Page 12 / IM 778
Unit Operation
The Mark IV/AC circuit board is an optional control sys-
tem with built-in features such as random start, compressor
time delay, night setback, load shed, shutdown, conden-
sate overflow protection, defrost cycle, brownout, and
LED/fault outputs. Figure 13 shows the LED and fault out-
put sequences.
The unit has been designed for operation with a 24 volt
mercury bulb type wall thermostat or a microelectronic wall
thermostat selected by the manufacturer. Do not operate
the unit with any other type of wall thermostat.
Each unit has a printed circuit board control system. The
low voltage output from the low voltage terminal strip can
be either AC voltage or DC voltage to the wall thermostat.
This is dependent on what terminals you use. R is A/C volt-
age output and F is D/C voltage output to the wall stat.
The 24 volt low voltage terminal strip is set up so R-G or
F-G energizes the fan, R-Y1 or F-Y1 energizes the com-
pressor for cooling operation, R-W1 or F-W1 energizes the
compressor and reversing valve for heating operation. The
reversing valve is energized in the heating mode. The circuit
board has a fan interlock circuit to energize the fan whenev-
er the compressor is on if the thermostat logic fails to do so.
Remember the output to the wall stat can be AC current
or DC current. Terminal (R) on the wall stat can be connect-
ed to terminal (R) on the PC board for AC voltage or to ter-
minal (F) on the PC board for DC voltage.
AC current
DC current
R to G = fan only
F to G = fan only
R to Y1 = cooling
F to Y1 = cooling
R to W1 = heat
F to W1 = heat
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 024 through 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 discon-
nect 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 thru 060.
The unit does not have to be reset on a condensate over-
flow detection.
The Mark IV/AC control board fault output sends a sig-
nal to an LED on a wall thermostat. Figure 13 shows for
which functions the fault output is “on” (sending a signal to
the LED).
Figure 13.
LEDs
Fault
Indication
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
*In heating mode only
The Remote Reset feature provides the means to
remotely reset automatic lockouts generated by high-pres-
sure and/or low-temperature (in heating) faults. When the
Mark IV board is in automatic lockout due to one of these
faults, and the cause of the fault condition has been allevi-
ated, energizing the O-terminal for 10 seconds or more will
force the Mark IV board to clear the lockout. A unit power
cycle can also be used to clear an automatic lockout if the
conditions causing the fault have been alleviated.
The Fault Retry feature helps to minimize nuisance trips
of automatic lockouts caused by high-pressure and/or low-
temperature (in heating) faults. This feature clears faults the
first two times they occur within a 24-hour period and trig-
gers an automatic lockout on the 3rd fault. The retry count
is reset to zero every 24 hours.
The Mark IV/AC control circuit 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. Fan operation terminates
and unit control reverts to 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
overridden for two hours by toggling the fan switch (inter-
mittently closing the R to O terminals) on the Deluxe Auto
Changeover thermostat. Day thermostat setpoints then
control the heating and cooling operation. The Mark IV/AC
control system also accommodates load shed and shut-
down operation on receipt of a “grounded” signal to the “L”
and “E” terminals, respectively, on the low voltage terminal
strip.
Figure 14.
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 a 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
pumps during unoccupied periods with a safety override
dependent, at minimum, on WSHP’s need. The P and C ter-
minals may be “daisy chained” between 200 units. See
page 19.
Field supplied relays installed on the input terminals W1,
W2, Y1, Y2 or G may introduce electrical noise. Never install
relay coils in series with the inputs.
Chassis
Ground
Unit
1
Unit
2
Unit
3
To
Additional
Units
Time
Clock
To activate the unoccupied mode for units on the same clock schedule,
a single wire can be “daisy chained” between units and simply ground-
ed through the time clock contacts. The same system can also be done
to activate the load shed and unit shutdown modes by running addi-
tional wires between units to ground.
Three types of units are available: Mark IV/AC control units or units equipped with a MicroTech™ 2000 or BACnet
®
Water
Source Heat Pump Controller.
Mark IV/AC Control Units
