EXAMPLE: (0—2000 ft altitude)
Furnace input from rating plate is 100,000 Btuh.
Btu heating input = Btu/cu ft X cu ft/hr
Heating value of gas = 975 Btu/cu ft
Time for 1 revolution of 2-cu ft dial = 70 sec
Gas rate = 103 cu ft/hr (from Table 11)
Btu heating input = 103 X 975 = 100,425 Btuh
In this example, the orifice size and manifold pressure
adjustment is within
±
2 percent of the furnace input rate.
SET TEMPERATURE RISE
Temperature rise must be within limits specified on furnace
rating plate. Recommended operation is at midpoint of rise
range or slightly above. Failure to follow this caution may
result in primary and secondary heat exchanger failure.
Determine and adjust air temperature rise as follows:
1. Place thermometers in return and supply ducts as close to
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.
3. Adjust temperature rise by adjusting blower speed. Increase
blower speed to reduce temperature rise. Decrease blower
speed to increase temperature rise.
Disconnect 115-v electrical power before changing speed tap.
Failure to follow this warning could result in personal injury.
4. To change blower motor speed selections for heating mode,
remove blower motor lead from control center HEAT termi-
nal. (See Fig. 33.) Select desired blower motor speed lead
from 1 of the other motor leads and relocate it to HEAT
terminal. See Table 12 for lead color identification. Reconnect
original lead on SPARE terminal.
Follow this same procedure for proper selection of COOL
speed selection.
BLOWER OFF DELAY (HEAT MODE)
The blower off delay has 4 adjustable settings from 90 sec to 180
sec. (See Table 13.) The blower off delay jumpers are located on
the furnace control board. (See Fig. 33.) To change the blower off
delay setting, move the jumper from one set of pins on the control
board to the pins used for the desired blower off delay. Factory
blower off delay setting is 120 sec.
Table 13—Blower Off Delay Setup Position
PINS
1 AND 2
2 AND 3
3 AND 4
4 AND 5
Time (in sec)
90
120
150
180
SET THERMOSTAT HEAT ANTICIPATOR
When using a nonelectronic thermostat, the thermostat heat
anticipator must be set to match amp draw of components in R-W
circuit. Accurate amp draw measurements can be obtained at
thermostat subbase terminals R and W. Fig. 49 illustrates an easy
method of obtaining these measurements. The amp reading should
be taken after blower motor has started and furnace is heating.
Connect ammeter wires as shown in Fig. 49. The thermostat
anticipator should NOT be in the circuit while measuring current.
If thermostat has no subbase, the thermostat must be disconnected
from R and W wires during current measurement. See thermostat
manufacturer’s instructions for adjusting heat anticipator and for
varying heating cycle length.
For an electronic thermostat, set cycle rate for 3 cycles per hour.
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 switch were all checked in the Start-up procedure section
as part of normal operation.
Step 1—Check Primary Limit Control
This control shuts off gas control system and energizes air-
circulating blower motor if furnace overheats. 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. As soon as limit control has shut off burners, return-air
opening should be unblocked to permit normal air circulation. By
using this method to check limit control, it can be established that
limit is functioning properly and operates if there is a restricted
return-air duct or motor failure. If limit control does not function
during this test, cause must be determined and corrected.
Table 12—Speed Selector
COLOR
SPEED
FACTORY-
SHIPPED
CONNECTION
Black
High
Cool
Yellow (When Present)
Medium High
Spare
Blue
Medium Low
Heat
Red
Low
Spare
White
Common
Com
Fig. 49—Amp Draw Check with Ammeter
A80201
R
Y W G
10 TURNS
THERMOSTAT SUBBASE
TERMINALS WITH
THERMOSTAT REMOVED
HOOK-AROUND
VOLT/AMMETER
EXAMPLE:
5.0 AMPS ON AMMETER
10 TURNS AROUND JAWS
= 0.5 AMPS FOR THERMOSTAT SETTING
FROM UNIT 24-VOLT
TERMINAL BLOCK
42
