Mystery hose in water heater circuit in PSC 31

[beetrootieee]

This isn't mine (mine is the 44). My uncle is asking me for help for his 1991 PSC 31. He noticed a coil of clear hose connected to a ball valve on the water heater side, near the cockpit locker. It's like part of the coolant loop but it's unclear if it's meant to be bled or used for filling. Well the coolant, as he says, has always been added through the expansion tank, but this setup is quite confusing.

 

[SeaConquest]

By your description, it sounds a lot like a bypass or a dedicated winterization or flushing point.
My best guess is it's an auxiliary bleeding point for air that gets trapped in the heater coil or perhaps for draining that specific part of the circuit for winterizing.
Since he's always filled via the expansion tank, I wouldn't mess with adding coolant through it unless specifically instructed by a manual or a PSC expert.
To confirm, I'd suggest tracing both ends of the hose to see where they connect in the system.
But let's also wait if others are for familiar with your uncle's boat model!

 

[Godofhunger]

Good advice from @SeaConquest, follow the tube to see where it leads. If you're having any doubts, I think it would be a good idea to consult a professional to make sure you're not doing something you shouldn't be and ending up with an expencive repair bill

 

[CrestlineCruiser]

Reminds me of when I helped my cousin sort out a mystery hose on his old Tayana 37, it turned out to be a DIY bleed line someone had added for the water heater loop. Looked sketchy at first, but once we traced it and cracked it open during a hot run, air hissed right out.