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In this article, we will be discussing methods to test the safety switches in a gas furnace using a multimeter. These switches include the flame rollout switch, thermal limit switch, and the pressure switch.
The gas furnace is designed with at least one flame rollout switch to detect flames or high temperature within the combustion chamber. The flame rollout switch is normally closed and opens on high temperature from the flame. The switch is meant to open if the flame is pushed backward away from the heat exchanger area where the flame should be traveling toward. When the switch opens, the control board stops sending power to the electrical gas valve. This stops the gas flow. The switch can only be manually reset and may indicate a problem such as high heat, a bad switch, air leaking into the combustion chamber, or a cracked heat exchanger.
The thermal limit switch is mounted in the air plenum of the furnace near the heat exchanger. The thermal limit switch is closed at normal ambient temperature and opens on a temperature rise. When the temperature of the air surrounding the heat exchangers is too high, the switch opens and the control board turns off the gas valve. The thermal limit switch closes automatically after the air temperature lowers to a safe operating range.
The pressure switch is a safety device that electrically closes only if the inducer motor is running, the condensate line is not clogged and there are no problems with the intake or exhaust. When the inducer motor is off, the pressure switch is open. Gas furnaces may have one switch or as many as four pressure switches, depending on the model.
Testing with a Multimeter
To electrically test these switches, two separate functions on the multimeter are used. One is for testing voltage while the switch is electrically connected. The other function is for measuring the electrical resistance in ohms across the switch while the electrical power is disconnected and the wires are removed.
Method 1: Measuring electrical resistance
To test switches using the resistance method, set the multimeter to the ohm (Greek letter omega) setting. The power to the unit should be turned off. The switches should be removed from the unit and the lead wires removed. There are metal tabs on each side of the switch, where the wires were connected. Use the alligator clips on each probe to easily connect to each tab. The red probe is mounted to one side of the switch while the black probe is mounted to the other side of the switch.
-A reading of 0.0 ohms indicates there is no electrical resistance across the switch and that the switch contacts closed.
-A reading of OL (Open Line) indicates an open circuit.
-A reading of .2 ohms or higher means that the contacts are closed but the contact tips are pitted. The switch is likely going bad due to high current melting the tips of the contacts.
(Pro Tip- Make sure that the multimeter reads 0.0 ohms from probe to probe before trying to measure the resistance of a switch.)
Method 2: Measuring for voltage with one probe on common/ground
To measure voltage using the multimeter, there is no need to remove or disconnect switches. Set the multimeter to the voltage setting (VAC). Start by identifying the common wire on the 24v transformer. This common wire is usually connected to the ground directly from the transformer or from the gas valve to the ground. Because of this, any part of the metal unit can be used as a ground for testing purposes. To conduct the test, touch one probe of the multimeter to the ground. This can be any metal part of the furnace. An easy way to accomplish this is to find an unused screw hole and rest the tip of the probe in the hole. Touch the other probe to the terminal on one side of the switch. The meter will indicate the voltage present at this point. Then, touch the probe to the contact on the other side of the switch. If the voltage reading on the first terminal is the same as the other terminal, this indicates that the voltage is present at both sides of the switch and that the switch is closed. If the switch has high resistance across the contacts, the voltage measurement on one of the terminals will be less than the terminal on the other side of the switch.
If the switch is open, 24v may be present when measuring with one probe on the ground and the other probe on a terminal. However, when the probe is moved to the terminal on the other side of the switch, the measurement will be zero volts.
Method 3: Measuring for voltage across the switch
In this instance, the pressure switch is tested by measuring the voltage across the switch. The pressure switch connected to the inducer motor can be tested using either of the two previously mentioned methods. However, a third method can be employed for testing switches as long as the power to the unit is on. When the inducer motor is off, the pressure switch is open. Set the multimeter on the voltage setting (VAC). Place one probe on one terminal and the other probe on the second terminal. If the switch is open, the meter will display 24v as the potential difference across the switch. If the switch is closed while the inducer motor is running, the potential voltage across the switch should be close to zero. If there is a .2 volt or higher potential difference, this means that the switch is closed but the contacts are pitted.
(I prefer methods 1 and 2 but some techs choose to test switches using method 3.)
As technicians, we need to know when the switch is supposed to be closed to determine if the switch is bad. The multimeter is our primary diagnostic tool used to understand why the furnace is not heating the building. We also need to know the temperature or pressure affecting the switch and if the temp is causing the switch to open at the correct rating of the switch. Testing the switch with a multimeter is only one part of the process as we will need a water column manometer for pressure and a digital thermometer for temperature readings in order to move through our next steps of troubleshooting!
I hope this has helped you better understand how to test Gas Furnace Safety Switches. If you are looking for more information on this topic, check out our video on this topic.
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Published: 04/21/2021 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 16 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/
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