Air blower not heating air

Hello,

The air blower in my Focus no longer heats up the air - just blows out
cold 
even with the switch turned to maximum on the red.

Bit of a pain trying to demist/defrost the car on these icy mornings with
no 
hot air to help!

Does anyone know what might be causing this? Haynes manual doesn't cover
it.

Thanks in advance.

Jim