what is this bypass for

We have just moved in to a 12 year old house with a system comprising of a gas boiler with header tank and hot water cylinder also with header tank, there are no TRV's
in the aiting cupboard around the 3 way valve is what appears to be some kind of bypass - highlighted in yellow - .... what is it for and how should it be set ?.... it is presently fully open and as such surely bypassing the cylinder heating coil, the boiler is cycling on and off fairly quickly and the water takes ages to heat up!

The idea is that if all the thermostatic valves on the radiators close there is somewhere to flow (given the hot water isn't on)

That makes sense.....except that there are no TRV's fitted !
I my previous installation, I left two rads without TRV's for that exact reason.....
so should the bypass be set to allow slight flow or fully closed........even with the HW "off" the 3 way valve will still be open to one of the circuits and here we have no TRV's

The gate valve should be opened just enough that when the 3-way valve is in its 'off position' the minimum flow specification for the boiler is met. This is all good theory but difficult to achieve in practice without looping in a temporary flow meter or a couple of pressure gauges.

Instead, I'd bring the system up to current Building Regulations (BR) standard, which is to replace the manual gate valve with an automatic bypass valve (ABV). While you're at it I suggest you (a) check the system for sludge and flush if necessary, (b) fit TRVs as required by BR and (c) consider sealing or semi-sealing the system.

davidwf said:

That makes sense.....except that there are no TRV's fitted !
I my previous installation, I left two rads without TRV's for that exact reason.....
so should the bypass be set to allow slight flow or fully closed........even with the HW "off" the 3 way valve will still be open to one of the circuits and here we have no TRV's

Click to expand...

By pass shut with system as described.