Winterizing water system - what's this valve?

[farmer.leo]

The big stainless steel tank is to the right, at the bottom is the feed to this "rectangle" with a Jabsco pump - the red Accumulator tank, and the closed loop with a valve in there.

Why this closed loop? What purpose does this valve serve?

I'm preparing to winterize, and intend to use a low power air compressor to blow out the lines. Not sure about this valve, and were the best place is to disconnect a water pipe and place the air compressor.

 

[davidwf]

Looks like its to bypass the pump should the pump fail. I have one in my system which allows a manual pump to be used taht is fitted below the sink.

 

[david_bagshaw]

Perhaps it is to let off the pressure from the system, back to the tank, such as when you leave the boat in the week, to help the accumulator live a longer life.

 

[catmandoo]

difficult to comment with out having a look at the whole system and relative elevations . Could be a tank drain or a means of flushing the tank

 

[alahol2]

It will serve 2 purposes, both of which have been mentioned:
1) bypass the pump and pressure vessel so water can be hand pumped.
2) de-pressurise the system.