Is this a "whole house fan" / how is it controlled?

[Jessica Seifert]

Hi,

Question about a new-to-us home (primary residence, 2-story SFR).

With summer rolling around here in WA - I was curious about that fan that sits behind a register in the ceiling of our upstairs hallway - and I cannot figure it out for the life of me.

House was built in 1993 and there is no switch to be found. Tried to trace the wire in the attic - but would need to dig up insulation to follow it. Talked to an original owner neighbor, they mentioned they had the same and it would come on by itself somehow but they didn't know what triggered it.

Before digging through our attic - I figured I'd ask here...

Could this be part of the overall heating system (gas furnace / forced air)? Is there some other sensor somewhere in the house that might be triggering this?

Is this a "whole house fan" or something else?

The fan itself is a radial EBM Papst fan, it has four wires connected to the unit and just a regular romex connecting to the unit based on what I could see. It's enclosed by a box in the attic and vented through the roof.

Would appreciate any pointers!

Thanks!