mysterious cross pipe in my central heating

[rkww]

It's a bit tricky to describe without a picture, but I found one close enough on the web...

So, referring to the illustration at:

http://www.gasman.fsbusiness.co.uk/system_basics.htm

I have a system just like the drawing, except there's an extra pipe from the output of the pump, before the motorized valve, (at the the tail of the red arrow below #4 in the drawing) across to the cold water outflow from the immersion heater coil (the tail of the blue arrow below #6 in the drawing.)

The extra pipe's 15mm diameter (the rest of them are 22mm) and it has a stop-cock on it, which is currently wide open.

Any idea what this pipe is for?

 

[AnonymousImport]

Its a manually adjustable system bypass valve. Its purpose is to allow a small flow through the boiler when the motorised valves, or Thermostatic Radiator Valves, are all closed. It can also be used to reduce the pump head at the radiators if pipe nise is a problem. If you rads are all heating up with the bypass valve fully open, then leave it that way. If not, then you might need to close it a bit to increase the circulation to the radiators.
Newer systems have an automatic bypass vavle, which opens and closes automatically to maintain a constant head at the rads. This is hugely beneficial if you are using TRVs. You can find more info on these at:

http://content.honeywell.com/uk/homes/Catalogue/Non Electric/5.1 DU145.pdf

 

[AnonymousImport]

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