We have just had a new worcester bosch system boiler fitted to a fully
pumped system. In the night we heard lots of noise from all the pipework and
the radiators so i went in the loft today to check something and found the
header tank which tops up the system has quite hot water in it !!

is this normal? i suspect not

what could this be caused by?

apart from the noise which only seems to happen at night, everything seems
ok

cheers

Daz

ps. it has just been converted to a standard Y plan system because we had
no independant control over the hot water so they added a 3-way valve