Loss of air flow through grates clogged with coals / ash during boil

[tgormley358]

I'm working on adopting more frequent firing on my 2x5 raised flue this year, from 15-20 minutes last year to closer to 7-10 minutes, based on what i've read here. Two days ago i was boiling for around 8 hours when i started to notice a slower boil in my flue pan despite frequent firing, nice dry hardwood and hemlock (50-50) which had been burning very well. I never figured it out that evening but it extended my boiling an hour or 2 beyond what i expected (late night sunday). Thinking back to what i was seeing, i recall seeing the fire box very full with a bed of red coals, 4-6" deep. I would load right on top of that, but i'm thinking now that i'd lost air flow through the grates due to that thick bed of coals clogging the gaps. If i let it burn down, then i won't be firing as frequently. Do i need to take a break in boiling at some point, let it burn down and clear the grates to bring back my air flow? Is there a way to avoid having to do that?

I have a blower i haven't installed yet, and guessing it might help with this by turbocharging the burn and blowing constant air through the grates. Do you agree? I don't have an airtight front -- double doors that have a 1/4" gap between them closed.

[Buddy 58]

You might want to try doing what I do, I have two farms , sugar operation is not at the one I live at . When I'm done for the day , fire is completely killed . ashes and coals completely cleaned out . Doors - draft door open , letting cool air in . when I leave I worry about nothing ! That way always have excellent draft , also letting ashes and coals build up , leaving thing hot can start your grates to start to sag after many seasons .

[Ghs57]

Adding AUF may help with the build up. My AUF keeps the fire hot from the bottom up, and also helps with removing ash via the chimney, but I still do a clean out before each firing. So try adding AUF using your blower. Adding air (both AUF/AOF) made a huge difference in my boil rate, but you also have to time your firings so that your boil remains constant. Also a probe thermometer in the chimney will let you monitor the heat going up the stack.

[lords sugaring]

Hey Tom, what is the thickness of your wood ? Is it split to wrist size ? Is your draft door all the way open ? If your putting in whole logs without being split to the proper size they're going to burn much slower

[tgormley358]

My wood is split to wrist size or thereabouts. I do clean out the firebox and grates very well before each boil, but not immediately after. That could lead to some even later nights ... I open the draft door variously during boil depending on what i'm seeing -- more open if the flue pan isn't boiling well, less if the front pan 's boil is weak. Think i've got that right. Since it's only happened once i'm not too concerned and will keep an eye on it in the future to see why it's happening. Meanwhile, i've got to cut open the back of the firebox for my blower.

Lords, are you using a blower on yours? I know we're using the same evaporator. If so, how have you gone about making the doors more airtight?

[Ed R]

Get a black iron fireplace poker or something similar to rake the top of your grates under your wood before you load your wood in each time you fire. Clears the bottom out so air can get up between your grates. I use the poker to open and close the fire doors, so I have it in my hand when I am firing anyway.

[Big_Eddy]

Mine does that if I over fire. If I only fire what burned it doesn't. Much more with hardwood than soft.
I try to keep the firebox half full only. Works better for me.
Sometimes I lie on the floor and rake the coals aside under the grates. Boil picks back up immediately

[tgormley358]

Thanks for the ideas. Today's boil went well, no issues like sunday but i'll try these ideas if it happens again.

[Michael Greer]

I have a narrow-bladed poker that I push in and out under the fire. Every couple hours will keep you burning hot.

[maple flats]

I also use a poker to rake the coals, not every firing but maybe once an hour. When I do, I get several of the openings between the grates opened to breathe. Then the fire does not get choked off.

[WhistlePig]

Big_Eddy hit on a question I have been wondering about in my second year of boiling. How high to fill the firebox. Should I pack it nice and full so the heat is right up under the pans or lower so I close the doors sooner. I feed it every 8 minutes. With nice wrist sized wood, it seems hard to keep it heaped up nice and full. The other question is how far back to push the logs. I have a 42" x 8' grimm with 3' deep grates and auf. Should I only fill the wood over the grates, or also the sloped section beyond the grates that slopes up under the flue pan?

[EBG18T]

I have almost no ash/coals or anything after the past couple cords of wood. I fire every 7 min, my firebox only gets filled 2/3 full. I only put wood over the grates since my drop flues would interfere if I went to much further in. I can hear the air flowing thru the box.


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[maple flats]

I fill my 3x8 raised flue to 2/3-3/4 full, every 9 minutes, mostly hardwood, but some hemlock slab mixed in.

[Super Sapper]

You had the right idea on firing more often but if you are getting a buildup of coals you are adding too much wood at a time. A blower will make a big difference as it will burn the wood faster. With natural draft you have only so much air for combustion so if you add too much wood it will not burn as well.

[Big_Eddy]

Big_Eddy hit on a question I have been wondering about in my second year of boiling. How high to fill the firebox. Should I pack it nice and full so the heat is right up under the pans or lower so I close the doors sooner. I feed it every 8 minutes. With nice wrist sized wood, it seems hard to keep it heaped up nice and full. The other question is how far back to push the logs. I have a 42" x 8' grimm with 3' deep grates and auf. Should I only fill the wood over the grates, or also the sloped section beyond the grates that slopes up under the flue pan?

I stoke about 1/2 to 2/3 full and pull wood forward, not back. Too much wood and air bypasses under it and the syrup pan does not boil. Wood at the back of the firebox or on the ramp is only heating half the evaporator so inefficient.
Add enough wood to replace what burned only. If you are getting coals drop to 1/2 full only.
Normally I burn ash and beech.
I was burning pine slabs for my first boil this year. I had to fill higher and reduce the interval but boy did it boil!

[Michael Greer]

The type of wood you're burning makes a huge difference too. I burn hardwood slab, and if I run into a section of the pile that's all Beech, my firebox fills with coals. Maple, Ash, and Cherry don.t do this.