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

SKU 3-20-6866
MPN 3-20-6866
In Stock
Notify Me When Available

Frequently Bought Together


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.

See our Helpful hints and instructions for troubleshooting information. 
Replacement For Part Number:
  • 3-20-6866

Click To See Compatible Models
UPC: 652174670475

Compatible Models


  • Absolute 43
  • Absolute 43-C
  • Absolute 63
  • Accentra 52i
  • Accentra 52i TC
  • Accentra Cast Free Standing
  • Accentra Insert
  • Advance
  • Allure 50
  • Allure 50-CE
  • HF60
  • P35i
  • P35i-C
  • P38
  • P38+
  • P43
  • P43-C
  • P61
  • P61-C
  • P61A
  • P68
  • P68-C
  • PB105
  • PC45
  • Pellet Pro II
  • Furnace & Boiler Systems
  • PF100
  • PF120
  • XXV
  • XXV TC

    Heatilator Eco Choice

  • BH105

Technical Specs

Normal Position Open
Closed Position .10 WC


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

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


Unfortunately, sometimes products do not work like they are supposed to. We offer a 90 day warranty on manufacturer defects. Some manufacturers make this a very simple and easy process unfortunately others do not. Items must be used in accordance with the manufacturers guidelines to qualify for warranty replacement. Please be advised that warranty claims for these parts may take up to 4 weeks for processing, and customers must cover shipping.

Shipping Info

This Item Qualifies For Free Shipping With Orders Over $199!

When Will My Order Ship

All orders are typically processed within 24 hours of receiving payment and are shipped from us same day if placed before our shipping cutoff time (Monday - Friday: 1PM, Saturday: 10AM, Sunday: No Shipping), orders placed after our cutoff time are shipped the next morning.

How Does Free Shipping Work?

Most orders over $199 qualify for free shipping! Orders which ship for free are shipped with either USPS or UPS based on our discretion. If you are using a PO box, free shipping orders will always go USPS.

Is Expedited Shipping Available?

We offer both overnight and two day air shipping services for all orders placed within the Continental United States. For information on shipping cut-off times, go to our shipping page.

Do You Ship To Canada?

We ship to Canada! We estimate USPS to take 4-10 days once receiving your package from us. We do not offer expedited shipping of any kind to Canada. Please allow an extra 2-5 days for all shipments to Canada. Free shipping is not available to Canada.

Do You Ship To Alaska Or Hawii?

We ship to both Alaska and Hawaii! We do not offer next day or two day shipping to Alaska or Hawaii. Free shipping is not available to Alaska or Hawaii.

Do You Ship Outside The US Or Canada?

Unfortunately, at this time we do not offer international shipping other than to Canada.

Shipping Time Estimations

For information on shipping time estimations, see the Shipping Information & Transit Times on the shipping page.

Read More About Our Shipping Information

Customer Reviews

Based on 7 reviews
Richard C.

Works great and easy to install

peter v.

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

William T.

Always Supper Service!

andrew D.

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

Tim A.
Harman Pressure Sensor Vacuum Switch

It is hard to rate a pressure switch. It either works or it doesn't and in this case, the switch - worked perfectly. It is easier to rate customer service. I gave my experience five stars because everything - worked perfectly. The web site is easy to navigate and ordering was painless. My order arrived on time and was securely packaged. While repeating one's self in writing is discouraged I have to say that from ordering to delivery and use everything - worked perfectly.