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SUF SERVICE HANDBOOK
TROUBLESHOOTING
Technical Training Department State Water Heaters
STC-077
26 Ashland City, Tennessee © 2004
PRESSURE SWITCH PERFORMANCE
STEP 9A: CHECK PRESSURE AT SENSING PORTS ON BURNER, BLOWER INLET AND GAS
VALVE.
Condition:
• All wires are reconnected to all pressure switches.
• On/Off switch is on.
• Call for heat is activated.
• Blower is running during trial for ignition.
Remove the air pressure sensing tubes from the sensing ports on the blower inlet and the burner.
Using a digital manometer take a pressure reading with the blower running during a call for heat at
both of these sensing ports. Take a supply gas pressure reading on the inlet side of the gas valve
as outlined in Step 13.
Record all 3 pressure readings and have them on hand. Continue to Step 9B.
AMBIENT ROOM AIR PRESSURE
NOTE: One side of the internal diaphragms on these pressure switches are vented to the
room atmosphere. Due to this construction a negative or positive room air pressure can cause
erratic switch operation and the water heater to shut down. If you suspect a pressure imbalance
between the equipment room and the outdoor atmosphere, “zero” or calibrate your digital
manometer to the equipment room ambient air pressure. With the manometer still on and
calibrated to the equipment room air pressure take the manometer outdoors. If the pressure
reading changes significantly you may have a pressure imbalance between the equipment room
and the outdoor atmosphere. This pressure imbalance may need to be corrected before the water
heater will operate properly.
Insure the combustion/make-up air openings into the equipment room are properly sized. Check for
leaks in ductwork on any nearby air handling equipment. If there is a kitchen vent hood installed in
the building, insure it is properly balanced. Missing or worn fan belts on make up air fans to vent
hoods can cause negative air pressure throughout the building.
Direct vent installations DO NOT eliminate this potential problem.
STEP 9A TAKE AIR AND GAS PRESSURE READINGS.
Blower prover and blocked outlet pressure
Blower inlet pressure