SET TEMPERATURE RISE
UNIT DAMAGE HAZARD
Failure to follow this caution may result in overheating the
heat exchangers or condensing flue gases in heat exchanger
areas not designed for condensate.
Temperature rise must be within limits specified on unit
rating plate. Operation is within a few degrees of midpoint of
rise range when setup switch SW1-4 is OFF.
Furnace must operate within ranges of temperature rise specified
on the furnace rating plate. Determine air temperature rise as
follows:
1. Place thermometers in return and supply ducts as near furnace
as possible. Be sure thermometers do not see heat exchanger
so that radiant heat does not affect readings. This practice is
particularly important with straight-run ducts.
2. When thermometer readings stabilize, subtract return-air tem-
perature from supply-air temperature to determine air tem-
perature rise.
NOTE:
Temperature rise can be determined for low-heat opera-
tion by placing setup switch SW1-2 on furnace control in ON
position. For high-heat operation, place setup switch SW1-2 in
OFF position and jumper R-W2 on furnace control. DO NOT
forget to return setup switch to OFF position and remove R-W2
jumper upon completion of testing. (See Fig. 37 for switch and
terminal location.)
3. This furnace is capable of automatically providing proper
airflow to maintain the temperature rise within the range
specified on furnace rating plate. If temperature rise is outside
this range, proceed as follows:
a. Check gas input for low- and high-heat operation.
b. Check derate for altitude if applicable.
c. Check all return and supply ducts for excessive restrictions
causing static pressure greater than 0.5-in. wc.
d. Ensure Low Heat Rise Adjust switch SW1-3 on furnace
control is in ON position when a bypass humidifier is used.
(See Fig. 37 for switch location.)
e. Check Troubleshooting Guide for Variable-Speed 2-Stage
Electronic Condensing Furnaces Series 170 and later.
SET THERMOSTAT HEAT ANTICIPATOR
When using a non-electronic thermostat, the thermostat heat
anticipator must be set to match the amp draw of components in
the R-W/W1 circuit. Accurate amp draw measurements can be
obtained only at the thermostat subbase terminals R and W.
The thermostat and anticipator should NOT be in the circuit while
measuring current. If thermostat has no subbase, the thermostat
must be disconnected from R and W/W1 wires during current
measurement.
Fig. 62 illustrates an easy method of obtaining thermostat amp
draw measurements. The amp reading should be taken after blower
motor has started and furnace is operating in low-heat.
1. To operate furnace in low-heat, turn setup switch SW1-2 to
ON position (See Fig. 37) and connect ammeter leads across
thermostat subbase R-W.
2. See thermostat manufacturer’s instructions for adjusting the
heat anticipator and for varying heating cycle length.
NOTE:
When using an electronic thermostat, set cycle rate for 3
cycles per hr.
3. Return setup switch SW1-2 to OFF position and replace
thermostat on subbase.
Step 7—Check Safety Controls
This section covers the safety controls that must be checked before
the installation is complete. The flame sensor, gas valve, and
pressure switches were all checked in the Start-up procedure
section as part of normal operation.
CHECK PRIMARY LIMIT CONTROL
This control shuts off the gas control system and energizes the
air-circulating blower motor if furnace overheats.
1. The recommended method of checking this limit control is to
gradually block off return air after furnace has been operating
for a period of at least 5 minutes.
2. As soon as limit control has shut off burners, a status code 33
will appear on furnace control.
3. The return-air opening should be unblocked to permit normal
air circulation.
By using this method to check the limit control, it can be
established that the limit is functioning properly and the furnace
will operate safely if there is a restricted return-air duct or motor
failure. If the limit control does not function during this test, the
cause must be determined and corrected.
CHECK PRESSURE SWITCHES
This control proves operation of the draft inducer. Check switch
operation as follows:
1. Turn off 115-v power to furnace.
2. Remove control access door and disconnect inducer motor
12-pin wire harness at inducer motor.
3. Turn on 115-v power to furnace.
4. Set thermostat to
″
call for heat.
″
When pressure switches are
functioning properly, status code 42 will flash on furnace
control approximately 20 sec after thermostat switch is closed.
If either a status code 31 or 32 is flashed when inducer motor
is disconnected, the furnace will shut itself down immediately.
Determine the reason pressure switches did not function
properly and correct the condition.
Fig. 62—Amp Draw Check with Ammeter
A96316
R
Y W G
10 TURNS
THERMOSTAT SUBBASE
TERMINALS WITH
THERMOSTAT REMOVED
(ANITICIPATOR, CLOCK, ETC.,
MUST BE OUT OF CIRCUIT.)
HOOK-AROUND
AMMETER
EXAMPLE:
5.0 AMPS ON AMMETER
10 TURNS AROUND JAWS
=
0.5 AMPS FOR THERMOSTAT
ANTICIPATOR SETTING
FROM UNIT 24-V
CONTROL TERMINALS
56
