PURPOSE: To provide a combined controller or control circuit for use in an HVAC system.

CONSTITUTION: At least two HVAC units 4, 5 connected with a common duct together with a system thermostat 3 are heating combustion chambers, air conditioners, fans, other forced air systems, or other apparatuses for heating and/or cooling environments. A combined controller 10 is operable in a single stage mode (single mode) or a double stage mode (double mode). The combined controller 10 includes a microcomputer for receiving an input from a system thermostat 3 and providing outputs 30, 31 to the HVAC units 4, 5 in order to effectively control temperature of an environment such as a life space. The controller 10 includes an air fan detection input for receiving a signal indicative of the states of air fans 6, 7 in the HVAC units 4, 5 and LED indicators 42, 44, 62, 66 providing a trouble code.