Hi all, I have an issue with my central heating system. My system has 2 zones (upstairs and downstairs) and works fine when either zone is selected on its own. The problem occurs when I set both zones "on". Upstairs heats up without any issues but the downstairs rads barely get warm.The house is 2 years old and the problem only started this week.I have an oil system and the boiler is a galvanised/outdoor warmflow system. Any help/advice on this matter would be greatly appreciated.