Golf initially came in for a coolant leak, and no hot air coming through the heater vents in the car. We identified the main fault as the water pump leaking. However, as a result of this the cooling system has been overheating, causing the silica bag in the expansion tank to burst. Silicon has contaminated the cooling system, blocking the heater matrix. This is a very common fault with the Mk7 Golf.
We replaced the expansion tank, heater matrix, water pump, and thermostat regulator. Refilled the cooling system with fresh antifreeze.
Once silicon is in the cooling system its incredibly difficult to extract completely.