Thermal mass question

Discussion in 'Wood Smokers' started by agonolin, Aug 23, 2017.

  1. So I own a CG offset and I love the thing to death but the temperature is so inconsistent side to side. I've done some of the easier mods and I have food safe rvt and some lava lock door gaskets on the way.

    I've been researching thermal masses and how they can help maintain and better distribute heat equally (I know some people disagree). So my question is where should I place them? Near the firebox or the other end of the cooking chamber? I have a couple bricks that I planned on using.

    So what makes sense to me is to put them on the side opposite the fire box so when they get hot they hold the heat on that side. But I was talking to a buddy about it and he said I should put it on the firebox side or else it will take forever for the bricks to warm up and just make the temperature difference even worse from side to side. So I was curious what the thoughts and consensus is from the people that have been doing this alot longer then I have.
     
  2. noboundaries

    noboundaries Smoking Guru OTBS Member SMF Premier Member

    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. 
     
  3. dward51

    dward51 Master of the Pit OTBS Member

    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.
     
  4. Would you guys consider 50 degree swings from one end to the other normal? And that's with baffle plates in place. I have two, one tight against the fb side and then the other so there is about 3 or 4 inches of a gap between them, which puts the second baffle in the middle of the cc. Is that a good placement?
     
  5. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    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.....   
     
    Last edited: Aug 24, 2017
  6. Okay thanks alot! I have some stuff on the way to help seal it up and then I'll have to try and play with it to see what helps. Thanks for all your help and I'll post results after I seal everything up
     
  7. keithu

    keithu Fire Starter

    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.
     
    Last edited: Aug 24, 2017
  8. sqwib

    sqwib Smoking Guru OTBS Member

    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.
     
  9. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    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.....
     
    Last edited: Aug 24, 2017

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