Vacuum Shut Down Switch. The vacuum hose from this switch connects to the Exhaust Blower of the stove. Will retro-fit to older (metal) type switches in older model pellet units.
2006 and later model pellet stoves use 1 ea CU-VS and 1 ea PU-VS. Pre 2006 models use 2 ea PU-VS.
NOTE: May replace PU-VS-A listed in the original owner's manual.