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.
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!
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local