36
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.
T
EMPERATURE
R
ISE
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.
SUPPLY
AIR
RETURN
AIR
Temperature Rise Measurement
Figure 49
C
IRCULATOR
B
LOWER
S
PEEDS
T
O
AVOID
PERSONAL
INJURY
OR
DEATH
DUE
TO
ELECTRICAL
SHOCK
,
TURN
OFF
POWER
TO
THE
FURNACE
BEFORE
CHANGING
SPEED
TAPS
.
WARNING
This furnace is equipped with a variable speed circulator
blower. This blower provides ease in adjusting blower 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 1225 CFM. The
display indicates 12. If the airflow being delivered is 1275,
the display indicates 13.
1. Determine the tonnage of the cooling system installed
with the furnace. If the cooling capacity is in BTU/hr divide
it by 12,000 to convert capacity to tons.
Example:
Cooling Capacity of 30,000 BTU/hr.
30,000/12,000 = 2.5 Tons
2. Determine the proper air flow for the cooling system. Most
cooling systems are designed to work with air flows
between 350 and 450 CFM per ton. Most manufacturers
recommend an air flow of about 400 CFM per ton.
Example:
2.5 tons X 400 CFM per ton = 1000 CFM
The cooling system manufacturer’s instructions must be
checked for required air flow. Any electronic air cleaners or
other devices may require specific air flows, consult
installation instructions of those devices for requirements.
3. Knowing the furnace model, locate the high stage cooling
air flow charts in the Specification Sheet applicable to
your model. Look up the cooling air flow determined in
step 2 and find the required cooling speed and adjustment
setting.
Example:
A DM96VC0603B* furnace installed with a 2.5
ton air conditioning system. The air flow
needed is 1000 CFM. Looking at the cooling
speed chart for DM96VC0603B*, find the air
flow closest to 1000 CFM. A cooling airflow of
1007 CFM can be attained by setting the
cooling speed to “C”.
5. Locate the blower speed selection DIP (S3) switches on
the integrated control module. Select the desired “cooling”
speed tap by positioning switches 1 and 2 appropriately.
Select the desired “adjust” tap by positioning switches 3
and 4 appropriately. To enable adjustments and select -5,
5, -10 or 10% trim, you must set dip switch S5-2 to
ON. If S5-2 is in the OFF position , you will receive 0%
trim. Refer to the following figure for switch positions
