The ins and outs of furnace pressure switches

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Now that we’re in the middle of heating season, we bet you’re making more and more service calls for malfunctioning or under-performing furnaces. During these calls, it’s important to make sure that the furnace pressure switch is functioning as intended.

The furnace pressure switch opens and closes to prevent harmful combustion gases from entering the living space and to prevent furnace fires and potential explosions. An elastomeric diaphragm within the switch remains open when the furnace is off. When the furnace is turned on and functioning correctly, the diaphragm senses the pressure created by the draft inducer motor. This completes an electrical circuit that allows the furnace to ignite for the heating cycle. If it senses negative pressure the switch shuts down because the furnace is not operating properly and could create an unsafe environment for residents and technicians.

When examining a pressure switch, start by using a CO detector to check for harmful gases to ensure you can work safely. Then, run the furnace through its cycle. If you hear a flapping sound when the furnace is on, that may indicate that the diaphragm or the pressure switch is old and could need replacing. Also, check to make sure that the furnace is properly vented.

If the furnace tries to cycle but stops during start-up and then shuts down, it may be because of a faulty pressure switch. In this case, check for a clogged hose port, corrosion or debris blocking the switch. A bad spring could also be the culprit. All of these should be examined.

To test a pressure switch, an ideal tool is the Fieldpiece SDMN6 Manometer Dual Port w/ Pressure Switch Tester. This versatile manometer allows a tech to use the ports to quickly connect to the switch while it’s still in place, verify that it’s operating correctly and even calibrate it to the manufacturer’s specifications.

When a pressure switch is at fault and requires a replacement, a good piece of advice is to keep a supply of universal pressure switches in the truck. This quickly helps get a furnace back to operating properly and eliminates delays during a call. When replacing a faulty switch with a universal switch, notify the property owner. They may request that an OEM part be used instead.

The pressure switch is an important piece of every heating system and knowing how to examine, test, calibrate and replace them is a critical job for techs and the Fieldpiece SDMN6 is the best tool for the job. See it and more tools like it at

Content provided by Fieldpiece Instruments

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