How do wood boilers avoid creosote buildup on heat exchanger?

monteville

New Member
Nov 23, 2019
47
Dallas
A wood stove could have creosote buildup inside the flue pipes.
Temperature of the heat exchanger is even lower than the flue pipes.
How do wood boilers avoid this creosote buildup problem?
 

hobbyheater

Minister of Fire
Nov 14, 2011
1,171
A wood stove could have creosote buildup inside the flue pipes.
Temperature of the heat exchanger is even lower than the flue pipes.
How do wood boilers avoid this creosote buildup problem?
On gasification boilers, the combustion takes place in the refractory chamber. The tars that make cresote are burned up before they get to the heat exchanger tubes.
 

Medic21

Minister of Fire
Feb 26, 2017
1,100
Northern Indiana
For mine the heat exchanger is basically the firebox. I have some tubes and a Multipass from Crown Royal but, the majority is the firebox walls themselves.
It burns pretty hot when calling for heat and stays pretty clean. Flakey, dry creosote.
 

E Yoder

Feeling the Heat
Jan 27, 2017
492
Floyd, VA
My G7000 (downdraft gasser) runs a really really low stack temperature so it is important that it gets a good reburn. So as was mentioned all the tars are burnt completely up.
And like Medic said a conventional unit just runs a hotter exhaust temperature so it cooks off any creosote that accumulates.
 
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PJ41

New Member
Oct 25, 2020
12
North East CT
I use Creosote Destroyer, by Meeco's Red Devil, couple of tablespoons on hot coals every third fire or so. Keeps the creosote build up down and I wire brush the fire box each summer. Been working pretty well on my old Tasso A3.
 
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maple1

Minister of Fire
Sep 15, 2011
10,691
Nova Scotia
A wood stove could have creosote buildup inside the flue pipes.
Temperature of the heat exchanger is even lower than the flue pipes.
How do wood boilers avoid this creosote buildup problem?
All boilers are not equal. Gasifiers burn the stuff that makes creosote, you only get some fly ash. Ones that aren't gasifiers still build creosote.
 

Medic21

Minister of Fire
Feb 26, 2017
1,100
Northern Indiana
All boilers are not equal. Gasifiers burn the stuff that makes creosote, you only get some fly ash. Ones that aren't gasifiers still build creosote.
And there is creosote with any and all wood with a boiler. Comes with the territory of having a water lined firebox. Keeps thing cooler and causes condensation no matter what you do.

Difference is the with my Multi Pass the creosote is basically in the fire box. I’ll take a pic when I clean the heat exchanger tomorrow. Mostly fly ash there.
 

maple1

Minister of Fire
Sep 15, 2011
10,691
Nova Scotia
And there is creosote with any and all wood with a boiler. Comes with the territory of having a water lined firebox. Keeps thing cooler and causes condensation no matter what you do.

Difference is the with my Multi Pass the creosote is basically in the fire box. I’ll take a pic when I clean the heat exchanger tomorrow. Mostly fly ash there.
Right. And the OP was talking about in the exchanger. I think. Yes, gassers will make some creosote in the firebox. But it is always burning off, never builds up or needs attention.
 
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hobbyheater

Minister of Fire
Nov 14, 2011
1,171
This is a picture of my chimney half way through a batch burn. The air coming out is just a little too hot to breath comfortably, but no odor of smoke - just air coming out! The boiler at the bottom of the chimney is a Jetstream; a early Gasification boiler(1983)which is burning at around 2000 F with the flu gas temperature running between 290 F to 325 F. Dry wood and Gasification with Storage is how you keep your flues clean!
IMGP3758.JPG
 

hobbyheater

Minister of Fire
Nov 14, 2011
1,171
I use Creosote Destroyer, by Meeco's Red Devil, couple of tablespoons on hot coals every third fire or so. Keeps the creosote build up down and I wire brush the fire box each summer. Been working pretty well on my old Tasso A3.
Is this a Tasso 3? I had one once coupled to 1,250 gallons of storeage with 2" steel pipe. It was a gravity system with no circulators. It heated the tank well but with no boiler protection it did plug up with creosote!
 

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maple1

Minister of Fire
Sep 15, 2011
10,691
Nova Scotia
Ah yes, boiler protection. Another important thing I forgot to mention. Need to keep return water entering the boiler, above 140°. Or else it will really build up in the firebox. A really important thing when using storage, or low temp radiant like infloor, that is almost always returning cool water.
 

GENECOP

Minister of Fire
Jan 31, 2014
713
Ny
Been running my Greenwood for 5-6 years now, the firebox with all the refractory burns around 2000 Deg, any creosote that develops from the cooler temps, burns off after the stove comes up to temp.
I also use an Rutland powder every week and brush the piping down when I clean the chimney..
The only complaint I have is the back section of the water piping is not easily accessible, I use a camera, compressed air, some long brushes, and a lot of patience..
 

peakbagger

Minister of Fire
Jul 11, 2008
5,993
Northern NH
i have an ancient Burham wood boiler. I havent seen anyone else on Hearth talk about them so I may be the only one running one. I got it used and have been running it full time for around 15 years. I have never seen a trace a creosote inside or in my chimney to the point that I inspect the chimney every year but have only cleaned the chminey twice in 30 years. Note its an ideal tall interior chimney and not advocating anyone to skip yearly chimney cleanings. I open the top cover on the boiler to clean the tubes every year but all I find is a layer of ash inside the tubes and on the turbolators. A light brushing is all it takes to clean them off.

Its a wood coal boiler so all the air goes up through a bottom grate. Above the grate is a refractory lined pit. There is about 12" of combustion space and then top a baffle that deflects the flames toward the front of the boiler and then above the baffle are 16 vertical heat transfer tubes with turbolators. There is an open chamber on the top before the exhaust gas exits into the stove pipe. I have 500 gallons of storage. I have never had creosote issues. I burn dry wood and burn it hot. It does not have a conventional return temperature protection valve but it does have a temperature probe on the water jacket at mid height. The probe controls the circulator pump that pumps water through the boiler. So when the boiler is cold and most prone to creosote the heat exchanger surfaces stay a bit hotter until the jacket temp is over 140 F. No doubt there is stratification in the water jacket but thermosyphoning probably kicks in.

My guess is the bottom air and refractory pit and the baffles creates a fairly high temperature zone once the boiler warms up and therefore its hot enough to burn any condensable gases that would form creosote. Its probably not the most efficient boiler but I have primarilly heated my small house in Northern NH with 3 to 4 cords a year with a 12,000 BTU(nominal) minisplit in the shoulder seasons for about 7 years. I could probably but in an economizer down stream of the primary heat exchanger tubes in the space above the tubes to up the efficiency. i cant really put heated secondary air unless I want to cut the water jacket.

Its a very heavy duty piece of construction and weighed that way when I got it. I doubt any company would build one as heavy these days. I wouldnt mind going with newer more efficient boiler but expect its going to keep running for as long as I need it. The only minor PITA is the fire is so hot it burns up my baffle plates. I just get new plates cut every few years and bend them back into shape a few times before replacing them.
 
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andym

Feeling the Heat
Feb 6, 2020
366
Hicksville, Ohio
i have an ancient Burham wood boiler. I havent seen anyone else on Hearth talk about them so I may be the only one running one. I got it used and have been running it full time for around 15 years. I have never seen a trace a creosote inside or in my chimney to the point that I inspect the chimney every year but have only cleaned the chminey twice in 30 years. Note its an ideal tall interior chimney and not advocating anyone to skip yearly chimney cleanings. I open the top cover on the boiler to clean the tubes every year but all I find is a layer of ash inside the tubes and on the turbolators. A light brushing is all it takes to clean them off.

Its a wood coal boiler so all the air goes up through a bottom grate. Above the grate is a refractory lined pit. There is about 12" of combustion space and then top a baffle that deflects the flames toward the front of the boiler and then above the baffle are 16 vertical heat transfer tubes with turbolators. There is an open chamber on the top before the exhaust gas exits into the stove pipe. I have 500 gallons of storage. I have never had creosote issues. I burn dry wood and burn it hot. It does not have a conventional return temperature protection valve but it does have a temperature probe on the water jacket at mid height. The probe controls the circulator pump that pumps water through the boiler. So when the boiler is cold and most prone to creosote the heat exchanger surfaces stay a bit hotter until the jacket temp is over 140 F. No doubt there is stratification in the water jacket but thermosyphoning probably kicks in.

My guess is the bottom air and refractory pit and the baffles creates a fairly high temperature zone once the boiler warms up and therefore its hot enough to burn any condensable gases that would form creosote. Its probably not the most efficient boiler but I have primarilly heated my small house in Northern NH with 3 to 4 cords a year with a 12,000 BTU(nominal) minisplit in the shoulder seasons for about 7 years. I could probably but in an economizer down stream of the primary heat exchanger tubes in the space above the tubes to up the efficiency. i cant really put heated secondary air unless I want to cut the water jacket.

Its a very heavy duty piece of construction and weighed that way when I got it. I doubt any company would build one as heavy these days. I wouldnt mind going with newer more efficient boiler but expect its going to keep running for as long as I need it. The only minor PITA is the fire is so hot it burns up my baffle plates. I just get new plates cut every few years and bend them back into shape a few times before replacing them.
So it is not a true gasser? How does it compare with the diagrams of the UNI wood boilers?
 

peakbagger

Minister of Fire
Jul 11, 2008
5,993
Northern NH
So it is not a true gasser? How does it compare with the diagrams of the UNI wood boilers?
It looks quite different. Its definitely not a gasser and would be miserable for modulating load. With storage I burn it hot and hard until the storage is up to temp then I bottle it up until the next time I need to charge up the storage. Even with it bottled up if carries the house load for several hous from the heat left in the boiler.
 
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Bad LP

Minister of Fire
Nov 28, 2014
1,404
Northern Maine
It looks quite different. Its definitely not a gasser and would be miserable for modulating load. With storage I burn it hot and hard until the storage is up to temp then I bottle it up until the next time I need to charge up the storage. Even with it bottled up if carries the house load for several hous from the heat left in the boiler.
So cast iron? How many gallons does it hold?
 

maple1

Minister of Fire
Sep 15, 2011
10,691
Nova Scotia
So it is not a true gasser? How does it compare with the diagrams of the UNI wood boilers?
Quite sure the Uni has no tubes? That's almost 3 strikes all at once.
 

peakbagger

Minister of Fire
Jul 11, 2008
5,993
Northern NH
It would take major digging to find what little documentation I have. Somewhere between 60 and 80 gallons sticks in my mind.

Below is a section through the Burnham. There are 16 tubes 2 -3/8" diameter. There is an insulated top cover that unbolts for access down through the tubes. The baffle is actually 4 strips of 1/4" steel that slide out. They are a weak point, they burn out and droop. I think the original ones were some sort of high alloy plate. I just pick up new plates every few years from local steel fabricator and when they start drooping I remove them and bend them straight. The air flap is controlled by a temperature switch, if the boiler goes over a high setpoint it closes. Its either open or closed. The goal is never have the flap close during operation. The jacket temp probe controls the return water pump. Below 140 F the pump is off, once the jacket goes over 140 F the pump turns on. There is also an overheat probe that if the jacket water exceeds a high high setpoint the system goes to a dump zone. There is also two safety relief valves, one of them are routed to the firebox. It was set at 13 psi with the main safety set at 15 psi but the originally 13 psi one is leaking and finding an affordable one has not happened so I have a 15 psi one on it The bottom grate used to be rotary shaker that could be accessed with a hand crank from the ash pit. The shaker was trashed when I got it, I have some pieces but a friend welded up a grate out of square stock that dropped in. The clearance between the baffle and the loading door is tight so not a lot of room to put in heated secondaries and controlling the secondaries from outside the boiler would be difficult. my guess is burn it hot and fab and install a economizer in the upper cleanout space between the upper tube sheet and the flue connection.

I think when new it was shipped with refractory shipped separately. It has a sheetmetal case. When I got it, it was a illegal install necked down from 8" to a tee in a 6" oil boiler flue tied to fairly short outside stack. The front of the wood boiler was black with smole above the loading door. It did not have storage. My guess is that it used a lot of wood and probably smoked the place out. My friend bought the place from a bank after it was foreclosed, she could not get a mortgage unless she had it removed and was going to pay someone to remove it. I offered to remove it for free and remove all the extra piping from the heating system and replacing the oil boiler flue. I had a friend help me. We stripped the sheetmetal case and everything else we could off the boiler and then rigged it onto a truck then hauled it to my place and dropped it in through a bulkhead. The controls were real basic. I had a winter on my hands while my employer was bankrupt so I reassembled it and got it working with no storage using mostly salvaged fittings . I didnt use it much as it was a PITA to use as it either burned full bore or overheated so it needed to be watched constantly. A couple of years later I got an American Solar Technics 500 gallon storage tank and then spent a fewmonths rigging up a lot more advanced relay based control system that mostly automates the operation of the wood boilerand the backup oil boiler. It is also designed to "fail safe" on the oil boiler. If I turn off the wood boiler master power switch the oil boiler works just like it originally did including all the thermostats and circulators. As long as the storage tank is above 140 F the system locks out the oil burner. I could automate it more but that requires a PLC and I did not want one as PLCs are notorius that the software goes out of date. I have a GE Fanuc system in my collection that suffered that fate. Relays may be "dumb" and complex but a ladder diagram, a multimeter and a jumper lead is all you need to diagnose. Its been 10 to 12 years since assembled and the only issues have been operator error.

The major limitation is my radiators. I have Slant fin and they are undersized for low supply temps. If I had low temperature emitters I would have a lot more hours of storage but the cost would pay for a lot of cords of wood. Maybe on the next house but hard to spend more money when I get the wood for "free" and use around 3 to 4 cords per year to heat the house in northern NH. I have solar hot water free from April to October and in the winter I use surplus heat in the wood boiler after the storage is at maximum temperature to heat up a hot water maker via my oil system. I bring it up to 180 F and then mix it down so the tank lasts awhile. This feature is not well integrated into the controls but not that difficult to run.

1612093727486.png
 
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andym

Feeling the Heat
Feb 6, 2020
366
Hicksville, Ohio
Quite sure the Uni has no tubes? That's almost 3 strikes all at once.
It does have heat exchanger tubes. I would describe them as similar to the drolet furnaces. Except smaller and more of them. In concept it appears similar to Peakbagger’s description of his.
 

maple1

Minister of Fire
Sep 15, 2011
10,691
Nova Scotia
It does have heat exchanger tubes. I would describe them as similar to the drolet furnaces. Except smaller and more of them. In concept it appears similar to Peakbagger’s description of his.
If it does, they dont show up in the cross section in the manual. And they aren't mentioned anywhere in it, including cleaning topics. That I saw. The broken English was a bit hard to read and left things to be desired though.
 

andym

Feeling the Heat
Feb 6, 2020
366
Hicksville, Ohio
I haven't checked recently, but a year ago there was some sort of picture on the MBTEK website that showed the inside of the clean out door. Kinda getting away from the original topic here though....there is a member here that is waiting to receive one of the Uni boilers. Hoping he starts a thread once he gets it operating.