How to Improve Old Log Wood Burners Increase Efficiency Clean Burn Gasification Secondary Combustion




About: Lateral thinker, Originator of Inclined Bed Therapy (IBT) Originator of Pocket Full Of Acorns Project. Originator of Operation OASIS.

How My old log burner / Boiler has been Improved beyond all expectations to match the efficiency of modern stoves! It cost less that £60 to bring this boilers performance up to match most new stoves with secondary combustion.
Many older Log burners were built to last a lifetime. These are being replaced because they are inefficient and emit too much smoke and this is a waste of irreplaceable craftsmanship. These modifications with a little bit of thought can be applicable to most older wood stoves.

Before you throw out your old log burner, watch how I improved the efficiency and combustion in an old Franko Belge Lorraine multifuel stove from being one of the most inefficient to one of the most efficient burners and achieved by installing two very simple modifications, which with a little thought can be adapted to most of these old faithful legends.

Before the modifications, our chimney used to collect creosote and required regular cleaning to remove it. Now that same creosote is burned in the fire box along with free timber collected during the year. In fact, Leylandii, which would normally blacken the glass and cote the inside of the log burner with thick black tar like creosote residues now burns as cleanly as any other wood.

We have plenty of free hot water and our home is heated burning scrap wood, mostly pine and if we are lucky some hard woods.

Our gas bill for the winter quarter is now a staggering £15.00 Yes you read that right. By opting for a non-standing charge tariff we have not only virtually eliminated our heating bill, but we now have an almost smoke free combustion in our wood burner and it is using only half the logs that it used to burn!

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Step 1: This Older Type Log Burner Was Good in It's Day, But Needed to Be Upgraded

Watch the full video of the upgrade in the intro.
This, wood stove, the Franko Belge Lorraine with back boiler for hot water and central heating, (see detailed drawing view) is fitted with a mild steel throat plate, that sits about half an inch above the back boiler pipe.

Combustion inside the firebox sends smoke, creosote soot and ash up the chimney as the fire roars it's way through an enormous amount of logs and scrap wood. So fast that burning soft wood in this unmodified log stove is labour intensive and it becomes impossible to keep a fire in, because very little ash ends up in the ash pan and maintaining embers to ignite the next log is hit and miss, often resulting in relighting the fire.

Even lighting the fire with kindling resulted in a very fast burn, which did not heat up the cast iron fire at all. (remember this)

That said, the fire did maintain heat and embers with hard woods and especially when coal or coke was added, but it still burned through a huge volume of fuel.

In fact, when we picked this fire up, the man who sold it to us second hand said he has had to upgrade to a new more fuel efficient and smokeless log burner and added he worked in the UK Environment Department.

Hi words where born out after only one season, when a huge stack of split logs vanished quickly and we needed to get more to see us through the winter. We had 4 rows of logs stacked up level with the top of the fence on the drive, which I thought should have easily seen us through the winter.

So after 3 years of excessive fuel consumption and creosote filled chimneys. It was time to have a look at how other's had modified their log burners and see if I could find a way to improve and simplify the upgrade to make my log burner as efficient and smoke free as a modern one.

Besides, I love this log burners features and it is designed to last a lifetime.

Step 2: Remove the Old Throat Plate and Replace With a 3 Inch Stainless Steel Perforated Tube

After a long time pondering how best this could be achieved, I decided the first move would be to slow down the exhaust gasses from the fire and improve the combustion. So Initially, I cut out notches in the throat plate to lower it down to around 3/8 of an inch above the back boiler tube that runs across the back of the fire box. While this did improve the fires performance it did little to address the fuel consumption. Image 1 and 2

The problem was and had always been this throat-plate, which aside from it heating up and buckling, used to get knocked off regularly when putting logs in the fire, and it was a pain to get back into place in a red hot fire. Trust me on this one :)
So the plate had to go and I got to thinking about what it could be replaced with, so eventually chose a perforated stainless steel 3 inch tube to fit loosely on the brackets that located the throat plate and to roll back and forward to draw the fire and to enable opening the doors to add another log or two.

3" 76mm Exhaust 450mm" Long Vehicle Exhaust Repair Pipe Perforated Stainless Steel Tube from Ebay
Around this tube, I rolled a length of stiff mild steel wire so that it protruded out of the fire through the gaps in the centre. The idea being that the tube would provide a vortex in the upper half of the fire and cause more heat to be emitted through the fire rather than up the chimney to the atmosphere.

The tube needed to be peened over at both ends using a hammer and something to hammer the ends over on in a vice. I used a large ring spanner as shown in image 2 and completed in image 3

Step 3: The Results From First Modification Greatly Reduced the Fuel Consumption and Induced Gasification

After running the fire over a season, with just the perforated stainless steel tube in place, it became clear that I was onto something, and having a mechanical engineering background, I could see a huge improvement in the furnace temperature and an equally huge reduction in consumption of logs.

Note: The perforated stainless steel pipe / baffle requires brushing regularly. I clean it before lighting each fire to make sure no carbon is blocking the holes.

The Wire mechanism for raising and lowering the baffle works well. Though would benefit from using stainless steel instead of mild steel. It enables opening of the log burners doors without letting smoke into the room, though on windy days this can still be a problem, as anyone who has a log burner will testify :)

Creosote from burning leylandii and other soft woods was still apparent by carbon deposits around the chimney top and we could hear lumps of carbon / creosote falling down the chimney occasionally, though I suspect this was the release of deposits accumulated prior to the initial upgrade, due to the increased heat from the gasification of wood.

Log burners run cleaner when burning at higher temperatures and this tube created a vortex which caused the flames to fill the whole of the upper half of the fire box. Given that this log burner runs our hot water and central heating for free, as I never buy wood or logs, we were able to determine the differences easily.

I also installed one of these for my friend who also has the same log stove as me. He also reported far more heat and far less fuel consumption over the winter season.

Step 4: Adding Super Heated Air to the Upper Furnace Chamber for Secondary Combustion

Having watched a number of videos on secondary combustion (Burning the exhaust gasses from the fire mixed with air) For which I am very grateful to those who uploaded their videos and shared their ideas and results, I set about introducing additional air to my old partly upgraded log burner.

While the stainless steel perforated tube caused gasification by retaining the heat in the fire where it belonged to release the gases from the timber being burned, it did not resolve the need for introducing more air for a cleaner burn.

Several of the videos I watched showed the need for a secondary source of air intake in the upper chamber, which consisted of a tube running up the outside of the fire and into the top to cause an up-draft to draw air up to mix with the carbon and smoke released from primary combustion.

I thought about an external tube, but decided it would be more effective if heated inside the furnace prior to reaching the gasses in the upper half of the fire.

To achieve this I had to drill a hole through the side of the cast iron fire, again shown more clearly in the video: Full instruction video of upgrading an old log burner

First a small pilot hole was drilled through from the inside, taking care to be above the front grates, allowing enough room to accommodate a 3/4" stainless steel bend and pipe and enough room to remove and replace the grates when the air feed pipe is installed.. Fig2

Then I increased the hole diameter using larger drills and eventually filed out the holes to the size of the tubing. This was then sealed using fibre glass rope and a brass compression joint threaded nut, which was filed out to fit over the end of the threaded tube, to act as a seal and washer. It worked well.

The pipe and components were assembled as shown in the images and video with a bend, 3 inch pipe and isolation valve on the outside to close off air before you open the door for refuelling and to draw air through the fire when lighting. Once heated sufficiently the valve remains fully opened.

I used 4 x 90 degree bends, two 6 inch lengths of threaded 3/4 tube, two 3 inch threaded tubes, a 3/4" nipple and a full bore isolation valve.

Parts list:

3/4" BSP Female 316 Stainless Steel Full Port Ball Valve Vinyl Handle WOG1000 CL x1
3/4" Male x 3/4" Male Threaded Pipe Fitting 150MM Stainless Steel SS304 BSP CL x3 3/4" Elbow 90° Angled Stainless Steel 304 Female Threaded Pipe Fitting BSP CL x4 3/4" Male x 3/4" Male Threaded Pipe Fitting 75MM Stainless Steel SS304 BSP CL x1 Seller : chaoling017 (8072)

The 90 degree elbow that is situated inside the fire close to the drilled hole, required filing on the inside edge to allow it to sit close to the mild steel boiler on the side. This was also filed slightly along the welded edge, again to allow the elbow to sit correctly.
I chose stainless steel because it does not corrode or deteriorate inside the furnace. So the threaded tubes can be removed and refitted with ease.

Step 5: Success.

Once the fires was hot enough, the valve was opened and the flames became more excited, filling the upper half of the firebox more completely, licking back around the front of the fire and dancing around the end of the heated air intake tube.

That was a year ago and since then the fuel consumption of this upgraded older type log burner and boiler has been conservatively reduced by 50%.

Now even the kindling heats up the top of the fire before any logs are added. Our water heats up much faster and our home is definitely warmer.

Step 6: Secondary Combustion and Gasification Achieved

Image 1 shows embers from soft wood combustion and gasification. Something we never saw before.

All the creosote from Leylandii burning was burned off the inside of the firebox, the colours inside the fire once allowed to go out, are light brown, indicating a clean burn. Ash falls into the ash pan now instead of being drawn up the chimney

Step 7: Chimney Is Almost Smoke Free, Even When Burning Seasoned Soft Woods, Including Leylandii

Almost a year since installing the super heated air supply.

The fire is running very well, even better than expected.

Sweeping the chimney less, and no creosote is present, see the ash pan.

The chimney smokes when first lit but when it is heated up the smoke is greatly reduced, by approximately 80% and often it burns smoke free, so I guess it also depends on the type of wood being burned.

Hard woods, leave embers in the bottom that emit clean blue flames.

Hope you found this instructable useful. I still wear my Instructable T shirt with pride :) Great site guys n gals keep it going.

Be sure to watch the video to see footage of fire in operation and more complete instruction.

Happy New Year

Kind regards Andrew

Step 8: The Evidence After a Winter Is In. Zero Carbon / Creoste in Chimney and Further Upgrade on Secondary Air Supply.

Yet another improvement to my old Franko Belge Lorraine multi-fuel stove / log burner that has enabled cleaner combustion and more fuel savings.

Since the latest modification with extended drilled stainless steel tube
as suggested by Gardenweld, the infra red thermometer has recorded temperatures off the scale of 400 degrees Centigrade. The fire was so hot it caused the water to kettle inside because the water pump was not running fast enough. Have now increased speed of pump and it has cooled the fire down better and boosted the radiator and hot water supply.



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    18 Discussions


    3 years ago

    Comment from Youtube:

    gardenweld4 days ago (edited)

    +Andrew K Fletcher Hi, Im loving this video, and if I may.. To help you further your project.. I manufacture log/wood burners, fire pits of all types for a living. I would like to believe that I know what Im talking about, My advice is based on that fact, ( im only trying to help).. Here goes, To achieve Secondary burn 1, You need a good amount of heat in your stove and good coals. 2, The secondary air you are looking for will only work well if you burn Dry Wood. Burning wet unseasoned wood will build up tar and acid in your firebox. To achieve the secondary burn you will need secondary Air. The second part of this video where you add the stainless pipe is almost there. If I may suggest you add a stainless steel tee and more pipe with 8mm holes drilled every 13mm extending out to the edges of your firebox cap the ends off., 2, when you close down the Primary Air ..The hot flue from your stove will pull Pre Heated Air from this stainless pipe relighting the smoke and un-burned gasses. Giving you a very slow burn and a spectacular dancing Gas like flame, you can open the Primarary Air to liven up/balance the burn rate. If i can help you in any way give me a shout: Gardenweld Shropshire.

    1 reply

    Andrew K Fletcher15 hours ago

    Thanks for your comment


    Have pondered introducing more stainless steel pipe to distribute the air to the sides of the fire box. As you say, end caps and 8mm holes is a good idea.
    My logic was that the perforated tube would draw in the air and gasses, and it does, but there is always more room for improvements.
    I will order a T junction and two more threaded stainless tubes with end caps added but to be honest, the stove is already performing beyond all expectations and the wood supply seems inexhaustible compared to previous winters.

    +Andrew K Fletcher Thinking about it, all I need is to extend the pipe to the opposite side rather than have a T junction and two additional pipes and cap the end off. Then drill the existing tube and the new one.

    One 3/4" stainless steel socket, one 6 inch threaded pipe and an end cap ordered from china via Ebay £5.60 inc postage :)


    Question 10 months ago

    Would this still work if the stove chimney is not at the back but at the top?


    1 year ago

    Little "slow" here .. so the final modification is just t have the 3/4" (guestimate) pipe with the holes drilled into it at the top of the burn box? Do you still have the perforated cylinder - from first video in there? WHy did you move that cylinder back and forth?

    Could you also pipe in air from outside rather than room air for the "after burner"(my name)?

    Thank you

    1 reply

    Reply 1 year ago

    Yes, the 3" perforated pipe with larger holes drilled around centre and both ends is still in place, and is fed air through the stainless 3/4" drilled stainless pipe. The 3" pipe acts as a damper, but also causes secondary combustion of the gasses / smoke. It is pulled in and out to allow smoke to clear when opening and closing the door.

    Yes you could bring in air from outside, but seems a lot of work for little gain?


    3 years ago

    hi im not sure I understand where the perforated tube is located and is this to replace existing throat plate? Great instructable, many thanks.

    1 reply

    Reply 3 years ago

    Yes, it replaces the original throat-plate.thanks for your comment


    3 years ago

    Does this system produce more dangerous fine dusts than tradizional burning?

    1 reply

    Reply 3 years ago

    On the contrary, more complete combustion means less soot particles in the chimney, less chimney sweeping and less creosote. In fact, the ash pan now contains more ash than before when burning soft woods, which confirms the above statement. Before this modification the light ash would be drawn up the chimney, the glass on the fire would become blackened, indicating lower temperatures and because of the uncontrolled updraught we would burn far more fuel so it is a win win situation. Great question, thanks.

    Just4Fun Media

    3 years ago

    Great instructable! Have you been able to calculate how much the heat efficiency has been improved?

    Have a great day! :-)

    2 replies

    Thanks Deirdre, with a little bit of thought, this could be applied to
    most log burners. If anyone needs advice on converting the inside of
    theirs, do let me know, send a photograph of the inside of your fire
    box and Ill fathom a way of introducing it.

    Might involve welding or bolting a couple of brackets to retain the stainless tubing in the right


    Hi, our only evidence is that our house is warmer, despite using 50% less fuel. It is possible to remove the perforated tube and replace the throat-plate and close the isolation valve to take a reading and then a comparative reading using the same fuel. I.E mostly Leylandii with some scrap soft wood to help combustion.. But after a year using far less wood than before, even taking into account that this winter has been very mild, the results are self evident. In fact, I waited a year before publishing the video to make sure it was running as stated in the video.

    deirdre 2

    3 years ago

    I love this video.makes so much sense.


    3 years ago

    I am a humbled jackass and owe you an apology!

    Only the intro, with one pic (in place of your video), through to the text "Step 1" loaded on my phone's browser and it appeared as though I had come across one of two things seen before at instructables: (1) a brag'ible, in which you only post a picture of what you made and tell us how great it is or (2) a bait-n-switch, in which the post started great, but then redirected us to a link for something completely unrelated, while failing to ever provide info related to the title of the post.

    I only realized that the error was all with me and my phone once I viewed this thread from my laptop today and saw the full post (which failed to load via my phone last night, even after I reloaded it a few times thinking it was my internet or browser)... ends up that my WiFi just didn't reach my room, providing a partial load of the webpage, and thus I only saw a small part of what you provided.

    Regardless of what was experienced on my end, I jumped to a conclusion and wrongly blasted your good efforts; for that, I apologize.



    2 replies

    Reply 3 years ago

    Despite technology, I forgot to follow some advice I was given a long time ago: "say what you mean, but don't say it mean". Corny I know, but a change in my initial tone would have yielded a completely different discussion... and with that, I will be removing my post of ignorance!