Valve between outflow and return pipes

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There is a valve connected between the outflow and return pipes just under my combi boiler. What is this for and should it be open or closed?
 
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CBW

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Without seeing it (photo would help) sounds like a bypass valve.

Depending on which Combi you have it might be needed?
 
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Possibly it's for filling and/or topping up the system. Should be closed unless system requires water. Could be a bypass. Need a pic please to say for certain.
 
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20190928_193123.jpg
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It's a bypass, just open it a quarter turn (using the spindle) and all should be ok.
 
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What would happen if it was fully open or closed?

There is no danger either way but it is a requirement for some boilers as mentioned above. If it is just slightly open you shouldn't have any issues.
 
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A bypass is fitted to allow some circulation through the boiler if all the rad valves close down. Leaving it fully closed will prevent that happening, and could possibly cause system damage. Having it fully open will allow a direct path for the flow from the boiler to take the easy option, and may result in the system taking longer to heat up. Personally would leave it well alone, valve might start leaking from the gland if you tamper with it!
 
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It is a by-pass as already said by others, it is set at install. if everything working as it should forget it is there and leave it alone
 
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