Standard 24Vac Sequence Of Operation - Zehnder Rittling VS Series Installation, Operation And Maintenance Manual

Standard 24VAC sequence of operation

Random start delay
When the unit is first powered "on" the control
microprocessor will generate a random number
to determine the start delay of the compressor
operation (3-5 minutes). This delay is used to prevent
multiple units from cycling "on" at the same time.
The purpose is to prevent a large power load on
the building electrical system after a power outage.
After the number, or delay time, is generated the
microprocessor will use it to determine the minimum
amount of time delay before the compressor is cycled
"on" after a demand is received from the thermostat.
Anti-short cycling delay
After the random start delay is generated the
microprocessor will use it to determine the minimum
amount of delay before the compressor is cycled "on"
after a demand is received from the thermostat. This
allows the refrigerant system to equalize in pressure
and prevents short-cycling of the compressor.
Minimum compressor runtime
The minimum compressor runtime of each cycle,
heating or cooling, is 90 seconds. Once the
compressor is energized it will not de-energize
until the minimum runtime is satisfied, even if the
thermostat input is removed.
Cooling 1st stage (Y1, O)
When the microprocessor receives (Y1, O) at the
24VAC thermostat input connection, the unit will
proceed with the cooling 1st stage sequence. The
microprocessor must receive these signals for 2
continuous seconds before it recognizes the inputs
as valid. Once the input signals are determined to
be valid the reversing valve will energize/de-energize
after 5 seconds. The microprocessor will then verify
that the anti-short cycling delay has been satisfied.
Once the anti-short cycling delay has been satisfied the
compressor will cycle "on." The blower will cycle "on"
in low speed 15 seconds after the compressor is
cycled "on."
Cooling 2nd stage (Y1, Y2, O)
When the microprocessor receives (Y1, Y2, O) at
the 24VAC thermostat input connection, the unit will
proceed with the cooling 2nd stage sequence. The
microprocessor must receive these signals for 2
continuous seconds before it recognizes the inputs
as valid. Once the input signals are determined to
be valid the reversing valve will energize/de-energize
after 5 seconds. The microprocessor will then verify
that the anti-short cycling delay has been satisfied.
Once the anti-short cycling delay has been satisfied
the compressor will cycle "on." The blower will cycle
"on" in high speed 15 seconds after the compressor is
cycled "on."
Heating 1st stage (Y1)
When the microprocessor receives (Y1) at the 24VAC
thermostat input connection, the unit will proceed with
the cooling 1st stage sequence. The microprocessor
must receive these signals for 2 continuous seconds
before it recognizes the inputs as valid. Once the
input signals are determined to be valid the reversing
valve will enertize/de-energize after 5 seconds. The
microprocessor will then verify that the anti-short
cycling delay has been satisfied. Once the anti-short
cycling delay has been satisfied the compressor will
cycle "on." The blower will cycle "on" in low speed
15 seconds after the compressor is cycled "on."
Heating 2nd stage (Y1, Y2)
When the microprocessor receives (Y1, Y2) at the 24VAC
thermostat input connection the unit will proceed with
the heating 2nd stage sequence. The microprocessor
must receive these signals for 2 continuous seconds
before it recognizes the inputs as valid. Once the
input signals are determined to be valid the reversing
valve will energize/de-energize after 5 seconds. The
microprocessor will then verify that the anti-short
cycling delay has been satisfied. Once the anti-short
cycling delay has been satisfied the compressor will
cycle "on." The blower will cycle "on" in high speed
15 seconds after the compressor is cycled "on."
Fan only mode (G)
The fan only mode can only be used with a 24VAC
thermostat and will energize the low speed blower
when a (G) input has been received at the 24VAC
thermostat input connection. When the input is
removed the blower will de-energize immediately.
24 volt thermostats
Non-programmable and programmable thermostats
use a control algorithm that uses both time and
temperature difference from the setting to determine
when to turn on stage 1 fan speed (low) and stage 2
fan speed (high). This logic therefore controls the room
temperature to plus or minus 1 degree from set point.
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