Citroen Xsara Picasso heater comes on when i accelerate

Could check the heater resistor and wiring .Almost sure it was on that car that there was problems with plug / connection melting .

do you mean hot air in car when driving but goes cold when car at idle speed check coolant level if low check for coolant leak rectify fill and blead out cooling system water pumps tend to leak not easy to check as behind timing belt cover