# Chimney longevity



## Tinstaafl (Jan 6, 2008)

From worst to best, which fuels have the most impact on chimney life?


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## griz (Nov 26, 2009)

Type of chimney/material?

Chimney for furnace or woodstove?


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## Tinstaafl (Jan 6, 2008)

I don't think the materials or method of burning are relevant. Those would be considerations in the planning stages, mostly to get bang for your buck. But the harshest exhaust is still going to be the harshest regardless.


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## fjn (Aug 17, 2011)

From harshest to least I would say it is coal,oil,wood, gas.


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## dom-mas (Nov 26, 2011)

Disagree. Gas is one of the worst. I see chimneys built in the '80s and 90s all the time that vent nothing except gas furnace and hot water heater. The liners are too often made of tissue paper type steel and disintegrate early. Wherever the liner is deteriorated there is a dissolved salt in the masonry (not sure what sort of a salt it is, but I'm sure it's an salt acid) and the brick is turned to junk. 

Just looked it up and gas fumes contain sulfuric and nitric acid. Apparently NG fumes also contain a lot of moisture

No experience with coal...I know I've worked on chimneys that vent oil fired furnaces but they haven't stuck out like the NG ones have. Wood smoke also contains acids but they are only present when the vapours condensate...I haven't seen near the problems with wood fired chimneys either although the same salts are found but not to the same degree.

My list would go...Coal (no idea really but I'll trust FJN...I doubt it's going to be used in a new build but who knows), NG/propane, wood, Oil (Only cause with oil furnaces nothing jumped out at me.)


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## fjn (Aug 17, 2011)

dom-mas said:


> Disagree. Gas is one of the worst. I see chimneys built in the '80s and 90s all the time that vent nothing except gas furnace and hot water heater. The liners are too often made of tissue paper type steel and disintegrate early. Wherever the liner is deteriorated there is a dissolved salt in the masonry (not sure what sort of a salt it is, but I'm sure it's an salt acid) and the brick is turned to junk.
> 
> Just looked it up and gas fumes contain sulfuric and nitric acid. Apparently NG fumes also contain a lot of moisture
> 
> No experience with coal...I know I've worked on chimneys that vent oil fired furnaces but they haven't stuck out like the NG ones have. Wood smoke also contains acids but they are only present when the vapours condensate...I haven't seen near the problems with wood fired chimneys either although the same salts are found but not to the same degree.


 




I would not have enough time before bedtime to type all the contaminants contained in coal,just ask someone that works at a coke oven. Also,coal,oil and wood leave deposits on flues that hold contaminants on surface of flue s so they are always "handy" to work their "magic" on the flues. Gas does not.


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## dakzaag (Jan 6, 2009)

Gas (propane or Natural Gas) fumes are the hardest on masonry chimneys. 

Wood is only hard on the chimney if the fire is burned slow and cold. A hot fire burns up all the nasty tars that create creosote. 

Not familiar with coal, but it might be the hottest fire which would then become the hottest chimney, but I don't know.

I would guess that someone burning the crap out of an oven or fireplace with a Stainless liner could wear it out over time, otherwise the SS liners are pretty solid. 

By the way, Gas exhaust is not that nasty, it is the condensate that forms in the chimney when the gas mixes with moist air on the outside that turns to acid and ruins the walls of a chimney.


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## hdavis (Feb 14, 2012)

fjn said:


> From harshest to least I would say it is coal,oil,wood, gas.


I'd swap it to coal, wood, oil, gas. Wood puts out some pretty harsh organic acids, lots of water vapor.

For coal, it makes a difference if it's low or high sulfur.... Low sulfur should be better than wood.

I'm betting there are EPA numbers out there.


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## dom-mas (Nov 26, 2011)

fjn said:


> I would not have enough time before bedtime to type all the contaminants contained in coal,just ask someone that works at a coke oven. Also,coal,oil and wood leave deposits on flues that hold contaminants on surface of flue s so they are always "handy" to work their "magic" on the flues. Gas does not.


But the oil contaminants aren't immediately detrimental to the chimney. The acid in the gas exhaust is. Like I say..never seen any real harm come from an oil fired furnace. I stand by gas being about the worst for masonry


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## hdavis (Feb 14, 2012)

I'll try again....

Just looking from the condensate and flue gas temp side, oil runs around 550F (old 82% burner efficiency) , wood can run from ~400-1200F or more, coal will burn even hotter than wood. No idea what the stack temp for propane and NG is. Higher stack temps keep condensate from forming in the chimney and eating away at it.


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## dom-mas (Nov 26, 2011)

Oil furnaces and gas furnaces both produce sulfuric acid (like diesel though oils sulfur content has been cut huge in the last couple of decades) The other acid that NG creates is nitric acid a strong acid, and the fumes condense quite readily once they get passed the roofline. Oils other acds is carbonic acid a very weak acid..the same one that's in coke that makes your teeth squeaky

Gas rots chimneys quick. Seen way too many to have any other opinion


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## hdavis (Feb 14, 2012)

dom-mas said:


> Oil furnaces and gas furnaces both produce sulfuric acid (like diesel though oils sulfur content has been cut huge in the last couple of decades)


Coal does too. Consumers can still buy high sulfur coal, but what I see around here is low sulfur coal.


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## Tinstaafl (Jan 6, 2008)

Not sure how relevant it is to chimney health as a whole, but when I bought a box store SS cap after rebuilding my chimney last year, I found it interesting that the warranty was significantly shorter for anyone burning coal, as opposed to any other fuel.


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## hdavis (Feb 14, 2012)

Tinstaafl said:


> Not sure how relevant it is to chimney health as a whole, but when I bought a box store SS cap after rebuilding my chimney last year, I found it interesting that the warranty was significantly shorter for anyone burning coal, as opposed to any other fuel.


A good indicator, IMO. The high sulfur stuff actually stinks when you burn it, so it's putting out a lot of sulfuric acid. I don't remember all the types you can buy, but the low sulfur anthracite is the best for home use. There are also special grades for blacksmiths.


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## hdavis (Feb 14, 2012)

Tinstaafl said:


> SS cap


Caps always take the worst beating, from what I've seen. They stay relatively cold, so they'll get condensation when most of the chimney isn't getting any.

Running oil here for ~30 years, you'll lose ~1.5" of mortar off of a mortar cap and out of the joints of any brick work above the top of the flue. All that can also see freeze / thaw cycles and spall. Clay liners aren't as susceptible. An old (ca 1700) chimney I took out was soft hand made brick, lime mortar, and had clay hand smeared on the inside as it was built. The part below the roof line was fairly pristine, but probably was only used for 100 years or so. Other unlined chimneys that were used for coal and then oil lasted for over 100 years before they were lined and were still OK below the roof line. Above the roof line is where the problems seem to happen first / fastest.

One common denominator about these old houses is they ran inefficient heating systems, and they weren't insulated, so the system was running a lot, and putting a lot of hot gasses up the flue. That meant the chimney was staying pretty warm.


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## dakzaag (Jan 6, 2009)

H makes a great point about stack temperature. While it is also an indication of efficiency, higher stack temps keep the chimney from experiencing as many freeze thaw cycles. 

Gas appliances generally have lower exhaust temps resulting in the condensation that occurs on the inside of the flue.

A well burning wood fire emits hardly any harmful gases into the atmosphere. Choke it down and let is smolder and you have something entirely different.


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## heavyc (Jul 2, 2013)

FMI Who actually uses coal?
Coal has fallen by the way side, not sure about it's relevance. 
Back in the 20th century I heard story's about pitching coal. As soon as Natural
Gas was discovered and piped in Coal as a primary heating resource disapated.
I know it is still mined daily, but where does it go? Steel mills or other commercial uses? I'm not familiar with it's residential application.


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## dom-mas (Nov 26, 2011)

Until recently more electricity was made from coal fired turbines than any other source. And like you say other industrial uses like steel production etc...but residential? While i find coal frequently when i dig around homes built as late as the '50's I can't see it being used in any new residential construction. But who knows...I've only lived here


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## dakzaag (Jan 6, 2009)

Try googling Anthrocite and you will see that it is alive and well. They cook pizza with coal in NY so it cannot be that bad.


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## hdavis (Feb 14, 2012)

Coal can be used in a lot of wood burning appliances. A friend of mine grew up burning wood in cool weather and coal in cold weather - it puts out more heat. You can still buy bags of stove coal here, and coal furnaces are still being made:

https://www.acwholesalers.com/US-St...-Ft/48491.ac?gclid=CNHu6JzMyMQCFeJj7AodzxwAow

It's use has dropped around here a lot over the years.


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## Tinstaafl (Jan 6, 2008)

I'd agree that the nationwide level of residential coal usage is considerably lower than it once was, but it's still very much in use in coal-producing areas (duh).

For our first few years in this house, I heated with wood. Then my wife popped out a couple of babies, and somehow a lot of spare time disappeared. I switched my stove's grates to handle coal, and have burned that for over 20 years.

When I rebuilt my chimney last year, it wasn't because of that. It was built with no foundation other than a slab on top of the ground, and was starting to lean away from the house. The clay flues didn't appear to be deteriorated at all. Don't think it has any bearing, but this is a fireplace adapted for a stove.


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## CJKarl (Nov 21, 2006)

May have been mentioned, but Gas because of it's high efficiency and low stack temps causes heavy condensation resulting in the aforementioned acids being deposited.


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## fjn (Aug 17, 2011)

Tinstaafl said:


> I'd agree that the nationwide level of residential coal usage is considerably lower than it once was, but it's still very much in use in coal-producing areas (duh).
> 
> 
> 
> ...


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## Fancis Casini (Jan 31, 2013)

CJKarl said:


> May have been mentioned, but Gas because of it's high efficiency and low stack temps causes heavy condensation resulting in the aforementioned acids being deposited.


And those shut down dampers they install to maintain boiler temp are absolute rain makers. I cured a few gas ''chimney attic water fall'' problems by cracking open the basement ash door.


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## superseal (Feb 4, 2009)

There's no clear winner here in terms of harshest...just depends on how things are configured. The proper materials and sizing should be of most concern 

All the fossil and solid fuels can be harsh on flues...usually it's the retrofitting of existing masonry/clay systems with modern day efficiency that causes problems.

The quickest way to destroy a healthy brick and clay stack is to pump an orphaned 35,000btu WH into a 9x9. Reline it with properly sized steel liner and it becomes harmless.


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## Tinstaafl (Jan 6, 2008)

superseal said:


> There's no clear winner here in terms of harshest...just depends on how things are configured.


I'll grant that a roaring coal fire is probably more damaging to a chimney than a handheld propane torch, but of course that's an unrealistic comparison. 

Nor is a furnace equipped with the absolute latest gee-whiz anti-emission doodads, though I admit the cutoff point might be difficult to nail down. I'm just curious about the average.


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## griz (Nov 26, 2009)

Without some parameters your question is akin to which saw should I use?


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## Fouthgeneration (Jan 7, 2014)

Around the Mid West, the 30-40 KBtu burner size on Natural gas hot water heaters just don't keep the chimney gases above 212, thus lots of condensation = acid damage. An adjacent flu chase for a 120 KBtu furnace will be in good shape.

Any Masonry chimney needs the stack gases to exit above the steam temp. for long term durability, I've actually insulated attic chimneys to keep the flue temps above 212 degrees when higher efficiency stoves and furnaces are used. 

Any oversized masonry chimney/under fueled chimney will suffer damage if there is condensation occurring within the flues after warm up. 

Our local coal was/is so sulfur infused it went out of use 40 years ago.


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## Fancis Casini (Jan 31, 2013)

Fouthgeneration said:


> Around the Mid West, the 30-40 KBtu burner size on Natural gas hot water heaters just don't keep the chimney gases above 212, thus lots of condensation = acid damage. An adjacent flu chase for a 120 KBtu furnace will be in good shape.
> 
> Any Masonry chimney needs the stack gases to exit above the steam temp. for long term durability, I've actually insulated attic chimneys to keep the flue temps above 212 degrees when higher efficiency stoves and furnaces are used.
> 
> ...



What would help is placing those auto dampers on the chimney tops not on the boiler pipe before the chimney. For many years I notced that flueless chimneys seem to last where as the flues disintegrate from sulpher and salt. IMO the flues harbor water and stay colder due to the air space between the warm brick.

I do burn my fireplace "alot'' and I dont use vents so when I have the fp going my boiler goes on constantly to maintain its temp.

The past two years that I've been using a 1:30 throat ratio it is about 100% less frequent however it still does happen, so now I merely shut the furnace off and heat the house with the f.p., down to about 20 for days straight, if the wind isn't too high.

I thought about the auto damper on the boiler but nope, it needs to be on top under the cap, if at all.


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## JBM (Mar 31, 2011)

I dont think I have installed a furnace flue in years. Direct vent is what most do now. My opinion would be that any gas up a clay flue is right up there in the #1 spot for damage. 

Ive seen some 12 year old chimneys with brick pieces in the flues to move them one way or the other with all the mortar missing- floating inside the chimney like a drunken sailor on a ship deck. Improper placement of the flues, lack of pinning inside the chimney, a rough chimney sweep not understanding this- causes more damage than the appliance imo.


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## hdavis (Feb 14, 2012)

Just some quick boiling point notes :

Nitric acid ~180F
Corrosive wood fire product (forget what the compound is) ~ 230F
Sulfuric acid > 600F

If you're buring wood, the general recommendation is to start the day with a hot fire to get the chimney up to temp, and end the day with another hot fire to evaporate out junk from the rest of the day's burn.


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## dakzaag (Jan 6, 2009)

If you want to burn wood, my recommendation is to burn a small fire in the morning in your masonry heater and then another small fire at night if needed. 
In the mean time, you can sit back and relax cause your normal wood pile is now three years worth of wood. 
But not everyone agrees...


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## fjn (Aug 17, 2011)

dakzaag said:


> If you want to burn wood, my recommendation is to burn a small fire in the morning in your masonry heater and then another small fire at night if needed.
> In the mean time, you can sit back and relax cause your normal wood pile is now three years worth of wood.
> But not everyone agrees...





Good points. It would be hard to argue with that logic.


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## S.U.M (Apr 17, 2013)

I lived in Ireland for 26 years all our fires where coal, my people light fires everyday, I never had to rebuild a chimney although I built plenty of new ones, 
I don't think I ever seen a natural gas fireplace in Ireland 
Logs very rarely used in my house, always coal and slack. 
Over here every time I rebuild a chimney it normally has a furnace liner of water talk heater venting through, a lot of time we notice holes in the liner or signs of rust on them. 

The weather here is extreme season to season and this plays a huge part also in the longevity of the chimney regardless of what it is used for.


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## fjn (Aug 17, 2011)

S.U.M said:


> I lived in Ireland for 26 years all our fires where coal, my people light fires everyday, I never had to rebuild a chimney although I built plenty of new ones,
> I don't think I ever seen a natural gas fireplace in Ireland
> Logs very rarely used in my house, always coal and slack.
> Over here every time I rebuild a chimney it normally has a furnace liner of water talk heater venting through, a lot of time we notice holes in the liner or signs of rust on them.
> ...





The coal you guys burned could also be low sulfur. In States I believe only L.S. coal comes from Pa.,the rest is soft,high sulfur.


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## dom-mas (Nov 26, 2011)

dakzaag said:


> If you want to burn wood, my recommendation is to burn a small fire in the morning in your masonry heater and then another small fire at night if needed.
> In the mean time, you can sit back and relax cause your normal wood pile is now three years worth of wood.
> But not everyone agrees...


How long have you been building heaters? Don't know if you know one of my professor...John McDougall...but he instilled a lot of interest in quite a few of us. And having Norbert as a guest lecturer and fireplace guru really helped as well.

Mc Dougall built a masonry hot water heater a few years back...pretty cool


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## dakzaag (Jan 6, 2009)

dom-mas said:


> How long have you been building heaters? Don't know if you know one of my professor...John McDougall...but he instilled a lot of interest in quite a few of us. And having Norbert as a guest lecturer and fireplace guru really helped as well.
> 
> Mc Dougall built a masonry hot water heater a few years back...pretty cool


I have not met John, but I know Norbert. If you are building fireplaces, you should at least check out the Masonry Heater Association. In my mind, this is the only fireplace worth building.


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## dom-mas (Nov 26, 2011)

I've checked out the MHA quite a few times. Not much recently. I've never been involved in a heater build although I would really like to. I agree 100% that they are by far the best, most efficient, and often the most attractive fireplaces out there. Unfortunately the few solid fuel fireplaces I do build (1 every 2 years or so) are mostly for show/romanticism. 99% of the fireplaces I'm involved with is just facing a gas or sometimes electric unit.


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## dakzaag (Jan 6, 2009)

I was the same way, and still do on occasion. I have customers looking at a masonry fireplace and then I gently steer them along this line.


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## wazez (Oct 25, 2008)

dom-mas said:


> Mc Dougall built a masonry hot water heater a few years back...pretty cool


This sounds like something I could use. I have tubes in my floor but no water heater.


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