RH-D AND RH-W ELECTRIC DUCT HEATERS
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INSTALLATION, OPERATION AND MAINTENANCE MANUAL
RENEWAIRE.COM
MAINTENANCE
7.4.4 Testing the SSR
7.4.5 Testing the Heating Elements
7.4.6 Testing the Transformer
Useful diagnostic/ testing process:
To force FULL power to the elements to verify the load current, operation etc. with out requiring
thermostat control.
1. Turn the Power ON.
2. Remove one of the air-flow sensor connections. The yellow (or red) light begins to blink quickly.
3. Short together the air-flow sensor connections (A-A) momentarily for 2-seconds. The program goes
to a “Diagnose Heater” process. The yellow (or red) light keeps blinking. The heater instantly turns
FULL ON, regardless of the 0-10V input voltage.
After 1 minute with FULL ON, the controller turns OFF the elements and the yellow (or red) warning LED
light flashes with a pattern of 1-second ON and 2-seconds OFF.
4. To put to normal operation: Remove the short, connect back the airflow sensor and then turn
controller’s power off and on (to reset).
SSR relays are Solid State Relays. They respond to an input signal of 4VDC to 32 VDC. That input is
sent from the controller in the unit. For this reason, it is the normal process to first test the controller
and then proceed with the testing of the relays. The typical failures of relays come from the fact that
those relays are designed to switch a certain number of times. After this, the contact in the relay will
eventually get stuck either in an open position or a closed position. If the contact is stuck closed, the
elements will eventually overheat and the thermal cut-outs will cut the operation of the unit. If the
contact is stuck open, the unit will never heat even when there is a demand for heat. Please note that
heat is the most common source of problem that can damage relays. This is why it is recommended
to have air circulate through the venting holes of the control box.
Refer to section “Useful diagnostic/
testing process:”
in the PSSR testing procedure to force relays on and off for troubleshooting purposes.
Make sure the automatic and manual thermal reset are in the closed position. Measure the resistance
between T on the SSR Relay and L2. If the reading is OPEN LINE, replace the heater.
Apply power to the primary side of the transformer and measure the voltage across the secondary side.
Voltage on the secondary side should be 24 VAC.
7.5 SEQUENCE OF OPERATION
7.5.1 RH-D Heaters
The heater operates on either 120 VAC or 240 VAC, 1 phase, 60 Hz.
The heater will maintain outlet air temperature set by a potentiometer on the controller whenever there
is airflow. Outlet air temperature is sensed by the Outlet Air Temperature Sensor, located on the outlet
side of the unit.
The heater is fully modulated, single stage that is enabled by the Outlet Air Temperature Sensor.
See
illustration in Section 2.1 of this manual.
The Outlet Air Temperature Sensor is identified as item 5, Duct
Sensor on the Repair Parts drawing.
The heat output of the RH-D is SSR controlled.
• When voltage is applied to the power side of the heater (L/N for 120 VAC, L1/L2 for 240 VAC), the
transformer will send 24 VAC to the control circuit and energize the D21-T controller.
• The Outlet Air Temperature Sensor detects the temperature of the air at the outlet of the heater.
• The controller reads the detected temperature and then modulates the current going to the heater
element as required to maintain the setpoint on the potentiometer. Modulation is accomplished by
the controller sending measured pulses of high voltage AC to the heater element.
• The automatic reset cut-out (125
0
F) and the manual reset cut-out (200
0
F) will cut off power to
the heating element if overheating within the heater module is detected.
• The Output Air Temperature Sensor (connected to terminals “S” and “S” on the D21-T controller)
must be installed on the outlet side of the heater.
• The electronic Airflow Sensor monitors airflow through the heater and will cause the controller to
reduce current flow to the heater element if there is insufficient airflow.
