In a 4 wire control system, which two terminals will have a jumper at the thermostat?

In a 4 wire control system for residential air conditioning, the presence of a jumper between two terminals at the thermostat is typically required to provide proper power and operation for the heating and cooling systems. The terminals in question, Rh and Rc, represent the power supply for heating and cooling, respectively.

When Rh and Rc are connected by a jumper, it allows the thermostat to draw power from a single transformer for both the heating and cooling functions, simplifying the wiring and ensuring that the thermostat can control either system effectively. This setup is particularly common in systems where a single transformer provides power rather than separate transformers for heating and cooling.

Understanding this configuration is essential for proper installation and troubleshooting within HVAC systems. In contrast, other combinations of terminals, such as R and Y or C and G, do not typically require jumpers as they serve different functions in activating the cooling or fan operations.