Thermal mass question

Inconsistent temps across the chamber are just a matter of the smoking life.  Heat flows out of the firebox into the smoking chamber, immediately rises, seeking the quickest path to the exhaust.  Your first priority should be redirecting the heat flow, not positioning thermal mass.  Placing baffle plates to distribute rising heat more evenly up into the cooking area would be a better use of your efforts than positioning thermal mass. 

The only way you could really use thermal mass in a horizontal offset would be to initially load it near the firebox to absorb heat, then move it to the other end.  Once you move it, it will start to cool.  As it gives off heat, the temp across the box will be more steady, but as it cools in the colder part of the smoker, you're right back where you started.  You'd have to reheat the mass near the firebox and move it again. 

In a horizontal smoker like an offset, I would think tuning plate would do more to even the heat distribution than thermal mass.  Thermal mass would however help to moderate temp swings and recovery from opening the pit.   Water as a thermal mass is well suited for that magic low 225* range in a smoker but unless the offset is built to hold a water pan, not so easy to use in an offset.

Water by it's nature wants to stay a liquid as heat/energy is pumped into it.  It takes a huge magnitude of extra energy to phase shift water at 212* to steam at 212*.  And you have to keep pumping that energy into it to keep it as steam.  Water is constantly wanting to fall back to it's liquid state and it actually sucks energy out of the air column in trying to do this.  It also absorbs a lot of energy as it falls back from steam at 212* to liquid water at 212*.   That's why water is almost the perfect heat sink and used in water smokers and works better at controlling an over temp spike than other thermal masses will (such as sand, fire bricks, etc...).  It just makes it easier to run in the 215-245* range a lot of folks want to be at.

One of the best solutions to even heating is having your smoker AIR TIGHT...  no air leaks anywhere...   Having adequate openings between the FB and CC...  adequate space under the tuning plates or RF plate... and an exhaust stack that allows for easy air flow.......  One additional item we found for even temps is an upper air inlet on the FB...   seen below....

...click on pic to enlarge...


When trying to adjust temps in the CC, if the fire, in the FB, gets too hot, and you have to close down the air inlet to reduce the heat, that step will severely reduce the air flow through the CC...  uneven temps are the result...   By opening the upper air inlet, the overheated FB will be cooled and air flow will still be maintained or even increased, evening out temps across the CC...   Some folks have found, leaving the upper air inlet partially open, significantly improves temps...   also reduces fuel usage by increasing temps in the CC so the air supply to the fire itself can be reduced....

This upper air inlet concept was introduced when folks measured the FB temp and found it to be 200, 300 or even 400 degrees warmer than the CC....   the fuel was being consumed without heating the food... 

Anywho, an airtight, really airtight cooker is the key to temp control.....   

Good discussion.

Sorry for the topic drift, but the comments about keeping it air tight got me thinking:

I have an OKJ Highland. I RTVed the firebox and chimney, and installed Lavalock gaskets on the doors. But I noticed I get smoke coming out of the drain hole at the far end of the cook chamber. Is this normal?

I know I could tap the drain and install a ball valve, but that seems like a bad idea.

Thermal mass is good, I use steel plates in my firebox and have cast iron grates on my reverse flow plate along with baffles from the firebox to the cooking chamber. Adding thermal mass helps out with "Heat Recovery" not necessarily evening out temps.

I don't sweat a tight smoker, sometimes when running the pit, my firebox door is left open a crack, like in instances where I just added splits and get a flare up.

Same with my smoke chamber, I don't sweat it being tight, if you have proper draft you shouldn't get smoke leaking from the cooking chamber or firebox.

Baffles will help with evening heat out a bit just like tuning plates.

What a lot of folks don't realize when using a stickburner is Fire Management and proper drafting can be quite effective against temperature fluctuations across a given area.

If your firebox always has a lot of flame and less coals, it's hard to maintain even temps, a heavy coal bed and proper drafting will go a long way in helping even out temps.

Keithu said:

Good discussion.

Sorry for the topic drift, but the comments about keeping it air tight got me thinking:

I have an OKJ Highland. I RTVed the firebox and chimney, and installed Lavalock gaskets on the doors. But I noticed I get smoke coming out of the drain hole at the far end of the cook chamber. Is this normal?

I know I could tap the drain and install a ball valve, but that seems like a bad idea.

Click to expand...

Smoke exiting other than the exhaust stack could indicate too small of an exhaust diameter and/or height...   The height of the exhaust stack is necessary to induce draft...   inside diameter of the exhaust stack is for adequate flow..  Try adding ~12" to the height...  you can use rolled up house flashing...   soup cans...  what ever you have to do a test...   It should be air tight where it couples to the original stack....

PS...  a wooden plug will work to plug the drain hole.....