Couple of things, 1st those fans aren't really up to the job, they are meant to suppliment the viscous fan which can shift an awful lot of air.
Ask Wizard about the switch, he sells a decent adjustable capiliary type that you can fit into any hose you choose, so it can detect cold water return or thermostat elbow hot, given how oong your pipes will be you might detect a difference at either end of each pipe.
Also, being in the back you will find the fans on more than noraml because you have removed them from the airflow.