The air up there

ACCORDING to Mark Lynch of Carcool Airconditioning, the problem is caused by a small vacuum leak in the heater control system. When driving normally the engine supplies enough vacuum to overcome the leak. But when you accelerate hard the engine vacuum drops to zero, which allows the system to default to demist. Firstly, try driving the car with the heater set to warm. If it is OK the fault is in the heater tap, if not it is behind the dash. A quick fix can sometimes be achieved by fitting a vac check valve at the engine supply hose.