Serious fire coming out the top of the stack

[Ntatar]

On my test run of my 2 x 3 Mason this weekend I had 12 to 18 inch flames like those out of a jet engine coming from the top of the stack. My fuel source is chopped up pallets - clean kiln dried white pine used to transport snowblowers (from local garden center). When I touched the stack with a piece of Dry wood it instantly caught on fire. Is this normal?

[Chickenman]

Mine does that if I don't throttle the AUF back a bit. I had to put a fire brick about 3 inches from the flue opening to get more turbulence to burn off the gases that did not burn in the fire box before they went up the flue.

[Ntatar]

I can definitely do both, turn down the fan and put a brick or two in. Just to confirm, these measures will improve performance and efficiency?

[raptorfan85]

With the shorter 3 foot arch you are getting a lot of hot unburnt gasses from the wood. Once it reaches the air at the top of the stack it reignites. There most likely isnt fire shooting all the way up the stack. Do you have a blower? If so turn it down a little and see if that helps. As long as the stack isnt glowing red and you have a good boil i wouldnt worry to much about it. At least you know you have a nice hot fire!

[Chickenman]

And a clean stack. And yes I did notice a better boil in the rear of the pan and less "after burner" effect.

[Cjadamec]

I can definitely do both, turn down the fan and put a brick or two in. Just to confirm, these measures will improve performance and efficiency?

That much heat going out the stack is just wasted energy and will over time shorten the life of your stack. Get a stack thermometer and try to maintain a constant stack temp between 4-600 degrees.

Adjust you blower to keep the fire under the pan and the stack temp where you get a good consistent boil.

You may also need to feed the fire less wood per feeding but to feed it more often so that you get complete combustion inside the arch. With the dry small wood you are feeding the arch its easy to over feed and turn you arch into a wood gasification unit. The blow torch out of the stack is a good sign that you are gasifying the wood.

[eustis22]

how tall is your stack?

[Ntatar]

6 feet (stainless steel) with no cap

[maple flats]

That is strictly all unburned wood gases that ignite as it gets new oxygen. I had the same thing until I added high pressure air over fire (AOF). Then it totally stopped. For a 2x3 it is likely not worth the expense of adding AOF, but is worth it if you graduate to a 2x6 or larger.
Read about AOF in a Proctor MRC publication about combustion efficiency.
Wood does not burn, just the wood gases burn. Light a match and look closely, the flame will hover a short distance above the match, that is the distance it takes for the wood gases to mix with the oxygen in the air.
You have intense heat in your firebox, which generates more gases than get burned in the arch, as those gases get new air, they ignite. It may help a little to get more air into the fire, but it will never be able to burn all of the gases until you can add high pressure air, blowing down into the fire from above. A squirrel cage blower is high volume but very low pressure. The right HP blower will be high pressure but not much volume relatively speaking.
Just a hair brained idea, but if you can duct some of the air from your blower to enter near the back of the firebox and up about halfway to the pan and elbow it to blow towards the door it might help burn some of that excess wood gas a little, maybe not.
Do you possibly have or can you come up with an old defective oil burner motor/blower? That is a HP blower, maybe you can figure a way to get it to blow air into the fire from above to burn those gases.
When I added AOF to my 3x8, my boil rate increased, my stack temp dropped , my refueling time went from 7 minutes to 9 minutes and I no longer had a flame at the top of my stack. I was then burning the wood gas under the pans where it made syrup.

[Sugarmaker]

Pallets burn very hot! I used them for years! You might just cut back on how much wood you put in each time. Watch the boil rate keep a good steady hard boil with just the right amount of wood added. Yes we over fired ours and the stack got red hot!
Regards,
Chris

[eustis22]

I should have asked also how much is the diameter of the stack? I also have a 2X3 Mason with AUF and while I think my stack temps are around 6-700, I never get flames shooting out. My stack is 8" X 8 feet and I may yet add another 2 feet.

That being said, I cannot say if a taller, wider stack would mitigate your issue. I leave that for the experts herein.

[steve J]

I started with mason 2x3 and now have a mason 2x4 with blower. My experience is that if you run blower full throttle for very long you will see your elbow turn cherry red to the point that if you dont back off you will melt your stack down. I installed a 3 speed switch and run it at the second speed. I will on occasion turn it up to 3 for short burst with my eye always on that elbow.