In many engines, the thermostat housing serves as the coolant outlet and is located on either the cylinder block or intake manifold. Coolant typically flows out of the housing, through a large hose, and into the radiator. These housings hold the thermostat that regulates the flow of coolant through the engine and radiator. They are typically made of a plastic composite, aluminum, pot metal, or other similar materials. A fiber gasket will typically be used to connect the thermostat housing to the block or intake manifold, though plastic and rubber gaskets also exist.
The main purpose of a thermostat housing is to contain the engine thermostat and provide a coolant outlet to the radiator. In some cases, each radiator hose will be connected to a similar looking housing, with one serving as a coolant inlet and the other as an outlet. The thermostat housing will typically be the one with a larger bulge where it meets the engine, as it has to have room for a thermostat while the other, empty housing is simply an inlet or outlet point.
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In addition to holding the thermostat, these housings may also serve as an air bleed point for the cooling system. A thermostat housing may have a bleed valve that can be loosened to allow the escape of any air that has been trapped in the block. Not every vehicle has this feature, though it can be very useful any time the cooling system is drained and refilled, or flushed out.
When a thermostat is suspected of failure and needs to be inspected or replaced, the housing must be removed. This is typically a simple operation, though some vehicles have thermostat housings that are very difficult or time consuming to reach. The best way to locate the thermostat is typically to follow each radiator hose to the engine and determine which connects to a removable housing. If both hoses connect to a removable component, there will typically only be one that appears capable of containing a thermostat.
In some cases, a thermostat housing may begin leaking and require replacement. Another reason for replacement is if pitting develops on the gasket surface, which may be noticed when the housing is removed to service the thermostat. As coolant breaks down over time it can become acidic and eat away at things like gaskets and hoses, though it may also attack metal, such as the internal components of the water pump or the mounting surface of a thermostat housing. When this happens, the housing typically has to be replaced.