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Hot box thermostat

post #1 of 29
Thread Starter 
I am building a hot box, and will use a small 110v resistance heater with a fan. I need a thermostat to control the heater. I tried a water heater thermostat, but it doesnt seem to react quickly enough to temperature swings.

I've read that you can use a thermostat from a kitchen supply store, but have no idea what I would need or how to wire it, and dont live anywhere near such a store.

Can someone suggest a cheap line-voltage thermostat that will work in the appropriate temperature range?

Thanks

Richr
post #2 of 29
Are you controlling the 110v with the thermostat?

Broan White Line Voltage Wall Thermostat
post #3 of 29
The problem is that thermostats such as that shown do not go to a high enough temperature to control a hot box.
Hot box temps are in the 120F - 130F range.
post #4 of 29
Quote:
Originally Posted by quickk9 View Post
The problem is that thermostats such as that shown do not go to a high enough temperature to control a hot box.
Hot box temps are in the 120F - 130F range.
Woops, didn't really think about that.


Letme give this some thought. Do you have a budget?
post #5 of 29

If you've already built it...

...you're on your own. I don't know of any way you can control temperature in this situation with a homemade hot box, which is why I chose not to try to do business this way:

http://www.rmmskiracing.org/articles...-03-Hotbox.pdf

http://www.rmmskiracing.org/articles...tbox-Part2.pdf
post #6 of 29
The trouble is that the fan powered heater comes on FULL blast as an on/off control with a standard thermostat. Then the issue is "how can you ensure the hot box has an EVEN temperature distribution?" The best way is to make sure that the heater is on all the time, just not running full blast.

You can purchase thermostats that will modulate the 110 volts to the heater, so that it will keep a steady low heat applied. This will help the hot-box temps stay uniform.

Take a look at the one at the bottom of this page. A Honeywell rep can tell you if this is the right one. If not, they have more. Your looking at about $200 each. Unless you find it on the industrial ebay.

https://www.neccdelivers.com/_PDF/Guidebook_233.pdf
post #7 of 29

tougher to build, but better control

A couple decades ago (yikes!) I used to do controls for a living.

The best controller would be one that has Proportional, Integral, and Differential control, or PID control. That will let your box heat up quickly but not over-shoot too much or oscilate once it reaches your desired temperature.

You can get these from industrial supply houses. Way back then, we used to get Watlow controllers, but most major brands work well (Fuji, Honeywell, etc.). My guess is that this kind of controller will run about $150. You'll also need a thermocouple to grab the temp.

Oh - if you do get one of these, get a self-tuning one unless you really want to dig into the details of PID control theory. Have the controller get its parameters through a few cycles without your skis.

I doubt that HVAC controls will fit because most aren't going to let you set the temp hot enough.
post #8 of 29
you can pick up a PID on ebay for cheap. (~$30-$50). a lot of coffee enthusiasts use them to to mod inexpensive espresso machines, so there is a bunch of info out on the internet on how to wire them, some of it should be applicable to what you are doing.
post #9 of 29
Do a search on Hotbox's here on Epic, controls and design have been discussed a few times.
post #10 of 29
Quote:
Originally Posted by BobHarwood View Post
I doubt that HVAC controls will fit because most aren't going to let you set the temp hot enough.
Aquastat.
post #11 of 29
Posts 7 and 8 are on the money. Only thing I have to add is that you probably need more fan than you think you need, and you should get some extra thermocouples/RTDs/whatever while you are at it.
post #12 of 29
Here's some information about my hot box:
  • Internal dims are L2000 x H450 x D350mm. The box has two levels. The longer top shelf takes skis up to 195cm length & the lower shelf up to 170cm length. I was limited by space so couldn't construct a W2200mm long box but very few people in the UK have DH or SG skis.
  • The box has a 50 x 20mm external timber frame with a 9mm plywood skin. It is lined with foil covered 12mm plaster/gypsum board. This gives a thin but very well insulated carcass. It's also heavy so the box is mounted on lockable castors. I originally had a hinged door as I was going to wall mount the box but now the box is on the floor it's best to have a detaching door with a number of catch around it's perimeter. Both the door & the inside of the box & rebated & have foam strips attached to give a very good seal & no air loss.
  • With the initial design I found that the heat rose too quickly up the centre of the box meaning that the middle third of the ski heated up too quickly & the tips/tails were noticeably cooler as the air did not circulate correctly. I revised the design by extending the top of the heater casing by a metre (cardboard covered in tin foil) to ensure that air entered the box & rose at the right hand end of the box (by the thermostat probe). This has cured the problem & the skis are an even temperature down their full length meaning that the air is circulating well & there are no hot spots.
  • The fan & heater element (see attached diagram) is housed is an outer casing at the left hand end of the box. The fan motor protrudes through the base of both the casing & the box for cooling. The outer casing is made from sheet metal but it could be easily fabricated from plywood or mdf.
  • The power switch & thermostat/control unit are housed outside of the box with the thermostat probe mounted in the right hand top corner of the box, ie at the opposite end from the heater.
  • The control unit is wired so that when the required temperature is reached the heater element is turned off but the fan remains switched on to keep the air circulating.
  • The wires used to connect the heater element need to be heat resistant to at least 100C.
  • The heater, fan & thermostat/control unit all came from Grainger in the US.
  • Here's the heater element spec & price http://www.grainger.com/Grainger/items/4E266
  • Here's the thermostat/control unit spec & price http://www.grainger.com/Grainger/items/3ZP77
  • Here's the fan spec & price http://www.grainger.com/Grainger/wwg/productIndex.shtml?from=Search&newSrch=yes&operato r=keywordSearch&search_type=itemnum&action=Go%21&Q ueryString=1TDP7&submit.x=17&submit.y=6
  • My cooking time is 4 hours at 140/150F. Every box runs at slightly different temperatures so do a few test runs starting at 120F for 30mins & check the skis to ensure the wax is fully melted along the full length of the skis. If not then up the temperature by 10F & re-test. There's no problem hot boxing skis with the bindings attached.

525x525px-LL-vbattach3945.jpg
post #13 of 29
Very nice job Spyderjon.
post #14 of 29

Thermostat for hot-box

Try http://www.rancoetc.com.

I just built a hot-box, using one of thier controls to regulate the temperature. The control allows for very precise temperatures, i.e. going up to desired temp, then cuts out the heating until the temerature drops 1 degree C, then starting heating again. Using two computer fans that runs all the time to ensure circulation.
post #15 of 29
post #16 of 29
Hi All - I'm a newbie to this but have been periodically watching some forums here over the past months. Mostly, forum(s) on hot boxes. Anyways, what up.

Well, I have now completed the guts on my hotbox, but I'm still working on figuring out what I want to do about the controls. From what I can gather, there really are two good approaches: (1) PID controller + SSR and (2) normally closed contacts of 140F range thermostat that has adjustable differential. Would those that have done it agree?

I was thinking of avoiding the PID approach and have been looking at a particular line level thermostat until I noticed that the differential is kinda poor (http://www.grainger.com/Grainger/items/1ZHC7) at 12 deg. Based on some of the recent posts, I'm thinking of going with the Ranco unit (http://www.grainger.com/Grainger/items/3ZP77), but I have one concern. It states the full load amp rating of the switch is only 5.8A (on normally closed). My heater is 1.5kW max.....has anyone used this switch above it's max rating? Maybe a different way to ask is, has anyone used this Ranco unit with greater than a 700W heater? If so, does it work well?

I'd appreciate any suggestions. Of course, I'd be curious to hear from those that have done the PID + SSR approach and how accurate of final temps it provides.

Cheers,
Troy
post #17 of 29
Thread Starter 
Quote:
Originally Posted by tdc5152 View Post
Based on some of the recent posts, I'm thinking of going with the Ranco unit (http://www.grainger.com/Grainger/items/3ZP77), but I have one concern. It states the full load amp rating of the switch is only 5.8A (on normally closed). My heater is 1.5kW max.....has anyone used this switch above it's max rating? Maybe a different way to ask is, has anyone used this Ranco unit with greater than a 700W heater? If so, does it work well?
I question whether you need a heater with that much power. I use a 1000 W space heater, and control it with a dimmer switch. I find that I have to throttle it way down to keep the temperature down below 140 degrees. If your box is well insulated, a smaller heater element should do the job.

Richr
post #18 of 29

I think it really depends...

...on how elegant you want to be. Spyderjon's is really awesome, and if I were going to do it all over again, I'd probably go in this direction. Mine's a lot more primitive...but it still works like a charm. You pay your money and you take your choices...

http://www.rmmskiracing.org/articles...-03-Hotbox.pdf

http://www.rmmskiracing.org/articles...tbox-Part2.pdf
post #19 of 29
Quote:
Originally Posted by tdc5152 View Post
Of course, I'd be curious to hear from those that have done the PID + SSR approach and how accurate of final temps it provides.
The last one I built was stable as measured at the control point to within 1 degree. Which is probably technically wrong, as I'm not sure the thermocouple or controller electronics are that repeatable/free of drift. In any case, the control works as advertised if you tune it right (and autotune works great by itself or as a starting point for you to refine.)

The more interesting data is how the temperatures of skis vary throughout the cycle in different locations in the box. Last design I worked on had variation up to 20 degrees during ramp and about 5 degrees steady state as I recall off the top of my head...was measuring this with a half dozen thermocouples I moved around to different locations over several cycles to datalog a bunch of locations...very shotgun type approach, I could have missed a particularly hot or cold spot but I think I got the gist of it.

I don't think ~1500w is excessive at all if you are building a fairly sizeable box and you want good control over the ramp rate. Richr mentions his is throttled down a lot...indeed this will be the case in steady state, but controlling ramp rate well requires a)lots of convection forcing and b) plenty of power.

I've never built one of these with a thermostat type control though, so that may be the way to go. The PID process control + SSR route just seemed obvious.
post #20 of 29
Thought I'd at least post the results. After testing, I determined that 750W is much more ideal than 1.5kW. As such, I unplugged 1 of the 2 heating coils (heater came from an old Qmark wall heater) to 1/2 the power output.

The PID controller + SSR approach works well. Note that most PID controllers (at least the cheap ones) do not "autotune" when using SSR on/off output. I don't think that's a big deal. I measured across the box and confirmed the maximum temperature deviation is ~4 deg F at the tail end of heating cycle from one end of the box to the other (~7' long) and near zero at the tail end of the steady state condition. The maximum drift is is +2F/-3F at the controlling thermocouple. It is fine for many, many hours.

If I had to do it again, I would do the following:
1. Place the heater element in the center of the heater box. Mine is a 3.25" x 10" duct that lies at the bottom with holes punched in it to try to even out the airflow. My thought is that after the heating cycle kicks off, leaving it more center would help prolong the steady state phase even longer. Then again, this could cause the "recirc" end of the box to cool off even faster. But worth trying IMO - especially considering #2 below.
2. Stronger fans. I experimented with this by adding another fan inside and it yielded slightly more uniform temperatures across the box. If the small 3" muffin fans go to h*ll, I'm replacing them with the highest CFM - same size models I can find.

I'm in the 3rd session of hotboxing now. Good luck.
post #21 of 29
Good to hear. Next box I build I'm going to do a proper plenum/plug fan for so I can run it hotter for other handy applications.

I should have some more results to post on this topic in a month or so.
post #22 of 29
Ive been hot-boxing all week....massive temp differentially throughout inspite of a 6" fan in front of the heating element....problem is the fan only works while the element is on I am going to add more fans with higher cfm that will be constant on.
post #23 of 29
Quote:
Originally Posted by Richie-Rich View Post
Ive been hot-boxing all week....massive temp differentially throughout inspite of a 6" fan in front of the heating element....problem is the fan only works while the element is on I am going to add more fans with higher cfm that will be constant on.
Richie, shoot for fans with CFM around 15x the volume of your box. Remember to use fans with adequate pressure margin else they'll stall and just make noise. If the fan is just in free air somewhere in the box you can probably get away with a plain axial fan. Haven't tried that kind of setup. The last box I built was very large, and only had about (edit) 9x volume in CFM. Thus the 20 degree gap between heater outlet and inlet. (the differentials measured on ski bases were much smaller, but significant)
post #24 of 29

hot box temps

After reading about 30 sites, I've come to the conclusion that many people who crank up hotboxes have no idea what c degrees and f degrees are. 49 degree c = 120 degree f. . And most sights suggest you do not (should not) go any higher than 120 degrees f. If the numbers are switched, you're going to cause a serious nuclear melt down. Lynn
post #25 of 29
Quote:
Originally Posted by speedvanish View Post
After reading about 30 sites, I've come to the conclusion that many people who crank up hotboxes have no idea what c degrees and f degrees are. 49 degree c = 120 degree f. . And most sights suggest you do not (should not) go any higher than 120 degrees f. If the numbers are switched, you're going to cause a serious nuclear melt down. Lynn

Well been doing it for 3 years now and no problems...that with capped and laminated skis ranging from entry level models to mid level metal topped Volants (that as you might imagine with all that extra metal can get quite hot) to top of the line race stock skis. I crank the heat up enough to make the wax melt, whatever that might be, it is usually above 120 but under 150. Total of 12 pais of skis and no issues save for the goop under the Vist WC plate on my GS skis, that stuff melts even if put into a warm room, but I have been assured by a few people including most recently a former WC tuner from Edgeweise, that's of no concern and that some racers actually scrape that stuff off.
post #26 of 29
Rich: please read what I wrote. There is a huge difference between centigrade and fahrenheit. Ie. 60 degree centigrade is about 150 degree fahrenheit. If any one is confused, which I'm convinced there are quite a few out there, and they dial in 160 degree centigrade, there will be a large mushroom cloud rising from the ignorant parties garage.
post #27 of 29
Thank you for all your posts. I have spent the last two days searching for your posts on hot boxes. Unfortunately the links to Grainger's website don't always coincide with the correct link. After several tries I have found everything but the fans part. Number and price. If you read this would you please respond to me with how to find it.

Thanks,

George
Journeysend@frazier.net
post #28 of 29
Quote:
Originally Posted by tdc5152 View Post

It states the full load amp rating of the switch is only 5.8A (on normally closed). My heater is 1.5kW max.....has anyone used this switch above it's max rating?

 

Using electrical equipment above or constantly at the rated maximum is a great way to burn your house down.  If you're lucky, the device is fused, and the fuse works - your device turns off.  Worst case - you set everything on fire and die destroy your quiver.


Edited by Magi - 11/21/13 at 9:09pm
post #29 of 29
Quote:
Originally Posted by Magi View Post

Using electrical equipment above or constantly at the rated maximum is a great way to burn your house down.  If you're lucky, the device is fused, and the fuse works - your device turns off.  Worst case - you set everything on fire and die.

Must be why we haven't seen TDC5152 since 2008 - his hotbox burned the house down.......popcorn.gif
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