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, condensate
overflow protection, defrost cycle, brownout, and LED/fault
outputs . Figure 11 shows the LED and fault output sequenc-
es .
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 voltage
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 when-
ever the compressor’s 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 con-
nected to terminal (R) on the PC board for AC voltage or to
terminal (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 units CCH
& CCW 019 thru 060, R-410A), (CRH & CRW 024 thru 070,
R-22) . 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 units CCH & CCW 019 thru 060, R-410A), (CRH
& CRW 024 thru 070, R-22) . The unit does not have to be
reset on a condensate overflow detection .
The Mark IV/AC control circuit fault output sends a signal
to an LED on a wall thermostat . Figure 11 shows for which
functions the fault output is “on” (sending a signal to the
LED) .
Figure 11.
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
On
Load Shed Off
Off
On
Off
Off
Unoccupied Mode
On
On
Off
Off
Unit Shutdown
Off
Flash
Off
On
Page 12 / IM 742
Unit Operation
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 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 then control the heat-
ing and cooling operation . The Mark IV/AC control system
also accommodates load shed and shutdown operation on
receipt of a “grounded” signal to the “L” and “E” terminals,
respectively, on the low voltage terminal strip .
Figure 12.
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 dur-
ing unoccupied periods with a safety override dependent, at
minimum, on WSHP’s need . The P and C terminals may be
“daisy chained” between 200 units . See page 21 .
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
Gr
ound
Unit
1
Unit
2
Unit
3
To
Additional
Units
Tim
e
Clock
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 unit shutdown modes by running additional
wires between units to ground .
Two types of units are available: Mark IV/AC control units or units equipped with the new MicroTech 2000 Water Source Heat
Pump Controller .
Mark IV/AC Control Units