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SECTION 2 - OPERATION
FORM 145.18-IOM2
ISSUE DATE: 06/17/2020
2
If at any time there is a call for both heating and cool-
ing, the heating operation is performed. Heating always
takes priority. If cooling mode is operating, it halts and
ends the call for cooling.
Continuous Blower
With the room thermostat fan switch set to AUTO and
the system switch set to either AUTO or HEAT, the
blower energizes whenever a cooling or heating opera-
tion is requested. The blower energizes after any speci-
fied delay associated with the operation.
The indoor blower energizes for a minimum runtime of
30 seconds. Additionally, the indoor blower delays for
10 seconds between operations.
When the room thermostat calls for cooling, the low-
voltage control circuit completes from R to Y and G.
The compressor and fan motor energize. After com-
pleting the specified fan on delay for cooling, the mi-
croprocessor control energizes the blower motor.
Once the room thermostat has been satisfied, it de-en-
ergizes Y. If the compressor satisfies its minimum run-
time, the compressor and fan de-energize. Otherwise,
the unit operates the cooling system until the minimum
runtime for the compressor completes. After the com-
pressor de-energizes and the fan off delay for cooling
elapses, the blower stops.
To be available, a compressor must not be locked-out
because of a high pressure limit switch, low pressure
limit switch, condensate overflow switch, or a freeze-
stat trip. The anti-short cycle delay (ASCD) must elapse.
Safety Control Reset
All VPCS heat pumps are furnished with a high pres-
sure limit switch, a low pressure limit switch, low wa-
ter temperature freeze protection switch, and conden-
sate overflow switch to prevent compressor operation
during abnormal conditions.
If any of these safety devices activate, a lockout relay
circuit engages. The circuit interrupts heating and cool-
ing operation even if the control contacts automatically
re-close.
This microprocessor driven lockout circuit must be
manually reset. Reset by momentarily moving the ther-
mostat control (system) switch to OFF, then back to
HEAT or COOL (or AUTO).
The lockout circuit can also be reset by opening and
closing the unit mounted disconnect switch.
If the unit must be reset more than twice
on consecutive operating cycles, check
the unit for a dirty filter, abnormal EWT,
inadequate or excessive water flow, or
refrigerant circuit malfunction. If the
unit continues to cutout, contact a trained
service technician.
Operation Errors
Each refrigerant system is monitored for operation out-
side of the intended parameters. Errors are handled as
described below. All system errors override minimum
runtimes for compressors.
High Pressure Limit Switch
If a high pressure limit switch opens, the microproces-
sor control de-energizes the compressor, initiates the
ASCD, and stops the unit fans (soft lockout). If a call
for cooling or heating is still present at the conclusion
of the ASCD, the microprocessor control re-energizes
the compressor and unit fan.
If a high pressure limit switch opens three times within
2 hours of operation, the microprocessor control per-
manently locks out the system compressor, requiring a
manual reset of the system (a hard lockout). To manu-
ally reset, either de-energize the 24-volt power to the
unit or turn the room thermostat to OFF, then back to
either HEAT or COOL as required. The microproces-
sor control flashes a fault code indicating the high pres-
sure lockout (see
Low Pressure Limit Switch
The microprocessor does not monitor the low pressure
limit switch during the initial 30 seconds of compressor
operation. For the following 30 seconds, the micropro-
cessor control monitors the low pressure limit switch
to ensure it closes. If the low pressure limit switch fails
to close after the 30 second monitoring phase, the mi-
croprocessor control de-energizes the compressor, ini-
tiates the ASCD, and stops the fan (a soft lockout).
Once the low pressure limit switch is proven (closes
during the 30 second monitor period), the microproces-
sor control monitors the low pressure limit switch for
any openings. If the low pressure limit switch opens
for more than 5 seconds, the microprocessor control
de-energizes the compressor, initiates the ASCD, and
stops the compressor (a soft lockout).
If the call for cooling is still present after the ASCD,
the microprocessor control re-energizes the compres-
sor.
