Tekmar 155 Difference Setpoint Control The Difference Setpoint Control 155 is designed to transfer heat from an intermittent heat source to a storage tank. It can be used in applications ranging from solar thermal to wood boiler systems. Two sensors are used to measure the temperature at the source and at the storage location, and energize a pump when there is a positive difference between the two respectively. This product uses 2 relays to accomplish this; relay 1 is a dry contact that can operate a pump (240 V (ac) dry contact), and relay 2 can be used for drain down solar installations.