Sticky thermostat?

I have a puzzling problem involving a thermostat.
I'm using an attic-fan thermostat (from Home Depot: http://www.homedepot.com/prel80HDUS/EN_US/diy_main/pg_diy.jsp? CNTTYPE=PROD_META&MID˜76&com.broadvision.session.new=Yes&CNTKEY=misc% 2fsearchResults.jsp) to control an exhaust fan. I have a small home theater with a CRT projector mounted in a sound-deadening box. When the projector heats up, the tstat runs the exhaust fan to pull hot air out of the box and out of the room.
Initially I used the tstat to trigger a 12V power supply that ran a 12V fan. There was virtually zero current on the trigger line. The tstat worked like a charm for this, and I've been using it for about 6 months that way.
But the 12V fan wasn't really up to the job (for various reasons I need a LOT of suction power on this fan), and my 12V source (an old PC switching power supply) didn't like driving that kind of load, so I finally scrounged an old shopvac and hooked it up. Now the tstat switches the shopvac. The shopvac draws 5.3A, and the tstat is rated for 20A.
Here's the odd thing: when the projector heats up, the tstat trips and turns on the vac just like it's supposed to. But when the projector cools down, the tstat doesn't open. The vac keeps running, and running, and running... for as much as 4 hours one time when I was testing it. If I unplug the vac and plug it back in again, it still runs. But if I leave it unplugged for about 8 minutes, the tstat opens and then it's back to its normal cool-and-turned-off state.
What would cause the tstat to "stick" like that until I remove power from it for 8 minutes? It's conceivable the tstat has gone bad, but it was working just fine for triggering the PC power supply. It's very hard to get at so I don't want to rip it out unless I'm sure that's the problem. A friend suggested maybe the current was heating the tstat internally, and it couldn't turn off until you removed the current and it cooled off. But since the vac is only drawing 1/4 the rated current, I can't believe that's it.
Help!? Gary
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Gary Fritz wrote:

Three possibilities. 1. The contacts may just be sticking. 2. The contacts aren't making a good connection, i.e. there is a resistance across them. The resistance would cause heat to be generated that could be sufficient to make the stat think the area is still warm. 3. The differential is too great.
Richard Perry
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Gary Fritz wrote:

What kind of 'stat? Wet mercury switch? Open contacts? Bi-metal strip? Direct control or indirect type? Does the projector box have enough holes for good ventilation? (can't suck air out of sealed box) I am surprised 12V fan couldn't do it.
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Sounds like your friend is right. Current through the thermostat is heating it. Test it with a 100 watt light bulb and then with a 1500 watt electric heater.
Al
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Gary Fritz wrote:

It would seem to me that what your would need would be volume not suction. I wonder if improving the air flow rather than just adding power, might not have been a better idea. That 12V fan may have been a bit too small for the job, but the shop vac is not likely designed for much continuous duty and I would think it is not the best tool for the job. I think I would have looked for a good bathroom exhaust fan, one that was quiet and moved a good amount of air.
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Joseph Meehan

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Or a large muffin fan.
Al
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Joseph Meehan wrote:

I'd use Muffin fan slowed down with capacitor hook up.
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Joseph Meehan wrote:

Let me add: reduce the static pressure difference. Allow for better (easier) air flow.
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Joseph Meehan

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Wow! Thanks for all the great suggestions. Let me address a few:

That seems unlikely given the tstat is rated for 20A, but it's certainly possible. I'll test that when I get a chance. (The logistics of this setup make tests like that difficult and time-consuming, but I'll get to it.)

I doubt #1 and #3, since it worked fine switching the 12V power supply. #2 is very possible.
snipped-for-privacy@sme-online.com wrote:

Again, since it worked fine with the 12V supply, that doesn't sound right.

The box doesn't say, but I'd guess bi-metal. It's not wet mercury, since there are no warnings about mounting orientation.
"Joseph Meehan" wrote:

There's the rub. I screwed up bigtime when I built the exhaust system. I used a long run of 3" vinyl/spring flexi-duct instead of some larger and smoother duct material. (The path from the projector to the fan is pretty tortuous, and getting rigid ducting in there would have been a nightmare. But this @#%@#$ 3" flexi-duct has been a nightmare too.) I would change it if I could, but I can't. I'd have to rip out several walls. It's not worth that.
So I'm stuck with it, and I just need a fan that can handle the resistance and still pull at least 50cfm or so. Judging by the airflow I get with "normal" fans, I'm guessing I have well over 1" of static pressure in the line. No ordinary fan can pull enough air through that -- I know, I've tried 6 of them. Many fans claim high CFM at >1" static, but that's for a 6" duct or whatever. Hook it up to 3" and those specs go out the window. (The approximation I've derived is that you have to derate the fan based on the cross-sectional area. A 6" duct has a cross section of pi*3^2 = 28 in^2, and a 3" is pi*1.5^2 = 7 in^2, so the 3" has 1/4 the area. So you have to divide the 6" cfm claims by 4 to see what a 6" fan can really pull through a 3" duct. It's probably not exact but it seemed about right in a few tests I tried.) If anybody knows of a fan that can **really** pull an honest 50-75cfm through a 3" duct with 1.25" or so of static, I'd like to hear about it!
The 12V fan I was using was a 3" "bilge blower," usually used to exhaust air from belowdecks on sailboats and the like. It had more suck at high pressure through a 3" line than any other fan I've been able to find.
But it was **barely** doing the job, and the PC power supply finally refused to drive a motor any longer. I could have bought a beefier linear power supply and contined with the almost-inadequate ventilation, or I could go with another approach. I went with the shopvac and a router speed control. The shopvac has plenty of power to suck air through the 3" line -- too much, which is why I'm slowing it down. The speed control is designed for universal motors so it should have no trouble controlling the vac.
But I still have to solve my tstat problem. I guess the most likely suspect is the "poor contact causing resistive heating" explanation. I can test that with Big Al's 100w bulb suggsstion, but I suspect I'll get the same result I got with the 12V power supply. So either I need to buy a new tstat and replace the old one (not a fun job, it's built up inside the projector box), or maybe I'll just wire up a relay. Then the tstat only has to switch the low-current relay winding.
Thanks guys! Gary
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I wrote:

Hm. Now that I think of it, I wonder if this might be an issue after all.
The set temp on the tstat is the same as it was with the old 12V fan. It kicks in when the temps inside the projector (NOT at the tstat, I don't have a temp probe there) reach about 93F, just like it did before.
However the new fan (vac) pulls a lot more air than the old fan, and it cools the projector a lot better. The projector's internal temps used to stabilize around 115-120F, but with the new fan it never gets much above 98F or so.
So that's one variable I hadn't considered before: the tstat is not heating as far above its set temp as it used to. Could that affect it?
I'll have to try slowing the vac down so much that it lets the projector heat up to 115F like it used to, and see if that changes the tstat's behavior.
Gary
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Gary Fritz wrote:

I would guess that is it. Attic fans seem to allow a very wide range of temperature swings.
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Joseph Meehan

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Doesn't look like that's it. I ran the projector with the vac unplugged until the internal temp got to 120F. Remember I don't have a temp probe at the tstat itself, but I assume it got at least as warm as it used to when the 12V fan was running and the internal temp was 115-118F. Then I plugged it in, and didn't watch the temps while I finished the movie. But it's been running for an hour since I turned it off, and the internal temp is in the high 70's. The tstat is still sticking on.
I suppose it's possible the contacts didn't get heated up because the vac wasn't plugged in for the first part of the test, but this experiment was trying to determine if the temperature swing I used to have was critical to making the tstat work. Doesn't seem to be.
Huh. That's odd. After letting it run for an hour or so, I went ahead and unplugged the vac. Then fooled with it for a few seconds and plugged it back in -- and it didn't turn on. The tstat "unstuck" almost immediately this time instead of after 8 minutes!? I'm sooo confused...
Gary
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wrote:

No, you just proved the load is heating the thermostat. Try the test with a 100 Watt bulb, and then with a 1500 Watt electric heater.
Al
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?? But I ran the projector for maybe an hour without the vac to heat it up, then ran the vac for over TWO hours waiting for it to shut itself off. If 2-3 hours of running didn't heat the thermostat, I don't know what would! Seems to me I proved the load DIDN'T heat the thermostat...
(Or I proved the @#$@# thing is just inconsistent...)
I'll try to do the bulb/heater test, but it will take a while.
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Gary Fritz wrote:

Yea, my error hit me within a few seconds of sending the message. I tried to delete the message but it will still show up on many servers.
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Joseph Meehan

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These t-stats normally have a switching differential of about 15 deg F. If you set to close at 105, it'll open at 90.
Raise the close set-point. (And thus the open set-point.)
It the stat is being heated by current, it has measurable resistance, and will fry, or has already. Not likely, with anything like normal load current.
Typical fan for attic draws 3-4 a.; ShopVac likely much higher. Not to mention noise. I'd check with Grainger or Marlin Jones (www.mpja.com) for suitable fan.
J
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