Just noticed that our 2 year old central heating system is not working correctly. When downstairs underfloor heating comes on, upstairs radiators go cold. Overcome this partly by setting underfloor heating to come on early hours of the morning so by the time upstairs radiators comes on, downstairs has reached desired temperature and underfloor heating has gone off. We have a condensing boiler which is not showing any faults and both heating systems work fine individually. Any idea what might be causing this?