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Hot box issues

post #1 of 14
Thread Starter 

Guys - I've built a hot box but I am having some issues with the heater and temperature control.  I have a Ranco thermostat (ETC-111000) that controls a Vulcan 900W heater element.  A fan runs full time when the heater is on or off.  I have the thermostat set to shut the heater off at 120 F.  The problem is - after the heater element has shut off (noticable click), the temperature in the box continues to climb to approximately 155 F.  The increase in temperature is observed throughout the box - not just in the vicinity of the heater element so I think that the air flow is adequate.  I've also shielded the heater element to help but didn't see much difference.  Why is the temperature continuing to climb after the heater is shut off?  Any ideas to resolve?  Thanks.

post #2 of 14
smile.gif. I recognize that problem, having just built mine and been getting it dialed in. I am using a 700w heater and PID controls, plus 2 fans. i have also built in a lot of insulation (fuller report to come). Bear in mind that when the thermostat kicks in, while it cuts off supply to the heater, there is still a lot of residual heat. If like mine you are well insulated, this continues to build. Sooo, you can either vent some of this or try setting the thermostat a bit lower. If it is a mechanical thermostat, this will be less successful as the temp may then cycletpo much. With the pid I have been playing around with the differentials before it kicks back in to keep it in a very narrow band but also thinking about adding an auto vent.

I think ( like me!) you may have used a bigger heat source and more insulation than necessary. Try adding a vent to start with. You don't want it at 150 +. ,
post #3 of 14
Thread Starter 

Thanks.  I am having the exact problems as you.  The temp continues to build from the residual heat.  I have been playing with it the last couple of days with thermostat settings. The problem with lower temperature settings is that the temp drops too much before the heater kicks back on and I have extreme highs and lows and not a constant temp from 120 - 140. 


I tend to agree with the heating element being way too big.  I am really considering downsizing it and just contacted the manufacture for a size they recommend at my fans rated air flow.  Right now, I'm thinking of cutting it to 300W.


Thanks again.

post #4 of 14

Ever heard of thermal inertia?

You are heating up the surrounding mass and the box overshoots from this heat after shutoff.

I'd need to see a drawing to make a recommendation.

My usual engineering consulting fee is $200/hr but for Epic Ski members I'm free.

post #5 of 14
Thread Starter 

How about a picture?


Hot box 1


Internal dimensions are 80"h x 16"w x 20.5"h.  Constructed of 2"x2" frame and 1/4 plywood skin.  1" foam board is between the framework and 1/2" foil faced foam board is overtop.  Fan is rated for 65 cfm.  Originally used a perferated dryer vent along the length of the box to move air.  Now just have a crude heat shield using the foil faced foam board.


Thanks for any input!

post #6 of 14

With that much insulation you don't need many watts.

The smallest heater that will bring things to temperature will slow temperature rise so it won't overshoot so bad.

If you controller is a three mode controller with proportional, derivative and integral settings you can fix this by cutting the proportional and increasing the integral rates.

If you have access to a varistat you can crank the heater voltage down until it is at a wattage level that won't overshoot so bad.

You can also play with the location of the temperature sensor.

Moving it as close to the heater as reasonable will help by shortening the on cycle time thus reducing overshoot.

Use a seperate thermometer to moniter the air temp in the middle of the box to get an average value.

Good hunting!

post #7 of 14
Good points by dakine.. You might also want to think about increasing the fan capacity. My box is slightly larger - 96*24*24 external, 90*20*20 internal and aim running 2 *110cfm fans. With the 36 heater centrally mounted on the bottom and the thermo meteor probes approx 24 inches from each end I am showing <1 degree temp difference at 130 degrees. Just continuing the fine tuning at the moment.

May also think about a much smaller heater as an ancillary, use the main one to bring up to temp and then use the controller to switch over to the auxiliary for tighter control.
post #8 of 14
Me and a buddy have a hot box we use to build bows.We use 4 100 watt light bulbs and dont have big temp.
diffrence.We messed around with bulb wattage to keep more consistent,we do have hooked to thermostat
post #9 of 14

The thermal inertia inside your box is too low for the size of your heater.    Long term you can reduce the size of your heater.    In the meantime, a very cheap way to reduce (not necessarily eliminate) overshoot  would be to put a giant pot of water inside the box.   

Originally Posted by edgerox View Post

How about a picture?


Hot box 1


Internal dimensions are 80"h x 16"w x 20.5"h.  Constructed of 2"x2" frame and 1/4 plywood skin.  1" foam board is between the framework and 1/2" foil faced foam board is overtop.  Fan is rated for 65 cfm.  Originally used a perferated dryer vent along the length of the box to move air.  Now just have a crude heat shield using the foil faced foam board.


Thanks for any input!

post #10 of 14

Inherited a very similar hotbox with a 900w strip heater in it and was pulling my hair out over the same problem.  My box has a heat shield over the heat element to spread the heat, and two fans (one low, and one high).  I've got 5 temp probes in the box and have finally got it to just a few degrees variance end to end and a fairly tight heat tolerance with pretty much zero overrun now.  To avoid the overrun I put a dimmer switch on the output of the digital controller before the heat element to bring the voltage to the heat strip down.  I tried the dimmer before the controller, but the controller likes full voltage.  It takes a while to heat up now, and stays pretty consistent.  Long term I'll probably calibrate the dimmer and move it to the outside of the box.  If you're thinking about switching heat elements, a friend just built a quick and dirty box with a roof heat (ice melt) cable for a heat element (also on a dimmer).  He says it's real consistent end to end as well without fans.



post #11 of 14

I have also been messing with hotbox design for the last few months and think I've got the thermal inertia thing licked.


My design has no insulation just 7 ply marine plywood and an electronic controller controlling on 350w baseboard heater and another 500w and fan on full-time. I experimented with the full-time heater sizing so it would take make within about 5 DecC of the desired temp but was incapable on its own of going any higher, this way the smaller heater is just making up the difference, I tend to get a 1 DegC overrun up the way and a dip of about 2 DegC which is all good for me.





post #12 of 14

My 2 cents.


A similar application is reflow ovens for PC boards.  There are several newsgroups and posting devoted to these projects and have pic controllers that can be programed to compensate for the the temp curves in toaster ovens.  These could also be programmed for the the curves that you are seeking.


Have fun. 

post #13 of 14

There's a time lag from the generation of the heat as the heater mass gets warm, then transfers heat to the air, which then heats up the thermostat until it switches off.

So the temperature rise of the heater always leads the temperature of the 'stat, and if the heater is oversized then it can way overshoot your desired temperature before being turned off.

When it finally switches off all of that saturated heat still radiates out and you overshoot the setpoint.


Two options:

  1. Reduce the power output of your heater so that at 100% duty cycle the box temp never gets more than a couple of degrees higher than the highest temp you'll ever want.
  2. Have the thermocouple read the temperature of the heater rather than the temperature of the air, and use a heater that doesn't have a lot of mass and can radiate the heat very quickly.


I'd take the first option because the second seems (at first glance) to pose a higher risk of overtemp and hot-spotting.

post #14 of 14

When doing reflow in printed circuit boards and components it is very critical that you don't cook them.  The programs and thermo couples can actual compensate for the thermal over run and match the curves or temps that you want.  Neat stuff actually.

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