Stoker on a UDS


 
J

JLeadbetter

New member
Greeting All,

I recently built a standard 55 Gallon UDS and have a question about using my 25 CFM Stoker fan. I bought the 25 CFM fan for my big pit...it's my whole hog cooker.

I tried the 25 CFM fan just to see what would happen. It worked to an extent. Over a 250 degree, 3 hour cook, the fan kicked on 3-4 times. However, as the fan kicked on it would drop the UDS temp 30 to 40 degrees and take 30 minutes to come back up to temp. Then when the fan kicked off the temp would spike 50 degrees. I understood what is happening, so off I went to figure out how to fix it without buying a new fan. The smoked wings weren't to bad either.

So I did some testing on ways to reduce the air flow to that of a 5 or 10 CFM fan. What I landed on was stuffing some course steel wool into the the silicone insulator tube. That reduced the time to fill a large black trash bag from 20 seconds to over 3 minutes. I thought it was a bit extreme but hey I was testing. To my disbelief it acted in almost the exact same way. So now I'm a little stumped.

How for the questions. How do WSM's act with a 5 or 10 CFM fan connected? Do they exhibit similar traits? Anybody know how to calculate CFM? Is there something I need to do to the Stoker or a setting in StokerLog? Any ideas, suggestions or personal experience? Aside from buying a new 5 or 10 that is...that is the last resort

stoker.jpg


Thanks,

Jason
 
You certainly tried the right thing given the overshoot.

The undershoot though, seems to me is caused by the fire probe not being placed right (i.e in the path of the blower air causing it to read incorrectly when the blower comes on ).

And no, stokerlog has no settings at all here. The blower control is all internal to stoker and not adjustable. One of the future projects is to take the control away from the stoker and have stokerlog manage it but that will probably happen by version 10
icon_smile.gif
.
 
That is a MASSIVE fan to have on a uds. I use a 5cfm and have had great success.

One of mine is a flat top UDS with a weber 4-hole opener (don't know the name) and the other is a UDS with a weber lid.

I start the fire as John at Rocks suggested with the fire on the opposite side of the blower opening (no pipe leading under the firebox as some do) and the temp probe is the opposite of that. That way the air flow isn't near the temp probe.

I have no spikes on the flat top, but the weber lid one does have spikes due to, I believe, the curvature of the lid.

But I also use a 5 cfm on a Superior SS-1. It works great as well.
 
I think I'd play with probe placement before I worried about a new fan or choking yours off more. Your experiment seems to show even with drastically reducded airflow you still have the problem. If the probe is being cooled by the air flow though it would explain why it is over-stoking your fire.
 
Thanks for all the comments. Below are a few more pics showing pit probe and fan placement as well as how my UDS is designed. The ash pan is not to scale. I wound up using using an 18" pizza pan.

PitProbeMod.jpg

BlowerPlacementMod.jpg

BarrelDesign.jpg
 
So where you doing an indirect cook? That is, there was a heat barrier between the blower down below and the probe up above? I am asking because that would stop the direct air path to the probe if you had a head deflector.
 
That is a bit subjective as to what you call indirect.

Direct in that the food is directly over the fire basket.
Indirect in that there is a >2 foot space between the food and the fire basket.

There is nothing between food and fire.

So I've been working on some calculations.

25 CFM Fan = .417 CF per Second
10 CFM Fan = .167 CF per Second
05 CFM Fan = .083 CF per Second

1 CF = 7.481 Gallons

55 Gallon Drum = 7.352 CF
33 Gallon Trash Bag = 4.411 CF <-- For sure less then 33 Gallons due to the bunching of the bag opening during testing and a lot of other factors.

7.352 CF / .417 CFS = 17.645 Seconds to Fill
4.411 CF / .417 CFS = 10.587 Seconds to Fill <-- I need to check my times or my fan is more like a 12.5 CFM

Anyway...based on the above numbers and my restricted fan filling a ~33 Gallon trash in ~180 seconds...I have a rough calculation of having created a 1.47 CFM Fan.
4.411 CF / 180 Seconds = .025 CFS or 1.47 CFM or 10.997 Gallons

Brain hurts now...haven't done anything like that in years.

So the consensus seems to be probe placement or shielding the probe. The probe is right in the center. I have observed and read that there can be a ~50 degree difference between the center and the edge of a UDS smoker...so I think I'll try to create a little "draft shield" out of an aluminum foil pan.

I'll keep you posted on what happens. Thanks for the help guys.

-Jason
 
When we use the term indirect we mean there is a real barrier between the fire and the food. Usually a pizza pan, cast iron skillet, etc. What you are doing is direct grilling.

I have not heard of direct grilling at such low temps. I suspect that is the source of your problem. The temp is low and gets impacted heavily by the fan blowing. Put some barrier in there and then it should maintain low temp nicely. At least that is my theory
icon_smile.gif
.
 
Amir is correct in that not having a diffuser between the charcoal basket and the temp probe is the problem. When the fan kicks on it is blowing cold(er) air into drum and it is going straight up to the probe.

A WSM would show the same behavior if you pulled the water pan.

Some UDS owners put bolts in the drum just above the charcoal basket so that you can put a clay saucer, pizza pan, or even aluminum foil to divert the heat around the edges of the drum and/or to keep meat drippings from dropping directly into the charcoal. The added benefit to a Stoker user is to keep cold air from blowing directly on the probe.
 
Jason, did you put this to bed? If not, I’d use a full port ball valve to restrict the flow from the 25 CFM fan. You will be able to turn it down as much as you like. You can get CFM estimates by blowing into a deflated bag of known volume and timing its filling. I'd shoot for approximately 5 CFM.
 
Here's my 2cents worth on this topic. I use a 5cfm blower, I either use a 1/4" steel plate with holes on the outside of the plate or a large Tera dish wraped in foil to seperate the fire camber from the food. The temp probe is about 6 inches from the top of the drum and I use a flat top style drum. I have no problem controling the temp with this set up and get about 7 hours to a load of charcoal and wood.[IMG:left]In Action.jpg[/IMG][IMG:left]ready for the lid.jpg[/IMG]
 
You must log in or register to reply here.

 

Back
Top