Bryant FA4A - Manual (Page 6)

AÏÏG/14/2008/THÜ 03:46 PM UTC TECH PUB FAX No. 317 240 5662 P. 006
fe. If Electric Hegt Stages Will Not Turn On But Fan Will
Turn On:
IF THERE IS NO HIGH VOLTAGE TO PCB:
1. Check plug connection between heaters and board. This
supplies power to transformer and fan. Be sure plug is
connected properly.
2. Check sequencer number 1 and plug wiring. Yellow wire
should be connected to pin number 9 of plug and to limit
switch. Black wire should be connected to pin number 7 of
plug and to sequencer number 1-
3- Check incoming high-voltage power leads, tf these arc not
receiving power, system c&undt function.
IF PCB HAS HIGH VOLTAGE APPLIED TO IT;
1, Check low-voltage transformer leads R (red) and C
(brown). Make sure they are wired to correct location. The
unit will not function without proper connections.
2. Check output voltage of transformer secondary side R (red)
and C (brown). If transformer output is low (less than 18
vac), reifer to items 3 and 4 of previous If PCB Has High
Voltage Applied To It section.
IF TRACES ARE OVERHEATED ON BACK OF PCB:
Usually whenever a trace is blown on PCB, it means cither there
has been a high-voltage short or high voltage has been applied to
low-voltage circuit. This can be prevented by making sure PCB is
wired correctly before PCB has power applied to it,
C, If PCB Fuse Keeps Blowing:
When loW“Voltage fuse blows, it means transformer would have
blown if fuse had riot been in circuit to protect it. The fuse usually
blows when there is a high Current draw on transformer, high
voltage applied to low-voltagc circuit, Or a direct secondary short.
When there is a high current draw on transformer, it is most likely
because transformer has been shorted or system is trying to draw
more va than transformer rating allows. When fuse blows because
of high voltage, the system has mixed high- and low-voltage
signals.
1. Check wiring of sequencers as shown in Fig. 4, Be sure
transformer is not shorting out because thermostat wires arc
miswired.
2. Check wiring of sequencers as shown in Fig. 4. Be sure
low-voJtagc and high-voltage wiring is connected to proper
sequencers.
3. Check va draw on transformer. If va draw is more than va
rating of transformer, fuse will blow. If this is the case,
replace transformer with one that has a higher va rating and
meets system spccifícationS-
D. If F^n Runs Continuously:
1. If PCB has no low-vohage power, check blue and black fan
leads. These may be switched at sequencer,
2. If PCB has lowwoltage power, check fan relay to see if it is ,
opening and closing. It may be stuck in the normally-closed
position due to debris in relay.
E- Transformer Failure:
1. Check 208-v and 230-v transformer connections. They may
be miswired.
ELECTRIC HEATER FUNCTION AND
TROUBLESHOOTING
This section describes KFA and KFB series electric heaters by
examining the functional operation of these heaters,
I.
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 clement radiant heat conditions.
The devices arc sized to remain on-line under heat pump condi­
tions (11 ST air off coil) and minimum CFM;, but trip to prevent
outlet air conditions above 200T or excessive component or duct
temperatures.
The device itself consists of a bimetallic disc, which when
overheated "snaps through" to open a normaliy-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 défective, NEVER
BYPASS THE LIMIT SWITCH. When replacing limit Switch,
ensure that it is replaced with a limit switch of identical opening
température and closing differential. Limits switches are typically
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 10 to 30 sec after application of control
signal and de-energizes 60 to 90 sec 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 (BTC), heat is supplied to a
bimetallic disc which "snaps through" and closes switch-
Tbe time required for PTC to heat to a suffiejeut 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.
These sequencers may be "ganged up" to 3 On a common mounting
plate to control up to 6 heater elements. In this situation, PTCs arc
different in each sequencer to provide a staged QN of a minimum
of 10 sec between each sequencer — 3 stages of 2 elements since
the KFA and KFB Series .Деа1ег8 use , sequencers in which both
switches of the 2-polc sequencer close simultaneously. Older
models used sequencers which had a minimum delay of fO sec
between each switch.
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-
с. Positive Temperature Coefficient (PTC)
The PTC heater is essentially a resistor. It will operate on either
AC or DC voltages. It should be noted that because resistance of
PTC increases as, it, heats, initial current inrush is higher than its
steady state condition.
II. TROUBLESHOOTING KFA AND KFB SERIES
electric heaters
A. Discolored Wire Insulation at Tormincl
Check quick-coimect 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.
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