Your water heater is the one HVAC unit you rely on 365 days a year to provide the heated water necessary to effectively wash dishes and enjoy a relaxing shower or luxurious bath. So, what do you do if the water heater isn’t heating your home’s water as it should? If you have a gas water heater using a hot surface ignition system, there’s a crucial component you can test to troubleshoot the cause of the problem: the pressure switch. Let’s take a look at how a gas water heater heats the water and the simple test you can perform that will determine if the pressure switch is functioning normally or not.
How gas water heaters ignite a burner to heat the water
Gas water heaters will ignite a burner positioned directly under the bottom of the tank using either a pilot or a hot surface ignition system.
How water heaters using a pilot ignite the burner
On water heaters using a pilot, the pilot and burner are regulated by a thermostatically controlled gas valve. When the valve detects a decrease in water temperature, it opens, allowing the pilot to light the burner and the water to be heated. Most pilot systems will use either a thermocouple or thermopile to monitor the pilot. If the pilot goes out, the thermocouple or thermopile will prevent the gas valve from opening. Water heater models with a thermopile will also incorporate a limit thermostat to prevent the gas valve from opening if the burner or the surrounding area gets too hot.
How water heaters using a hot surface ignition system ignite the burner
A hot surface ignition system also uses a thermostatically controlled gas valve along with a control board. In addition, a power vent blower fan will exhaust the fumes generated by this system to the outside of the home. When the gas valve detects a decrease in water temperature, the control board begins a timed ignition sequence by sending 120 volts of alternating current to a draft inducer fan motor which turns on to draw air into the burner. Once the draft inducer fan motor reaches its maximum speed, a pressure switch closes, and the control board sends 120 volts to the hot surface igniter. Next, the control board energizes a solenoid on the gas valve which allows gas to flow into the burner and be lit by the igniter. A flame sensor is used to monitor the burner flame to ensure it is lit while gas is flowing through the valve.
A malfunctioning pressure switch will stall the ignition process
If the water heater’s pressure switch is malfunctioning, it may be unable to close properly, even with adequate air flow. This will stall the ignition process resulting in the water heater not lighting and not heating the water.
How to test a gas water heater’s pressure switch
If your gas water heater uses a hot surface igniter to ignite the gas, you can test the water heater’s pressure switch for continuity using a multimeter while the draft inducer fan motor is running. This will determine if the switch is malfunctioning and causing the water heater to not heat or not light. Here are the 7 steps you should follow to successfully test the pressure switch:
1) Shut off the power to the water heater – Before you begin the test, be sure to unplug the water heater’s power cord from the wall outlet or switch off a circuit breaker to interrupt power to the unit.
2) Disassemble the water heater’s blower assembly control housing – The pressure switch is normally located in the blower assembly control housing close to the draft inducer fan motor. To reach it, you will need to detach the control housing cover (often secured with a screw) and move it out of the way.
3) Disconnect the pressure switch wires – There will be two wires connected to terminals on the pressure switch. Disconnect both wires.
4) Restore power to the water heater – With the wires disconnected, you’re ready to restore power to the water heater by plugging the power cord back in or switching a circuit breaker back on. Now that voltage is reaching the unit, be very careful not to touch any exposed electrical components with your bare hands.
5) Set the multimeter to the lowest setting for ohms of resistance – Rotate the multimeter’s range selection dial to the lowest setting for ohms of resistance to get an appropriate reading when testing the pressure switch.
6) Contact the meter leads to the pressure switch’s terminals – If necessary, wait until the draft inducer fan motor is running, then contact the black meter lead to one of the pressure switch’s terminals and the red lead to the other terminal.
7) Observe the multimeter display – Pay attention to the multimeter display. If the pressure switch has electrical continuity and is functioning, the meter display should indicate close to zero ohms of resistance. However, if the meter display doesn’t change when the leads come into contact with the switch terminals, you’ll know the pressure switch has no electrical continuity and will need to be replaced with a new switch to fix the problem.
If the troubleshooting test has shown that the water heater pressure switch is faulty, you’re going to need a new pressure switch to fix your water heater. Importantly, that switch should be a genuine original equipment manufacturer (OEM) part to ensure the repair is successful. Repair Clinic makes it easy to find the exact pressure switch that matches your water heater model, whether you’re repairing a Rheem, A.O. Smith, Bradford White, Bryant, Carrier, Coleman, Honeywell, or Lennox unit. Just enter the full model number of the water heater in Repair Clinic’s search bar to see a complete list of OEM parts that are fully compatible with that model. You can then use the “Part Category” (“Switch”) and “Part Title” (“Pressure Switch”) navigation filters to identify the specific pressure switch you need.
Repair Clinic has free water heater repair help
As your repair partner, Repair Clinic wants to make sure you’ve got access to the help you need to fix a water heater yourself. Our “Videos & Articles” content library provides free instructional videos such as this one demonstrating the correct way to replace a pressure switch on the model GPVH40100 A.O. Smith gas water heater. You can also find model-specific schematics, diagrams, and step-by-step repair guides for all of your home’s heating and cooling units in addition to all major home appliances and outdoor power equipment.