Found this on another size, not sure if it works:
There is an additional valve in the coolant circuit to the heater matrix.
This valve stops flow of coolant through the matrix when the heat control is set to fully cold.
It is a round disc shaped object, about 2" dia., with a single vacuum pipe coming out of the top, and is mounted on one of the two coolant pipes where they run through the bulkhead (engine side) and into the heater box.
Try disconnecting and plugging this vacuum hose and seeing if the heater then works.
If it does, then the vaccum control system for this valve has a fault.
If it doesn't, then either the valve is stuck or there is a blockage or air lock in the heater matrix or pipes to it.
Jarvster, November 2010