Chapter 4 Sequence of operation
40 CNT-SVX05B-EN
Auto mode
A communicated request of Auto or the controller default operation (Auto)
can place the unit into cooling mode. A zone temperature input is
required for discharge air temperature control when auto heat/cool
changeover is desired. When the controller automatically determines the
heating or cooling mode using auto mode, the unit switches to the desired
mode based on the control algorithm and the relationship between zone
temperature to the configured daytime warm up start and stop setpoints.
See daytime warm up in configuration section of this manual.
When the controller first powers up or after a reset, it makes an initial
determination if the discharge air temperature control mode should be
heating or cooling. The discharge demand for a cooling-only unit is always
cooling. The discharge demand for a heating only unit is always heating.
A unit that can heat or cool initially starts in cooling mode.
Power-up sequence
This sequence applies to both space temperature control and discharge
air temperature control.
When 24 Vac power is initially applied to the AH540/541 controller, the
following sequence occurs:
1. Green Status LED turns On.
2. All binary outputs are controlled to their de-energized state, and ana-
log outputs are set to the normally closed output voltage.
3. The controller reads the inputs to determine initial values.
4. Power-up control wait feature is applied. The controller waits
300 seconds to allow ample time for the communicated control data to
arrive. If after 300 seconds, the controller has not received any com-
municated control data, the unit assumes stand-alone operation.
5. Normal operation begins assuming no diagnostics have been
generated.
Manual output test can be initiated at any time in the power-up sequence
or during normal operation. Refer to the “Performing a manual output
test” on page 84.
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