A.O. Smith AWH-35, Instruction Manual

Page: 29 / 48

CONTROL SYSTEM
29
CONTROL BOARD
The heart of the control system is the Control Board which is a printed circuit board (PCB) installed inside the control box. See Figure
13 below, Figure 14 on page 3 0 and the Wiring Diagrams on page s 32 to 36.
The Control Board is powered by 24 VAC from the transformer. The F4 fuse on the Control Board protects the secondary winding of
the transformer.
The Control Board has four (4) proving circuits that must be closed by either a factor-installed jumper wire or the dry contacts in an
external control to enable HPWH operation. The Control Board also has one (1) thermostat circuit that is factory wired to the HPWH
Terminal Strip. The factory supplied Digital Tank thermostat (see page 19) or a field supplied standard tank thermostat is wired to
this thermostat circuit to regulate water temperature in the storage tank. See Figure 1 5 on page 37 and Figure 1 6 on page 38 .
SERVICE NOTE: All five (5) of these circuits must be closed circuits or the HPWH will not operate. See items 1-5 in Figure 13 below.
If the HPWH unit does not begin a heating cycle within 5 minutes (allowing time for the anti-short-cycle timer) after power is applied
ensure the thermostat is calling for heat (closed contacts). is unresponsive when power is applied
PRESSURE SWITCHES
The Control Board monitors two refrigerant pressure switches, a high and a low pressure switch. If the contacts on either switch open
during a heating cycle the control system will lockout and the compressor will be disabled. The blower and the pump will continue to
operate as long as a call for heat is present. The cause of the pressure switch contacts opening must be corrected and the control
system must be reset before the HPWH will resume compressor operation. See Table 1 1 for pressure switch activation pressures.
Low and High pressure switch LED lights on the Control Board will be energized during this fault condition as a diagnostic aid to
indicate which switch has cased the fault condition. See Figure 14 on page 3 0 . The low pressure switch is on a time delay (Time
Safety By-Pass) to prevent nuisance lockouts during start up when the return (inlet) water may be below . This delay is set for 300
seconds from the factory.
TABLE 11
MODEL
LOW PRESSURE SWITCH
Normally Open - Close On Rise
HIGH PRESSURE SWITCH
Normally Closed - Open On Rise
AWH-35 - AWH-170
Contacts Close Contacts Open Contacts Close Contacts Open
35 PSI (241 kPa) ± 5% 15 PSI (103 kPa) ± 5% 250 PSI (1,724 kPa) ± 15% 350 PSI (2,413 kPa) ± 15%
CONTROL BOARD
LP Switch
Remote Alarm
(dry contact)
To HP
Switch
From Transformer
24 VAC Power Supply
To LP Switch
Time Safety
HP Switch
Remote Alarm
(dry contact)
2
4
ON/OFF
Switch
To HPWH Terminal Strip
Tank Aquastat (thermostat)
Push
Button
Reset
To Pump Contactor
To Blower Contactor
To Compressor Contactor
Remove the jumper when Phase
Monitor is used.
1
1
Remove the jumper when Push Button
Reset (NC contact) is used.
2
Remove the jumper when Compressor
Overload Module is used.
3
3
Remove the jumper when ON/OFF
Switch is used.
4
NOTE: 1-4 are all proving circuits for external
control dry contacts. These circuits must be
closed to enable operation. If any of these
circuits are open HPWH operation will be
disabled; compressor, blower and pump
will not run. Jumpers must be installed for
proving circuits 1-4 when external controls
are not in use. Tank Aquastat (thermostat)
must be connected to #5.
Figure 13