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SECTION 2 - OPERATION
UNIT CONTROLS
The control system microprocessor board is specifi-
cally designed for water source heat pump operation.
The control system interfaces with a conventional type
thermostat.
• The unit is complete with a self-contained low-
voltage control circuit.
• The unit incorporates a lockout circuit that pro-
vides reset capability from a hard lockout at the
space thermostat or base unit if any of the follow-
ing standard safety devices trip and shut off the
compressor:
• Low pressure limit switch (loss of charge)
• High pressure limit switch
• Freeze protection switch (unit shutdown on
low water temperature)
• Condensate overflow switch
• The unit operates with conventional thermostat
designs and has a low voltage terminal strip for
easy hook-up.
• Unit control board has on-board diagnostics and
fault code display.
• Standard controls include anti-short cycle and low
voltage protection.
• The control board monitors each refrigerant safe-
ty switch independently.
• The control board has a random start feature.
• The control board retains the last five fault codes
in nonvolatile memory that cannot be lost in the
event of a power loss.
Sequence of Operation
The room thermostat makes a circuit between R and
Y1 for cooling.
The call passes to the unit microprocessor control that
determines whether the requested operation is avail-
able and if so, which components to energize.
For heating, the room thermostat makes a circuit be-
tween R and W1. The microprocessor control energiz-
es the compressor and fan, enabling the unit to run in
heating mode.
40
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 Y1 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-
energizes Y1. If the compressor satisfies its minimum
runtime, the compressor and fan de-energize. Oth-
erwise, the unit operates the cooling system until the
minimum runtime for the compressor completes. After
the compressor 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.
FORM 145.18-IOM1
ISSUE DATE: 8/30/2018
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