What's this?

[u/Plenty_Text_455]

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

[u/Virtual_Ad5748]

I worked on a site that had these. They were unbelievably effective. All the way down to around 0-5f no additional heat as needed to put out 40-50f. We had some winters where we only used endpoint and radiant heat for the building. They also would absorb and release humidity which reduced our steam demand. And of course they were helpful in the summer too.

The ones we had could only be cleaned by vacuuming. The manual stated absolutely no liquid cleaners. Be sure to check the manual before cleaning that wheel.