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Relay status LED
• The green LED on the front panel illuminates when the SPDT output relay is energized and the LC
and LNO contacts are closed.
Parameter codes and modes of operation
• Relay Off temperature (OFF) Select the temperature at which the output relay de-energizes, the LC to LNO relay
contacts open (cutout), and the green LED goes off. The range of usable temperature values is -40°F to 212°F (-40°C
to 100°C) in 1° increments.
• Relay On temperature (On) Select the temperature at which the output relay energizes, the LC to LNO relay
contacts close (cut in), and the green LED lights. The range of usable temperature values is -40°F to 212°F (-40°C to
100°C) in 1° increments.
• Heating or Cooling mode of operation When you select your required On and OFF values, the control automatically
determines the mode of operation and displays the proper mode icon on the Main screen.
Note: The A421 Series controls do not have jumpers for setting up the heating or cooling mode.
• The heating or cooling mode is determined by the On and OFF value relationship as follows:
• OFF > On = Heating mode = Flame icon
• OFF < On = Cooling mode = Snowflake icon
• Important: During normal operation, adjusting just the On value or just the OFF value on the A421 control changes
the differential between On and OFF, and can potentially change the mode of operation from heating to cooling or
cooling to heating. To maintain a constant differential between on and off, you must adjust both the on and off
values by an equal number of degrees, or set up the control in the Restricted Adjustment mode. See Restricting
User Adjustment.
• Anti-short cycle delay (ASd) Select the minimum time that the output relay remains off (de-energized) before the
next on-cycle can start. The ASd interval overrides any load demand (On) and does not allow the output relay to go
on until the selected ASd interval has elapsed. See the following figure.
• Anti-short cycle delay is typically used for refrigeration applications so that the system pressure can equalize before
restarting the compressor.
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