Can someone please tell me the purpose of the 'by-pass pipe' between the DHW flow and return - white line in the picture - and the gate valve and why the outlet from the cylinder has been made 15mm?
It is a three-port valve (above the pump) fully pumped system.
The bypass is used to provide a minimum flow across your boiler.
The 15mm pipe is used to restrict the flow through the cylinder when both CH and HW are operating together. If you have TRV's fitted to your system, then the bypass needs to be set up. If there are no TRV's then it is not needed and can be closed. A bit more system info would help.
But it is only between the inlet and outlet of the cylinder coil.
I.e. it only reduces, or not, flow through the coil when HW on.
I could understand if it were between other pipes.
The 15mm pipe is used to restrict the flow through the cylinder when both CH and HW are operating together.
If you have TRV's fitted to your system, then the bypass needs to be set up. If there are no TRV's then it is not needed and can be closed. A bit more system info would help.
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