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Operating information
Electronic control overview
The primary control mechanism for the electronic control
system is an electronic process controller which is installed
in the control box. The controller utilizes feed forward and
PID (Proportional Integral Derivative) algorithms to provide
precise control of the water heater outlet temperature. Outlet
temperature control is accomplished by modulating the open/
closed position of the control valve actuator.
The controller continuously monitors the water heater outlet
temperature via an RTD located in the outlet port. The
controller also receives a signal from the temperature sensor
mounted in the mixed water inlet as shown in FIG. 3-1A. The
mixed water sensor monitors flow changes through the water
heater and provides a signal to the controller proportional to
the change in flow. The mixed water temperature provides an
instantaneous response for the specified flow range of the unit.
The controller provides a 0 - 10 VDC output signal to the
control valve actuator. This 0 - 10 VDC signal proportionally
modulates the control valve position from fully closed (0 VDC)
to fully open (10 VDC). The control signal varies as necessary
to maintain the set point temperature programmed into the
controller. Under normal conditions, the APW water heater
outlet temperature is maintained 4°F of the desired set
point based on a load change of 50% or less.
Over-temperature control and safety
features
The control box contains an over-temperature switch which
continuously monitors the outlet water temperature of the
heater. Normally, the over-temperature switch is set to 20°F
above the set point temperature for the water heater. If the
programmed over-temperature limit is exceeded, the over-
temperature switch is activated. This in turn disconnects
power from the control valve actuator moving the control valve
to the full bypass position (B- AB). It should be noted that
the over-temperature switch has a slightly slower response to
temperature changes than the temperature controller.
The control valve actuator also incorporates a "Fail Safe" feature
which automatically closes the valve if there is a loss of the 0 - 10
VDC control signal or loss of input power. The 24 VAC power
supply transformer has an integral 3 amp circuit breaker to
protect the control valve electronics.
An electronic control system coupled with a control valve
results in a highly responsive system which provides virtually
constant hot water flow at the selected set point temperature.
Accessories included with the APW water heater include:
• Boiler water and domestic water Y-strainers with blow-
down valves
• Pressure & Temperature relief valve
• Isolation valves and domestic water drain valve
• Integral domestic water circulator pump
The following paragraphs provide a top-level functional
overview of system operation.
Sequence of operation
The cold water enters the heat exchanger through the inlet
connection and strainer. Cold water flows through the plate
heat exchanger, where it is heated by hot boiler water, and
then discharged through the Domestic Hot Water (DHW)
outlet connection. A portion of hot water is mixed with the
cold water entering the heater.
Boiler water supplied to the heat exchanger passes through
the inlet connection and strainer. The boiler water heats
the colder domestic water, and is then discharged through
the boiler water outlet connection. The boiler water flow is
controlled by a 3-way valve.
For APW double-wall heat exchanger models, heat transfer
plates are positioned together to form one assembly with
an air space between them. This protects against leakage
of boiler water into the domestic water. If one of the plates
should develop a leak (boiler water or domestic water,
whichever is leaking), the water will enter the air space and
exit to the atmosphere. It will flow through leak detection
channels alerting an operator that a leak has occurred.
Indirect Plate Water Heater Installation & Service Manual
