The 928 heating system uses a vacuum operated shut-off valve at the rear of the engine. It controls the amount of hot coolant that flows through the heat exchanger in the center console. By default the valve is open, it is shut off when no heat is needed by applying vacuum. The heating valve is known for two problems: one is the vacuum pod that operates the shut-off leaks, causing the valve to be completely inactive and a full flow of hot air is always present. The second problem is that the valve works, but leaks. So a partial hot air flow is always present. Not nice if you try to keep the interior cool with the AC while the heating is actively heating up the cabin. The only solution is a new valve. (You may want to replace the short rubber hose that attaches to the engine as well, they often deteriorate and start to leas quite suddenly).