system 26 power conductors 16 and 18 are connected to electrical power such as a 120 volt circuit power conductors 16 and 18 also provide power to control system 14 control conductor 20 receives a signal from control system 14 that drives relay coil 22 causing a controllable connection of relay contact 24 thereby allowing power to flow from power conductor 16 through heater system 26 to power conductor 18 heater system 26 can be the controlling pump of a hydronic heating system 26 or an electrical heating element 26 now additionally referring to fig2 there is shown a control circuit 30 which is part of control system 14 control system 14 also includes moisture detector 32 and temperature detector 34 moisture detector 32 includes a moisture grid that is a spaced apart interdigitated set of conductors exposed on the top of control system 14 moisture in the form of water ice snow and or sleet on the surface of moisture detector 32 is detected by a current flow between fingers of the interdigitated conductors prior moisture detectors measured the conductivity between the interdigitated conductors using an uninterrupted supply of a dc voltage this causes electrochemical problems including polarization and copper electroplating that reduces the life expectancy and reliability of the sensor polarization occurs when dc current flows through the grid when wet the water from melted snow and ice becomes an electrolyte due to atmospheric contamination and the ions therefrom are positioned due to the constant electro potential on the interdigitated fingers however the circuit and method employed by the present invention reduces this problem to a negligible proportion by employing an active sensing technique that reduces the current through the moisture detection.