20
changed to a smaller size. The chart below shows
the proper gas orifice to use at various altitudes for
either natural or LP (propane) gas.
High Altitude Gas Orifices
Altitude above sea level in feet
0-
2000
2000-
4500
4500-
8000
8000-
9000
9000-
10,000
Natural
Gas
#42
.093
#43
.089
#44
.086
#45
.082
#46
.081
LP Gas
#54
.055
#55
.052
#55
.052
#56
.046
#56
.046
Never attempt to modify this furnace – fire,
explosion, or asphyxiation may result. If
malfunction is apparent, contact a qualified
service agency and/or gas utility for assistance.
How to Determine Gas Input Rate
Where gas is metered, the input rate may be
determined by the following method:
Contact the gas supplier, Public Utility Company or
LP (propane) gas distributor to obtain the calorific
gas value of the gas being used. When checking the
gas input rate, any other gas burning appliances
connected to the same meter should be completely
off. The furnace should be allowed to operate for five
minutes before attempting to check the gas-input
rate.
To check flow rate, observe the one cubic foot dial
on the gas meter and determine the number of
seconds required for the dial hand to complete one
revolution (seconds to flow one cubic foot).
To determine the number of seconds per cubic foot
that is necessary to achieve the correct input rate,
use the following formula: GAS VALUE X 3600
÷
DESIRED INPUT = SECONDS NEEDED
Example: 1000 BTU gas, furnace input 100,000
BTUH
Seconds for one cubic foot = 1000 X 3600
÷
100,000 = 36 seconds.
When clocking the meter, if the one cubic foot dial
makes a complete revolution in less time than was
calculated that it should, the furnace is overfired and
should be derated. If it takes more time for the meter
to make one revolution than was calculated, the
furnace is underfired.
The orifice size must be changed to correct an
overfired or underfired condition. If it is determined
that different orifices are needed, please contact
your distributor for assistance in selecting the correct
replacement.
Minor Input Adjustment
The input may be adjusted slightly by adjusting the
pressure in the gas valve in order to change mani-
fold pressure.
To adjust pressure regulator, remove cover screw
(See location on Figure 13) on valve. Turn adjusting
screw counter-clockwise to decrease pressure, turn
clockwise to increase pressure. IN NO CASE
SHOULD THE FINAL MANIFOLD PRESSURE
VARY MORE THAN
±
.3" W.C. FROM THE SPECI-
FIED REGULATOR PRESSURE SETTINGS (3.5"
FOR NATURAL GAS AND 10" FOR LP, PROPANE
GAS).
Measuring Air Temperature Rise
For proper furnace operation, the air temperature
rise must be within the range shown on the furnace
rating plate.
Operating the furnace outside of the proper air
temperature rise range may cause poor heating
performance and shorter heat exchanger life.
1. Place thermometer in the supply air plenum
about two feet from the furnace outlet. If an air
conditioning coil is installed, the thermometer
should be placed downstream of the coil. The
thermometer tip should be as near as possible
to the center of the plenum for proper measure-
ment.
2. Place thermometer in return air duct about two
feet from furnace, with the tip of the thermome-
ter as near as possible to the center of the duct.
3. Set thermostat to highest temperature setting
and allow furnace to run for at least five minutes
before checking temperatures.
4. Calculate air temperature rise by subtracting
supply air temperature from the return air tem-
perature.
5. If temperature rise is not within the range shown
on the furnace rating plate, the blower speed
may need to be changed.
6. If the blower speed is changed, recheck the
temperature rise by repeating steps 1 through 4
above.
7. If the air temperature rise is not within the proper
range even after changing blower speeds, check
ductwork design with a qualified heating engi-
neer. It may be necessary to re-size the duct-
work.
