Gas Input Rate Measurement—Natural Gas Only
The 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, time and record one complete
revolution of the smallest gas meter dial.
3. Calculate the number of seconds per cubic foot (sec/ft
gas being delivered to the furnace. If the dial is a one cubic
foot dial, divide the number of seconds recorded in Step 2 by
one. If the dial is a 2 cubic foot dial, divide the number of
seconds recorded in Step 2 by 2.
4. Calculate the furnace input in Btu per hour (Btu/h). Input
equals the sum of the installation's gas heating value and a
conversion factor (hours to seconds) divided by the number
of seconds per cubic foot. The measured input must not be
greater than the input indicated on the unit rating plate.
Example: Installation's gas heating (HTG) value: 1,000 Btu/ft
(Obtained from gas supplier)
Installation's seconds per cubic foot: 34 sec/ft
Conversion Factor (hours to seconds): 3,600 sec/h
Input = (Htg. value x 3,600) ÷ seconds per cubic foot
Input = (1,000 Btu/ft
x 3,600 sec/h) ÷ 34 sec/ft
Input = 106,000 Btu/h
Minor changes to the input rate may be accomplished through
manifold pressure adjustments at the gas control valve. Refer to
"Gas Manifold Pressure Measurement and Adjustment" in
"Start-Up Procedure and Adjustment" section for details.
NOTE: The final manifold pressure cannot vary by more than ±
0.3" W.C. from the specified setting. Consult your local gas
supplier if additional input rate adjustment is required.
5. Repeat steps 2 through 4 on high stage.
6. Turn on the gas supply to all other appliances turned off in
Step 1. Be certain that all appliances are functioning properly
and that all pilot burners are lit and operating.
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
1. Operate furnace with burners firing for approximately
2. Ensure that all registers are open and all duct dampers are in
their final (fully or partially open) position.
3. Place thermometers in the return and supply ducts as close
to the furnace as possible. Thermometers must not be
influenced by radiant heat from the heat exchanger.
4. Subtract the return air temperature from the supply air
temperature to determine the air temperature rise. Allow
adequate time for thermometer readings to stabilize.
5. Adjust the temperature rise by adjusting the circulator blower
speed. Increase blower speed to reduce temperature rise.
Decrease blower speed to increase temperature rise. Refer to
"Circulator Blower Speeds" in "Start-Up Procedure and
Adjustment" for speed changing details.
Temperature Rise Measurement
Rise = Supply air temperature - Return air temperature
A. Heat exchanger radiation
"line of sight"
B. Supply air
To avoid personal injury or death due to electrical shock,
turn OFF power to the furnace before changing speed taps.
This furnace is equipped with a multispeed circulator blower. This
blower provides ease in adjusting blower speeds. The
Specification Sheet applicable to your model provides an airflow
table, showing the relationship between airflow (CFM) and
external static pressure (E.S.P.), for the proper selection of
heating and cooling speeds. The heating blower speed is
shipped set at B, and the cooling blower speed is set at D. These
blower speeds should be adjusted by the installer to match the
installation requirements so as to provide the correct heating
temperature rise and correct cooling CFM.
Use the dual 7-segment LED display adjacent to the DIP
switches to obtain the approximate airflow quantity. The airflow
quantity is displayed as a number on the display, rounded to the
nearest 100 CFM. The display alternates airflow delivery
indication and the operating mode indication.
Example: The airflow being delivered is 1,225 CFM. The
display indicates 12. If the airflow being delivered is 1,275,
the display indicates 13.
1. Determine the tonnage of the cooling system installed with
the furnace. If the cooling capacity is in Btu/h, divide it by
12,000 to convert capacity to Tons.
Example: Cooling Capacity of 30,000 Btu/h
30,000/12,000 = 2.5 Tons
2. Determine the proper airflow for the cooling system. Most
cooling systems are designed to work with airflows between
350 and 450 CFM per ton. Most manufacturers recommend
an airflow of about 400 CFM per ton.
Example: 2.5 tons X 400 CFM per ton = 1,000 CFM
NOTE: The cooling system manufacturer's instructions must be
checked for required airflow. Any electronic air cleaners or other
devices may require specific airflows, consult installation
instructions of those devices for requirements.
C. Supply air temperature
D. Return air temperature
E. Return air
Circulator Blower Speeds