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Set Temperature Rise

CAUTION:

Temperature rise must be within limits

specified on unit rating plate. Recommended operation is
at midpoint of rise or above. Failure to follow this caution
may result in condensing or overheating the heat ex-
changers.

Furnace must operate within range of temperature rise specified on
the unit 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

temperature from supply-air temperature to determine air
temperature rise.

NOTE:

Temperature rise can be determined for low-fire opera-

tion by placing setup switch SW-2 on control center in ON
position. For high-heat operation, place setup switch SW-2 in OFF
position and jumper R-W2 on control center. DO NOT forget to
return setup switch to OFF position and remove R-W2 jumper
upon completion of testing. (See Fig. 25 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 unit 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 restric-

tions causing static pressure greater than 0.5-in. wc.

d. Ensure bypass humidifier switch (BPH) setup switch

SW-3 on control center is in ON position when a bypass
humidifier is used. (See Fig. 25 for switch location.)

e. Check

Troubleshooting

Guide

for

Variable-Speed

2-Stage Electronic Condensing Furnaces.

Set Thermostat Heat Anticipator

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 at the thermostat subbase terminals
R and W.

Fig. 50 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 SW-2 to

ON position (See Fig. 25) 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 SW-2 to OFF position and replace

thermostat on subbase.

CHECK SAFETY CONTROLS

CHECK PRIMARY LIMIT CONTROL

This control shuts off the combustion 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 33 fault code

will appear on control center.

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 supply or motor
failure. If the limit control does not function during this test, the
cause must be determined and corrected.

TABLE 14—GAS RATE CU FT/HR

SECONDS

FOR 1

REVOLUTION

SIZE OF TEST DIAL

SECONDS

FOR 1

REVOLUTION

SIZE OF TEST DIAL

cu ft

10
11
12
13
14

360
327
300
277
257