Which terminals should always measure 24 volts if power is connected to the system?

In a standard heating and cooling system that operates on a 24-volt control circuit, certain terminals have specific functions that dictate the expected voltage readings. The terminals labeled TH (Thermostat Heat) and TR (Thermostat Relay) are integral to the operation of the heating control circuit. When power is connected to the system, these terminals should consistently measure 24 volts.

The TH terminal is typically connected to the heating relay or component, while the TR terminal serves as a common connection point for the thermostat. When the thermostat calls for heat, it provides an electrical signal to these terminals, resulting in the measurement of 24 volts. The continuity and proper functionality of this circuit are essential for the heating system to activate and operate correctly.

Other terminals, such as R and W (generally related to heating and cooling calls), may not always show 24 volts unless specifically engaged in a calling scenario. Similarly, terminals MV and C (associated with various components and functions) can also fluctuate based on system demand or specific operations. Thus, TH and TR consistently reflecting a 24-volt measurement indicates proper power supply and readiness of the heating system.