john wrote in message
...
Around my central heating pump there appears to be what looks like a
by-pass
loop (with 15mm pipework) containing a gate valve. What's the purpose of
this loop and what should be the setting of the gate valve (fully open,
fully closed or somewhere in between) ?


MID
=== POSITION ===
VALVE
||
||
||
||--------------
|| |
PUMP O - Bypass loop with gate valve
|| |
|| || =======
|| ||
Flow || || Return



You are sure that you've got a single mid-position valve - and not two
separate 2-port valves?

A by-pass loop ensures that hot water from the boiler *always* has somewhere
to go - under the circumstances where the hot water and heating demands are
both satisfied and the boiler shuts down, but the pump continues to run
until the boiler has cooled down to below its over-run thermostat
temperature.

Under most circumstances, with a mid-position valve, you shouldn't need a
by-pass loop (I certainly haven't got one!) because the valve can never
close both outlets at the same time - unlike two 2-port valves which *can*
both be closed and thus make a by-pass loop essential.

I suppose if all your rads have thermostatic valves - or if you are in the
habit of turning them all off manually - a by-pass could have its uses.
Otherwise, I'd be tempted to close the gate valve on this loop and see what
happens.

Roger