Heater + controller

That's not a brand name.....it means Single Pole Dual Throw. I obviously don't know for sure but it sounds like Tangster made his own.

 

It probably works similar to a mercury thermostat, where the bi-metallic spring moves and opens or closes the circuit to send/cut current to turn the heater on and off. Only that one has an extra throw that also works if temp is too hot, turns on the cooling fans and chiller. Is this correct?

 

I'll leave it to Roger to know how his is set up. SPDT can be mechanical like you are describing or it can also be solid state (100% electronic)

 

Good info! Omard did a thread on this http://www.3reef.com/forums/i-made/diy-fan-thermostat-ugly-but-works-49547.html so I'm guessing this is the SPDT that Tangster is talking about...

 

I took a old heater controller rated for 15 amp and wired it to a SPDT relay and wired the heater controller where you would plug the heater into I plug that SPDT into and then I have that on the open to run a fan or chiller and on the close it works the heater. The advantage is the sensor in in the water and not subjected to differential loss through contact with glass . A double Ranco is 200.00 I built mine for a lot less and about a hr of time.. Even at my old labor rate of 90.00 Per Hr it was still a large savings