Here in Canada its hard to find an attic fan thermostat if you find a store that has one it will be very expensive. After searching so many stores I found one, but it was prices at CND $70.00 witch is not reasonable for bi-mental thermostat. Where electronic cooling and heating thermostats are available for under $50.00 CND. However, we can't use this for our projector.

Finally, I thought why can't we convert a heating thermostat to a cooling thermostat.

There is two ways to do it.
(1) Using a bi-metal heating thermostat and a relay to achieve a reverse affect. However, this is complicated.

(2) Using a bi-metal heating thermostat and changing the bi-metal bending strip to the opposite direction to create the reverse effect. This is the easiest and cheapest.

I used the second option to create my cooling thermostat. I've tested it and is working fine with my PJ. Find below some pictures showing how to change a heating thermostat to a cooling thermostat.

This the Heating Thermostat that I bought from Home Depot for CAD $12.99. DIMPLEX Built-in Electric thermostat for baseboard. MODEL # DTK-SP Single Pole.

A --> SWITCH

B --> BI-METAL STRIP

C --> NUT

[B]- bi-metal strip bends opposite direction of the switch when it get heated and gets off contact from the switch when it meets the set temperature to OFF the switch.

If we change the side of the bi-metal to bend towards to switch then its going to be a cooling thermostat, switch will be ON when its heat, That's what we are going to do next.

Remove [D] cover and [C] Nut and take the Bi-Metal strip out,

after taking off the cover.

Bend the strip bent area to opposite direction of the same angel.

assemble back the way it was, Only difference now the strip is going to bend towards the switch when its heat

Now if you like you can do a calibration before installing in PJ.


To do calibration you will need hair dryer to produce heat, multi-meter, thermostat to read temperature.

Mark a line to use as a guide.

Connect the thermostat to the multi-meter as shown below. Set the multi-meter to ohms. Set thermostat probe closer to the bi-metal strip to get the closest heat reading of the bi-metal strip.

Mark the adjusting knob according to heat reading you want to set. This part is very tricky, since probe only takes the air temperature which will not be the exact temperature as the bi-metal strip. I noticed there was upto +3 to -3 degree variation when installed in the PJ. I set it to switch off the fan when the heat in the PJ reaches 25 degrees. I tried 3 times, on two attempts it switched off around 24 degrees and the third time it switched off at 26 degrees. I think it will all depend on where you set your thermostat inside the PJ. Closes to the light you might get the best results.

I marked my thermostat knob with 3 different temperature readings. This will be a rough guide for the temperature settings.

Looks like a good solution. You could even make it failsafe by adding a relay and using it the way it was as a heat controller.

If you wire your fan through a relay to always be on, you could use the controller to turn the relay off when the temp gets low enough. That way your fan will stay on all the time if your controller goes out. Being a mechanical switch you could use 12 volt or 120 volt.

Thanks for the idea !

Twisted,

I was thinking the same thing about using the relay with the heating thermostat before I start striping the thermostat, reason I did not use that method was complicated and will cost another $15-20 and have bad experience with relays I did a circuit using a relay to install a car starter to a manual transmission car. after 2 years really started failing that's because when you leave the relay on for long period of time the contacts get sticked on even after switching off power to relay.

Skanmyth, I bought one of these DTK-SP thermostats tonight to try this.

I was attempting to follow your instructions but when I get to the step where you say to bend the area to the opposite direction of the same angle, I'm not sure what you mean. When you re-assemble yours, the after picture appears to be the same as the before picture. In your Post #6, the bend appears to be going in the same direction as previously. Did you physically bend the metal strip or did you simply flip it over so that the bend was on the other side?

Hey Quasi I don't think were going to get a reply from Skanmyth (you probably didn't realize that it was from 2004) but I am quite interested in this as well. I did the first thing he mentioned with the relay, but the relay burned I think and stopped working. Now the fans are just on a switch. If anyone else with more knowledge could help out it would be greatly appreciated.

The picture in post 6, looks like the completed mod.
When you disassemble the side of your switch you should be able to see the original bend direction of the bi-metal strip.
The bi-metal strip would have to be placed back in the assembly in the original orientation with the bend on top of the bar, or so it looks to me.

You need to flip it, that way the heat will make it "warp or bent" tward the switch contact so it will on in a hot state and not off.

^^^Thanks for the replies and help

Neil

Neilrp, you're right - I thought the date was Oct 2007.

So... if I understand correctly, the metal bar does not need to be physically bent back the other way, it only needs to be flipped over.
Is mine correct in the picture below?

Yes, that is correct.

I could be wrong, but I think the point was that after flipping it over, which then allows the bi-metal strip to 'bow' opposite what it would normally, you have to also bend the section of the bi-metal strip that is angled the opposite direction, the same amount as it is bent originally.
The bent section is what rests against the cam on the Temperature setting dial shaft.

At least that is what I get when reading the OP's instructions, and it makes sense to me, since the bend is what allows the temp to be adjusted.
When you turn it over, it is now bent differently then it was originally.
The turning over of the strip simply changes the direction the strip bows in relation to heat/cold.
For example: if it turned the switch on as it got colder originally, it will now turn it on as it gets hotter, but the relation of the bend has to remain the same as it was originally to operate correctly with the dial.

It sounds like you're saying that it has to be physically bent backward in addition to flipping it over.
Is this right?

I give up.
I'm just going to buy one from TESCORP.

Like fmerrill and I said, the strip needs to be put back in the way it came out. With the exception of making the bend.
A bi-metal strip is made the way it's named. One piece of metal laminated to the other both having different reactions to temperatures.

That's a pretty good mod! Well, if the attic thermostat that I got off eBay doesn't work, I can always try this.