Central Heating Pump

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
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It is balance pipe John. It is there to make sure that there is always a flow of water through the boiler, even if it is somewhat restricted, when all the motorised valves are opening and closing and the pump stops and starts, and it just makes sure that there is that flow to stop the water pressure, mind it's heated and expanded, from hammering the system to bits. Start it at about half way, turn it all the way in till it stops. Now turn it all the way out but count how many times one of the head stocks passes a certain point. Now turn it in half that amount. So if it did 6 full rotations, then turn it back in 3 full rotations.
If you hear the system giving a thump when the demand for heating or hot water is asked for, then give it one turn out. If it seems as though it is taking to long to heat the water, then turn it one in, and so on.
Hope this helps.
--
BigWallop

http://basecuritysystems.no-ip.com
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Yes, your explanation has been extremely useful. Many thanks.
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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
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