My first overnight HM brisket adventure

[Dave McK]

First, let me say thanks to Bryan and the rest of community here for this cool project. And a special thanks to Ralph Trimble for his help and patience in getting my Rotodamper together and installed.

Like most of you here, I am in pursuit of a restful night of sleep while the HeaterMeter takes care of business. The business this past weekend was a 15-lb brisket to feed a group of friends and family. Unfortunately, sleep was not part of the deal for me this time. Everything started out fine but as the night progressed things seemed to deteriorate. I'm hoping the wisdom of this community has some pearls for me.


Setup:
KJ Classic (stock charcoal grate, daisy-wheel 3/4 open) - all ash completely vacuumed out before cook
Ozark Oak lump + several chunks of oak smoke wood randomly throughout (baseball size)
HM 4.2.4 (stock PID, servo + blower, blower on only at 100%)
Rotodamper (w/ stock fan)
Maverick ET73 thermistors

Set temp: 225
Lid open detect: 6%

Ramp-up looked good so I was initially encouraged. About 2 hours after putting the brisket on, strange oscillations appeared but the HM seemed to keep it under control. I was hoping it would smooth out more but it never did. My confidence was still intact but a sliver of worry crept into my brain. During one of my short periods of sleep the temp dropped and it entered lid-open mode. Fortunately, it recovered by itself. This happened twice more but both times I noticed in time to manually cancel lid-open and allow it to recover. Around 8am, the temp suddenly plummeted and despite the "bwoop! bwoop! give her all she's got!" efforts of the HM, the temp continued to dive. Intervention was required. I opened the top vent completely and thankfully that was enough to recover the fire. I returned the vent to 3/4 daisy but at this point I wasn't sure what to do because the oscillations were wide and I worried that I wouldn't finish in time for our guests. I decided to push up the target to 235 but that seemed to make it worse so I backed down to 230 briefly before returning it to 225. In the end, the brisket was delicious and my guests were very complimentary. However, I was less enthusiastic due to the lack of sleep and baffling behaviors of my setup.

Thoughts:
1) Looking at the output levels of the servo/fan on the graph it would suggest that the grill heat responsiveness to vent changes dropped over time. I see a lot more 0% and 100% later in the cook.
2) Lump was loaded with larger pieces at the bottom. I didn't dump anything in. It was all hand-placed in an effort to eliminate voids and keep a nice distribution of smoke wood. Maybe my technique could be improved?
3) The stock KJ coal grate might need to be replaced with the 2xSmokeyJoe grates to allow more airflow and less ash accumulation (mentioned in another thread here)
4) I wonder if the stock PID settings are appropriate for running servo + blower with blower on only at 100%. I tried to search the forum for posts that might suggest that but it seems like this should be the last thing to adjust.

Ideas?

 

[Steve_M]

With a servo based damper, I leave the small daisy wheel holes fully open on my KJ classic. The first thing to check is that you're not allowing air to enter the bottom vent through any other means other than the rotodamper. This is critical for the HM to be able to effectively control the natural and forced convection process. If I recall correctly, the stock PID settings are setup for fan + servo, not for the fan set at 100% only mode.

Regarding the stock KJ fire grate, I ran into problems a few times at the 10+ hour mark. The switch to the 2 x Smokey Joe grates really helped with this.

 

[Dave McK]

Steve_M, do you run normal fan+servo with full daisy and default PID? Did you make any other KJ mods to improve performance for low&slow cooks?

I think my grill is reasonably sealed. I made my lower vent adapter out of sheet aluminum and a compression pipe fitting as suggested by Ralph. When assembled, the pipe fitting sticks out the back so there is no way to slide it into place. I ended up mangling it to get it in the opening. I plugged the resulting gap with foil for this cook (no time left to get creative before the cook). I need to improve it so it will be quick and easy to install/remove. Maybe I can reduce the size and put some flexible gasket heat seal material around the edges as a "frame" so it can flex fit into place? How did you attach/seal your Rotodamper?

 

[RalphTrimble]

Steve gave you some good advice already, I will add that on your adaptor plate if you cut it just the right size (top to bottom) you should be able to insert the plate into the top slot of the vent (all the way to the top) and just be able to slip it into the bottom slot. If you can't do this then shave down the plate a little until you can. The top lip should be a little longer than the bottom so it should still hold in place and the slider should also aid in holding it firmly. Once you've got that worked out you can move on to sealing it up better, you can line the outer edges of the inside of the plate with nomex gasket material (get the kind with the adhesive back), the nomex is like high temp felt and will seal your plate into the vent.
On your cook, hard to tell exactly what caused the drops having not been there, how much coal was left in there when you started getting big dips? If there was plenty did ash build up on your grate and restrict the air flow, or perhaps the coals were setup in a way that after the bottom layer burned it didn't ignite the top layer right away? Also, the blower started out running really high, so it seems Steve was on target telling you to open the top vent more, cause that will allow air to flow easier and the damper should then run a bit less open, running somewhere around 30-50% is around the best place to have it. On the big dips, when running with the blower at max only, if you have your grill setup so the damper is running nearly open like your graph, each time the HM hits 100% the blower will kick on and the air flow will increase dramatically and bring the pit temp down very fast, you can see this effect on the biggest dip in your graph. To prevent this you need to get your grill holding temp stable with the damper opened less (30-50% as stated above), also, the latest HM software has a new setting for "Startup Max" that can be helpful with this. Set the Startup Max to 100% and the blower will run at 100% until it hits the target temp the first time, after that if the blower kicks on it will run at the speed set for MAX (rather than Startup Max). So, set your MAX speed much lower and the blower wont blow so hard if it has to kick on during a cook, which will reduce the amount of air flow through the grill and minimize the cooling effect that has (like seen on your largest dip on your graph)

 

[Dave McK]

Thanks Ralph. I'll make the adapter plate mods as my next step. I'll also make sure I update to the latest to get the "Startup Max" setting. I'm currently running 20140630B.

I didn't open the grill once during the cook and even if I did I couldn't see past the heat deflector plate used during low&slow cooks. So I'm not sure how much was left when the big dips started. Also, my fire was lit at the top of the pile in the center. I'd never heard of lighting a kamado from the bottom. Is that a normal technique?

Checking the coals this morning it appears the fire didn't consume much fuel from the edges. I guess I need to pay more attention to how I load the lump next time to get a more even burn. Judging from the ash in the bottom, it sounds like the 2xSmokeyJoe grate replacement will be necessary.

 

[RalphTrimble]

If you have ash build up that is hampering the air flow you should see better long term stability if you change your grates to prevent that. The deflector can make it difficult to manage the fire during the cook, if your coals aren't collapsing down to ignite the unlit coals maybe you can use something like a coat hanger to snake in there and make the pile collapse? My bet is if you solve the ash buildup issues and correct the air flow a bit you wont have to worry though. I use a lot of different techniques with my coals depending on what type of cook I am running, but I do generally light from the bottom with half of a weber starter cube on my (steel) kamado. That might be an unorthodox technique, don't know, works great for me....