McQuay FDD Series, Installation & Maintenance Data, Page: 15 / 24

IM 407 / Page 15 of 24
Unit Operation
Figure 28.
INDICATION
LEDS FAULT
YELLOW GREEN RED OUTPUT
Normal mode Off On Off Off
High pressure fault Off Off Flash On
* Low temperature fault Flash Off Off On
Condensate overflow On Dim Off On
Brownout Off Flash Off On
Load shed Off Off On Off
Unoccupied mode On On Off Off
Unit shutdown Off Flash Off On
General
Each unit has a printed circuit board control system. The low
voltage output from the low voltage terminal strip on the
control board is always 24 volts DC (direct current). Termi-
nals R and C on the low voltage terminal strip supply 24 volts
AC power. The unit has been designed for operation with a
24 volt mercury bulb type wall thermostat or a microelec-
tronic wall thermostat selected by the manufacturer. Do not
operate the unit with any other type of wall thermostat.
Two types of units are available: electromechanical con-
trol units or Mark IV/AC control units. Both have 24 volt DC
circuits on the output of the low voltage terminal strip. Each
requires its own unique automatic changeover thermostat.
Mark IV/AC control units
The Mark IV/AC circuit board is an optional control system
with built-in features such as random start, compressor time
delay, night setback, load shed, shutdown, condensate
overflow protection, defrost cycle, brownout, and LED/fault
outputs. Figure 28 shows the LED and fault output se-
quences.
The 24 volt low voltage terminal strip is set up so R-G
energizes the fan, R-Y1 energizes the compressor for cool-
ing operation, R-W1 energizes the compressor and reversing
valve for heating operation. The reversing valve is set up to
be energized in the heating mode. The circuit board has a fan
interlock circuit to energize the fan whenever the compressor
is on if the thermostat logic fails to do so.
The Mark IV/AC control board has a lockout circuit to stop
compressor operation if any one of its safety switches opens
(high pressure switch and low pressure switch on unit sizes
048 and 060). If the low temperature switch opens, the unit
will go into the cooling mode for 60 seconds to defrost any
slush in the water-to-refrigerant heat exchanger. After 60
seconds the compressor is locked out. If the condensate
sensor detects a filled drain pan, the compressor operation
will be suspended only in the cooling mode. The unit is reset
by opening and closing the disconnect switch on the main
power supply to the unit in the event the unit compressor
operation has been suspended due to low temperature
(freezestat) switch, high pressure switch, or low pressure
switch on unit sizes 048 and 060. The unit does not have to
be reset on a condensate overflow detection.
The Mark IV/AC control board has a fault output signal to
an LED on a wall thermostat. Figure 28 shows for which
functions the fault output is “on” (sending a signal to the
LED).
The Mark IV/AC control board has built-in night setback
operation. A “grounded” signal to the “U” terminal on the low
voltage terminal strip puts the unit into the unoccupied mode
for night setback operation. The fan shuts off and the unit is
put under control from the night setback terminal on the
thermostat, W2; day heating and cooling operation is locked
out. R-W2 energizes the compressor and reversing valve for
heating operation. Night setback operation can be overrid-
den for two hours by toggling the fan switch (intermittently
closing the R to O terminals) on the Deluxe Auto Changeover
thermostat. Day thermostat setpoints the control the heating
and cooling operation. The Mark IV/AC control system is also
set up for load shed and shutdown operation on receipt of a
“grounded” signal to the “L” and “E” terminals, respectively,
on the low voltage terminal strip. See Figure 29.
The P and C terminals of the Mark IV/AC board are used
for pump restart. These terminals pass a voltage signal
whenever the unit compressor is turned on. This signal is
detected by a pump restart relay board providing an N.O. or
N.C. set of contacts for heat pump loop circulation pump
control. When used with the Loop Water Controller, the relay
operation accommodates turning off circulation pump con-
trol. When used with the Loop Water Controller, the relay
operation accommodates turning off circulation pumps dur-
ing unoccupied periods with a safety override dependent on,
at minimum, one WSHP's need. The P and C terminals may
be “daisy-chained” between 200 units. See page 19.
To activate the unoccupied mode for units on the same clock schedule, a single wire can be
“daisy-chained” between units and simply grounded through the time clock contacts. The
same system can also be done to activate the load shed and emergency shutdown modes
by running additional wires between units to ground.
Unit #1 Unit #2
Time Clock
Ground
Figure 29.
To
Additional
Units
Unit #3
* In heating mode only.
14-Position Terminal Strip
Pin Designation Description
1 C Transformer ground (Ovac)
2 R Transformer supply (24vac)
3 V -DC power connection
4 P Pump request output
5 A Alarm fault output
6 U Unoccupied input
7 L Load shed input
8 E Remote shutdown input
9 F +DC power connection
10 Y1 Occupied cooling mode input
11 W1 Occupied heating mode input
12 G Fan only input
13 W2 Unoccupied heating mode input
14 O Tenant override input
Mark IV/AC Sequence of
Operation