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19. MANIFOLD PRESSURE ADJUSTMENT
You will need a 0 to 15 inch water manometer with
0.1 inch resolution and a 1/8" NPT manual shut-off
valve to measure actual manifold pressure.
WARNING
Correct manifold pressure is necessary for
proper ignition and burner operation. Failure to
accurately adjust pressure could cause heat
exchanger failure.
1. Turn off gas at equipment shut-off valve in gas
supply line just ahead of furnace.
2. Remove plug from outlet pressure tap in gas
control or gas manifold.
3. Install 1/8" NPT manual shut-off valve in hole
vacated by plug. Make sure shut-off valve is in off
position.
4. Attach manometer to 1/8" NPT manual shut-off
valve just installed.
5. Slowly open equipment shut-off valve in gas
supply line just ahead of furnace. Start furnace
following "Operating Instructions" on front door.
6. Slowly open 1/8" NPT manual shut-off valve
leading to manometer.
7. Read manifold pressure on manometer.
8. Adjust manifold pressure by turning gas control
regulator adjusting screw clockwise to increase
pressure or turning counterclockwise to decrease
pressure. Manifold pressure must be within
allowable range below:
•
Natural gas manifold pressure must be between
3.2 and 3.8 inches W.C. Rated pressure is 3.5
inches.
•
Propane gas (LP) manifold pressure must be
between 9.7 and 10.3 inches W.C. Rated
pressure is 10 inches.
NOTE: For natural gas, if gas flow rate can't be
properly set within these pressure ranges
then you must change main burner orifices to
obtain proper gas flow rate.
9. Shut off furnace. Turn off gas at equipment shut-
off valve in gas supply line just ahead of furnace.
Install outlet pressure tap plug in gas control. Turn
on gas.
10. Check regulator adjustment cover screw and gas
control plug for gas leaks. Use a commercial soap
solution made for leak detection.
Manual 2100-365
Page 20
20. CHECKING GAS INPUT RATE
It is the installer's responsibility to see that the BTU
input rate of the furnace is properly adjusted. Under-
firing could cause inadequate heat, excessive
condensation or ignition problems. Overfiring could
cause sooting, flame impingement or overheating of
heat exchanger.
WARNING
Failure to adjust furnace to the proper firing
rate could cause heat exchanger failure.
Depending on your local gas heating value and
elevation, you may need to adjust manifold pressure
or change orifices to get proper gas input rate. Check
with your local gas supplier to determine heating
value (BTU/cu. ft.) of natural gas in your area.
NOTE: If furnace is being installed at an altitude of
more than 2000 feet above sea level, you
must derate the furnace. See Section 18
"Standard Orifice Sizing and High Altitude
Derate".
NATURAL GAS INPUT RATE
Natural gas heating value (BTU/cu. ft.) can vary
significantly. Before starting natural gas input check,
obtain gas heating value at your location from local
supplier. You will need a stopwatch to measure
actual gas input.
1. Gas supply pressure must be between 5 and 7
inches W.C. for natural gas.
2. Turn off all other gas appliances. You may leave
pilots on.
3. Start furnace following "Operating Instructions" on
front door.
4. Let furnace warm up for 6 minutes.
5. Locate gas meter. Determine which dial has the
least cubic feet of gas and how many cubic feet per
revolution it represents. This is usually one-half,
one or two cubic feet per revolution.
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