House heat will either have a mechanical valve on the radiator or an electrically driven valve with a wall mounted thermostat.
A car will have it in the or on the engine where the outlet hose connects (this is most common)
The thermostat is in the radiator. Open the radiator lid and pull the thermostat out.
The coolant is pumped from the thermostat housing into the top of the radiator but not until the thermostat opens.
The thermostat is located in the upper radiator hose. The radiator hose will need to be removed to get to the thermostat.
Follow the lower radiator hose from the radiator to the thermostat.
The thermostat is located on the front bottom of the engine. You can follow the radiator water hose from the radiator to the thermostat.
Follow the top radiator hose from the radiator to the engine, where it connects is the thermostat housing, the thermostat is inside.
Drain the radiator into a container. Remove the upper radiator hose at the engine and remove the 2 bolts in the thermostat housing. Remove the thermostat and clean the surfaces. Install the new thermostat ,upper radiator hose and refill the radiator.
Open the hood, locate the upper radiator hose on the radiator, follow the hose from the radiator, it will lead you to the thermostat housing on the engine, the thermostat is located under the thermostat housing.
Drain the radiator. Remove the lower radiator hose from the motor. Unbolt the hose inlet, and remove the thermostat. Replace the thermostat and the inlet, the hose, and refill the radiator.
Follow the upper radiator hose from the radiator to the engine. It is attached to the thermostat housing. Removing the thermostat housing will expose the thermostat.
Follow the upper radiator hose from the radiator towards the engine. The upper radiator hose is connected to the thermostat housing on the engine side. The thermostat is under/inside the thermostat housing. Remove the upper radiator hose from the housing then remove the two bolts holding down the thermostat housing then remove the thermostat housing to expose the thermostat.
Follow the upper radiator hose from the radiator to the engine. It will lead you to the thermostat housing. Drain coolant from radiator, remove upper radiator hose from engine, remove the two bolts holding the thermostat housing to engine, remove housing, remove thermostat.