Think I've just realised what that tap is for. After isolating the inlet it should allow the pipes between there and the tap/boiler to be drained instead of water pissing everywhere when the compression fitting is undone.
@Brun started
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Think I've just realised what that tap is for. After isolating the inlet it should allow the pipes between there and the tap/boiler to be drained instead of water pissing everywhere when the compression fitting is undone.