Why are the downstairs radiators cold when my heating is on?

I have just moved into a three storey house with a Vaillant boiler and a Telford Tornado water tank. When I have the heating on, the radiators in the top two floors are nice and warm, but the ones on the ground floor remain totally cold - including the delivery pipes; there is simply no heating coming downstairs. I have hot water throughout the house, however. Someone said I probably have a zoned heating system. How do I track down the zone controller, if this is the case, or does anyone have any other bright ideas?