I have a problem. The closet in my home where I have my server equipment is getting quite hot. My solution has been to leave the closet door open but my servers need to be cooler and the equipment noise has become too much of an irritation. An exhaust fan is needed to get rid of the excess heat coming from the server closet. So I decided to install a ceiling exhaust fan to blow the hot air out of the closet and into the attic.

Exhaust Fan

My first task was to select an appropriate ceiling fan. The fan needed to be powerful so as to quickly cool the closet. Noise was not so much an issue as the closet door would be closed. Oh and did I mention cheap as possible? My choice was the NuTone 210 CFM, 7.5 Sones from Home Depot. For $75.00 dollars this fan will suck 210 cubic feet per minute. It's a bit utilitarian looking but it does it's job well.

Thermostat

Second was to select a thermostat. This is where I diverted from the norm and chose to go a fancier route. If I wanted to make it easier I could have chosen an attic thermostat like the Master Flow Adjustable Power Thermostat. This thermostat is designed for cooling attics and can be simply installed. However I wanted to see a digital readout of the current temperature and more exactly adjust the set temperature. Enter the 24V programmable thermostat. This type of thermostat is the most common type of thermostat on the market. Used for controlling your standard heating and AC systems. I chose the $25.00 dollar HoneyWell PRO 2000 Thermostat because it fits a standard size electrical receptacle and looks nice upright.

Fan Relay

For the thermostat to control the fan I needed a way for the opening and closing circuit on the thermostat to open and close the power connection of the fan. Running the 120V power through the thermostat circuit would certainly fry the thermostat as it is only rated for 1 Amp at 24 Volts. What I needed was an electrical relay. The relay would open the 120V circuit to the fan when the circuit was open on the thermostat. At Radio Shack I found a relay with a 12VDC rated coil and a 5A at 240VAC rated contact. The part number was 275-249.


The thermostat circuit doesn't have any power in itself to trip the relay. In a typical system it uses the heating or cooling system for power. So I needed to provide a source of power. The relay coil was rated for 12V so I bought a 12V DC adapter with the smallest amperage I could find.

Putting it all together

I first connected the positive power adapter wire to the RC labeled terminal connection on the thermostat. Next I ran a wire from the G labeled thermostat connection and soldered the wire to the relay coil circuit. I finished out the circuit by soldering the negative power adapter wire to the relay coil. At this point I could actuate the relay with the power adapter plugged in by turning the thermostat fan on and off. For the fan connection I soldered two flexible 14 gauge wires to the relay contacts and wrapped the whole mess relay and all together in black electrical tape. After turning off the power breaker to the closet I cut the black power wire to the exhaust fan and striped it's ends. Next I connected the ends of the black power wires to the two relay contact wires using electrical twist connectors. Finally I turned the breaker back on. You can see this all in the schematic diagrammed below.

Additional Resistors

Okay so this did work but I had one problem. The relay was running hot. Not good. I looked at the specifications for the relay and the coil was rated for 60mA. Hmmmm? I was running 500mA through the relay. That could account for the burning circuit. So time to do some home work. I needed a way to convert 500mA to 60mA. What I needed was a resistor! Remembering Ohm's Law, Voltage = Amps * Resistance.

My target amperage is 0.06mA so...

12V = 0.06A * R
R = 200 Ohms

I needed 200 Ohms of resistance. So I was on the hunt for the proper resistor. What I found at Radio Shack was 100 Ohm (10 Watt) pair of resistors.

Perfect! I could connect both of them in series to achieve 200 Ohms. Also Watts = Voltage * Amps so for my circuit so...

12V * 0.5A = 6 Watts. 

The 10 Watts resistance provided would be enough wattage to resist the 6 watts of power coming from the power adapter.

Schematic

Here is the final schematic of the exhaust circuitry.

Conclusion

My server closet is now much cooler with the fan installed. Plus I can now close the door! It seems to kick on about every half hour or so and will run for about ten minutes until the closet is cool. I set the thermostat to 80 degrees and it keeps it between about 78 to 82. I am pretty satisfied with this setup and it only cost me $125 dollars. Hooray!