The coolant housing or the thermostat housing is the connection point between the engine and the radiator of a water cooled engine. The coolant flows out of the engine, through the coolant housing and into the radiator where it is cooled by a steady stream of air as well as auxiliary fans. As well as acting as the interface between the engine and radiator, the housing contains the thermostat which regulates the flow of the coolant. Coolant housings are made from plastic or aluminum.
The purpose of the coolant housing:
The coolant housing serves multiple purposes; it is the physical connection via a hose between the engine and radiator and it houses the thermostat. Depending on the vehicle manufacturer, the design may be such that there are two housings; one as an inlet for the coolant and the other for the coolant outlet. The housing that contains the thermostat will always be the larger of the two.
As well as housing the thermostat and a connection point for the radiator hose, it may also serve as a point where air can be bled from the system. Periodically air can be trapped in the engine block cooling channels, this can happen when the cooling system is flushed, drained and refilled.
Replacing the thermostat or a damaged housing:
If the thermostat fails the coolant housing has to be removed. For your mechanic this is a straight forward task although, once again, based on the design of the engine, it can be located in an awkward position.
Although thermostat failure is most common, the coolant housing can also fail. The housing could break, perhaps the metal will begin to pit and the gaskets will no longer seal properly. Over time, coolant can break down and begin to attack metal and even rubber. Replacement coolant housings are inexpensive and readily available.