34
Gas Input Rate Measurement (Natural Gas Only)
The actual gas input rate to the furnace must never be
greater than that specified on the unit rating plate. To
measure natural gas input using the gas meter, use the
following procedure.
1. Turn OFF the gas supply to all other gas-burning
appliances except the furnace.
2. While the furnace is operating at high fire rate, time
and record one complete revolution of the gas meter
dial, measuring the smallest quantity, usually the dial
that indicates ½ cu. ft. per revolution. You will use this
number to calculate the quantity of gas in cubic ft. if
the furnace would consume if it ran steadily for one
hour (3600 seconds).
3. If the ½ cu. ft. dial was used, multiply your number x 2.
EXAMPLE:
If it took 23 seconds to complete one
revolution of the ½ ft. dial (23 x 2 = 46).
This tells us that at this rate, it would take 46 seconds to
consume one cu. ft. of gas. 3600 / 46 = 78.
This tells us that in one hour, the furnace would consume 78
cu. ft. of gas.
The typical value range for 1 cu. ft. of natural gas is around
1000 BTU. Check with your gas utility, if possible. In this
example, the furnace is consuming 78,000 BTUH.
NOTE:
The final manifold pressure cannot vary by more than
± 0.3” w.c. for Natural and + 0.5” for LP from the specified
setting. Consult your local gas supplier if additional input
rate adjustment is required.
4.
Turn ON gas to and relight all other appliances
turned off in step 1. Be certain that all appliances
are functioning properly and that all pilot burners are
operating.
OUTLET
INLET
White-Rodgers Model 36J22
Connected to Manometer
Figure 40B
Mode
.
Measuring Inlet Gas Pressure (Alt. Method)
Figure 41
Temperature Rise
Temperature rise must be within the range specified on the
unit rating plate. An incorrect temperature rise may result
in condensing in or overheating of the heat exchanger.
An airflow and temperature rise table is provided in the
Specification Sheet applicable to your model. Determine
and adjust temperature rise as follows:
1. Operate furnace with burners firing for approximately
ten minutes. Ensure all registers are open and all
duct dampers are in their final (fully or partially open)
position.
2.
Place thermometers in the return and supply ducts as
close to the furnace as possible. Thermometers must
not be influenced by radiant heat by being able to
“see” the heat exchanger.
3.
Subtract the return air temperature from the supply
air temperature to determine the air temperature rise.
Allow adequate time for thermometer readings to
stabilize.
4.
Adjust temperature rise by adjusting the circulator
blower speed. Increase blower speed to reduce
temperature rise. Decrease blower speed to increase
temperature rise. Refer to
Startup Procedure and
Adjustment - Circulator Blower Speeds
for speed
changing details.
