Heater valves are water control valves that open or close to allow more or less heated coolant into the heater core. They can be mechanical, vacuum or electronically controlled.
If the heat on your vehicle is regulated by a heater control valve, when it fails you may notice either no heat at all or constant hot air coming from the vents.
Replacing a heater control valve should not be a very difficult repair, but it will require draining the engine coolant below the level of the valve to prevent a mess!
We stock and sell a wide variety of quality heater valves from these manufacturers: