Offset airflow tuning + firebox sizing question on co-specced build

[David from the UK]

Hi all,

About a year ago, I commissioned a traditional offset smoker here in the UK and co-specced much of the design myself. I specified the look, cook chamber volume, and materials (8mm steel throughout), but I left the firebox sizing to the builder, assuming it would be scaled up to match. In hindsight, I’m now wondering whether the firebox may be undersized for the size of the chamber, and whether that’s the root of some fire performance issues.

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Specs:

• Cook chamber: ~538L / 142 US gal / 118 UK gal internal volume (55" long, 27.5" diameter or approx. 140cm x 70cm)
• Material: 8mm steel throughout
• Firebox: 23.6" deep x 21.6" diameter (60cm x 55cm), semi-insulated design
• Stack: 5" diameter (approx. 12.7cm), 47" high (120cm) from collector, damper-controlled
• Fuel: Air-dried English oak (~11" splits), moisture typically 9–12%, some larger pieces closer to 18%
• Fire management tools: 4 Tel-Tru-style thermometers, Thermopro digital probes (grate and meat), anemometer to track airflow at the stack

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What I'm experiencing:

• Fire behaves well early on after establishing a good coal bed (I use lump charcoal + kindling to start)
• Temps initially reach and even exceed 250°F / 121°C, but the chamber then tends to "settle" toward 200°F / 93°C unless I'm constantly intervening
• Interventions include breaking up the coal bed, stoking frequently, or even using a leaf blower to kick splits into flame
• Even with decent coal mass, splits sometimes smoulder rather than catch cleanly
• Anemometer shows airflow in the 1.7 to 2.6 m/s range with the stack 65% open and the firebox door cracked to the 4th notch
• Letting airflow exceed ~2.6 m/s often causes temps to drop, which led me to reduce stack draw slightly, but that still doesn’t solve the need for babysitting

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What I’ve tried / learned from Reddit and experience:

• Running stack fully open appears to be the standard advice, and I plan to do that more consistently now
• Some have suggested my fire may be struggling due to sitting directly on the base of the firebox, so I’ll likely add a raised grate or expanded metal basket to improve air under the fire
• Ash buildup may be limiting airflow over time – I’ll monitor that more closely next time
• I’ve already been experimenting with split size and timing, but the "peak-and-wane" fire cycle still feels tight and unforgiving

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My core question now: Do these symptoms sound like the firebox is simply too small for the chamber volume?

If so:

• Would you expect heat loss / inconsistent chamber temps despite good fire technique?
• Could a higher-volume firebox have helped the pit sustain temp more naturally?
• Any suggested retrofit options (e.g. increased intake, deeper coal bed)?

Happy to answer any follow-up questions or post additional photos/measurements. Just trying to understand whether I’m fighting technique issues, design limitations, or both.

Thanks in advance!

 

[Heart of Dixie]

Try opening the stack all the way, leave the fire box door cracked to wide open. If cracked, have the vents on the door wide open. Build a good coalbed, and see how it flies. I see you already pre-heat your splits which I do even in the summer. I have owned my old offset for over 20 years and always leave the stack wide open. I hope you get it worked out. Beautiful pit by the way.

 

[chopsaw]

That's a looker for sure .

 

[Redicans]

Beautiful looking Pit! I would try the raised grate just to see how it works for you. I always leave my stack wide open and my fire box door closed but the vent wide open. Your fire looks more toward the cook chamber. Have you tried building it closer to the fire box door? I'm no Pro, just some things to try.

 

[FoxmanNC]

Good fire management video (Warning: I'm not an expert) That is one beautiful pit. The optimal firebox is 1/3 of the cooking chamber, from what I have learned. Stack wide open, firebox door open (monitor and adjust) splits diagonally across the curved firebox, creating a natural airgap above the coles, adjust the height of the split as needed for airflow. I use roughly 2x2x10 splits, make sure the moisture percentage of the wood is well below 20%, mine does better in the 10%-15% or below range (Moisture meters available cheaply on Amazon), use a poker to push and pull the split and or coals where needed for maximum airflow and temps. I doubt it is the size of the firebox.