Tackling the buildup of coals in a T5

[KeithO]

I have read of many folks having a problem with the coal buildup from 24/7 burning. I have been adjusting to my T5, after last season burning a Morso, which has a cast iron grate built into the stove bottom. By contrast, the T5 has a firebrick floor with a small "ash chute" that I don't use because the ash drawer fits pretty badly and the concept of the ash chute is pretty weak to begin with. How do you convince all the ash to make its way to an opening smaller than 2x2" at the front middle of the stove ?

So I have similarly had the issue of a big buildup of coals and the issue of separating ash from coals when making space for wood again. So here are a few of the rules I have learned regarding how my stove works:

1) When setting the stove up for an extended burn (which is what most of us non retired people do..), we do the usual process: break up any big coals, spread the coals around to "level the field", remove any obvious ash deposit on top and at the front from the previous burn. Pile on the new splits, open the primary air all the way to char the new load and then reduce air to the "extended burn" mark and leave.
2) With the primary air at the "extended burn" point, I find that the vast majority of air entering the stove comes through the secondary combustion system, thus being directed top down onto the wood below and burning off any smoke that may come off the wood. It is the fact that air impingement is primarily from above that I believe is the cause of the large coal bed. It doesn't take long to form a layer of ash on top of the coals, which blocks access of the air to the remaining fuel, thus slowing the proces down and preserving the coals for a long time. Many of the coals are buried to the point that they die out and sit in charcoal format, just like many have said.

I think this situation is exacerbated by too high a draft, since the stove has stops built in to force a certain level of draft, even with all of the draft levers fully closed under "minimum" chimney conditions (low draft). The result is that in practice in northern latitudes with low temperatures, steep roofs and tall chimneys we need to operate our stoves at "minimum air" almost all of the time, that is if we do not need supplemental dampers to reduce chimney draft to a sane level. Under these conditions, minimal primary air is supplied, with the bias strongly toward secondary air only, over which we have no (legal) control.

So by evaluating this picture, the right way to address the situation would be to balance the airflows of primary and secondary air, but none of us has a stove equipped to do this, since at the start of the burn one needs a lot of secondary air to burn off smoke, whereas at the latter (60%) of the burn one needs more primary air, directed under the fire to enable efficient burning of the coal bed. This is no secret, just look at any stove designed to burn coal and you will find the air directed below the fuel, as well as the grate to allow the small ash particles to fall through so as not to impede the flow of air.

So, at the risk of being long winded, here is how I have dealt with the situation. First of all, since ash tends to be fine, it also tends to end up at the bottom of the heap in the stove. After making a mess trying to rake live coals to the side to get to the ash, I tried a different way. I take my long handled ash shovel and simply shove it in the stove, trying to keep it directly on top of the firebrick. I then lift it up (pushing live coals to the side as it come up) and if required, I may give it a slight shake to make remaining live coals fall off. Out it comes and dump it in the ash bucket. Shove it in another time, this time just push it further back under the coals, lift it up again with the same result. I find that I get out almost all of the ash and carry out only a few small live coals. Repeat this procedure from one side of the stove to the other, working the front, middle and back (the T5 has a major depth dimension) then spread out the remaining coals (which are now exposed to air and glowing brightly). Load up the new spilts, char them and reduce the air as usual.

If I am home (mornings and evenings) after getting out the ash and loading a new load of splits, I open my ash chute. This lets in air below the base of the coal bed. I am generally able to reduce the primary air to the fully closed position, but now the air will be entering below the coals, rather than over the top. There is still plenty of secondary air, so no problem with smoke from the fresh load. Visually, the fire looks different, since the coal bed is glowing bright red, whereas with the ash chute closed it is dull. Under these conditions the coal bed is burnt down progressively at a faster rate than it builds up from the new wood which is added. If I go to bed or leave, I close the ash chute and burn with the primary air in its "regular" position and of course I will over time get the same buildup as before. That said, I no longer have any problems with making space in the stove to fit wood, nor do I have to shovel out charcoal or live coals.

I know all readers here do not have the same type of stove, so one has to figure out what could work in your situation. Perhaps an additional small "window" is added to the ash door to provide air under the fire (much safer than cracking the ash door).

Keith

 

[madison]

?? Next to last paragraph ...' If I am home (mornings and evenings) after getting out the ash and loading a new load of splits, I open my ash chute. This lets in air below the base of the coal bed....'

I have considered this, but was concerned with the possibility of NOT being able to close the ash chute (ie jammed coal or malfunction) and having a "runaway fire". Is the primary air shut down while you have the ash chute open? Any concerns about not being able to close the ash chute?

At current I am raking all coals to the front, and using the mound to lift the front edge of the N-S loaded spits. I have also had some success reducing the coal bed by raking the coals into a front AND center mound, two larger N-S splits on either side of the mound, and three E-W spits on top of the two N-S base layer. The channel creates an inferno that takes care of the coals. Though this is with the T6, which is wider than the T5.

I do not have any softwood, all semi seasoned hardwood. Though I may be keeping my eye out for pallet wood etc to help with the hot burn off of the coals.

 

[KeithO]

I don't find the draft through the ash chute to be dramatic at all. I think the air has to flow through the bed of coals, which adds its own resistance. Compare this to open primary air or a cracked window or bad rope seal which lets air in without any further impediment. The closure spring on the ash chute door is pretty strong.. If it is not going to close properly, it would be because the opening was blocked by ash or coals, which in turn also block the flow of air. By comparison, my Morso would turn into a rocket if I opened the ash door for more than a few seconds. The draft would become so strong that an acoustical resonance was set up so that the chimney became a sort of organ pipe "whistling" at a low frequency like 30-100hz. It scared the bejebas out of me the first time it happened.

 

[InTheRockies]

I think all non-cat EPA stoves suffer from this, at least during the heart of heating season when we're all burning more wood. I didn't have a coaling problem during shoulder season, only recently as our temps have really started falling and I've started burning more wood. Not having an ash pan, I found the best thing for me to do is modify my burn behavior--now toward the end of the burn cycle I've started opening the air up to burn the coals. If I still think my coal bed is too big, I follow the advice given on woodheat.org, which someone else had linked in an earlier post on coal buildup--(broken link removed to http://www.woodheat.org/tips/toomuchcharcoal.htm) The advice in the link really works. I find it best to use a very small split when following their instructions. We all just need to be flexible and perhaps tweak our stove operating procedures slightly to account for changes throughout the heating season.

 

[Todd]

I suggest raking all your coals forward, reload and burn with a higher air setting longer before dampering down. This should turn those coals into ash and produce new coals towards the back when its reload time.

 

[precaud]

Use Todd's advice, plus use piece size to your advantage. You're looking for a balance between coals production and coals burnoff. Coals are burned off by air washing over them over time. But that's also what stimulates the load to burn hotter, which isn't necessarily what you want. If you stuff the stove to the gills, chances are the coals are going to build up. The solution is to use smaller loads of larger splits, which will gasify slower.

I can burn my Quad continuously on loads of two 6"+ splits (actually, rounds are better) at about two hour intervals. If I add in smaller pieces to fill in the gaps, they gasify quickly, stimulating the fire and upsetting the balance.

This technique doesn't work with the Nestor Martin. With it's tall-and-wide geometry, the floor area is so small that it fills up with coals after every load. Fortunately, it has a grate and separate control for under-fire air. So it's a simple matter of cutting off the main airwash and secondary air, opening up the underfire air, and burn down the coals just like you'd do with a coal stove. Works great.

 

[KeithO]

To Madison's post, I checked carefully and am only running with the ash chute cracked, not fully open. So his concern of an overfire with the ash chute open is justified.

Below are pictures of the flue temperature (probe type) and what the coal bed looks like while running with the ash chute cracked. No-one will complain that not enough heat is produced !!

Since I don't use the ash chute for any other purpose other than burning off coals, I think I will replace the swing down door with a fixed plate with a draft control built in, this will enable precise (risk free) control of air to the coal bed.

I think burning off the coal bed with primar air open is just a great way to send heat and energy up the chimney. Some of the primary air gets to do its job burning coals, but the majority just absorbs heat and goes up the stack. By getting air below the coals, you are able to combust the coals without adding splits or adding unneccersary primary air that essentially turns your stove into a sort of open fireplace.

 

[madison]

To Madison's post, I checked carefully and am only running with the ash chute cracked, not fully open. So his concern of an overfire with the ash chute open is justified.

Below are pictures of the flue temperature (probe type) and what the coal bed looks like while running with the ash chute cracked. No-one will complain that not enough heat is produced !!

Since I don't use the ash chute for any other purpose other than burning off coals, I think I will replace the swing down door with a fixed plate with a draft control built in, this will enable precise (risk free) control of air to the coal bed.

I think burning off the coal bed with primar air open is just a great way to send heat and energy up the chimney. Some of the primary air gets to do its job burning coals, but the majority just absorbs heat and goes up the stack. By getting air below the coals, you are able to combust the coals without adding splits or adding unneccersary primary air that essentially turns your stove into a sort of open fireplace.

I used the ash chute today for the first time to clean out the stove. Previously i had just used a shovel. Interestingly, after I removed and emptied the ash pan, I inspected the ash chute flap and it had not completely closed, though i believe it was occluded with ash. So I personally would be a bit hesitant to use the ash chute technique.

I would also weakly argue that if you are concerned with sending heat up the chimney, i don't personally think that it matters if you use the ash chute or primary air, both would introduce fresh air.

With 24/7 burning of NOT the best seasoned hardwood fuel, I have only needed to clean out the box about once per week. Though I am letting the stove cycle completely, and raking the coals front and center and the end of the burn cycle a couple times as well as opening the primary air gradually during this process.

How would you modify the swing down door with an adjustable plate ?? A drawing of your design would be interesting to see.

Thanks

 

[KeithO]

Here is todays picture after I cleaned the glass a little (the second time this season..)

 

[branchburner]

...my Morso would turn into a rocket if I opened the ash door for more than a few seconds. The draft would become so strong that an acoustical resonance was set up so that the chimney became a sort of organ pipe "whistling" at a low frequency like 30-100hz. It scared the bejebas out of me the first time it happened.

It's like feedback from an amp, and it gets real loud real quick. Part of me wants to see how loud it can get, but the bigger part of me is afraid it will vibrate the entire chimney right down on top of me! ("I'm givin' her all I can, Cap'n -she won't take any moore!")

 

[Elfin]

We haven't used our ash pan at all, nor have we cracked it to help burn the coal bed. I wonder if others have an outside air kit? We do, and find that our biggest problem is that this stove wants to burn too hot... it seems to get plenty of air, and we frequently damper down all the way. Therefore I'd be really nervous to open the ash door... I think things could get very scary very quickly if we introducted extra air to the fire.

 

[KeithO]

Elfin, there are several air intakes on this stove. If you remove the ash pan, the primary air intake is easy to see. On my T5 it is a circular opening just under 2" in diameter and a welded stop prevents the control from closing off the last 1/4". I was surprised how big the "minimum" intake was. Then on the left underside there is a hole about 1" in diameter for the air wash. And at the back of the stove is the secondary intake hidden under a U shaped steel cover (welded in place) that I found was more than 1/4"x1.5" wide.

If you add these 3 surfaces together, it makes for a lot of airflow into the stove. I plugged the air wash intake with a small piece of sheet metal held in place with furnace cement. Given that I today cleaned the glass for the first time since I started burning, I don't see the problem. I blocked about 80% of the permanently open space at the primary air intake (what would be open with the primary control closed all the way). I decided to leave the secondary alone, since on a low burn with risk of smoldering, this is where almost all the air will come in and burn the smoke (taking into account my modifications).

The result is much better control of the burn, given my draft conditions. Now, when a big coal bed has built up and the fire is in the coaling stage, the primary problem people mention is insufficient heat output when they try to burn off the coals. The volatile stage of the burn is over and just the carbon is left. Not much risk f overfire at this point, providing you don't add new fuel or let in too much air. I have tried all the recommendations with burning with the primary air full open and the coals raked to the front, but the fact is that all that air does not penetrate into the coal bed. I find that no matter how much air the top of the coals gets, the flue temperature will not rise higher than 300F, which is explained by all of the excess air, just like a fireplace. As I have shown, if you direct the air below the coal bed, a much higher % will actually react with the coals and burn them. In the picture I posted earlier , those weird orange geyser looking things from the coal bed are in fact the iridescence of the ash which is being carried out of the coal bed by the flow of the air. I have watched "proper" coal stoves burn and you get almost the exact same effect when they are running right, although the coal bed is much deeper of course.

 

[madison]

Experimented with the ash chute while the stove was burning.

Rather scary, as I had just added two spits (first mistake). The blow torch from the ash chute and goal bed torched off the two spits. Then the coals/ash temporarily jammed the ash chute partially open (second mistake). It took a bit of wiggling to get the ash chute clear and completely closed. The sound of the ash chute blow torch was strikingly similar to the sound from the turkey fryer burner on full blast.

Experiment over! I'll stick with the rake and small split routine if needed.

Though it would be nice to have some pinhole openings in the floor of the stove that you could open and close on demand to feed some air to the coals....

 

[Elfin]

Yikes, madison! I'll take your word for it! Though I think your idea is interesting, Keith, I'm too chicken to try it... I guess I'm just a rule-following, non-risk taking sort of person I'm really interested by what you wrote about the amount of airflow going into the stove... it really explains the quick hot fires I get in the Alderlea. Good luck with your experiments in adjusting it so it works better for you!

 

[KeithO]

I am just going to make a replacement adjustable air intake that bolts in place of the ash chute. Initially I couldn't believe that the ash chute was really bolted in (since everything else is welded), but checked with the flashlight and there are really 3 bolts that hold it in place.

The present design is certainly inclined to jam partially open due to coals being half in and half out. I would dispense with the flapper thingy altogether and replace it with a plate that had the usual triangular opening. Then add a pivoting control arm that starts uncovering the small end of the V first and when fully open the whole V is exposed.

I would use a full firebrick over the top of the opening, but groove the undersides of the firebricks to allow the incoming air to disperse across the base of the fire. The idea being that the air would enter the fire from the joints between the firebricks, of which there are quite a few. Using the ash chute as is tends to blow a hole in the coals right where the opening is, and consequently one has to keep migrating the coals over to the ash chute to burn them away. For unattended operation, the air needs to be well distributed. The inrush of air tends to keep ash out of the way, I have not had the system plug up yet, but with the new system that has to be proven.