vizo 24 combi boiler problem ?

I have got a vizo 24 combi boiler fitted and it works fine when i turn the hot tap on, but goes off when i turn on to the rads when the boiler is reset and i turn it on to the rads the pilot light comes on and it fires up for arount twenty seconds and then cuts out but hot water still comes from the taps.
does anyone know what the problem is please