HM keeps overshooting and then fire dies and needs to be relit at 225.


 

JasonL

TVWBB Member
This is on an akorn. It's a HM with a rotodamper 3. I start my fire by filling up the box with lump and using a cotton ball to light it. I put the heater meter on immediately. The temperature steadily rises and shoots past 225 and eventually levels off around 255-260. The temp drops and eventually gets below 225 and the fan starts blowing but the fire seems to have died. I have to restart it by taking everything out and relighting.

Any ideas? I've played around with the settings without much luck
 

Steve_M

TVWBB Guru
is the plate you attach the rd3 to the grill with creating a tight seal? What PID settings are you using?
 

JasonL

TVWBB Member
Yes I believe so. I have the BBQ guru adapter for the akorn and I used nomex gasket to try and seal everything.

I will post the settings when I get home. Apparently the fire didn't die because I see it going up. It went all the way down to 195 before the temperature made its way back up.
 

JasonL

TVWBB Member
here's the graph. can see how its fluctuating so far around 225.

http://imgur.com/K2xZsf6


settings are:
Proportional
4
Integral
0.01
Derivative
6
Fan output on above
65
% min
5
% max
30
% startup max
75
% Invert output
Servo pulse duration
500
us -
2400
us, fully open at
60
% Invert output
 

Steve_M

TVWBB Guru
I'd go back to the servo fully open at 100% and fan output on above 0%, which is the stock settings.

If that works ok, then you might need to play with the PID values to run with the custom servo and fan parameters you've chosen or maybe raise the fan output on above value from 60% to 95% or even 100%
 

JasonL

TVWBB Member
Thanks. Fortunately my ribs turned out ok even with the uneven temperatures. I'll play around with the settings again the next cook or do a test burn at some point.

The thing I'm not really understanding is what is actually happening in the grill when it drops below the target temperature, the fan turns on full blast, and it still continues dropping another 15 minutes down to like 195 before it starts climbing up again. I would think it should start heating up immediately once the fan starts, but maybe the circulation isn't good and the air isn't getting to the lit coals? The fire definitely wasn't snuffed completely given it eventually starting rising again.
 

John Bostwick

TVWBB Wizard
When you see the temperature drop, turn off the blower and see what happens.

If you see the temperature go up, then your blowing to much into the Akron, and you will need to adjust the maximum percent of the blower, down until you see the temperature rise.

If you see the temperature to continue to drop, then there might some other restrictions, or the blower is not blower is not enough.
 

JasonL

TVWBB Member
When you see the temperature drop, turn off the blower and see what happens.

If you see the temperature go up, then your blowing to much into the Akron, and you will need to adjust the maximum percent of the blower, down until you see the temperature rise.

If you see the temperature to continue to drop, then there might some other restrictions, or the blower is not blower is not enough.


awesome thanks. i'm doing a test turn right now. just turned it on so will play with settings and see what happens.
 

Dan Francis

TVWBB Fan
I am a long time Heatermeter user and just had this same thing happen for the first and only time ever recently. I am using a ceramic cooker also (BGE). I will bet it is the way you lit your lump. I have always used a torch and lit the outer parimeter of the lump and never had the spike and die problem. A couple cooks ago I was doing ribs and for some reason just lit the center of the lump, I had the temp spike then coals die at least 3 times during the cook - it was a complete PIA! I realized what I had done differently afterwards and went back to lighting the entire outer perimeter and never have had this problem again. Put cotton balls all around the outer perimeter and I think your problem will go away.
 

JasonL

TVWBB Member
I am a long time Heatermeter user and just had this same thing happen for the first and only time ever recently. I am using a ceramic cooker also (BGE). I will bet it is the way you lit your lump. I have always used a torch and lit the outer parimeter of the lump and never had the spike and die problem. A couple cooks ago I was doing ribs and for some reason just lit the center of the lump, I had the temp spike then coals die at least 3 times during the cook - it was a complete PIA! I realized what I had done differently afterwards and went back to lighting the entire outer perimeter and never have had this problem again. Put cotton balls all around the outer perimeter and I think your problem will go away.

interesting, i do light with a single cotton ball in the center. i wonder if its because the charcoal up top is being lit, and when the air flows the air cant actually stoke the coals since the coals at the bottom are blocking access to the lit coals. another variable to try out, thanks!
 

JasonL

TVWBB Member
When you see the temperature drop, turn off the blower and see what happens.

If you see the temperature go up, then your blowing to much into the Akron, and you will need to adjust the maximum percent of the blower, down until you see the temperature rise.

If you see the temperature to continue to drop, then there might some other restrictions, or the blower is not blower is not enough.

Removing the blower didn't help. Once the temperature reached a little above 225 (230 or so, so this time didn't overshoot as badly), the temperature started to drop again with the fan going 100% (tried a few max fan %, 20,30,40,50% none seemed to help). I removed the fan which didn't help either. I pushed around the coals before lighting to hopefully get some airflow but it wasn't optimal since I just ended up pushing the coals to 1 side vs creating a hole in the middle.

I opened the top vent a little and my temps shot up pretty quickly (and kept climbing until I shut down the test since it's getting late), so thinking maybe that is the next place I focus my testing. Thinking maybe I need to find the sweet spot on the top vent. Maybe the airflow from the blower needs somewhere to escape otherwise you're just moving air around and it's cooling things down.
 

John Bostwick

TVWBB Wizard
If you are closing you top vent, that's more then likely your problem. On my UDS, I have have it fully open, if I close it down the chamber does get warmer but also, the blower has to work a I'd harder to push air through. The Akron is more airtight then a UDS so it needs that air flow and you are probably closing it to far. Open it up and see what you get. If it gets to hot, then maybe close it a tad at a time.
 

JasonL

TVWBB Member
If you are closing you top vent, that's more then likely your problem. On my UDS, I have have it fully open, if I close it down the chamber does get warmer but also, the blower has to work a I'd harder to push air through. The Akron is more airtight then a UDS so it needs that air flow and you are probably closing it to far. Open it up and see what you get. If it gets to hot, then maybe close it a tad at a time.

Thanks ill try a test burn again at some point this weekend and try cracking the top vent a bit more open to see if helps.
 

JasonL

TVWBB Member
during a test burn now. left the vent a bit more wide open than before (20-30% open, previously it was barely enough to stick a toothpick through.

Temperature rose pretty steadily. Fan has been at 0% (with damper completely closed) for the last 10-15 minutes and temps are still rising (slowly).
http://imgur.com/ap9uJga

Here are my current settings. Essentially was trying to only use the fan at startup and go damper only once I got it up to temp:
PID Output Parameters
Proportional
3
Integral
0.005
Derivative
5
Fan output on above
95
% min
0
% max
20
% startup max
30
% Invert output
Servo pulse duration
500
us -
1900
us, fully open at
95
% Invert output
A/C input line noise filter

is this a sign I have the top open too much still? Or that i have leaks letting too much air in from the bottom? I have tried sealing it with nomex gasket tape but I know there are still some small leaks (though seem to be only leaking when the fan is blowing and the top vent is almost completely closed).

Thanks!


EDIT: 30 minutes later and its sitting around 260 and seems pretty stable.
 
Last edited:

JasonL

TVWBB Member
I had to close the top vent a bit since it wasn't dropping below 260 even though the fan was at 0% for an extended period of time.

here's the latest http://imgur.com/a/remVn.

I let it run for the last few hours while I was out and it seems to be pretty stable at 225 +/ 5 degrees. Should I be able to shoot for than +/- 5 degrees fluctuation, or should I consider that pretty good and focus on how to start up scratch and get it to fall into this pattern. I think the key is going to be adjusting the top vent at some point between start up, and when its close to achieving target temperature. Guessing more open during startup, and close it to only slightly open once I'm near the target temp.
 

Steve_M

TVWBB Guru
Realistically, the controller should be able to keep you within 1 degree, but the food really doesn't care if the pit is fluctuating within 5 degrees. Looking at your graph, the main concern is that you are hovering between 80-100% PID output to maintain 225F. That's way too much output for such a low temp. Either the inlet for incoming air is too small, you don't have enough fuel or the air isn't properly reaching the fuel you have.
 

JasonL

TVWBB Member
that's a concern even with the fan set at not starting up until 95%? I think it should just be the servo doing the work right now. I can take a look to make sure the intake is able to reach the lit coals (i kind of just threw the coals in and pushed them a bit to 1 side so there would be airflow from underneath through the coals. I have the servo pulse duration set set to 500 -1900, where 1900 is like 5/6 of the way open. I'm not exactly sure where 500 is in the spectrum of closed (barely closed or way past closed). guessing i should have it set to min = barely closed, and max = fully open?

Edit: I opened the top damper a bit more to see if that would help.
 
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RalphTrimble

TVWBB Diamond Member
The HM allows a LOT of flexibility in the setup, with a blower and damper there's a few ways to go about it...

The easiest way to hit and maintain a low and slow target temp is to let the servo damper and blower work in tandem, both working the full range of output, with the top vent open only a crack. (just past the dimples on an Akorn). This will give you a forced air stoking scenario with the top vent and damper clamping down hard on air flow when the pit hits the target temp. This setup is the most forgiving of PID settings, however, you still need to have the PID tuned reasonably well or you could get the over stoke and choke response if the PID settings are too slow to respond.

Press the "P" key while the HM web page is selected and the PID info dialog will pop up which can give you some incite on where your PID settings might be too aggressive or too laid back. You can set the PID so the response is laid back and only reacting after the setpoint has been achieved, or you can set it so it is more aggressive where it starts to react as the temp approaches the setpoint.... It looks like you want the PID set so it starts to react before the target is achieved, in which case the HM will start to slow the fan and close the damper earlier which will reduce the overshoot, then as the temp starts to fall it will open up the damper and start the fan before the temp dips below the setpoint which will prevent the choking.

The other more difficult way to control the pit is with the setup you have been attempting to dial in, using the fan for stoking and the damper only for the cook. This is my preferred method because I feel it gives a better cook with less air flow to dry out the food and less stale smoke trapped in the pit. However, in this scenario it will be much more sensitive to the PID and top vent settings and you must be much more attentive to the way you build the fire. Using this method you generally need to clamp down on the top vent (to the dimples) while the fire is first stoking, then after the target temp is achieved and starts approaching the setpoint you should open the top vent up a bit to allow convection air flow to do it's thing. In this scenario you need to make sure to light only a very small fire because the larger the fire the more convection air flow which can continue to stoke the fire during overshoot. When you are first trying to figure all this stuff out a little cheater trick you can use to prevent the "stoke and choke" is to pop the lid open briefly after the temp starts to fall toward the target temp after the initial overshoot. In your initial test run it looks like the fire built up too large initially then was choked out too small before the HM kicked back in. This situation can be eliminated with PID tuning, but until you figure that out popping the lid as noted above will provide some oxygen to the fire to prevent it from choking and release some heat from the pit and kick the HM into action to stoke the pit back to the setpoint. The HM will eventually regulate the fire to the appropriate size to maintain your target, the more accurate your PID settings are and the more appropriate size fire you light initially the faster the HM will get things under control. Once things are rolling along at the target temp (in damper only mode) you want to adjust the top vent so the HM is holding the servo damper open about 30% so it has plenty of room to stoke or reduce the fire and compensate for grease and wood chunk flare-ups etc. You wont need a large load of lump coal to do a low and slow cook with damper only control, limiting the amount of coals you put in and how many of them you light is a good way to get started out with the right size fire. It only takes a tiny fire to heat an insulated kamado up to low and slow temp and maintain it, if you get too many coals burning you will get more initial overshoot and have more overshoot issues each time you open the lid.

Sorry for being long winded, but I hope some of what I said is helpful to you.
 

JasonL

TVWBB Member
Thanks Ralph. I really appreciate the thorough response. Yea, I actually got the thought to try and go damper only from 1 of your posts from last year. Seems to make sense and now I think about it, it does make sense that the top vent is much more important.

I had opened the top vent a bit more and the temp jumped and wasn't falling so I closed it a bit. When you say you have the vent open a bit wider in damper only mode after the target temp is reached, how wide are you talking about? Mine is open past the simple, but not by too much (if from closed to dimple is 0 to 1, I probably had it at a 1.5, maybe 1.75.

I wouldn't be surprised if when I started the fire it started too big because I wasn't very careful with coal placement. I did just start it with 1 cotton ball, but once the fan started blowing I don't know where it ended up by the time I reached the target temp. In the damper only mode, does it matter if you light the coal up top or should it be near the bottom?

Thanks again!
 

John Bostwick

TVWBB Wizard
How does the Akron have it's coal. Is it on a grate of some kind and the damper below, like a weber. Or do you have them on the bottom of the Akron. I would do a minion method of lighting the coals. You shouldn't have to push the coals to one side of the smoker.
 

 

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