Here's a heat exchanger demo using the open source temperature controller.
Full heat exchanger available here
Intelligent controller, schematics, and code available here
The open source temperature controller allows you the flexibility to control DC appliances based on the temperature of two thermistor inputs. The processor is reprogrammable with an AVR ISP. To get you started, we include a temperature comparison program for an automatic heat exchanger. The controller has both green and white LEDs to indicate status.
The default program allows you to experiment with a wide variety of temperature control applications out of the box.