Can't get heat from new UDS build

[BudFrog99]

First post, so be gentle. I have a theory as to my cause, so if you care to skip down.

I built a "micro UDS" last year from a 15 gallon drum, and have used it with great success. Pertaining to this thread, I have no problems getting temperatures well over 300F and in fact the trick is to try to stay at 225 during hot summer cooks. Air intake is four 3/4" holes, and I regulate them with 1" disk magnets. Exhaust is through the bung hole in the middle of the drum lid. Fire basket is expanded metal, about 12" diameter to fit into ~13" diameter drum.

I obtained a 55 gallon drum, and made a full-size unit last weekend. Four 3/4" holes for air intake; exhaust through the Weber-clone recycled dome lid.

First cook of some chicken thighs. Target of about two hours at heat at about 325F or so. I loaded up my 12" fire basket (circular; with small can in the middle and a baffle so the fire burns around) with charcoal topped with cherry chunks as normal.

But I couldn't get the temperature much above 200F. Opened all four intakes full--no help. Cracked the lid--no help. Piled charcoal higher in the burning area--no help.

I was very confused. Yes, 55 gallon has more surface area. But with temperature in the 70s and in sun and black barrel and moderate breeze, I didn't expect this problem.

I now have a theory. I used my 12" fire basket from the small UDS, and the diameter of the large drum is 22"-plus. With the centered fire basket, there is 5" on all sides between the outside of the fire basket and the inside of the drum.

I'm speculating that the air coming through the intake holes is more-or-less immediately rising in that large space, and not really getting to the fire basket for combustion.

I'll test that theory by using the cut-out drum top as a baffle over the fire basket. It should reach nearly to the sides, and then the air will draw towards the center (where I'll try maybe a 10" diameter hole).

(I guess another quick test would be to put the fire basket into one "corner" near the drum side between two intake holes, and close the other two... [Why didn't I think of that when I was cooking?] )

Any thought on that, or other reasons for lack of heat? TIA

 

[Bludawg]

Just a Guess but a 12" basket is pretty small to heat that much volume. Most everyone builds thew from a weber charcoal grate meant for a 22.5" weber. Mine works flawlessly.

 

[BudFrog99]

I hear what you are saying, but (in my limited experience) I don't think that is the problem per se. The 12" basket burns faster/hotter with more air in the small unit. The actual fire area remained quite small indicating to me it wasn't getting enough air for some reason, and my theory is "bypass".

Lessee-- Weber charcoal grate for a 22.5" kettle is 17" diameter. So that would leave it 2"-3" from the barrel sides if built straight up? My smaller basket, when centered, is twice that distance from the sides. I'm still guessing chimney effect and "bypass".

 

[LarryO1947]

Welcome to the Forum.

 

[SmittyJonz]

My basket is 18" Diameter. Did the charcoal burn well? Only ash left?

 

[Dave M]

Welcome. When I'm going for high heat for chicken I don't try for a circle burn, I light the whole basket. I think you may be right about the air rising too. Sort of like a SJ with the holes on the sides.

 

[BudFrog99]

It burnt OK. The active fire spot was nicely hot, and was proceeding around the ring a bit. But very slowly, given the open intake area. I shut it down after a couple hours and finished the chicken in the oven. [Only a "semi-fail", as there was good smoke flavour and enough heat to crust the rub. But not enough to complete cooking the chicken.]

 

[Fwismoker]

Build a mini wsm, that cooks chicken for me 350-400* standing on it's head.

 

[SmittyJonz]

I'd try an 17-18" diameter basket.

Was the charcoal old? I stock up every year on KBB on half price sale but since it's been raining everyday for a month my garage stays humid almost damp and I had some that took forever to get burning good in my a Kettle this past Monday. I use lighter fluid and I had to spray and light it 3 times when normally only takes one try.

 

[BudFrog99]

Build a mini wsm, that cooks chicken for me 350-400* standing on it's head.

As I said in the background info in the OP, my 15 gallon unit generates plenty of heat. But sometimes the grate area (Smokey Joe-sized) is limiting, and stacking two grates is OK but a bit of a pain to tend.

So I wanted to also have a "full size" 55 gallon unit also.

And indeed, eventually I'll want another fire basket. Either for the reasons in this thread, or to be able to run both at once.

Now, unlike many of the beautiful units that I've seen in the build threads, I made true UDS units. For both the 15 and 55 gallon:

-- Drum contained automotive oil. Free except to buy the donor a drink or two.
-- One-time outlay for cheap step-drill set from Harbor Freight. About $15.
-- Six 1" circular magnets, about $2.
-- Recycled cooking grates from SmokeyJoe and Weber kettles.
-- A few stove bolts from the junk bin.
-- ManLaw $10 smoker gauge (I don't think they have the same model anymore. And contrary to most opinions I've seen here it is pretty close in temperature readings.)

No paint; no grinding. Now, my big outlay for the 15 gallon was a sheet of expanded metal from a hardware store, over $20. I couldn't find a junk source.

I did one thing that I haven't seen on these forums. I used pilaster strips mounted on the inside of the 15 gallon for adjustable cooking grate height.
http://www.menards.com/main/storage...sories/60-zinc-pilaster/p-1802490-c-12645.htm
One 60" strip, cut into three, $2 and another $2 for a pack of clips. These are thin strips and clips as would be used in built-in bookcases and the like. I plan to do that to the 55 gallon as well. I like them.

=======================
As I'm a partner in a small electronics engineering firm, I cannot just use the well-working 15 gallon UDS as-is. I'm refining my own electronic heat control. I looked at reviews of many of the units and wasn't satisfied overall.

So:
-- $5 RTD100 temperature probe from China, stainless; 6" probe; sheathed cable.
-- $10 RTD-to-voltage converter from China
-- $2 1" 5VDC mini cooling fan
-- "Surplus" microcontroller board from work, with LCD display and input/output capabilities
-- Some piping and fittings to mount the fan in a 1" conduit pipe
-- $5 pair of wireless modules so I don't have to go outside to monitor
-- "Surplus" SLA batteries from work as the power source. Recharged with automotive battery charger.

The subsystems above work quite well. I still need to refine my control algorithm a bit, but the tiny fan pushing air via conduit through one of the 3/4" intake holes (with the other three covered) is more than enough for even hot cooks. At 90% duty cycle (on for 27 seconds out of each 30) temperatures were well over 400 degrees. For a 225F target, about 10% to 15% duty cycle (3 to 5 seconds out of each 30 seconds) is about right. (the actual closed-loop value depends on weather conditions and how the particular fire was built and the like.)

=========================
Summary: I learned a lot on this site before doing my first build last year. Thanks, all. I'll try the experiments above (move basket closet to sidewall; add a baffle to direct intake air through the fire basket) and report back.

 

[SmittyJonz]

Also more wood chunks = more heat. I mix about 60/40.

 

[BudFrog99]

"more wook chunks, more heat"

Agreed. Placing the wood chunks around my small-diameter circle arrangement is a bit of an art I'm still trying to master. A dry wood chunk will go into a hot flame and cause a temperature spike. Even worse, the flaming wood chunk causes some of the charcoal to be "bypassed", leading to some unburnt charcoal and shorter cook times. I'm still experimenting with chunk size and placement.

Side note: If I artfully built my little 12" basket properly and don't end up bypassing some of the charcoal, I can get at least 10 hours out of a burn in summer. Very satisfied...

 

[Fwismoker]

I'm a more 90/10 type of guy. Sticks, small chunks

 

[BudFrog99]

"I'm a more 90/10 type of guy"

Which ratio? 90% wood, and a few charcoals?

For a UDS, I'd think "mostly charcoal" as it is harder to tend a wood fire in a UDS once loaded, right?

 

[Fwismoker]

"I'm a more 90/10 type of guy"

Which ratio? 90% wood, and a few charcoals?

For a UDS, I'd think "mostly charcoal" as it is harder to tend a wood fire in a UDS once loaded, right?

90 charcoal

 

[Bob C Cue]

Build a mini wsm, that cooks chicken for me 350-400* standing on it's head.

You have a mini wsm? Who knew? Also can you explain the benefits of cooking with the mini in the upside down position?

 

[Fwismoker]

You have a mini wsm? Who knew? Also can you explain the benefits of cooking with the mini in the upside down position?

You really haven't seen my mini in action have you? Upside down it works on heat exchanging concept!

 

[funugy]

Do a dry run. Don't put the coffee can in the center of the basket and do a "Minion" method burn. Since your basket is on the small side, the burn won't last as long as those of us with bigger baskets, and will take more work to get the heat. Think of a match vs a log on fire. Both are hot as hell within the flame, but the log has more area for the heat to radiate from.

Try this-- For higher temps, lite about 20-25 coals, get them to ash over and then add them to the basket. Keep all vents open until you are within 20* of target temp, then start closing off some intake. If the temp starts dropping open the vents and remove the lid for about 10 minutes, replace the lid and re-assess the temp.

Where is your temp gauge? This will have an impact on your temp reading. Even with bigger baskets the heat is higher directly above the basket and less toward the outer edges of the barrel. Thus the reason some of us use diffusers.

With such a small diameter basket your higher temp area is greatly reduced. The difference between a 12" diameter and 17" diameter basket is HUGE! The 12" basket only has ~113 sq. in. of surface area, whereas the 17" basket has a surface area of 220 sq. in. Thus allowing more coals to be lit with more air contact creating more heat. The volume ratio between the two diameter baskets is just as staggering. Assuming both baskets are made 12" deep, just one is 12" dia. and the other is 17" dia. The volume is 678" vs 1094" allowing more coals and greater area for air to reach the fire. More fuel + more air = more heat.