Puzzled about pipe from Indirect coil?

[jirams]

I have Baxi BBU 552 with gravity fed indirect cylinder. The Boiler 28 mm flow enters top connection to coil in cylinder and a 22 mm continues on up from it into attic which I presume is the expansion overflow. What is puzzling is me that a 15 mm pipe is connected to the 28 mm coil return where it leaves the cylinder. This 15 mm rises vertically into attic and has a hand stop valve on it.

Any suggestions as to what this 15 mm pipe with manual valve is for??

 

[PlumbingSucks]

cold feed

 

[jirams]

Please forgive my very senior moment in asking such a stupid question - cold feed it must be !!!

I'll get me coat !!