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SERVICE AND MAINTENANCE
ELECTRIC HEATER FUNCTION AND
TROUBLESHOOTING
FEM4, FCM4, FSA4, FEA4, FXM4, FVM4,
WAPM, WAPT, WAPL, WAHM, WAHT, WAHL,
WAXM, WAXT, WAXL, REM4
This section describes EHK and EHC series electric heaters
by examining the functional operation of these heaters.
DESCRIPTION OF ELECTRIC HEATER COMPONENTS
A. Limit Switch
The limit switch is a temperature sensitive control whose
function is to prevent system from overheating in abnormal
conditions. The temperature settings often vary from heater to
heater due to variations in airflow patterns and element
radiant heat conditions.
The devices are sized to remain on−line under heat pump
conditions (115_ air off coil) and minimum CFM, but trip to
prevent outlet air conditions above 200_ F or excessive
component or duct temperatures.
The device itself consists of a bimetallic disc, which when
overheated "snaps through" to open a normally closed
high−voltage,
high−current
temperatures cool sufficiently, the switch will automatically
reset to its closed position. Normal failure mode for this switch
is open.
If a limit switch has been determined to be defective, NEVER
BYPASS THE LIMIT SWITCH. When replacing limit switch,
ensure that it is replaced with a limit switch of identical
opening temperature and closing differential. Limit switches
are typically color coded to identify their range.
B. EHK and EHC
Electric Heat Relay
EHK and EHC electric heater packages have relays
controlling the heater elements instead of sequencers. A
small rectifier PCB is mounted to each relay which converts
the incoming 24−Vac control signal to DC.
In addition to the rectifier circuit, the second and third stage
relays contain a time−on delay circuit of five seconds for
second stage, and eight seconds for third stage. When the
control signal is removed from the relays, all relays will open
with no time−off delay.
TROUBLESHOOTING EHK and EHC
SERIES ELECTRIC HEATERS
A.
Discolored Wire Insulation at Terminal
Check quick−connect terminal at discoloration. Connection
may be loose, creating a high resistance through connection
point.
B.
Fuse Failure
1.
Check for shorted wire. Replace wire. Never try to fix
wire using electrical tape.
2.
Check shorted element. If element is shorted, replace
heater.
C. No Heat
1.
Check fuse for failure. If fuse has failed, refer to Fuse
Failure section.
2.
Check for faulty transformer. Check output voltage of
transformer secondary side R (red) and C (brown).
Make sure output is between 18Vac and 30Vac. If
output voltage is low and input voltage tests normal,
replace transformer.
3.
Check for miswired heater plug harness.
4.
Check limit switch or sequencer failure. These switches
should have failed in open position. If output voltage is
zero volts, replace switch.
496 08 8001 00
switch.
When
system
Specifications subject to change without notice.
5.
Check heater relay and PCB (EHK and EHC heaters
only). Control voltage input to PCB should be 24Vac.
Output to relay should be 18−Vdc minimum. If input is
present but no output, replace PCB. If output is
present, replace relay.
D.
Heater Will Not Turn Off
1.
Check low−voltage wiring for miswire.
2.
Check for shorted elements to ground.
3.
Replace sequencer/relays. They may be stuck closed.
E. Nuisance Trips
1.
Check for low airflow due to dirty filters, blocked
registers, or undersized duct.
2.
Check blower motor and wheel for proper operation.
Excessive current draw of motor will cause internal
overload to trip.
3.
The fan speed may be low.
FMA4, WAMA, WAXA
This section describes EHK2 series electric heaters by
examining functional operation of this heater.
Service can be completed with heater in place. Shut off
power before servicing.
DESCRIPTION OF ELECTRIC HEATER COMPONENTS
A. Limit Switch
The limit switch is a temperature sensitive control that's
function is to prevent system from overheating in abnormal
conditions. The temperature settings often vary from heater to
heater due to variations in airflow patterns and element
radiant heat conditions.
The devices are sized to remain on-line under heat pump
conditions (115_ F air off coil) and minimum CFM, but trip to
prevent outlet air conditions above 200_ F or excessive
component or duct temperatures. The device itself consists
of a bimetallic disc, which when overheated "snaps through"
to open a normally closed high-voltage, high-current switch.
When system temperatures cool sufficiently, the switch will
automatically reset to its closed position. Normal failure mode
for this switch is open.
If a limit switch has been determined to be defective, NEVER
BYPASS THE LIMIT SWITCH. When replacing limit switch,
ensure that it is replaced with a limit switch of identical
opening temperature and closing differential. Contact FAST
for
replacement
part.
Limits
color−coded to identify their range.
B. Sequencer
The sequencer is essentially a thermally−activated time-delay
relay normally activated by low-voltage control signals from
thermostat. The typical sequencer is a 1- or 2−pole normally
open device which energizes within 30 to 70 seconds after
application of control signal and de−energizes 60 to 90
seconds after control signal is removed.
In simplistic terms, the sequencers which we use are nothing
more than normally open limit switches which sit on top of a
small resistive heater. When voltage is applied to this heater,
a positive temperature coefficient resistor (PTC), heat is
supplied to a bimetallic disc which "snaps through" and
closes switch.
The time required for PTC to heat to a sufficient point controls
ON timing of device. The time required for disc to cool down
when power is removed controls OFF time of device. The
PTC can be varied to provide varied timing. Typically a short
ON equates to a long OFF.
Because this is a thermally−activated device, ambient
conditions affect the ON/OFF cycle. Higher ambient
temperature means shorter ON times and longer OFF times.
FAN COILS
switches
are
typically
21
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Table of Contents
Troubleshooting
