DIY Secondary Combustion / Air

[Turbo_B]

I want to share what I have learned from hours of reading forums and watching youtube - an attempt to repay those who laid the groundwork for me.

I have a Suburban Manufacturing Wood Chief / Coal Chief stove insert that has faithfully heated my house(s) since 2006ish. This is been a great stove, 4cf fire box, ash tray, two speed blower with snap disc, and two small windows.

The stove makes good heat but began to bother me with the amount of heat lost up the chimney - there are NO baffles in the stove. Not wanting to continue being wasteful I decided to buy a Quadra Fire in the summer/fall of 2020. Gotta save up for these expensive things! As I read the manual for the new stoves, none of them have ash pans - I don't want to shovel ash from my stove! Seriously, the lack of an ash pan was the straw that broke the camel's back on this new stove idea!

I did a fair amount of reading on retrofitting secondary air to old wood stoves. After some measuring and thinking I took a stab at making my own. All parts came from Lowe's and Tractor Supply.

I used 1" black pipe to make the secondary air hoop around the top of the stove. The air distribution bars are 1/2 black pipe. Air control to the secondaries is by a 5" long tube that I plasma cut the sides out of, then turned the threads out of a 1' on the lathe to make a shutoff valve. I made a "tray" to hold 1.5" thick fire brick baffle on top of the air manifold. There is a 1" drop from the front of the baffle to the rear. The gap between the firebrick and the top of the stove is the same area as my 8" flue liner.
the pictures will probably make a lot more sense than my text.

Final result? I believe I am successful! I get rolling yellow to blue flames across the baffle, once up to temp smoke from the chimney is nearly gone, and this thing is MUCH less hungry than before. I have no problem burning a load through the night. Just yesterday I relight a fire 14 hours after the last load. Stove temps come up to 750 right after loading and settle in around 550-600 (through the top air vent), 450 on the doors.

I watched a bunch of youtube videos, and get the impression that some of them are for show. I occasionally get long yellow jets of fire, but that doesn't last long. Light blue rolling flame across the baffle occasionally lasts for hours!

I had to reseal my doors and make new hinge pins before this thing really started to work. My primary air comes from two hand wheels on the ash pan door, I believe it would be better if my air came from the back of the stove, but that is a modification for another day!

I dont' see a hyperlink button, so here is a link you may need to copy for a short video - really it is a summary of the attached images.



Thanks for the help from those before me.
-Robert

 

[begreen]

Cool project. Looks like the stove has a new life. Is there a full liner on it to the chimney top?

 

[Turbo_B]

Sure is, 8" 316Ti single wall liner to the top of the chimney.

 

[begreen]

Good deal, that is helping pull air through the secondaries.

 

[Turbo_B]

For years I ran stoves direct venting to the fireplace. This past spring the stale smoke smell was bad enough that I decided to install liners. (easier to plug the liner at the stove when not in use)

After installing adapters to convert the square firebox opening to round, I ran 8" 316Ti liner to the top of the chimney with a plate(sheetmetal termination) at the top. It is AMAZING how much heat was being lost to the open chimney, and how much better the stove drafts!

I have two pre-EPA wood stove inserts in this house. With equal loads the modified stove will burn twice as long as the unmodified.

 

[evanwhat]

This is excellent, thank you for sharing. What kind of burn times would you typically get before the modifications?

I've been learning to heat our home with wood this year and have an old Earth Stove (circa 1977) and I'm lucky to get 3 hours from one load. Anyways, I'm pretty sure I'll end up burning at least 5 or 6 cords of wood this year to keep this house warm, which is a lot of wood (and time processing it). I keep going back and forth between trying to add a baffle to my own rig and possibly secondary combustion tubes as well. My biggest hesitation to giving it a shot is the fact that my stove doesn't have a window on the door.

Ever come across anyone that's added secondary combustion to their stove and done so blind?

 

[Turbo_B]

I probably got 2 hours of hot burn, refueling at 3 hours. I could stuff it at night and choke it down and have coals in the morning- but blowing close to cool air.

I get closer to 4-5 hours of hot burn now, then coals last for much longer. I can load it at night, turn the primary air down and have a nice coal bed in the morning

Tonight I started the fire at 5pm, came home at 9 and still had strong coal bed. Tossed in three fresh logs and it took right off.

 

[Turbo_B]

I have been using the modified stove for about two months. It makes great heat and seems to burn longer than it did in its original configuration. The glass sure stays cleaner!

I found that it would take close to 45 minutes of a hot fire to get the secondaries to light and stay light, even if it was a restart from coals it still took that long. After a lot of reading I decided the hard fire brick I used was transferring heat instead of reflecting it back into the firebox.

I ordered some 1900 degree vermiculite insulation board from the pottery shop Euclids in Ontario Canada. They were the cheapest place I could find...

Link for anyone who wants it

Results, the secondaries definitely light faster which was my goal. The secondaries also burn more evenly left - right.

Learnings - hard firebrick works as a baffle, but takes a long time to heat up. Vermiculite insulation is much better for the secondaries.

 

[Turbo_B]

Someone asked about my burn times. Here is my coal bed after 7 hours overnight burning year old Live Oak.

 

[Turbo_B]

I also started learning Arduino and C++ for projects at work, which made me curious if I could add automation to my antique stove. I put together the code and hardware to switch the fan from LOW to HIGH based on firebox temp, and close the air inlet valve based on flue temp. I installed the flue thermocouple two days ago so I am watching this closely until I feel it can be trusted.

I added wood to the fire this morning, hit the "Load Wood" button and let it do it's own thing. Here is the result. The firebox temp sensor is about in the middle of the firebox wall, stove top with IR gun is 650-700.

There is just the slightest wisp of grey smoke from the chimney but the camera won't pick it up.

 

[begreen]

Very cool. That old stove is getting pole-vaulted from 1980 to 2020 in a hurry.

 

[blades]

Back before the big internet hack, there were many accounts of secondary conversions on the older stoves and furnaces ( even some fairly new furnaces). One thing that did stand out was the the black iron pipe had a tendency to get a bit too hot and start to droop. Stainless tubing was used for repacements of tubes in the stove- withstood the higher heat without drooping ( somehere in the 900-1200 degF range up at the top of the firebox). That said I congratulate you on your conversion.

 

[Turbo_B]

Thanks Blades - I had exactly the concern you mentioned. This black iron will glow brightly at times and is showing signs of wear from the heat. I was also concerned about the frame I made to hold up the baffle. Luckily no bending yet, but as parts fail I will replace with stainless.

The wood burner runs too well with the modifications to go back to "stock."

 

[Corey]

Neat project. Thanks for sharing. I've followed a similar path - retrofitting my old '80s smoke dragon with secondary air and air wash for the glass. Working on arduino temp monitoring for several key points on the stove and ultimately in the future, some sort of air control. So would definitely be curious to see how your arduino air control comes along.

I did go the stainless route - found an old industrial kitchen/butcher table at the scrap yard, so I suspect it's just generic 3-something stainless. Ran 2"x2" box beams for the 'manifold' part and 1" x 0.090 wall pipe for the burn bars. My hope was that the large volume of the manifold would mean slow air velocity and a lot of heating, and the thin burn bars would mean fast heat transfer and quick heat up. So far, ~15 years of use on the stainless - often at orange-hot heat and no visible degradation.

One thing that always struck me is the fact that from a 'wide open air' start, the stove will generally get hotter as you close down the air until a peak temp is reached, then it will start to get cooler as you continue closing the air. I always say the stove is like a carburetor, but instead of controlling fuel, you control air. Too little air and you get a smoky burn, too much air and you flush a lot of heat up the flue before it can be transferred to the room. It is similar with the secondary air system. Too little air and it's not doing anything, too much air cools the pipes and baffle and can knock you out of secondary burn. At one point, I even considered putting a wide-band oxygen sensor in the flue to help monitor excess oxygen!

The other thing is the hysteresis of the whole system. Any change in airflow may take 5-10 minutes (or more) to stabilize in the whole system. So the arduino needs to be a 'patient' controller!

 

[checkthisout]

Excellent, thoughtful mods. The arduino will make things even more interesting.

This is also a good opportunity to buy some stainless tube and learn how to weld.

Curious to see how much better it burns once you have insulating firebrick. The tinkering becomes addicting because you are now intimately familiar with just how much fuel was going up the stack.

 

[Turbo_B]

My current Arduino is pretty "dumb". It knows the firebox temp, it knows the flue temp, and uses that to change the blower between high and low. At 550 F in the flue it begins to shut down the (original) air valve, about halfway through the valve closure temps get up around 650 and then come back down around 450-500 once the valve closes fully. It isn't smart enough to reopen the valve. My current code is clunky using the ULN2003 and a 28BYJ from the Arduino kit. I have a TB6600 and a unipolar stepper for the future of the air valve - substantially easier to control than the ULN2003!

 

[mark cline]

I made a retrofit secondary air for my Fisher Grandma Bear a few years back, I hope it was helpful in building yours. It’s still working beyond expectations.

 

[Turbo_B]

Mark - it is very possible I read your posting! I made this post in hopes that I can help out someone else the way your posts helped me.