FCM4, FEV, FEM4, FJM, FMA4, FSA4, FSM4, FSU4, FVM4, FXM4, REM4, WAH, WAM, WAP, WAX, WBHL: Service and Maintenance Instructions
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
21
ELECTRIC HEATER FUNCTION AND
TROUBLESHOOTING
FCM4, FEM4, FEV, FJM, FSA4, FXM4, FVM4,
REM4, WAPM, WAPT, WAPL, WAHM, WAHT,
WAHL, WAXM, WAXT, WAXL, WBHL
This section describes EHK and EHC series electric heaters by
examining the functional operation of these heaters.
NOTE: 2023 models (FJM) are not compatible with EHK or legacy
EHC series electric heaters. Check to assure that the heater selected is
compatible with the fan coil being installed.
Description of Electric Heater Components
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
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. Limit switches are typically color coded to identify their
range.
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 24VAC 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
Discolored Wire Insulation at Terminal
Check quick-connect terminal at discoloration. Connection may be
loose, creating a high resistance through connection point.
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.
No Heat
1. Check fuse for failure. If fuse has failed, refer to Fuse Failure on pg
21.
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.
5. Check heater relay and PCB (EHK and EHC heaters only). Control
voltage input to PCB should be 24VAC. Output to relay should be
18VDC minimum. If input is present but no output, replace PCB. If
output is present, replace relay.
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.
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
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 switches are
typically color-coded to identify their range.
Sequencer
Early production EHK2 heater kits have sequencers controlling the
heater elements instead of relays. 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.
Application of these devices is such that the first switch ON not only
turns on first heater element, but also ensures that indoor fan is
energized, because first ON is last OFF. This ensures fan remains ON
until the last heater de-energizes.
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