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Need technical advice on homemade elec. smoker

ztugdriver

Newbie
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Joined Jul 1, 2016
Hello all.  I have built a electric smoker from an old refer and have used it to make smoked pork, salmon, pepperoni, and ribs.  Over the past year I have been struggling with the temp from top to bottom.  The top of the smoker is 15-25 degrees cooler than the bottom.  I know I should expect some difference due to the fact that my heating elements are on the bottom but as I smoke my meats the bottom sometimes burns before the top is near done.  I tried rotating but that just makes the temp spike higher because I opened the doors.  I was hoping for some input on how to even the heat out and get some technical advice on whether or not I have the temp probe in the correct spot and the controller set up properly.  

Basic description of the smoker:

It was an old commercial fridge that I gutted and removed all the foam.  I then added rock wool to all sides and reinstalled all the panels.(Pain in the ass)  There is a drain center bottom of smoker for drippings, a vent on the left side which I hardly use and a port for my cold smoke generator on the right.  There is a 4" chimney that starts at the top center and exits out the top right side of the smoker.  There are 2-3000 watt elements running off single phase 240V.  I have a PID controller for temp control which runs 2-40A solid state relays.  I added a third 40A variable output relay to the right burner in an attempt to equalize the output to the left burner.  This relay is in series with the main SSR relay for the right burner.  My temp probe is at the top back of the cabinet in the center about 8" down. If you guys need more info please let me know or if you have questions on how I did something please ask.  This may be a bunch to swallow at once but if you guys can help I would be grateful because I'm ready to scrap this thing and start all over again.  Nah not really Im just frustrated that I cant figure it out.

So my questions are:

1-Would a heavy gauge steel plate or a layer of bricks over the heating elements help with keeping temp spike down as the PID turns the elements on either after a door is opened or when I change the temp?  Would the plate help with evening out the temp in the smoker from top to bottom?

2-My temp probe is at the top of the smoker should it be at the bottom?  If so the temp at the top would never get to set the point.

3-I bought two of the same elements at the same time but the right side is 10-15 degrees hotter than the left.  I have tried switching the control relays and the elements but the right element is always hotter so its not the relays.  Has anyone ever come across identical elements putting out different wattages?

4-Do you think I should have a PID for each burner?


Here you can see I tried to manage the temp difference from right to left with some tin foil over the right element.


Here are the two 3000 watt elements which I have covered with 18 gauge sheet metal.


PID controller and chimney exhaust.


The element with the heat shield.  Yes I had a flare up when doing smoked pork.


Here is a video of the smoker.

Thanks in advance for the help!
 

forluvofsmoke

Smoking Guru
OTBS Member
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Joined Aug 27, 2008
Looks and sounds like a pretty sweet build...just needs a little tweeking to make it perform better, right?

This may require a bit of trial and error to get the size correct, but I think you need a baffle that has nearly equal distance from the cabinet on all four sides, and possibly higher from the elements than where you're at now....that's my first impression on correcting the top to bottom temp differences.

Here's what happened to me with a charcoal bullet smoker when modded to add twice the height to the original smoke chamber: my grate temps were cooler on top than the lower grates, and eventually I removed the original pan and added one with larger diameter (less gap from the pan to the outside/walls. This improved my grate temp variances from top to bottom from nearly 40* to 10-15*. My theory was that with the larger baffle the heat had to rise closer to the sides and farther up along the outside of the smoke chamber before it could converge towards the center of the grates, thus forcing more heat towards the top than was previously allowed. It's sort of a balancing act finding the right size of gap to create the right conditions on each grate level. You may lose some of your lowest grate positions, as they may be too cool, but as it is now, you're too hot to use the space down low, so nothing is really lost by cooling that area down and moving that thermal energy up to the top. If you really fine-tune it, you can get a lot more usable grate space than you probably have now.

From what I'm seeing in your photos I think your baffle is too close to the heat source...it may be acting as a radiant heat source, with hot-spots directly above the element...hence being very hot down lower in the smoke chamber. They still will act on spreading out the heat to some degree, but I think they could perform better. Adding mass such as fire-bricks will only help for shorter smokes (should read spikes)...given enough time they will also radiate a lot of heat. They would not help much with temp spikes due to the high output of the heat source, unless there were a lot of mass. The downside to this is a lot longer pre-heating time before you could hot-smoke. Also, I see little to no gaps on the left or right sides of the baffle...maybe that's just the angle of view in the photo. You may need 6-8" space between the baffle and heat source, and/or, add a second baffle above the existing one, only a bit smaller in size (still maintain the smaller gap from the walls to the baffle). Also, to avoid flare-ups with drippings a pan of some sort will trap them and keep them from dripping off the edge and catching direct heat. The second baffle could be fashioned as a drip-pan with a drain tube leading close to the center drain on bottom (passing through the lower baffle) could reduce your flare-ups a lot. A double-baffle design might help balance the top/bottom temp variances, but if drippings were caught on the upper baffle they wouldn't be as likely to flare-up as there would be far less direct heat potential.

Your foil baffle on the right to balance out the left/right side temp variances is on the right track, but if you can balance the heat output from each element would be best, long-term, as you're eating up some potential smoke chamber space, if that matters to you.

Just my observations...maybe not the right solutions for your application, but that's all I can find from what you've shown and stated...which was very detailed...well done on that.

As for temp spikes, you heater elements carry a lot of residual heat after powering down, so having your controller temp probe closer towards the bottom will reduce the peak temp by allowing the probe to sense the shut-down temp sooner (in theory)...depending on many variables with internal configurations and the resulting heat flow. Positioning about 1/3 the distance up from bottom to top, and, about 3-5" from the back wall, should enhance the probe sensitivity. Note that with indirect heating in a vertical system the bulk of the heat needs to travel upwards close to the walls for much of the distance, so the hottest areas should be close to the walls. If you temp probe is in that path of heat it will likely cause premature shut-down from set temp, and, more rapid cycling. Ideally, until you find exactly where the probe works best for a good balance of reduced temp spikes while providing more accurate chamber temps, I would suggest being able to adjust the depth of the probe's protrusion into the cabinet, by a few inches.

You may want to determine what your cyclic times are for given chamber temps (if you haven't already) and if they cycle slowly (long on/off times) you could run with lower output from your elements. Also, if your controller will allow tighter on/off set-points may help, but could be hard on other components from excessive heat due to short repeated cyclic switching. If you see a lot of peaks and valleys with the doors closed for extended periods, less output from the heat source will help to resolve this. Replace the off-time with more on-time to maintain the same target chamber temp. Your insulation should be sufficient to not be much of a factor in temp swings.

A separate variable output for both elements could possibly achieve better balance of temp swings and side to side temp variances, and would allow for higher output potential as needed, such as during undesirable weather events. Control of the output potential could be manually adjusted, or automatic with the proper circuit controls. Maybe PWM controllers would be adequate in conjunction with your PID...not sure on that...not sure where to get one for this application, either. The side to side temp variance could be used to your advantage with different meats that you may want to smoke at different temps...same goes for top to bottom temp variances...but not if you're loading it up with the same thing...the way I understand it you want as close to zero temp variances as possible.

One final thought: there are three possibilities that I can think of at the moment which could be causing your side to side heat element outputs to be different. (1) If there is a difference in the wire run length from left to right elements you have more resistance on the longer run...that causes less output potential on the longer wire run due to lower terminal voltage. To correct this make both wire runs identical in length, and be sure the wire size is adequate for the current potential. (2) One heater element could actually have more internal resistance than the other. If you don't have a multi-meter capable of testing the actual heater element resistance, simply switch the positions of the elements from left to right and recheck the side to side temp variance without adjusting anything else. I re-read your post and could not find if you've made these determinations yet or not. (3) Defective wire termination: make sure all terminations are tight and of adequate size hardware. Any loose terminations cause increased resistance, therefore, lower circuit voltage. Inadequately sized termination hardware will result in the same conditions as undersized wire. Come to think of it, these would be my first steps before doing anything else...termination checks, wiring checks, element checks.

Remember that if you do too many things at once before checking operational conditions again it will be difficult to determine what the actual cause of the problem is, if that will matter to you, especially when dealing with multiple issues.

Vertical smokers and horizontal smokers each have their own set of pros and cons regarding cooking, and each have their own set of basic needs for optimal performance. Understanding what your vertical rig wants or needs isn't always easy, but I hope I shed a little insight. If your rig is as tall as I think it is, you need fairly close heat baffle gaps on all four walls of the cabinet (maybe 1.5" or slightly less) to carry the heat up high before it starts to spread out towards the grate centers. Then, a bit slower heat production should reduce temp spikes. I think you're pretty much thinking along the same lines, from I've read thus far. Hope this makes sense, because I think I'm getting too tired to think real clearly anymore.

Well, that's about all I can come up with for now. You've tossed up a couple known issues...hope I addressed them all.

Good luck with the work you have ahead of you!!!

Eric

EDIT: I forgot to ask - what is the size and orientation of the bottom drain hole? If it's not centered on the bottom plate that could cause a slight front to back temp variance. Also, if it's the only intake in use and is offset, that can cause some turbulence in the air flow through the smoke chamber. It is possible that you need to add a small intake on each side, front, and back to get just a little more upwards flow going through the smoke chamber. This may help with top to bottom grate variances, along with the baffle issues. I've seen a few smaller electric rigs with a small drain hole centered in the bottom, but they have no where near your internal capacity or overall height.

Also, looked again at the element orientation...they're close against the back wall...may run hotter towards the rear...can't really change that, but possibly a very slight tilt on the lower baffle (upwards towards the front) and less gap on the rear from the wall than front could possibly help reduce that temp variance, if there is any. But, right now I think your temp swings are biggest issue, while front to back temps are of lesser concern...work that out if/when the need arises.

Just another thought: separate temp probes for each side to control each element on a separate circuit as a possible solution for the side to side temp variance, with probes oriented slightly more towards the sides than the center. Do this ONLY after completing the previously described trouble-shooting with terminations, wiring, etc, and not finding any probable root-cause, or, performance not changing for the better.
 
Last edited:

ztugdriver

Newbie
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10
Joined Jul 1, 2016
Thank you very much for all the input Eric but its late here and I gonna let my brain rest too.  I will post back once I have made some checks following your advice.  I want to fire back with ideas but i gotta get some sleep.  Thanks again.
 

daveomak

SMF Hall of Fame Pitmaster
OTBS Member
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Joined Nov 12, 2010
Eric has some great ideas....   I would add a couple of broiler pans above the elements and fill them with pea gravel as some extra thermal mass...    I have rigged my smoker with a dimmer switch so the heating element NEVER turns off...   I adjust the output with the dimmer...

Maybe try using 1 (one) heating element to have less "off" time...   less fluctuation...     Also, I prefer a LOT of air flow through my smoker...   here's a video I just did to answer some folks questions..

 

ztugdriver

Newbie
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10
Joined Jul 1, 2016
OK guys well I did some fiddling around and some thinking too which might sound scary but I figured a couple of things out.  First I grabbed a bunch of my wireless thermometers and placed 2 at the bottom where my lowest racks would be then I placed one next to the PID controller temp sensor is.  Secondly I installed a variable output 40A relay in line with the hot right side element.  Thirdly I set the maximum output of the elements to 60%.  I was amazed at what I found.  First the PID sensor was 20 degrees off from what my known good sensor was reading.  So I went into the PID menu and offset the sensor 20 degrees to match the known good one.  I then fired up the smoker unfortunately with nothing inside but the racks.  I set the temp to 110 and watched all the sensor as the temp climbed.  I noticed that while heating there was a 10 degrees difference from top to bottom and only a 2-5 degree difference between left to right.  I let it settle and reset the temp to 140 then to 160.  Each time watching the temp sensors.  Each time the temp varied from top to bottom only 10 or so degrees and the side to side was pretty much dead on. 

So I guess my issue with the side to side is solved with the variable output relay.  I also solved the high temp spikes by limiting the output to 60% which I may play with a bit more on higher temp smokes.  Running two elements off one PID would be fine if they had the same output I guess. Probably could have gone with 2000 watt elements instead but I was worried I would not be able to do briskets and such.  The last pork run I did I had it up to 265.

The top to bottom temp is a bit better at least I can live with that.  I understand there will always be a difference just because of the design and the heat coming from the bottom but I don't think that's bad at all.  Besides once the box gets to temp all the sensors read within 5 degrees after 5-10 min and the PID has no problem pulsing the elements to maintain the temp. in fact they on pulse every 5-10 minutes once the temp is reached. 

I think I may put a 1/4" steel plate over the existing heat shield just to help dissipate some of the radiant heat and to help with recovery times when I have to open the doors.  As far as a drip pan I have put some thought into that because I don't like fires in my electric smoker.  

Still dint know if I should relocate my PID temp sensor. 

I also noticed in some vertical smokers there is a baffle at the exhaust vent opening directing the air around and possibly keeping some of the heat from going right up the vent I might try adding one of those but making sure not to slow the flow of air. 

I also realize that there was nothing in the box during this test and the results will change with meat in the box and at higher temps.  I'm gonna come up with an excuse to smoke something here in the next week and I will let you guys know how it goes.  

To clarify some of your questions:

My drain is dead center of the bottom.  I'm not sure if it helps or hinders the air flow but its an 1-1/4" tube. 

I think the wire length might have been the issue or maybe just a different internal resistance of the elements but the variable SSR fixed that.  I'm still gonna check tho.

All the connections were solid.

The temps on the sides of the smoker are pretty consistent all around with the exception of the front being slightly cooler so I just flip the rack 180 degrees half way during the process.  Hopefully its a lot better now so I wont have to swap top to bottom too.  I will play around with the gaps on the heat shield. My first heat shield covered the entire bottom with only 1/2" gaps on the sides but it made it hard for the heat to travel up and the got super hot.

Lesson learned today don't trust your temp sensor out of the box!

Here is a pic of the updated control panel.

 

forluvofsmoke

Smoking Guru
OTBS Member
5,170
405
Joined Aug 27, 2008
ztugdriver, glad we could be of assistance. Dave's MES, even though it's a smaller project, I found it to be very innovative...too bad manufacturers don't work-up solutions like this for a bigger share of their retail market. Some might do a custom job, for a premium cost, of course...but I guess that would take a lot of the fun and creativity away from us back-yard smokers...well, that's my story and I'm sticking to it...LOL!!!

Sounds like the little bit of digging you went through revealed some interesting secrets that your smoker was holding out on you. I haven't used PIDs, and haven't researched them much, so I wasn't aware that you could program them (or at least yours) for pulsed output, or, reduced maximum potential output...that's pretty cool to know. A PWM controller wouldn't really do you any good coupled to the PID, as they pulse in Hz/KHz/MHz ranges and if one could find a suitable PWM it would do the same job as your PID with overall output potential limiting...via adjustable rapid pulse output. PWMs have their place, but it sounds like your PID will do a bit more for you than most PWMs are capable of.

I've been out of the loop on a lot of the electronics advancements for the last 16+ years, so I feel a bit inept. I was a technician for 8-1/2 years inspecting and maintaining industrial safety equipment, including electronics, back in the day. I always enjoyed trouble-shooting/testing, circuit board and wiring repairs, calibrations, but this was mostly gas and fire detection instrumentation...handheld gas detectors with electro-chemical sensors and fixed gas detection systems at gas plants/refineries and field sites. I also ran across some fixed infrared fire detectors, and some fixed gas detectors which used a laser reflecting off of a gold-plated concave mirror and passing through a oscillating flag which alternately passed the laser beam through 2 different micro-thin, high precision windows made of rare minerals (the red one was ruby, can't remember what the green one was) to determine the presence of certain combustible gases. These were intriguing to work on, but extremely accurate and sensitive with near zero false-alarms. I have never seen anything like them, and probably never will again. I did run into some very interesting equipment at times and had to learn on the fly (with electronics I was ~90% self-taught), but these were in class of their own and nothing like what we're speaking about here. Just wanted to toss that out at you so you knew a bit more about my background, so if some of my ideas seemed "a bit out there", now you know why...sometimes I over-think the problem and have been known to miss the solution, at least at first.

I think you're on the right path, though, and with what you've found so far you'll have more answers, and probably even raise more questions you haven't thought about yet when you start your test-runs again. I could sense your earlier frustration...been there...sometimes I just walk away, forget about it...do something else for a few hours, or even days...come back with a fresh outlook and maybe some new ides that rolled around in my head after I took a break from it. Anyway, glad we could pass on some different view-points, if nothing else. Being your temp variances are closer than you originally thought, and you identified and corrected your PID probe temp reading issue, you can focus more on temp stability for now. With reducing your heat output potential, I think things will look a lot better for your next smoke. See how it runs with a load in it, make some notes and see what you'd like it to do better. Smoke with your digital probes to monitor while smoking, just to be sure your grate temps and temp swings are acceptable. Oh, I know everybody wants nice steady temps, but especially with electric, at some point many are forced have to decide what is really an acceptable differential. If it's only 3 or 4* temp swing, maybe call it a day. You do have the hardware to make temp swings nearly unnoticeable, so being you've taken that extra step, may as well make it work to it's fullest potential...I get that, 100%...I like minimal temp swings just as much as the next guy, even with propane or charcoal.

By all means, keep us up to date. I'd like to read about this rig taking your smokes to another level. Then, you can focus on refining your methods, recipes...well, you know all this, but I'd like to hear about it.

I think you will nail down the problems and be able to really enjoy that monster smoker conversion...I drool just looking inside that beast, and I'm not even a watt-burner...I roll with charcoal!!!

Keep on keepin' on!!!
 

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