Heating stages are controlled by the W1 through W3 thermostat inputs.
- A W1 input energizes the H1 output.
- A W2 input energizes the H2 output.
- A W3 input energizes the H3 output.
When the ignition safety verification process is complete, the ignition module energizes the gas valve and provides a 24 V input to the MV terminal on the UCB. This does not apply to units with electric heat.
The FAN ON HEAT DELAY timer starts as soon as 24 V is present on the MV terminal. When the timer expires, the FAN output for the indoor fan operation energizes.
When the thermostat heat inputs are lost and the 120-second Minimum Heat Run Timers have expired, heating outputs stage off. The Fan Off Heat Delay timer starts when all heating outputs are off. When the timer expires, the FAN output for the indoor fan operation de-energizes.