In a heating system, what voltage reading indicates that a thermostat is potentially malfunctioning?

A reading of 0 volts at the thermostat indicates a potential malfunction. In a typical heating system, especially those that operate on low voltage control systems like a 24-volt setup, the thermostat acts as a switch that controls the heating device (like a furnace or boiler).

When the thermostat is functioning correctly, it should produce a voltage reading that corresponds to its operation, typically around 24 volts in these systems, when the thermostat is calling for heat. If you measure 0 volts, it means there is no power reaching the thermostat or that the thermostat is not completing the circuit, suggesting a failure either in the thermostat itself, the wiring, or the transformer supplying power to the system.

The other readings mentioned (110 volts, 24 volts, and high resistance) do not indicate a malfunction in the context of a typical low-voltage thermostat. A reading of 24 volts is expected when the thermostat is operational and calling for heat, while 110 volts is atypical for residential heating controls and could signify a different issue if observed. High resistance might indicate a problem but does not directly point to a thermostat malfunction in the same clear manner as a 0-volt reading does.