Home Heating Systems?

I've turned my Vaillant Turbomax boiler onto summer mode but when I open a tap the system fires up and stays that way and the flue outside is like it is during the winter when the central heating system is on. the only way I can stop this is to switch the boiler off then on again and hope nobody uses a tap in the house. any ideas why, it usually only fires up now and again during the summer.