A typical heating and ventilating system, with heater ducts to the windscreen and to the front foot-wells.
The plenum chamber is a box fitted on the bulkhead beneath the windscreen. It takes in fresh air from a grille on the outside of the car.
Inside the chamber, movable flaps connected by cables to the heating and ventilation controls of the car divide and direct the air to the different parts of the interior.
Incoming air is warmed by passing it over the fins of a small radiator, called a matrix, which takes hot water from the engine. The matrix is usually inside or next to the plenum chamber.
Sometimes the supply tubes slip off their connections, or the interior lining of a tube may collapse and block it.
There is usually a simple valve at the base of the plenum chamber which allows water to drip out — either through a flap or a rubber tube with lips that just touch each other. Remove the valve or the tube to clear leaves or debris from the chamber.
To check the trunking, pull it off in sections and look through it. If it is a loose fit on its connector stub, push the trunking well home then bind the joint with sticky tape.
Alternatively, use tape to build up the stub, which will then make a tighter fit with the tube. Trunking that is loose fitting reduces the efficiency of the heating or ventilation.
An incorrect flow of water through the heater matrix may be due to stretching in a cable which operates a water valve on the engine.
Ask a helper to work the hot-cold lever in the car while you check that the lever on the valve is turning through its full travel.
A kink in the heater hoses can also restrict the water flow. If a hose feels squashy it may have collapsed internally, and should be replaced.
An airlock in the heater system or a build-up of sludge in the matrix also reduces heating efficiency.
Disconnect the heater-return hose at the bottom of the heater or at the water pump and run the engine. There should be a good flush of water. A trickle means the matrix is blocked.