Mike PleziaInside service technician, Clark Associates · Commercial Equipment, HVAC, and Refrigeration professional

Gas furnace pressure switch

10 Oct 2024 · 2 min

Watch on YouTube

2:24

In this instructional video, the speaker from A Furnace Maintenance introduces newcomers to the HVAC trade to the pressure switch component of a gas furnace. He explains that the pressure switch contains a flexible rubbery baffle that is pulled by negative air pressure created by the inducer (draft) motor when it runs. This movement closes two contacts in the switch, completing an electrical circuit that signals the control board that proper airflow is present through the flue. The pressure switch therefore acts as a safety interlock: during furnace startup, the inducer motor first runs, the system checks the pressure switch, and only if the switch is closed (indicating airflow) does the furnace continue its ignition sequence. The speaker uses a drum analogy to describe the flexible baffle, emphasizes the safety purpose, and invites viewers to ask questions.

Chapters

Good morning, techs, over here at A Furnace Maintenance. And for those of you who are new to the trade or considering a trade career or possibly considering HVAC, just wanted to highlight for you, uh, this part today. You know what that is? No. So if you don't know, that is the pressure switch on your gas furnace. The other question is, how does it work? Well, if you imagine the surface of a drum and how it's flexible, and that vibrates, that's how it makes the sound that a drum makes. Inside of that little disc is a flexible baffle. It's more of a rubbery baffle, but it could be a papery substance like a drum, but usually it's more of a rubbery baffle. As the Inducer motor turns and blows air up through the flue, it creates negative air pressure. That negative air pressure sucks that baffle in into the, into the pressure switch. That baffle is connected to 2 contacts of a switch. Now, when there's no power, that switch is open, okay, meaning current can't flow through the switch. But when the inducer motor or draft motor is turning, that switch closes due to the negative air pressure, completing the circuit, which allows the safety to be bypassed, or rather, which functions as a safety. So as the air is pulling on that baffle, that is confirming for the control board that there is in fact airflow. Moving through the flue. Okay, so it's all about safety. So this is one of the safety devices in your furnace. So when your furnace starts up, your inducer motor turns on. The first thing it's going to check for after a few seconds is, do we have airflow? If that pressure switch is closed, that proves airflow. If you have any questions, feel free to message me, and I look forward to discussing with you, uh, later. Otherwise, have a good day.