Carrier FA4ANF Service Manual page 50

HEATING (COMPRESSOR AND ELECTRIC HEAT)
If heat pump alone cannot satisfy heat requirements of structure,
room thermostat calls for additional heat by way of an input to W2
terminal of control. When this occurs, control begins sequencing
on electric heaters by energizing the first sequencer/heat relay.
Indoor fan should be running at this time.
If room temperature requirements cannot be met, additional stages
of electric heat are energized
sequencers/relays, at 2.
The logic of control determines how much and how soon these
additional stages are brought on. By observing LED status light,
the number of operating stages of electric heat can be determined.
The control also varies indoor fan speed depending on number of
energized stages of heat. In general, the more stages of electric
heat, the higher the fan speed.
There is no hard and fast rule for determining exactly when stages
will be energized since microprocessor in the smart control
constantly monitors duration of previous heating cycle, setting of
room thermostat, and number of stages of heat needed in previous
cycle to satisfy room thermostat. Rather than try to determine what
should be operating when, service technicians should check LED
status light to see how many stages of electric heat are energized
at a given time and confirm operation of sequencer/relay using a
voltmeter. If a quick check of all stages is desired, set room
thermostat as high as it will go. With a constant signal on W2T
terminal, sequencers/relays energize every 10 minutes until all
heaters are on.
The previous paragraphs described how to check for low-voltage
inputs to control by placing voltmeter leads across low-voltage
terminal strip and checking for 24v. The call for electric heat can
be confirmed by placing voltmeter leads across W2T and C.
To check for operation of sequencers/relays, check directly across
appropriate coil for 24 volts DC.
NOTE: All sequencer/relay coils are powered by 24 volts DC.
This voltage is generated internally by the smart control.
In this example, sequencer/relay 1 is being checked. The procedure
is the same for all other sequencers/relays. If 24 volts DC is not
observed, check the following:
• Is room thermostat calling for electric heat? This can be
confirmed by checking for 24 volts AC between low-voltage
terminal strip terminals W2T and C.
• Is LED flashing appropriate code for number of
sequencers/relays energized? In this example, 2 flashes should be
visible, indicating 1 sequencer/relay is energized. Indicator tells
how many sequencers/relays are energized, but does not tell which
ones are.
• Is 9-pin connector for heater package tightly secured to
control board?
CHECKING SEQUENCER OPERATION
To check for operation of power circuit of sequencer/relay, place
voltmeter leads across sequencer/relay
sequencer/relay 1 and terminal 4 of limit switch 3. The reading
should be 208/230v, and heater should be energized.
If 208/230v is not observed at terminals specified, check the
following:
• Are fuses FU3 and FU4 good?
through the other 2
terminal 4 of
50
• Is limit switch LS3 open? If so, check fan coil air filter or air
distribution system for blocking or restriction that might reduce
airflow and cause limit switch to open.
Operation of heaters can be confirmed further by using a clamp-on
ammeter in circuit shown to verify current is flowing through
heaters. Amperage readings will depend on size of heater. The
procedure for checking other sequencers/relays in system is
identical to that just described.
EMERGENCY HEAT
Emergency heat mode requires that room thermostat be placed in
EMERGENCY HEAT position. This locks out and prevents
operation of heat pump outdoor unit. In this mode, heat is provided
exclusively by electrical heaters in fan coil.
In this mode, room thermostat, at 1, provides an emergency heat
input signal to smart control at terminal W2T. If jumper JW1, at 2,
has not been cut and an outdoor thermostat installed to control
electric heat stages, that same input signal is applied to terminal
W3. For this example, assume that jumper JW1 has not been cut.
Eventually, the total number of heaters is cycled on. In emergency
heat mode, LED indicator signals 7 flashes.
The amount of emergency heat available depends on whether
jumper JW1 is cut and the number of heating elements. Table 13
illustrates results under various conditions. The heaters cycle on
two elements initially and an additional element every 5 minutes
until maximum level is reached.
Table 13—Effect of JW1 on Emergency Heat
JW1 cut*
4 elements
50%
JW1 not cut
4 elements
100%
* Outdoor thermostat opens
With JW1 cut, an outdoor thermostat (if closed) could cycle on
additional heaters and system would perform as if JW1 was not
cut. In effect, closed outdoor thermostat replaces cut jumper.
DEFROST
In defrost mode, LED indicator signals 6 flashes. Defrost is
initiated by heat pump and is signalled to smart control by an input
to W2D terminal. During defrost, outdoor unit switches to cooling
mode, making it necessary to cycle on electric heaters to temper
indoor air.
If electric heaters were not energized prior to defrost, smart control
energizes sufficient elements to temper air. If heaters were
energized prior to defrost, additional heaters will be cycled on to
provide additional heat.
A defrost initiation signal can be confirmed by attaching voltmeter
leads across low-voltage terminals W2D and C. If 24v is not
observed, check the following:
• Is 24v available across R and C? See Ready-To-Start section
for items to check if 24v is not available.
• Is there continuity in thermostat leads between heat pump
and smart control?
• Is defrost control in heat pump operating properly?
3 elements
66.7%
3 elements
100%