Harman Differential Pressure Sensor Vacuum Switch, 3-20-6866-OEM

A pressure switch is often referred to as a vacuum switch. This sensor is looking for airflow within the stove to close the circuit to the feed system and the igniter. If your stove is feeding pellets, your pressure switch is ok. For the switch to work properly, the hopper and the front door must be making a tight seal. Almost all of the time, the switch is good but there is an air leak or point of failure somewhere else within the stove. For example, a clogged vent or burnt-out combustion blower motor.

Replaces part numbers 3-20-9301 & 3-20-3433

This switch is calibrated to close the ignition and feed circuit at .10 WC PF.

• 3-20-6866

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Compatible Models

The switch is located in the rear of the stove. There is a white wire coming from the controller to one side of the switch and there are two light blue wires connected at the terminal on the other side of the switch. These light blue wires run to the igniter and to the feed motor. To change the switch, disconnect the white wire and double blue wires from the switch, disconnect the air tube, replace the switch and then connect the wires and tube back to the switch.

If the stove is not feeding and you are getting a vacuum error, 99.99% there is usually nothing at all wrong with the switch. The switch is simply sensing an airflow problem and is doing the job it was designed to do. The switch is normally open, which means no power can flow through the switch until the switch senses enough airflow to close the contacts.

The most important thing to know about the pressure switch is:

• if there is a problem with the switch, the stove's feed system will not dispense pellets because the auger motor is not getting power.
• unlike most brands, there is no vacuum error code on Harman Pellet Stoves

Common things that cause the switch to remain open are:

• Excessively Dirty Stove
• Clogged vent
• Burnt out of worn out combustion blower
• Door Gaskets: poor seal on stove door or ash pan door
• Door Handle: Handle mechanism is broken and the door cannot close properly
• Bad Hopper Gasket
• Missing hardware or broken latch on hopper lid
• Ash pan must be in stove to create proper air flow
• Combustion motor cover must be installed and seated properly
• Fines collection box cover must be installed and tightly sealed
• Units with a hopper lid switch, the switch can fail
• Units that take the pressure reading from inside the hopper (early versions of the XXV, Accentra and Accentra Insert) can have a broken filter at the end of the vacuum hose or the vacuum hose may have fallen off the end mount
• Accentra Inserts - Hopper lid can be hinge bound and not seating

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