As some of you guys know, we have a non-profit that teaches kids about fire safety. We received a DHS/Assistance to Firefighters Grant this past year that allowed us to build a new fire safety trailer. I'm working on the heated doors right now. Part of what we teach the kids is to feel the door with the back of thier hand before opening it to determine if there's fire on the other side, hence the 'heated doors'.

I have what amounts to a heating pad that's made out of two layers of silicone with a heating grid sandwiched between the two layers, (it's basically an industrial drum heater). One side has sticky 3M stuff on it so the pad can be stuck to something, in this case the interior of the aluminum door. My problem is how to regulate the heating pad. It will achieve a temperature of approximately 300* F in about a minute and a half, and this is the smallest/lowest heat pad that I could find, and it only draws a few amps, (it only has 20 gauge wires feeding it).

Here's my problem; I need something like a line voltage thermostat with a remote sensor that will cut the power to the heating pad when it reaches about 110* F and turn it back on around 90* F or so. Does anyone here have any HVAC experience that might point me in the right direction? Maybe I'm not going about this in the right way either and I need to do something totally different in regards to a thermostat. ANY suggestions would be appreciated.

chaikwa.

 

I am assuming when you say line voltage you mean 110V. There are several types of thermostats for Knipco heaters and such that have adjustable thermostats depending on the temperature, that will shut off the power to the unit. I can't say that they go up to 100* or more, but the fact that there are some means you can surely modify it or find one that suits your needs. Wish I was more help........I'll do some searching.

 

http://www.sbt.siemens.com/sbttempla...pdf/155019.pdf

Look like what you are after?

 

Kinda, but no. If the remote sensor were a wire instead of a liquid filled tube, it'd work great I think. But the remote has to move with the door while the thermostat will be mounted on the wall behind the door. I think the liquid filled tube would just break after the door opens and closes a few times. Thanks tho, appreciate the help!

chaikwa.

 

http://www.grainger.com/Grainger/items/4E636

or

http://www.grainger.com/Grainger/items/3ZP77

or here is a whole list! Good luck Chaikwa, its good to see people involved in educating kids on safety!

http://www.grainger.com/Grainger/wwg/productIndex.shtml

 

Wow! The second one you listed is exactly what I'm lookin' for I think. THANKS!

chaikwa.

 

We use a chromolox immersion heater on our equipment. they have an external thermometer that is feed by a small copper probe
Here is a link

http://www.chromalox.com/products/prodinfo/en/pit.html

 

Scott, couldnt you wire in teh control from an electric skilet?
sounds like its made to do what you are wanting

 

This is what I used to control an un-repairable hot tub, it has a remote thermistor and is easy to program.
Have the relay operate a contactor for the high current requirements.

Also you might want to install a Snap switch in series and mount it to the door surface as a High Limit.

http://www.grainger.com/Grainger/items/3ZP77
Jim

 

I don't know, I've never pulled a skillet apart to find out what's in there... heck, I've never USED an electric skillet. People see me cook once, then keep me away from anything hot!

I was thinking of that thermostat too after vzdude posted it. I'd still need to have it run a contactor even tho it's a line voltage thermostat? And could you expand on the 'snap switch'? I'm not familiar with those and really that's all I want to do is shut off the power when the door reaches a certain temperature.

chaikwa.

 

I was kinda thinking of a thermostat like off snowmobile handwarmers, like for the cupholder going on my tractor...

 

Off fromthe subject, but....

"This is what I used to control an un-repairable hot tub,"


We are in the process of hot tub shopping right now. From what i've seen and read, other than a cracked shell, there no such thing as an unrepairable hot tub. I found a website with all the parts you'll ever need if your interested.

 

chaikwa.[/QUOTE]


I would use this electronic temperature control and mount the remote thermistor in contact to the doors surface.
http://www.grainger.com/Grainger/items/3ZP77

The relay contacts from the control would actuate the relay of this universal fan center. The purpose of this is to have all of your control circuits work at a safe 24 VAC so there is no shock hazards. This is a nice ready-made transformer/ continuous duty relay package ready to mount on any 4S electrical box.

http://www.grainger.com/Grainger/items/3TZ56

I would then connect this Snap action switch in series between the controller relay contacts and the relay in the fan center.
This is a High Limit switch, in the event something went wrong with any of the controls as soon as the surface temperature gets above the set temperature, the bi-metallic disc will snap and open the circuit shutting off the power to the heating element, as soon as the temperature gets below the differential temperature it will reset itself.
You would want this switch mounted to the door and have it with a good thermal contact to the surface, thermal paste would be good.

http://www.grainger.com/Grainger/items/2FBU9

This controller is very versatile and it can be programmed to either Make or Break on rise and you can set the Differential

Quote:
My problem is how to regulate the heating pad. It will achieve a temperature of approximately 300* F in about a minute and a half, and this is the smallest/lowest heat pad that I could find, and it only draws a few amps, (it only has 20 gauge wires feeding it).

Do you mean the door or the heating pad? being in contact with the metal door should sink some of the heat.

If it is heating up this quickly and you are trying to maintain the temperature around 120* this circuit will be cycling probably about every minuet or so as the temp raises and the power is cut to the element.

The trick would to have bought one meant to operate on 220 volts, you did not mention how many watts the element is but operating at 1/2 the voltage would have dropped it by about ½. You could connect it to a Variac and run it down around 90 volts if it was uncontrollable.

I hope I a not confusing you because in my mind it is really simple but it a bit harder trying to explain it.

Other than the temperature controller, the rest of the parts can be found at any HVAC supplier like Johnstone’s or Burke Engineering most people can walk in off the street and get it cash/ tax.
Just act like you know what you are talking about.



Quote:
Off fromthe subject, but....

"This is what I used to control an un-repairable hot tub,"


We are in the process of hot tub shopping right now. From what i've seen and read, other than a cracked shell, there no such thing as an unrepairable hot tub. I found a website with all the parts you'll ever need if your interested.

an un-repairable hot tub?

This was how I feel about it also; there is not much of anything that I cannot repair.

The problem was the thermostatic control circuit for the Teledyne Larrs electric heater was bad and it would not send a solid voltage to the heating contactor, it was a 120 VAC coil causing the coil to drop out from low voltage and chatter then the contacts would burn from the arcing. Without tearing into the circuit that was sealed into the deck controls it was much easier to redesign the control circuit into something I knew would work. I still used the air switch to control it but the temperature was maintained by the electronics underneath, the thermistor fit into the original location in the well of the element. It was then a matter of entering the desired water temperature, preset the differential and close the door.
To have it repaired would have cost more than the spa was worth having an obsolete heater. I found a replacement deck control for around $250.00 and it would of had to be modified to work with the relay pack in the heater.

All of the parts cost me about $100.00 and now it was controlled by an industrial grade part.

So like I said, there is almost nothing that cannot be repaired just how much money you want to spend.

This was a friends Spa and he in no way had the knowledge to repair it.

Jim

BTW what was the name of the website? I found a good one also.

 

I'll reply/ask questions in red to make this easier;

THANKS FOR ALL YOUR HELP!

chaikwa.

 

I'll reply/ask questions in red to make this easier;


Quote:
Originally Posted by Jim Lane

I would use this electronic temperature control and mount the remote thermistor in contact to the doors surface. Understand. This was my plan too.

http://www.grainger.com/Grainger/items/3ZP77

The relay contacts from the control would actuate the relay of this universal fan center. The purpose of this is to have all of your control circuits work at a safe 24 VAC so there is no shock hazards. This is a nice ready-made transformer/ continuous duty relay package ready to mount on any 4S electrical box. But then I'd hafta get a transformer, right? And I wasn't planning on using ANY fan. Or are we just talking about using a fan control?

No, the fan center comes with it's own 24VAC transformer built into the package. It is just called a fan center because it is used to control blowers but is a nice unit because all you have to do is to supply the input power and it provides a low voltage switching unit.

http://www.grainger.com/Grainger/items/3TZ56

I would then connect this Snap action switch in series between the controller relay contacts and the relay in the fan center.
This is a High Limit switch, in the event something went wrong with any of the controls as soon as the surface temperature gets above the set temperature, the bi-metallic disc will snap and open the circuit shutting off the power to the heating element, as soon as the temperature gets below the differential temperature it will reset itself.
You would want this switch mounted to the door and have it with a good thermal contact to the surface, thermal paste would be good.Could I just use this snap switch and forget about the thermostat then? All I really need is a high limit control anyway.

No it would not be advisable, this switch in works by way of 2 dissimilar metals bonded together and as it heats up it will flex and the deflection is what breaks the circuit. I do not think they were designed to do this repeatedly maybe 100’s of times per session.

http://www.grainger.com/Grainger/items/2FBU9

Do you mean the door or the heating pad? being in contact with the metal door should sink some of the heat. The door will heat up that fast.

If it is heating up this quickly and you are trying to maintain the temperature around 120* this circuit will be cycling probably about every minuet or so as the temp raises and the power is cut to the element.Is this a bad idea?

No I do not think so if you used the proper equipment.

The trick would to have bought one meant to operate on 220 volts, you did not mention how many watts the element is but operating at 1/2 the voltage would have dropped it by about ½. You could connect it to a Variac and run it down around 90 volts if it was uncontrollable.I didn't see any that were operable on 220.

I know that you have already got one but this one has an adjustable thermostat and has a range from 50* to 400* farenheit, since you only need to heat the portion of the door they will be feeling then this should work.

http://www.grainger.com/Grainger/items/2YA60


I hope I a not confusing you because in my mind it is really simple but it a bit harder trying to explain it.No, not confusing me at all really. I'm just not used to these electrical/control things.
THANKS FOR ALL YOUR HELP!

chaikwa.