kind of life should I be expecting from a liner? If it doesn't show signs of breaking down, is it still prudent to replace the liner for safety reasons based on age? Would installing a new, insulated liner give me a noticeable performance benefit over my existing set-up? I've attached a photo of the Tee to try to capture what looks like is maybe corrosion?

- The existing set up hasn't been up to snuff in regards to hearth floor fire protection and is definitely a project I'm relieved to be tackling. My plan here is to remove the square tiles and another ~20" of wood flooring and re-build with cement board and then brick tiles. Anyone see anything else I should be considering?

The AS will have a left-hand door and be rear vented.

View attachment 277849 View attachment 277850

A heavy wall liner like yours should last 30 years unless abused. Cleaning with a steel brush may have taken a few years off the life though. So if it was insulated I would say have it inspected and it will probably be perfectly fine. But it should really be insulated. If it was newer I would say yank it out insulate it and drop it back in. But I am not sure I would reinstall a liner that old.

 

[CharlieTuna]

After a million years of waffling around and spending my woodstove budget on other parts of the house, I finally got around to ordering a new stove (WS Absolute Steel-- thread to come). Now my issue is figuring out the details of remodeling my hearth and determining the health of my flue liner.

- I believe the flue liner is ~20 years old (to match the double door F400). It is uninsulated and runs ~24' from the cleanout to the chimney cap. For many of those years, it was cleaned from the top via metal brush and I've recently starting cleaning it from the bottom via a poly Sooteater. From what I can see when I clean it out, the liner appears to be in ok shape to my uneducated self. What kind of life should I be expecting from a liner? If it doesn't show signs of breaking down, is it still prudent to replace the liner for safety reasons based on age? Would installing a new, insulated liner give me a noticeable performance benefit over my existing set-up? I've attached a photo of the Tee to try to capture what looks like is maybe corrosion?

- The existing set up hasn't been up to snuff in regards to hearth floor fire protection and is definitely a project I'm relieved to be tackling. My plan here is to remove the square tiles and another ~20" of wood flooring and re-build with cement board and then brick tiles. Anyone see anything else I should be considering?

The AS will have a left-hand door and be rear vented.

View attachment 277849 View attachment 277850

I just completed the project of replacing an insert. I can't see from your pics if you have a register plate. Make sure you have a register plate and that it is well insulated. I used Rockwool insulation and a lot of aluminum sheet metal. Aluminum is a better choice than steel because it reflects 95% of radiant heat. Also, if galvanized steel gets too hot, the zinc coating turns into a poisonous gas. I don't know how likely that is to happen, but why take the chance? Because the fireplace is in the basement on an outside wall, I put aluminum sheet medal all around the back and floor of the fireplace to keep as much heat as possible in the room. I put the stove up on three bricks to further reduce heat loss to the floor, which in the basement is a heat sink.

Is the chimney liner steel or stainless steel? Regular steel rusts, stainless steel has a longer life. Unless it is heat damaged somehow, I don't know what else would shorten the life of stainless steel. Give it a good visual inspection to look for damage or holes. If it looks good, I can't think of a reason to replace it. Since its inside the chimney, why does it need to be insulated? (I assume the chimney is masonry.) Unless that is required to meet some code requirement or fire safety reasons, I don't see the point of it.

 

[begreen]

Register plate meaning block-off plate in the damper area, correct?

 

[CharlieTuna]

Yes, there may be a technical meaning, but just make sure something keeps all the heat from going up the chimney.

 

[Whirled Peas]

The install does have a a steel block off plate a few inches above the bottom of the lintel. I'm unsure of what's above it. When I rap on it with my knuckles, I'm still undecided on if it feels like there's insulation sitting on it or not. I would guess not.

It's (STILL!) snowing here in southern Vermont but as soon as the roof dries I'll climb back up there with some tools and take a few more photos of the chimney and liner with the stove removed. The liner is stainless and was painted black in the area below the lintel.

For what it's worth, the chimney is interior and climbs ~9 feet but then runs the remaining 15' through an unconditioned attic. I had the chimney brick above the roof line rebuilt last summer. It's lined with a rectangular terracotta liner and the mason who redid the bricks gave that a thumbs up at that time. I think I'm leaning towards ripping it out and putting in a new, insulated liner, but I wanted to make sure I wasn't needlessly throwing money at a non-issue. I sometimes get a little... enthusiastic with my projects.

 

[CharlieTuna]

The install does have a a steel block off plate a few inches above the bottom of the lintel. I'm unsure of what's above it. When I rap on it with my knuckles, I'm still undecided on if it feels like there's insulation sitting on it or not. I would guess not.

It's (STILL!) snowing here in southern Vermont but as soon as the roof dries I'll climb back up there with some tools and take a few more photos of the chimney and liner with the stove removed. The liner is stainless and was painted black in the area below the lintel.

For what it's worth, the chimney is interior and climbs ~9 feet but then runs the remaining 15' through an unconditioned attic. I had the chimney brick above the roof line rebuilt last summer. It's lined with a rectangular terracotta liner and the mason who redid the bricks gave that a thumbs up at that time. I think I'm leaning towards ripping it out and putting in a new, insulated liner, but I wanted to make sure I wasn't needlessly throwing money at a non-issue. I sometimes get a little... enthusiastic with my projects.

The insulated pipe keeps more heat inside it and out of the chimney. So your present pipe does a better job of radiating heat to the chimney masonry, and thus into your house. With a masonry chimney, I don't see any safety issue, so no reason to change it. I'd call it throwing money away on a non-issue and decreasing the thermal efficiency of your chimney. I'd take out the steel register plate, check or replace the insulation, and consider adding aluminum when reinstalling. From the stainless steel I've been working with, it's relatively indestructible. Even drilling it resulted in the metal deforming more than getting cut by the bit until it failed from metal fatigue. I don't see the point of replacing it without a good reason.

 

[bholler]

The insulated pipe keeps more heat inside it and out of the chimney. So your present pipe does a better job of radiating heat to the chimney masonry, and thus into your house. With a masonry chimney, I don't see any safety issue, so no reason to change it. I'd call it throwing money away on a non-issue and decreasing the thermal efficiency of your chimney. I'd take out the steel register plate, check or replace the insulation, and consider adding aluminum when reinstalling. From the stainless steel I've been working with, it's relatively indestructible. Even drilling it resulted in the metal deforming more than getting cut by the bit until it failed from metal fatigue. I don't see the point of replacing it without a good reason.

There is absolutely a safety issue unless the chimney has the required 2" clearance to combustibles from the outside of the masonry structure. It is required by code for a very good reason and the only way to make a chimney without that clearance compliant and safe is with an insulated liner. And believe me stainless is not indestructible. I pull out a few failed liners a year.

 

[bholler]

The install does have a a steel block off plate a few inches above the bottom of the lintel. I'm unsure of what's above it. When I rap on it with my knuckles, I'm still undecided on if it feels like there's insulation sitting on it or not. I would guess not.

It's (STILL!) snowing here in southern Vermont but as soon as the roof dries I'll climb back up there with some tools and take a few more photos of the chimney and liner with the stove removed. The liner is stainless and was painted black in the area below the lintel.

For what it's worth, the chimney is interior and climbs ~9 feet but then runs the remaining 15' through an unconditioned attic. I had the chimney brick above the roof line rebuilt last summer. It's lined with a rectangular terracotta liner and the mason who redid the bricks gave that a thumbs up at that time. I think I'm leaning towards ripping it out and putting in a new, insulated liner, but I wanted to make sure I wasn't needlessly throwing money at a non-issue. I sometimes get a little... enthusiastic with my projects.

The only way to know if the liner is in good condition is by having a pro inspect with a camera. You just can't see enough any other way

 

[CharlieTuna]

There is absolutely a safety issue unless the chimney has the required 2" clearance to combustibles from the outside of the masonry structure. It is required by code for a very good reason and the only way to make a chimney without that clearance compliant and safe is with an insulated liner. And believe me stainless is not indestructible. I pull out a few failed liners a year.

Maybe it's changed in some places, but that wasn't the code a few years back when our chimney liner was installed. Other than where it goes through the damper, it doesn't touch the chimney anywhere and has more than 2 inches separating it from the masonry inside the chimney. The liner itself is an improvement over burning straight through the chimney with a "slammer".

So when you remove failed stainless steel liners, how do they fail? Overheating? Metal fatigue? Damage during installation? I don't think that squirrels could chew through them.

 

[bholler]

Maybe it's changed in some places, but that wasn't the code a few years back when our chimney liner was installed. Other than where it goes through the damper, it doesn't touch the chimney anywhere and has more than 2 inches separating it from the masonry inside the chimney. The liner itself is an improvement over burning straight through the chimney with a "slammer".

So when you remove failed stainless steel liners, how do they fail? Overheating? Metal fatigue? Damage during installation? I don't think that squirrels could chew through them.

It has been code in PA since the early 80s.

The liners usually fail due to corrosion. Some are over heated some just burnt out from years of heat. They have a predicted lifespan of between 15 and 30 years depending upon thickness. The liner in question is heavy wall so it is usually a 30 year lifespan. Obviously if it isn't used full time it will last much longer

 

[CharlieTuna]

It has been code in PA since the early 80s.

The liners usually fail due to corrosion. Some are over heated some just burnt out from years of heat. They have a predicted lifespan of between 15 and 30 years depending upon thickness. The liner in question is heavy wall so it is usually a 30 year lifespan. Obviously if it isn't used full time it will last much longer

There are different kinds of stainless steel alloys. Some may last better than others as chimney liners. I am always leery of buying the cheap stuff.

The older houses weren't built to the modern codes. Personally, I'd rather open the walls and replace the wood with metal studs, etc. than decrease the thermal efficiency of the chimney. I did that for a relative. Where I couldn't replace the floor or ceiling joists, I used aluminum as a heat shield. I am all about safety. I thought the insulated chimney liner was only needed in a chimney that wasn't masonry, like the ones the modern builders use made out of wood and covered with siding.

 

[bholler]

There are different kinds of stainless steel alloys. Some may last better than others as chimney liners. I am always leery of buying the cheap stuff.

The older houses weren't built to the modern codes. Personally, I'd rather open the walls and replace the wood with metal studs, etc. than decrease the thermal efficiency of the chimney. I did that for a relative. Where I couldn't replace the floor or ceiling joists, I used aluminum as a heat shield. I am all about safety. I thought the insulated chimney liner was only needed in a chimney that wasn't masonry, like the ones the modern builders use made out of wood and covered with siding.

Yes as a chimney pro I know there are different alloys of stainless liners used for different purposes. But 304 is all that is needed for wood.

Yes old houses were not built to modern codes but if you install a liner in an old chimney you are required to do it to code. And those codes were put in place because many of those old houses caught fire.

Insulation increases the thermal efficency of a chimney. A chimneys job is to safely carry the products of combustion out of the house as fast as possible while loosing as little heat as possible. You should be heating with the stove not the pipe or chimney.

Liners insulated or not are only for use in masonry chimneys. In the case you are referring to you need to use prefab chimney pipe not a liner

 

[CharlieTuna]

It has been code in PA since the early 80s.

I had to look this up because we first got an insert in the fireplace in 1980. It didn't have a liner. It was just a "slammer". We had a chimney sweep clean the chimney once a year, and not the same one every year. Then in 2000 we got another insert which was installed by the local Vermont Castings dealer. It was also a "slammer" with no chimney liner. Again we had a chimney sweep come once a year to clean the chimney, and not the same one every year. Sometime after that, we had the top half of the chimney rebuilt. Not a single person in all of this time ever said that we needed an insulated chimney liner. Eventually, the bottom of the chimney cracked, and instead of rebuilding the entire chimney, we just put a chimney liner in to save on the cost. As far as I can see, it isn't insulated. So for you to be right, all of these people had to be wrong, and they missed the opportunity to make more money.

Now according to the PA Dept. of Labor and Industry's website,
"Chapter 401 (Uniform Construction Code Training and Certification of Code Administrators) was first approved in April 2002 and took effect on July 12, 2002. This chapter was amended and took effect on October 1, 2018. "

Department of Labor and Industry | Department of Labor and Industry | Commonwealth of Pennsylvania

The Department of Labor & Industry (DLI) administers benefits to unemployed individuals, oversees the administration of workers' compensation benefits to individuals with job-related injuries, and provides vocational rehabilitation to individuals with disabilities.

www.dli.pa.gov

I did try to actually read the code, but it is proprietary and not available online. The matter of copyright protection for public laws and regulations has not been resolved in the courts. What this means is that the average homeowner needs to go to the library to know what is really in the code. So, an unscrupulous contractor looking to make more money on a job can BS people easier by saying, "I have to do it this way. It's in the code." Few people will go to the trouble to check.

 

[CharlieTuna]

Insulation increases the thermal efficency of a chimney. A chimneys job is to safely carry the products of combustion out of the house as fast as possible while loosing as little heat as possible. You should be heating with the stove not the pipe or chimney.

I am not the only one who disagrees with this:

Thermal mass is great, but insulation between the fire and the room is bad (and the firebrick is both of those things).

But firebrick bellies pay us back by making secondary combustion work better, and that means more heat from the same fuel!

There's also a whole class of stove that cover the entire firebox with insulation (soapstone), which is a dead loss. People like to talk about them having the same heat output but gentler.... but these are not the kind of people who could explain to you what the difference between thermal capacity and thermal conductivity is. They probably have nice clean chimneys with all the BTUs going up them though!

And then there's heaters where the thermal capacity is the whole point (masonry heaters). You use a small hot fire to warm up a large central mass of stone. It may sound iffy to those of us raised on steel stoves designed to maximize thermal conductivity, but it is still used as primary heat in some very cold places in the world!

How do EPA stoves efficiently release heat to room, despite the insulating bricks?

EPA stoves have insulating bricks around the combustion chamber, theoretically it blocks heat from being released into room, so more heat is kept in flue, but EPA stoves actually have higher fuel efficiency compared to classic wood stoves. Seems contradictory. Anyone could explain to me?

www.hearth.com

 

[jetsam]

I am not the only one who disagrees with this:

I actually agree 100% with what bholler said. (Don't tell him I said so, or he'll be impossible for the rest of the week... )

The insulation is needed to keep the inside surfaces of the flue above creosote condensation temp. It also adds draft, and reduces the risk of fires in structures that have combustibles too close to the chimney (common hazard in older houses with inserts).

With an insulated flue, my stove is a lot more flexible, as I can burn extremely low and still keep the top of the flue above condensation temp. I'd need to run it like a tube stove if my flue wasn't insulated.

Let somebody else process firewood to heat the outdoors with. Mine is for heating the indoors.

 

[MissMac]

actually agree 100% with what bholler said.

this.

 

[bholler]

I had to look this up because we first got an insert in the fireplace in 1980. It didn't have a liner. It was just a "slammer". We had a chimney sweep clean the chimney once a year, and not the same one every year. Then in 2000 we got another insert which was installed by the local Vermont Castings dealer. It was also a "slammer" with no chimney liner. Again we had a chimney sweep come once a year to clean the chimney, and not the same one every year. Sometime after that, we had the top half of the chimney rebuilt. Not a single person in all of this time ever said that we needed an insulated chimney liner. Eventually, the bottom of the chimney cracked, and instead of rebuilding the entire chimney, we just put a chimney liner in to save on the cost. As far as I can see, it isn't insulated. So for you to be right, all of these people had to be wrong, and they missed the opportunity to make more money.

Now according to the PA Dept. of Labor and Industry's website,
"Chapter 401 (Uniform Construction Code Training and Certification of Code Administrators) was first approved in April 2002 and took effect on July 12, 2002. This chapter was amended and took effect on October 1, 2018. "

Department of Labor and Industry | Department of Labor and Industry | Commonwealth of Pennsylvania

The Department of Labor & Industry (DLI) administers benefits to unemployed individuals, oversees the administration of workers' compensation benefits to individuals with job-related injuries, and provides vocational rehabilitation to individuals with disabilities.

www.dli.pa.gov

I did try to actually read the code, but it is proprietary and not available online. The matter of copyright protection for public laws and regulations has not been resolved in the courts. What this means is that the average homeowner needs to go to the library to know what is really in the code. So, an unscrupulous contractor looking to make more money on a job can BS people easier by saying, "I have to do it this way. It's in the code." Few people will go to the trouble to check.

You really think there were no building codes in pa before 2002? Just because a "pro" did something doesn't mean they bothered to follow codes.
Here is the applicable code. Read r1003.18

CHAPTER 10 CHIMNEYS AND FIREPLACES - 2018 INTERNATIONAL RESIDENTIAL CODE (IRC)

ICC Digital Codes is the largest provider of model codes, custom codes and standards used worldwide to construct safe, sustainable, affordable and resilient structures.

codes.iccsafe.org

As far as making more money goes I could make much more money by dropping 2 or 3 bare liners a day than I do doing one a day correctly. There is really not an increased profit margin by going with insulated liners. In many cases it is far more work.

 

[bholler]

I am not the only one who disagrees with this:

How do EPA stoves efficiently release heat to room, despite the insulating bricks?

EPA stoves have insulating bricks around the combustion chamber, theoretically it blocks heat from being released into room, so more heat is kept in flue, but EPA stoves actually have higher fuel efficiency compared to classic wood stoves. Seems contradictory. Anyone could explain to me?

www.hearth.com

No one in that thread is saying that the chimney should be used as a heater. There is mention of masonry heaters but those are very different from a modern stove and a chimney

 

[CharlieTuna]

You really think there were no building codes in pa before 2002? Just because a "pro" did something doesn't mean they bothered to follow codes.
Here is the applicable code. Read r1003.18

CHAPTER 10 CHIMNEYS AND FIREPLACES - 2018 INTERNATIONAL RESIDENTIAL CODE (IRC)

ICC Digital Codes is the largest provider of model codes, custom codes and standards used worldwide to construct safe, sustainable, affordable and resilient structures.

codes.iccsafe.org

As far as making more money goes I could make much more money by dropping 2 or 3 bare liners a day than I do doing one a day correctly. There is really not an increased profit margin by going with insulated liners. In many cases it is far more work.

Of course there were building codes in PA before 2002. They were all local codes, and even today the code is normally enforced and interpreted locally, unless some rural township won't enforce it. Those local codes didn't require what you claim, nor does the present state wide code.

Thanks for the link. I tried doing a search using the link on the PA state site, but kept getting a demand for a subscription to use it. The link proves you are wrong. The diagram doesn't show a metal liner in the chimney. R1003.11.1 Residential-type appliances says that a clay flue liner is acceptable. So, if one has an acceptable clay flue liner, why are you selling people something else? You wouldn't work for me.

 

[bholler]

Of course there were building codes in PA before 2002. They were all local codes, and even today the code is normally enforced and interpreted locally, unless some rural township won't enforce it. Those local codes didn't require what you claim, nor does the present state wide code.

Thanks for the link. I tried doing a search using the link on the PA state site, but kept getting a demand for a subscription to use it. The link proves you are wrong. The diagram doesn't show a metal liner in the chimney. R1003.11.1 Residential-type appliances says that a clay flue liner is acceptable. So, if one has an acceptable clay flue liner, why are you selling people something else? You wouldn't work for me.

Where are you getting your information? Please provide links showing my claims are incorrect. I have spent countless hours in training from industry pros and code officials regarding the codes applicable to the chimney industry.

You do realize that the info you linked to is just about training about codes not adoption of those codes. You really need to do more research if you want to debate this topic.

And yes clay liners are perfectly acceptable if built to code with the required clearances. Unfortunately very few are. As far as working for you that's fine I will only do jobs to the standard required of me by code and what I feel is safe and appropriate. If someone doesn't want that they can find someone else who will do an inferior job for them. It won't be me.

 

[CharlieTuna]

I actually agree 100% with what bholler said. (Don't tell him I said so, or he'll be impossible for the rest of the week... )

The insulation is needed to keep the inside surfaces of the flue above creosote condensation temp. It also adds draft, and reduces the risk of fires in structures that have combustibles too close to the chimney (common hazard in older houses with inserts).

With an insulated flue, my stove is a lot more flexible, as I can burn extremely low and still keep the top of the flue above condensation temp. I'd need to run it like a tube stove if my flue wasn't insulated.

Let somebody else process firewood to heat the outdoors with. Mine is for heating the indoors.

Well please check the actual building code before buying anything from him. It doesn't read as he claims. A clay flue liner in a masonry chimney is perfectly acceptable in PA if done right. No need for a stainless steel liner, and certainly not a more expensive insulated one. That is a belt and suspenders approach, and quite expensive.

I think that you have conflated two different issues, or at least conflicting engineering challenges. One is keeping the exhaust temp high enough to burn off creosote, the other is keeping as much heat as possible in the house. If your firewood is for heating your house and not the outdoors, then you want to keep as much heat in your house as safely possible and not send it all up the chimney. That way you burn less wood. Again, I think it's better to upgrade older houses if they have combustibles too close to a chimney than to just send more heat out the chimney and waste wood. That also isn't environmentally friendly as it results in more greenhouse gases being released into the atmosphere.

 

[bholler]

Well please check the actual building code before buying anything from him. It doesn't read as he claims. A clay flue liner in a masonry chimney is perfectly acceptable in PA if done right. No need for a stainless steel liner, and certainly not a more expensive insulated one. That is a belt and suspenders approach, and quite expensive.

I think that you have conflated two different issues, or at least conflicting engineering challenges. One is keeping the exhaust temp high enough to burn off creosote, the other is keeping as much heat as possible in the house. If your firewood is for heating your house and not the outdoors, then you want to keep as much heat in your house as safely possible and not send it all up the chimney. That way you burn less wood. Again, I think it's better to upgrade older houses if they have combustibles too close to a chimney than to just send more heat out the chimney and waste wood. That also isn't environmentally friendly as it results in more greenhouse gases being released into the atmosphere.

I never said send more heat out the chimney. You should be running your stove with the exhaust temp as low as possible to avoid sending extra heat out the chimney. In the case of this thread the op is going from a tube stove to a cat stove. That means they will be running much lower exhaust temps and will need insulation to keep that exhaust above the condensation point untill it leaves the chimney.

 

[brenndatomu]

A clay flue liner in a masonry chimney is perfectly acceptable in PA if done right. No need for a stainless steel liner, and certainly not a more expensive insulated one. That is a belt and suspenders approach, and quite expensive.

Insulation doesn't add that much cost to a liner...$300 maybe, even if a little more, that is pretty cheap insurance to know your good to go on the clearance to combustibles (CTC) around the chimney...plus the chimney will perform better on a modern stove, especially a cat stove like you are looking at...they can have very low flue temps at times...tend to work poorly on a clay flue many times. Insulated liners stay cleaner, and for longer too.
It would be foolish to install a new liner without insulation in my opinion...UNLESS, it was a flue of the appropriate size (most are over sized for modern appliances), internal to the house, AND able to verify the CTC for the full height of the chimney...which is almost impossible to do without tearing into the walls on many/most houses...