FCM/A5, FEV, FJM, FEM4, FHMA5, FMA4/5, FM(C,U)5, FS(A,M,U)4, FTMA5, F(V,X)M4, F5M4, REM4, WA(H,M,P,X) WB(G,H)L, WC(G,H)L: Service and Maintenance
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
23
Electric Heater Function and Troubleshooting
FM(C,U)4, FED, FEU, FEM4, FEV, FCM, FMA4,
FMA5, FSA4, FTMA4, FXM4, FVM4, F5M4, WAPM,
WAPL, WAPT, WAHL, WAHM, WAHT, WAMA,
WAXA, WAXL, WAXM, WAXT, WBGL, WBHL,
REM4
IMPORTANT: 2023 model fan coils (FJM/5, FTM/5, FCMB4) are
designed to use KFFEH electric heaters, and are not compatible with
KFCEH heaters – they use different and incompatible plugs and
polarities.
This section describes KFC, and KFD series electric heaters in exclusion
of Smart Heat by examining the functional operation of these heaters.
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.
EHC and EHK Electric Heat Relay
EHC and EHK 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 EHC and EHK 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 p23).
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. 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. Limits switches are typically color-coded to identify their
range.
Sequencer
Early production EHK2 heaters have sequences controlling the heater
elements. 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
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