What is this valve behind my furnace?
That appears to be a damper (given the location I'd call it a &quot;main&quot; damper, as opposed to the ones for various smaller branch ducts) and having it &quot;wide open&quot; may be overloading your furnace fan motor, causing it to overheat and shut down.
This involves one of the counterintuitive engineering principles that the internet has trouble with because it's counterintuitive, but anyone can spew their intuition on the internet.
A centrifugal blower (which most furnace fans are) is moving less air, and thus doing less work, and thus drawing less power when it is restricted, rather than &quot;wide open.&quot;
When a properly trained and equipped HVAC technician &quot;balances&quot; or &quot;tunes&quot; your hot air furnace system, one thing they will do is use an ammeter to check the load on the fan motor while adjusting the damper setting(s) to keep the load within what the fan motor can actually push. The right setting is generally neither &quot;on&quot; nor &quot;off&quot; but somewhere in the middle.
When a homeowner or other person starts fiddling with the settings, they are inclined to follow intuition and &quot;OPEN IT UP!&quot; Physics being physics does not care what their intuition says, and the motor (may) overheat, and if it's protected in the usual fashion, shut down.