284806-YIM-A-0107
50
Unitary Products Group
3.
Using the number of seconds for each revolution
and the size of the test dial increment, find the
cubic feet of gas consumed per hour from the Gas
Rate - Cubic Feet Per Hour Table 28.
If the actual input is not within 5% of the furnace rating
(with allowance being made for the permissible range
of the regulator setting), replace the orifice spuds with
spuds of the proper size.
NOTE:
To find the Btu input, multiply the number of
cubic feet of gas consumed per hour by the
Btu content of the gas in your particular locality
(contact your gas company for this information
- it varies widely from city to city.)
Example: By actual measurement, it takes 13 seconds for the hand on the 1-
cubic foot dial to make a revolution with just a 300,000 Btuh furnace running.
Read across to the column in the table above, headed “1 Cubic Foot”, where
you will see that 278 cubic feet of gas per hour are consumed by the furnace at
that rate. Multiply 278 x 1050 (the Btu rating of the gas obtained from the local
gas company). The result is 292,425 Btuh, which is close to the 300,000 Btuh
rating of the furnace.
ADJUSTMENT OF TEMPERATURE RISE
The temperature rise (or temperature difference
between the return air and the heated air from the fur-
nace) must lie within the range shown on the rating
plate and the data in the Gas Heat Application Table 4.
After the temperature rise has been determined, the
cfm can be calculated as follows:
After about 20 minutes of operation, determine the fur-
nace temperature rise. Take readings of both the return
air and the heated air in the ducts (about six feet from
the furnace) where they will not be affected by radiant
heat. Increase the blower cfm to decrease the temper-
ature rise; decrease the blower cfm to increase the
rise. Refer to the Blower Motor and Drive Data
Table 23.
BELT DRIVE BLOWER
All units have belt drive single-speed blower motors.
The variable pitch pulley on the blower motor can be
adjusted to obtain the desired supply air CFM.
TROUBLESHOOTING
COOLING TROUBLESHOOTING GUIDE
TABLE 28: GAS RATE - CUBIC FEET PER HOUR
Seconds
for One
Rev.
Size of Test Dial
1/2 cu. ft.
1 cu. ft.
4
6
8
10
450
300
228
180
900
600
450
360
12
14
16
18
20
150
129
113
100
90
300
257
225
200
180
22
24
26
28
82
75
69
64
164
150
138
129
CFM
Btuh Input x 0.8
=
108
.
.
x F Temp Rise
o
Troubleshooting of components may require
opening the electrical control box with the
power connected to the unit.
Use extreme
care when working with live circuits!
Check
the unit nameplate for the correct line voltage
and set the voltmeter to the correct range
before making any connections with line termi-
nals.
For troubleshooting of optional VFD, discon-
nect all power to the drive. Be aware that high
voltages are present in the drive even after
power has been disconnected. Capacitors
within the drive must be allowed to discharge
before beginning service.
When not necessary, shut off all electric power
to the unit prior to any of the following mainte-
nance procedures so as to prevent personal
injury.
Label all wires prior to disconnection when ser-
vicing controls. Wiring errors can cause
improper and dangerous operation, which
could cause injury to person and/or damage
unit components. Verify proper operation after
servicing.
