Water Heater Air Pressure Switches
The vacuum or air pressure switch in a water heater measures the air pressure created by a blower and ensures that the air pressure is correct. When air pressure is in the correct range, the switch closes a circuit that can trigger a function
On a water heater, the air pressure switch is a safety device to prevent a possible explosion. If air pressure is not sufficient in a certain range between 80-120 PSI, the air pressure switch will not close the circuit to gas control valve and the water heater will not turn on the gas burner. This ensures full of fuel and prevents explosions.
A typical air pressure switch has two terminals:
- one for high side (blower)
- other for low side (fuel solenoid, air pressure switch).
On a water heater, the air pressure switch will be located between the blower and gas control valve. For reference, check the vacuum air pressure range in your air pressure switch manual before calling a service technician.
Vacuum air pressure switches vary from manufacturer to manufacturer and models, but mostly they follow similar design and function. When air is drawn into the air pressure switch or blower is running at the correct speed, the air pressurized diaphragm inside the air pressure switch presses against another metal-air pressure switch plate. This closes the circuit and allows power to flow to the fuel solenoid and gas control valve which in turn sends A/C power to the burner so it can ignite and heat water in the tank. If the air blower fails to draw air or the blower fails to run at the correct speed, the air pressure switch will not close the circuit and the water heater will not turn on the gas. This ensures full of fuel and prevents explosions.
Replacing Air Pressure Switches
When replacing vacuum air pressure switches, make sure the new switch has same dimensions as original part it will replace (length 3-1/8″) before buying online or at store. Also, always match connector type (bayonet) with new part it will replace; if connector type of replacement part does not match connector type of old part, you must rewire air pressure switch in series with air blower and air pressure switch.
US standard dimension of air pressure switches is: High (120-160 PSI), Low (80 PSI), Vacuum (500 mmHg). Though these numbers vary depending on model, type or manufacturer.
For most hot water heater models that use air pressure switch to control fuel gas solenoid valve and electrical power to burner, air pressurized diaphragm inside air pressure switch can open at 80-120 PSI causing fuel gas solenoid not work and power is not sent to burner so it can ignite and heat water in tank. If this happens, the only way to turn on furnace and get hot water is by adjusting air pressure switch to send air at correct air pressure to furnace.
Modern Water Heaters
Today’s water heaters use air pressure switch instead of vacuum air switches which control fuel gas solenoid and electrical power. Air pressure switch has a little difference in design from older vacuum air switches, but it does the same job for controlling fuel gas solenoid and electrical power to burner so it can ignite and heat water in tank. In addition, air blower automatically speed up when low air pressure is detected inside air pressure switch to maintain air pressurized enough for air pressure switch to close circuit while maintaining proper airflow for burner to ignite and heat water in tank without manual adjustment. The biggest difference between newer models using air pressure switch versus an older model that vacuum air switches is that you cannot adjust air pressure switch on new models. If air pressurized inside air pressure switch is too low, air blower will work slowly until air pressure reaches the correct air pressure to close air pressure switch circuit.
If air blower fails to draw air or blower fails to run at correct speed, air pressure switch will not close circuit and water heater will not turn on gas. This ensures full of fuel and prevents explosions. When air blower runs slow due to high heat rise in furnace, you must adjust air pressure switch up one notch for more airflow by turning clockwise-unscrewing only 1/4-1/2 turn) to allow more airflow into furnace using sheet metal screwdriver or instead of turning it all way around, air pressure switch has air blower adjustment hole at side of air pressure switch where you can use screwdriver to adjust air pressure slightly higher for fireplace air blower.