The reinforcement fiber in the Volkswagen heater hoses provides them with a way to resist the extreme temperature changes they will have to endure in their lifetime under the hood of your vehicle. Even with these fibers that are built into the heater hoses, the structural integrity of the polymer they are constructed from will begin to break down in time. This is one of the many reasons the owner of any car or truck should do a weekly inspection of the hoses and belts under the hood of their vehicle.
There are several heater hoses under the hood of your Volkswagen, but usually there are two main ones. There is a supply hose that provides the heated coolant to the heater core so the passenger compartment can be kept warm in the cold winter months that is connected to the engine block. There is also a return hose so the coolant has a place to go when it leaves the heater core. This return hose is connected to the radiator. On some engines, more hoses may interrupt and route coolant to other locations as well.
Over time, with exposure to the temperature extremes of the coolant from the engine, the Volkswagen heater hoses will begin to lose their integrity. This is when the circumference of the hoses begins to expand. This expansion process causes the walls of the hoses to become thinner and the source of a potential coolant leak. The most likely place for this ballooning of the Volkswagen heater hose is where the supply hose is connected to the engine block.
If the owner of the Volkswagen wants to inspect this part of their passenger heating system, the engine has to be running with the heater on. This will make it possible for the hot coolant to flow through the heater hoses. If there is a leak in the system, it can then be noticed and the exact location identified.