Pellet Tech 101 – Vacuum Switch, Pressure Air Switch

Welcome Back to Pellet Tech 101!

Our video above will cover the basics on general vacuum switches, also referred to as a pressure switch, air switch, or draft switch. In the pellet tech world there is a distinct difference between a vacuum switch and a pressure switch. Not all replacement switches can do both, so being specific is important when ordering the correct replacement part. Most commonly used in pellet and corn stoves, the vacuum switch is detecting negative air pressure in the stove’s firebox and the pressure switch is detecting for excessive positive pressure in the exhaust vent flue pipe. Designed as a safety switch, the vacuum switch and pressure switch are either in-line with auger feed motor circuit, and will shut down the auger motor, or monitored directly by the control board if it trips. Vacuum switches connect to the stove side of the combustion fan and are normally open and close with the proper amount of negative pressure. Pressure switches connect to the vent side of the combustion fan and are normally closed and will open with excessive vent pressure (blockages). The vacuum and pressure switches are vital components to the safety and proper operation of your appliance.

Most vacuum switches will have a positive and negative nozzle port where the vacuum switch tubing hose connects to. Most pellet and corn stove appliances are under negative pressure in the firebox and positive pressure in the exhaust vent flue pipe. It’s important that the vacuum hose is placed on the correct nozzle port; we recommend paying attention to the one you are replacing and installing the new vacuum switch in the same way. Some models will use tubing on both nozzle ports, however the majority of pellet and corn stoves will have the tubing on the negative nozzle port. Vacuum switches will vary in size, mount, and WC rating. It’s important you are replacing your vacuum switch with an OEM or aftermarket switch that matches the specs of your existing. The vacuum switch is a normally open circuit and will close when proper negative air pressure is met completing the electrical circuit to the auger feed motor. If at anytime during operation there are air losses or air blockages, the vacuum switch will trip and shut down the auger feed motor.

common reasons for a vacuum switch to trip:

  • Vacuum switch tubing hose is cracked or not connected properly to the nozzle ports.
  • Heavy ash build-up in the stove and venting.
  • Poor or weak gasket seals – Firebox door, glass, ash pan door, hopper door (Not all units have ash pan door or hopper door gaskets).
  • Weak or failing combustion blower/exhaust fan.
  • Faulty vacuum switch.

General Tips :

  • Closely inspect the vacuum tubing hose with a good light. Replace if there are cracks, breaks, or if it’s becoming stiff and brittle.
  • Thorough cleaning of the burnpot, firebox area, ash traps, heat exchange tube area, exhaust fan manifold, and exhaust vent pipe.
  • Inspect and test your gaskets. With the stove off, use a dollar bill and place half in and half out of the door, close the door and pull on the dollar bill. There should be heavy resistance. Check in multiple places around your firebox door and if applicable your ash pan door and fuel hopper lid. If there are any areas with minimal resistance, air gaps, indents, or heavy fraying, we recommend gasket replacement. Visually inspect your glass gasket; if you notice air gaps, fray, or notable signs of wear, we recommend replacing the glass gasket.
  • Inspect and test your combustion blower/exhaust fan. You can use a basic voltage meter to monitor the voltage on start-up and in normal operation mode on various heat settings.

A vacuum switch can be tested and bypassed for TROUBLESHOOTING ONLY. We never want to leave a safety switch bypassed in our appliance. If you have checked all the basics and are still running into issues with a tripped/tripping vacuum switch, follow along with our video below on how to further troubleshoot to determine if the switch requires replacement.

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