Hotly recommended: our cooling circuit tips

If there are problems with the temperature of the cooling water after changing the thermostat, it is usually because of air in the cooling circuit. Before you focus your suspicion on the new part you’ve just installed, you should always first ventilate the cooling circuit.

Falsely accused: the new part

When the cooling circuit is open, cooling water escapes and air enters the system.

If insufficient heating and cooling capacity is experienced after replacing a coolant thermostat, suspicion frequently falls on the new thermostat—often wrongly so, since the defect may also have arisen during the repair itself! When replacing the thermostat, the cooling circuit of the engine needs to be opened, causing cooling water to escape and air to enter the system (see Figure 1). If these increasingly complex systems are not completely ventilated after the repair, entrapped air remains in the circuit. This not only results in inadequate cooling water circulation and thus poor heating and cooling performance, but can even ultimately lead to serious engine damage. This is because the trapped air can potentially cause local overheating.

Reliable assistant: the vacuum-venting device

By means of a standard workshop compressed air connection, a vacuum-venting device (see Figure 2) creates a vacuum in the cooling system (see Figure 3). The air is thereby completely evacuated from the circuit and new coolant is sucked in free of bubbles by the vacuum. This ensures that all isolated air traps are removed from the cooling circuit.

  • Standard vacuum-venting device.
  • A vacuum is created in the system by means of the Venturi effect.

You will find more helpful information about the assembly of thermostats and our product range here:

Technical poster [PDF; 682 KB]
Product brochure [PDF; 1405 KB]

Should you use your air conditioning system in winter?

Definitely! It might seem a strange thing to do at first, but it makes sense. Why? Here are four super-cool reasons:

Clear view

Air conditioning systems don’t just cool, they also dry. The moisture contained in the air condenses on the surface of the evaporator, thus preventing fogged-up windshields—important for road safety.

Perceived temperature

With the majority of air conditioning systems, you don’t need to worry about the windows icing up. Temperature sensors measure the outside temperature and prevent the air conditioning system from running under 4°C and the condensation water from freezing.

Running smoothly

Turning the compressor on ensures that all parts in the air conditioning circuit are lubricated with oil. The gaskets and hoses remain flexible.


For the system to work reliably, it is important not only to service the A/C regularly, but also to change the cabin air filter annually. This prevents contaminants, such as mold, from forming on the evaporator.

Whether it’s winter or summer

Turn off your air conditioning system five to ten minutes before you arrive at your destination. This allows the evaporator to dry and minimizes the formation of bacteria.


We regularly provide technical tips relating to the powertrain, thermal management, and mechatronics in automobiles.


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