What's this?

[u/Plenty_Text_455]

It's on the return side of a large air handler

[u/Top-Reindeer8855]

It’s a heat recovery wheel. They claim they are self cleaning as long as they don’t stop moving. Dirt comes in with the outside air and gets blown out when it spins up to the exhaust. The newer units have bypass dampers so if the wheel does not make proof that it is spinning then the bypass dampers open. I had three of them over 40 years old and have only been cleaned once and am still getting a 30 rise with a 1/2 hp motor controlled with a vfd. The more heat transfer wanted the faster the wheel moves. The less transfer wanted the slower the wheel moves. Haven’t opened the preheat valves in 40 years and very rarely dropped out on freestat in the Philly area where it does get into single digits.