AERCO Electronic Controls System (ECS) - Manual (Page 33)

Electronic Control System (ECS) and Type CXT-E Valve
5. FUNCTIONAL DESCRIPTION
VD2: 02/28/14 AERCO International, Inc. • 100 Oritani Dr. • Blauvelt, New York 10913 • Phone: 800-526-0288 Page 33 of 82
AC-105 OMM-0003_0D
5. FUNCTIONAL DESCRIPTION
5.1 INTRODUCTION
The AERCO Electronic Control System (ECS) and Electronic Control Valve Type CXT-E
provide a highly responsive control system which provides virtually constant hot water flow at
the selected setpoint temperature. A simplified functional block diagram for the ECS and CXT-E
is shown in Figure 5-1. The following paragraphs provide a top-level functional overview of
system operation.
5.2 FUNCTIONAL OVERVIEW
The primary control mechanism for the ECS is an Electronic Process Controller which is
installed in the Control Box. The Controller utilizes 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 CXT-E Control Valve
Actuator.
The Controller continuously monitors the Water Heater outlet temperature via a thermocouple
located in the outlet port. The Controller also receives a feed-forward signal from the flow
sensor mounted in the cold water inlet as shown in Figure 5-1. The flow sensor monitors flow
changes through the Water Heater and provides a feed-forward signal (0 to 5 VDC) to the
Controller proportional to the change in flow.
Using PID algorithms, the Controller provides a 4-to-20 mA output signal to the CXT-E Control
Valve Actuator. This 4-to-20 mA signal proportionally modulates the Control Valve position from
fully closed (4 mA) to fully open (20 mA). The control signal varies as necessary to maintain the
setpoint temperature programmed into the Controller. Under normal conditions, the Water
Heater outlet temperature is maintained within ±4°F of the desired setpoint based on a load
change of 50% or less.
5.3 OVER-TEMPERATURE CONTROL AND SAFETY FEATURES
The Control Box also contains an Over-Temperature Switch which continuously monitors and
displays the outlet water temperature of the Heater via the input received from the thermocouple
installed in the hot water outlet port. Normally, the Over-Temperature Switch is set 20°F above
the setpoint temperature for the Water Heater. If the configurable over-temperature limit is
exceeded, the Over-Temperature Switch alarm is triggered, which sounds an audible alarm,
disconnects power from the Control Valve Actuator and energizes the Solenoid Valve located in
the Water Heater top head, expelling over-heated water from the Heater.
The Solenoid Valve in the top head remains open until the water temperature drops below the
over-temperature limit setting. Once the water temperature drops below the limit, the Over-
Temperature Switch resets automatically, silencing the audible alarm, de-energizing and closing
the Solenoid Valve, and reapplying power to the Control Valve Actuator.
It should be noted that the Over-Temperature Switch has a slightly slower response to
temperature changes than the Temperature Controller. Therefore, slightly different temperature
readings may appear momentarily in their respective displays, particularly during system start-
up.
The Control Valve Actuator also incorporates a “Fail-Safe” feature which automatically closes
the Valve if there is a loss of the 4-to-20 mA control signal or loss of input power.