Update after 3m of use: works as it should. This summer was hot and my installation of the booster fan with this switch paid for itself triple. I saved a lot of energy cost because now the master bedroom gets colder much faster. Great solution - install it and you wont regret it!Original review: As many owners of two story houses with cathedral ceilings I have a problem with air pressure in my master bedroom hvac registers. Put simply - the air pressure is almost non-existing, the length of ducts is extremely long and my brand new furnace has not enough juice to push air from basement 3 stories up. Solution? A booster fan! Thoses with smart guts are expensive. So, I opted for a basic fan and separate pressure switch - this thing. It works perfectly. I did not drill 1/2” hole as manual suggests, instead, I drilled a tiny 1/4 hole under duct and inserted plastic 90° angle sensor (comes with switch) into it. Wiring is relatively easy - you just need to cut hot wire and connect each end to two separate contacts, ground and neutral pass the switch without cutting them (ground is screwed in). I connected the switch to in-line 6” booster fan and it now turns when furnace kicks on, and turns off when furnace turns off. Switch is installed into main duct, while the in-line booster fan is about 5’ down the duct, in a branch 6” duct. I was worrying that the pressure that fan creates will be enough to trip the switch but my worries were baseless. Even on maximum speed this 240cfm fan allows the switch to turn off when the furnace is off. I cannot be happier with how everything worked out.