Building an offset cold smoker- a little advice please.

[stringman]

Hi all I have a very basic offset smoker which has served me well. its basically a wooden box 2ft high 2ft depth 2.5ft width with a removable lid/roof. (no chimney)
A small firebox with a pellet maze linked by a 4" tube.

I am hoping to build a slightly bigger one 3ft cubed
I will be using a small bbq (with clipped on lid) as the firebox and again was hoping to use 4" pipe to fit to the chamber.
for the chimney I was thinking of something like this

Pardon our interruption...

I wondered if there was a calculator or formula to work out the size of pipes I need for the size of the chamber?
I also wondered how to calculate the height of the chimney?

Many thanks in advance

 

[Jonahind]

Upgrade to 6" for proper airflow and match the chimney diameter to that. Make the chimney about 1.5 to 2 times the smoker’s height (4.5–6ft) to get good draft without overpulling heat. Smaller pipes choke airflow; taller chimneys can cool your smoke too fast.

 

[in. pred]

edit oops i didnt read the "cold" part in my comment. lol the only cold smoking o do is with cheese and i just use the tube in grill or smoker for that task.

 

[philpom]

Hi all I have a very basic offset smoker which has served me well. its basically a wooden box 2ft high 2ft depth 2.5ft width with a removable lid/roof. (no chimney)
A small firebox with a pellet maze linked by a 4" tube.

I am hoping to build a slightly bigger one 3ft cubed
I will be using a small bbq (with clipped on lid) as the firebox and again was hoping to use 4" pipe to fit to the chamber.
for the chimney I was thinking of something like this

Pardon our interruption...

I wondered if there was a calculator or formula to work out the size of pipes I need for the size of the chamber?
I also wondered how to calculate the height of the chimney?

Many thanks in advance

Since the whole thing is essentially a vertical chimney the height of your stack need only be a nub. I'd go 6" on the diameter but 4" would be fine and use something like this on top:

Many smokers like this don't even have a stack and just use gaps at the top to vent. I suggest piping the smoke in directly to the center of the bottom and using some kind of diffuser or plate to force the smoke to rise up from all over. The most important consideration is the temperatures you want to be able to achieve. This is controlled by the fuel source, the size of the fire, ambient temperature and the length of the pipe leading from the smoke source to the smoking chamber. If you have a big enough fire chamber then you can control this greatly based on the size of the fire. For 3' squared chamber and something like a 14" kettle for the fire chamber I'd start around 2 to 3 feet in length. You want it to run at about a 4" rise per foot of length. If you can't get the temps where you want them then shorten it to get it hotter and lengthen it to cool it down. Of course you have fine control sizing the fire and using your vents.

Good luck, it'll be cool I'm sure!

 

[stringman]

Since the whole thing is essentially a vertical chimney the height of your stack need only be a nub. I'd go 6" on the diameter but 4" would be fine and use something like this on top:

View attachment 722311Many smokers like this don't even have a stack and just use gaps at the top to vent. I suggest piping the smoke in directly to the center of the bottom and using some kind of diffuser or plate to force the smoke to rise up from all over. The most important consideration is the temperatures you want to be able to achieve. This is controlled by the fuel source, the size of the fire, ambient temperature and the length of the pipe leading from the smoke source to the smoking chamber. If you have a big enough fire chamber then you can control this greatly based on the size of the fire. For 3' squared chamber and something like a 14" kettle for the fire chamber I'd start around 2 to 3 feet in length. You want it to run at about a 4" rise per foot of length. If you can't get the temps where you want them then shorten it to get it hotter and lengthen it to cool it down. Of course you have fine control sizing the fire and using your vents.

Good luck, it'll be cool I'm sure!

Many thanks for the reply.
My old smoker didnt have a chimney and i just moved the lid to let the smoke out. That one had a very small firebox 10" with a maze smoker and about 3ft tube. I will use the maze smoker with pellets and similar length tube/
I will probably put a small chimney on the top with a cowl on top.
I am wondering with a chimney extension If I could do hot smoking (very low temp) as the chamber will be made of softwood

 

[radioguy]

Welcome to SMF. Take a look at this calculator. This was done by Dave [RIP] and will get you in the ballpark.
You can also Google "Feldon BBQ calculator "

have fun!

RG

Reverse Flow Smoker... How to calculate build tutorial...

> Reverse Flow Smoker... How to calculate build tutorial... By DaveOmak This is a tutorial and you will need a calculator and paper to note desired dimensions... This tutorial has been proven to make an excellent operating Reverse Flow smoker... Here's to "Alien BBQ" and the folks that first...

www.smokingmeatforums.com

 

[stringman]

Welcome to SMF. Take a look at this calculator. This was done by Dave [RIP] and will get you in the ballpark.
You can also Google "Feldon BBQ calculator "

have fun!

RG

Reverse Flow Smoker... How to calculate build tutorial...

> Reverse Flow Smoker... How to calculate build tutorial... By DaveOmak This is a tutorial and you will need a calculator and paper to note desired dimensions... This tutorial has been proven to make an excellent operating Reverse Flow smoker... Here's to "Alien BBQ" and the folks that first...

www.smokingmeatforums.com

That looks very interesting.
The problem I have is that 6" may well be unwieldy. I had already bought 4 inch pipe so may give that a try ( can always make a hole larger!!....)
So my smoker will look similar to the image attached. but with a longer pipe going to the chamber to cool the smoke.

Looking at the mass of stuff I bought when I was hoping to build a commercial fridge smoker, see posts from 2022. (that didnt work well- bought the fridge for £15, £30 on fuel,£ 30 on tools, abandoned idea, paid £100 to get recycled!!!) I have some sensors for heat and humidity. some heaters from a wine cooler/heater and various fans.
So I am thinking of:
on the inside untreated whitewood for walls and ceiling. Cement board for the floor.
large difusser plate sitting over the smoke inlet
Insulation between the inner and outer wall: lambs wool insulation
Outer skin, treated softwood shiplap.
on the outside a waterproof box with electrics to supply :
100w fin heater (for initial draw and/or hot smoking)
Led lighting
fans to circulate the air ( I know they will have a limited lifespan)
Temp and humidity probe
Indirect chimney fan to assist draw
Small chimney ( 1ft tall?) with indirect fan if needed
There will be the ability to move the lid if required.
With regards to the smoke chamber.
It will be exactly like the first attached pic with a maze pellet smoker.
The lid already has the 4 hole/sliding cover that is standard.
But I have also bought ( 3 years ago) two flip sink plugs. So I guess I had intended to fit these at the bottom of the bbq , as I know that wood needs air from the bottom. Obviously the rubber seals would be removed /replaced.
Any Thoughts on this set up would be much appreciated.
THanks

 

[stringman]

That looks very interesting.
The problem I have is that 6" may well be unwieldy. I had already bought 4 inch pipe so may give that a try ( can always make a hole larger!!....)
So my smoker will look similar to the image attached. but with a longer pipe going to the chamber to cool the smoke.

Looking at the mass of stuff I bought when I was hoping to build a commercial fridge smoker, see posts from 2022. (that didnt work well- bought the fridge for £15, £30 on fuel,£ 30 on tools, abandoned idea, paid £100 to get recycled!!!) I have some sensors for heat and humidity. some heaters from a wine cooler/heater and various fans.
So I am thinking of:
on the inside untreated whitewood for walls and ceiling. Cement board for the floor.
large difusser plate sitting over the smoke inlet
Insulation between the inner and outer wall: lambs wool insulation
Outer skin, treated softwood shiplap.
on the outside a waterproof box with electrics to supply :
100w fin heater (for initial draw and/or hot smoking)
Led lighting
fans to circulate the air ( I know they will have a limited lifespan)
Temp and humidity probe
Indirect chimney fan to assist draw
Small chimney ( 1ft tall?) with indirect fan if needed
There will be the ability to move the lid if required.
With regards to the smoke chamber.
It will be exactly like the first attached pic with a maze pellet smoker.
The lid already has the 4 hole/sliding cover that is standard.
But I have also bought ( 3 years ago) two flip sink plugs. So I guess I had intended to fit these at the bottom of the bbq , as I know that wood needs air from the bottom. Obviously the rubber seals would be removed /replaced.
Any Thoughts on this set up would be much appreciated.
THanks

Here is a couple of photos on the build.
The inside is all finished as is the roof, back and one side.
I am leaving the other side as I unsure (hoping for some feedback ) on whether it is worth installing the fan(s) and heater. I have also found a nice little device that measures temp and humidity so I might put that it as well.
Any Thoughts?

 

[stringman]

Well the build took forever (mainly due to the electric devices ( that I probably dont really need)
I am waiting for a blade fuse (all the electrics are 12v) before I can do the full test. But in the mean time I did a test/season smoke last night. It seems to work pretty good, in that there was smoke coming out of the chimney and if I opened the chamber it was full of smoke!
One thing I forgot to make was a smoke diffuser.
Now after the test I am not sure if i need it but I have the base from the bbq (that the coals would have sat on) so I could use that.
My question is what size should the diffuser be?

I have a 4 inch pipe going into the chamber.
the bbq plate is 12inches (round) with the center(6 inches) solid and the outer part with holes.
Would that work?
My last smoker just had a solid metal plate about 6 inches square that sat over the pipe.
Also what sort of height above the pipe should it be?
This is for a cold smoker only

Many thanks in advance

 

[philpom]

Looking good, I suspect that 12" round you have would work perfectly, I'd place it about 6" over the intake with minimal obstruction around it so maybe allthread bolted on as legs.

 

[stringman]

Looking good, I suspect that 12" round you have would work perfectly, I'd place it about 6" over the intake with minimal obstruction around it so maybe allthread bolted on as legs.

Thanks for the advice.
My only worry is that the smoking chamber itself is quiet small (162 d, 26"w, 27"h) with the first shelf only 7 inches above the base. I was wondering if I could get away with 4" above the intake?
If so, would I be better to make it slightly smaller?
good idea on the allthread!
Thanks

 

[philpom]

Thanks for the advice.
My only worry is that the smoking chamber itself is quiet small (162 d, 26"w, 27"h) with the first shelf only 7 inches above the base. I was wondering if I could get away with 4" above the intake?
If so, would I be better to make it slightly smaller?
good idea on the allthread!
Thanks

Ok, based on this information, I'm not sure the density or pattern of holes in the round plate but assuming 1/2" holes spaced roughly 1 1/2"-2" apart I would include holes in the center and set the plate as low as 3" over the intake. Holes included in the center (not too many) so the lower shelf doesn't get shielded from the smoke in any way. A pic of the plate may be helpful.

 

[stringman]

thanks for the reply
I did put the disc over the inlet, but thought it might be too big.
photos attached

 

[philpom]

Size looks good to me but I'm seeing a photo and not in front of it. Very simple to replace it if you decide you don't like the performance later. Just get those legs on it.

 

[stringman]

Size looks good to me but I'm seeing a photo and not in front of it. Very simple to replace it if you decide you don't like the performance later. Just get those legs on it.

Thanks for that,
Do you think I should drill holes in the centre part?
I may cut off the front and back edge to make the diffuser the same ratio as the box.
The other idea I have is some 30 mesh.
I need to experiment

 

[philpom]

Thanks for that,
Do you think I should drill holes in the centre part?
I may cut off the front and back edge to make the diffuser the same ratio as the box.
The other idea I have is some 30 mesh.
I need to experiment

Looking at your photos, I'd put 3" legs on it just like it is and run it and observe it. You can always make those changes later but you can't go back. Test tweak test tweak

Put raw canned biscuits in it and run it, the sticky dough will collect smoke. You can inspect for obvious spots where smoke is very light or very heavy. I think it'll be fine. You only need to do this on the bottom rack.