On Dec 4, 3:56 pm, jaime wrote:
Hi.

I've spent a few hours recently up in the loft, trying to understand how
my central heating/hot water plumbing works. One point that baffles me
is a copper pipe connecting the flow pipe (from the boiler) back to the
return pipe (to the boiler), situated about a meter before the two pipes
enter/exit the (indirect) copper tank. This "connection" has a
wheel-handled valve half-way along it (I assume, from the wheel-handle,
that it's a gate valve).

Do you have thermostatic radiator valves (TRVs) on all your radiators?
The pipe is a bypass to allow some flow when all of the TRVs are closed
but the boiler hasn't realised yet. The alternative is to have one
radiator (e.g., in the bathroom) that is always on and doesn't have a
TRV.

It needs to be open a little. I can't tell you how much "a little" is,
but I'm sure someone will be along shortly.

MBQ