A controller for a water heating system has an electrical interface, a housing, and logic. The electrical interface receives an electrically conductive wire that is covered by a coating composed of electrically insulating material. An exposed end of the wire is inserted into the electrical interface. The housing has a hole through which the wire passes. The electrical interface and a wall of the housing are separated by a gap so that the coating can pass through the hole and reside in the gap without passing into the electrical interface. The logic is configured to control an operational state of a heating element.