Project: 170mm (6.7inches) ceramic batch box bed and cooker

[Orange]

tnx satamax , opening to chimney is 22x14cm=308cm2 (system size is 170mm diameter, so CSA is 227cm2). I will enlarge it if necessary but first I'll try your startup fire method.

Forsythe - I have some insulation around firebox so back and right side of the metal bell don't get hot. I might insulate chimney pipe but what if chimney temperature becomes constant 200C - in that case I'm throwing heat out. I'll need to find balance.

[Forsythe]

Oct 8, 2021 6:31:58 GMT -8 Orange said:

I have some insulation around firebox so back and right side of the metal bell don't get hot. I might insulate chimney pipe but what if chimney temperature becomes constant 200C - in that case I'm throwing heat out. I'll need to find balance.

That’s a good point. If it gets that hot in the chimney, there is still heat that could be given to the room. But, you also don’t want to harvest all of the heat from the flue gas.There always has to be some heat loss up the chimney for there to be good draft through the system. A clean burn will mostly only exhaust CO2 and water vapor — which are both heavier than ambient air [when at the same temperature] — so in order to make those heavier exhaust gasses rise, they have to be hotter than the air around them.

You’re definitely right: Balance is key.

My thinking was to encourage draft in the early part of the burn, before the flue gasses have gone all the way through the heated bed / bench…so maybe a bypass damper would be the solution here? With the damper open, you could warm the chimney and get it to draw early — then close it once draft is established.

[Forsythe]

Another thought just occurred to me, which might be something to double-check for:

With all the opening-up of the ceramic brickwork to make recent modifications [and that central wall removal] — is there a chance some room air might be seeping into the bed/bench/bell where some mortar might have come loose?

You may already have good draft in the stovepipe and chimney, but if there’s a place where the system isn’t gas-tight anymore —even a small one— that could reduce the amount of draw exerted on the firebox unit at the front-end of the system, slowing down the pull of combustion gas into the heated bench/bed/bell.

One old-timer trick for leak-testing is to slowly and carefully pass a candle flame around the outside of joints while the system is running. [Provided you have still air in the room] the candle will flicker and pull towards the joint in places where it’s sucking room air in through cracks or seepage spots.

Just a thought, and it might be an outside chance…but if it were my system (and my own handiwork 😅) I’d check for that, as a potential cause.

Cheers

[satamax]

Oct 8, 2021 6:31:58 GMT -8 Orange said:

tnx satamax , opening to chimney is 22x14cm=308cm2 (system size is 170mm diameter, so CSA is 227cm2). I will enlarge it if necessary but first I'll try your startup fire method.

Forsythe - I have some insulation around firebox so back and right side of the metal bell don't get hot. I might insulate chimney pipe but what if chimney temperature becomes constant 200C - in that case I'm throwing heat out. I'll need to find balance.

227cm² x 1.5 at the minimum usually for the transition area, you're looking for 340cm². Absolute minimum.



Where you have written 1.4 system size.

Basically, at that spot, you have drag from the walls, from the floor, and the top lip.

Admitting that it is perfectly square.

That's about 17.55 x 17.55 centimeters.

Retract of that 3cm from each wall, which the usual thickness of the boundary layer in atmospheric conditions. Might change with gases density.

But for a rough explanation, that should do.

You then have 11.55x11.55 centimeters of full speed flow.

133.40cm². Of free flowing gases.

Far from your required 227cm².

I'm not absolutely sure it is where the problem is. But has proven to be the case quite often in the past.

[Orange]

satamax your method of fire startup is working well, there's no smoke comming back

Oct 8, 2021 13:30:58 GMT -8 Forsythe said:

My thinking was to encourage draft in the early part of the burn, before the flue gasses have gone all the way through the heated bed / bench…so maybe a bypass damper would be the solution here? With the damper open, you could warm the chimney and get it to draw early — then close it once draft is established.
...
You may already have good draft in the stovepipe and chimney, but if there’s a place where the system isn’t gas-tight anymore —even a small one— that could reduce the amount of draw exerted on the firebox unit at the front-end of the system, slowing down the pull of combustion gas into the heated bench/bed/bell.

with bypass open (1/3 of CSA) chimney temp quickly goes to 150-200C.

But even with those temps I feel like draft isn't good enough for the first part of the burn - that might be due to upper part of the chimney nor fully heated and small opening to the chimney that Satamax mentioned.

Later in the burn with bypass closed chimney temp stays above 100C so there's no need to insulate or I'll be pumping heat out.

Bench and firebox are airtight but metal bell isn't perfect as there are some metal on metal joints but there should be too much air leak.

[Orange]

lighted full batch, after the middle of the burn CFB box seems to be glowing:

 - CFB is rated at 1250C max, I've coated it with waterglass + clay

 - there is "burnt" smell, I really hope it goes away (not sure but think it comes from CFB as perlite and air-contrete are pretty inert and iron is uncoated)

 - bench gets too hot to sit on ranging from 80C to 150C, I have to burn less or increase ISA (I wouldn't like to increase bench height)

 - I've recalculated ISA an it's now around 5m2 (according to the specs it should be around 6,80 m2)

 - overal design seems great - enough power, most heat radiate low and towards the room, bench heats up quickly (magnetite brick are hotter than surrounding clay bricks)

[satamax]

You should be all right soon.

When it is cold.

My chimney temps are 60/80 celcius at the first burn, 120 at the second, 180 at the third usually. Taken at standing height.

Either start your fire early in the morning, or in the evening, when the sun has set, and temps cool. Lighting it during the day in the autumn or spring sometimes is troublesome.

Like you have 16c° inside, 18c° outside, you're bound for trouble.

You have the typical batch burn pattern inside, the soot is burned at an angle.

When you do more fires, it goes away, then comes back, in spring or autumn.

[fiedia]

Orange,

Regarding the bench which is too hot:

It seems that you have just one layer of brick on top of the bench. Am I right ?

You can add one or two layers on top (5 to 10cm). It will take more time to warm up but also to cool down (it stores more heat). Start with one layer and you will judge if you need it thicker.

[Orange]

Oct 14, 2021 9:20:40 GMT -8 fiedia said:

It seems that you have just one layer of brick on top of the bench. Am I right ?

You can add one or two layers on top (5 to 10cm). It will take more time to warm up but also to cool down (it stores more heat). Start with one layer and you will judge if you need it thicker.

you're right, one layer and I envisioned that it heats up quickly and that edges are warm etc. Like you said - thicker bench's disadvantage is more time to warm up and also increased height is not ergonomical.

I'm thinking of adding thin wood or metal grill so I can put matress on top so it doesn't start burning.

[fiedia]

I would worry about wood vs temperature. If you put a mattress + wood on top of your bench, you will insulate the bench and brick temperature will rise up. Since you already measured 150°C, direct contact with wood is risky.

[Orange]

with full flames upper window is interesting but not in the other stages of the burn:

[satamax]

So, how is it running now? Two things, you're not really supposed to open it while running. Just at the ember stage, to refill.

And i find you're not filling that firebox much. On mine, it's crammed in there. So air goes through all the gaps.

[Orange]

it's running well but I'm not ovefilling firebox because it seems that CFB is still off-gassing. Remember Solomon measured over 1300C in J-tube so I'm not sure what will happen here. CFB is coated with waterglass and clay and clay is peeling off in the riser. Also space is only 16m2 so I don't need so much heat at this point.

I prefer the bricks to have "wet look", might try coating them with waterglass or something:

[Orange]

more wood, more fire:

I'm not sure if CFB is glowing or the clay lining on it

Startup is easy with colder temperatures and satamax method. After like 20min it doesn't smoke back when the door is opened.

[Orange]

a few updates for this season:

CFB clay lining got peeled a bit in the riser so I painted it again (it's also coated with waterglass):

brick on the right started leaning inwards - as perlite is settling down it's pushing the brick so I tried to prevent that by adding tile on top but it didn't work.

I also added the sloped bricks in the bottom made from air-concrete + tiles (for low mass). This also prevents side bricks from leaning inwards:

Sloped bricks are holding without issues, can't really say if combustion is better or not but the firebox volume is smaller, I have to clean ash after around 3 batches:

Steel chimney also painted with clay, looks more natural and clay doesn't emit any VOCs (I hope):

welded some decorations for more pleasant look and I also want metal bell to disperse heat faster: