A heater control valve is simply a water valve which allows more or less water to circulate through the heater core, depending on how much the valve is opened.
When the engine warms up it heats coolant (mostly water) in the water jacket of the engine. To keep the engine from overheating, the water pump circulates water through the radiator once the engine temperature reaches the set-point of the thermostat.
When it's cold outside the engine STILL reaches a temperature that allows you to make use of the engine heat when you open the water valve of the heater.
Heater control valves take many forms but basically they all do the same thing; they open to allow heated engine coolant to circulate through the heater core or they close to stop the coolant from flowing.
Heater control valves can be manual (cable controlled or even lever controlled), vacuum (which allows the engine vacuum to control the position of the heater control valve based on where you set the control knob) or electrical which allows for greater flexibility and even allows the auto manufacturer to connect the heater controls to a computer or other electronic control device.
faulty control valve on heater or air in the lines
Sometimes, the best way to fix a heater control valve on a 1996 Town Car is to put in a new heater control panel. A used heater control panel can be found in an automobile salvage yard for less than the cost of a new one.
If your not getting any hot air in the car it will be the control valve, its about 28 quid. very common failure
the heater core, heater core hoses or heater control valve is leaking
It partly depends upon what sort of heater it is, but if it's a car heater, the valve alters the amount of hot water (from the car's cooling system) entering the heater matrix. This would also be true of a water-filled central heating system.
Defective heater temperature control valve.
a "t" in the hoses at the rear, top of the engine has a restrictor built into it no "valve" and no bleeder on this car.
Check for a thermostat that is stuck open. Also can be a clogged heater core, or defective heater temperture control valve.
For a car as old as a 1985 model, it could be that the heater blower motor control module has gone out. A new one for a 1985 Corvette is around $165 to $175 at auto parts stores.
bad heater core, heater hose blockage, low coolant, bad heater valve, bad control head. With my car, it was Heater matrix clogged with radiator sealant. The workshop cleaned it and it's start working fine!
Does the fan work? if not, check fuses. if does work, check coolant level first. Next, check heater valve in engine compartment, on fire wall, has ~ 1 inch hoses connected between the engine and to the heater core inside the passenger side. Have someone turn temperature knob inside car and observe cable on Heater Valve is moving. (Can be simple as the screw on top of heater valve loose or cable maybe stuck). Heater valve can be stuck closed. Check hose gets hot between valve and heater core.
The best bet is that the valve that controls the flow of water to the heater core is not functioning, but on some cars it's the flaps that control the air flow through the core.