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Having trouble getting my Reverse Flow/Offset working properly


dobson156
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Hi everyone.

I recently bought a used hand made offset, I think it's supposed to be a Reverse flow design.

TphIeWx.jpg

However I have been really struggling to use this with any sort of success.

I think there a few issues which I could do with some input on these are:

  1. The firebox doesn't seem to be able to get enough air to it with the door closed - when the door is open it seems better but a lot of the heat is the lost though the door.
  2. The baffle system used to distribute the smoke seems to have a significant damping effect on the exhaust path.

The design of the smoker is internally is as shown in this diagram.

SX5XNGC.jpg

I tried using it as I had hoped it would work. I loaded a starter chimney into the fire box and opened the intake vent wide open. White smoke poured from the chimney the but temperature gauge showed it really struggled to get up to a decent smoking temp.

I tried significantly increasing the size of the fire, it got hotter (but not really hot enough) but the smoke was thicker

I then experimented removing some of the channeling which makes the smoker a reverse flow smoker. First I removed the 'perforated floor plate' (2 in diagram) which distributes the smoke inside the cooking chamber and the temperature rose significantly - however the smoke was still thick.

I then removed the angled plate (3 in diagram) and the temp went up and the smoke went clearer.

Finally I threw some logs onto the fire and burnt them with the firebox door cracked open. The fire was then able to burn with a flame and the exhaust was clear with small amount of light smoke!

I was able to smoke a chicken spatchcocked chicken on the part of the cooking grate (1) directly above the firebox inlet and below the chimney. My goal is to be able to smoke a whole Brisket on this thing which I don't think I'd be able to do right now.

My question:

Is there something obvious that I am doing wrong?

I am starting to think that I need to get the grind out and increase the size of the air intake vents to improve airflow to the fire.

I am also starting to think I should try and convert it from a reverse flow smoker into a normal offset by take off the chimney and welding it onto the other end and not bothering with any of the floor plates.

More photos

https://imgur.com/a/1T15WcG

PS: this might sound weird but if anyone live in the South Manchester/Stockport area and would be interested in having a look at it that'd be awesome - I'd pay travel, food, fuel and beer! 

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Replying to myself here, but I've been thinking overnight that it makes sense to go from the least invasive to the most step my step.

This is the current air intake

 

3jmvaRo.jpg

It seem to me that I could just grind out the 'triangles' so that the opening becomes a solid circle. That would probably double the the max air flow through that vent.

I might also try and convert it into a more traditional reverse flow smoker by using heat proof tape and taping over the preforations in the floor plate:

 

geQQxk8.jpg

And then ensuring there is a decent sized gap at the far end. I'll probably also tape over all of the joints between the floor and the angled baffle plates to ensure there is a tight seal and an obvious channel for the exhaust to follow.

Finally I think it might make sense to install more temperature gauges but lower down, just above the cooking grill. One at each end.

The current on is here:

TphIeWx.jpg?1

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@dobson156  Welcome to the forum matey.....I don't run a reverse flow myself,  but I've read up on them.  Going by a general rule of thumb of how bullet, kettles, and offset works I would say you do need to cut a bigger hole for the air intake.  Have you asked the previous owner how he got on with using it?? 

If you do have any bad gaps between the body and the lid you will need to use fire tape has you have said...link...https://www.amazon.co.uk/Stove-Adhesive-Black-Glass-Reeds/dp/B0772XYKVQ/ref=sr_1_12?keywords=fire+tape&qid=1559825494&s=gateway&sr=8-12

 

Ice.

 

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Cheers. I've actually got some of the fire door seal already as some had fallen off in transit. I used it on the firebox door. But I will apply it to the door of the smoking chamber too.

I think the first step is:

  1. Seal all the joints.
  2. Use heat proof tape to tape up the perforated floor baffle ensuring there is a wide gap at the far end
  3. Increase the size of the intake vent. 
  4. Add more gauges

I will then see if I can get decent fire going.

Question: when an offset is working as intended - should the wood in the firebox be burning with a flame? 

Right now if the door is open (and the baffles are removed) then so long as there is enough fuel I get a flame and I get clear exhaust.

If I close the door but leave the vents wide open then any coals (charcoal or wood embers) burn dimly and any wood i put on there smoulders and chars (goes a deep brown).

I don't want to contact the previous owner too much because he was selling it because he was too busy to make use of it. I did ask how big the fire needed to be before and he said:

Quote

it takes about couple of hours to build up enough embers to reach a suitable temperature So a good fire for long periods

So I am guessing he just built up a massive fire and then smoked in the residual heat - rather than use an active fire required for a long smoke. I think he used it to smoke things that didn't have too much cook time like fish.

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Don’t start grinding bits off yet, it looks like it has been well used so the previous owner may have not had trouble, although I suspect he may of, where is the extra bit of flue? Was this added to increase airflow?

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Hi Mick,

Which bit of extra flue are you talking about? The piece that connects the firebox to the smoke chamber?

RUuLoD9.jpg

I think that is there to allow the firebox door to open without hitting the chimney. He worked with flue and other combustion related industries so I think he had a lot of this sort of material around.

Edited by dobson156
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Also, I would like to avoid taking a grinder to it if possible. But do you think it's be a good idea to install some more gauges on it but lower - just above the smoking grill regardless as to whether any modifications are needed to be made.

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51 minutes ago, Icefever said:

If I can find it there's a site the works out the sizes of firebox to cooking chamber, this may give you some ideas on what could be wrong with yours.

ps...I've found it....link...http://www.feldoncentral.com/bbqcalculator.html

 

Ice.

I've just filled that in with the measurements off the top of my head. And they are in the right ball park.

The flue pipe has a diameter of 13.5inches and the I think the smoking chamber is roughly 2.5-3x longer than the firebox

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16 minutes ago, dobson156 said:

Also, I would like to avoid taking a grinder to it if possible. But do you think it's be a good idea to install some more gauges on it but lower - just above the smoking grill regardless as to whether any modifications are needed to be made.

I like to know the temp where my food is, that is where I want my temp probe so a ink bird type one would be very helpful

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6 minutes ago, dobson156 said:

Oh yeah, haha, sorry

I had to cut the chimney down to get it in my van.

I took some photos of it then, then put the rest of the flue on.

All of the above experiments I've detailed above has been with chimney at its full length.

Oh lol, I get it

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Cheers,

I will get some digital thermometers, some more door seals and some heat. 

So do you I suggest I do next?

I am conscience about how many pounds of oak & charcoal and hours of time I am investing in dud burns.

The last time I used 2 Chimneys of charcoal (started with one) and then added wood on the top. With the door close the wood would just smoulder.

Also even if I get the Digital thermometers with the door close the smoke is thick and white - which I think will impart an acrid tastes on the meat.

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3 minutes ago, Mick said:

Try to sort the problem out before adding wood, you should have a natural through draft, is the exhaust that you put back sealed?

You mean sealing the chimney flue onto the stump? It's a tight fit and doesn't leak and exhaust.

Like I said when I remove the baffling and open the door it works 'fine'. 

There is not specifically blocking the flow. 

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3 minutes ago, Mick said:

What happens if you just crack the door open a small amount

Without the baffles (I don't think I tried doing this with the baffles in - because I experimented with that first - then with cracking the door) then I think it burns with a flame.

But if it's just cracked a bit and there is a lot of fuel then the smoke might be quite thick.

This is what the inside of the firebox looks like:

xywRsgv.jpg

Note that the intake is below the ash grate. Just the angle of the photo makes it look higher.. 

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18 hours ago, Phlashster said:

I read in Franklin BBQ that the height of the chimney affects the draw through the chamber. Maybe that's the problem. 

Yes definitely does a longer chimney will pull a lot more air through

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