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When the temperature outside begins to drop, it’s important to have a functional heater inside of your car. On some vehicles, a component called the heater control valve plays a vital role in heater operation—and keeping your cabin warm. 

How Does a Heater Control Valve Work?

heater control valve diagram
An example of a heater control valve diagram.

Some vehicles (mostly older models) have a heater control valve. As its name implies, the heater control valve controls the flow of coolant through the heater core. 

To have a better understanding of the heater control valve, it’s important to first know how your car’s heater works. 

How Your Car’s Heater Works 

Your car’s heater relies on hot coolant—from the engine’s cooling system—to warm the cabin. A typical cooling system has four primary components: the water pump, radiator, thermostat, and heater core

car front revealing large radiator and intercooler
A typical cooling system has four primary components: the water pump, radiator, thermostat, and heater core.

When the vehicle is running, the engine-driven water pump moves coolant through the engine (where the coolant picks up heat) and toward the thermostat. The thermostat blocks coolant flow to the radiator until the engine reaches operating temperature. Then, the thermostat opens, allowing coolant to flow from the engine to the radiator. After being cooled in the radiator, the coolant returns to the water pump to repeat the cycle. 

The water pump also forces hot coolant through the heater core during the cooling system’s cycle. A pair of hoses connect the heater core to the cooling system. When the driver turns on the climate control system’s fan, a blower motor forces air across the heater core, transferring heat into the cabin.

car water pump in focus
When the vehicle is running, the engine-driven water pump moves coolant through the engine and toward the thermostat.

The Role of the Heater Control Valve 

On most vehicles, coolant flows through the heater core whenever the water pump is turning. A blend door, which routes airflow toward or away from the heater core, regulates the temperature of the heat coming out of the vents. 

But there are also some vehicles that use a heater control valve to manage the flow of coolant through the heater core. Depending on the system design, the heater control valve may be normally closed (blocking coolant flow until activated) or normally open (allowing coolant flow until activated). 

Some heater control valves are operated via a cable or engine vacuum, while others are electronically controlled. 

The video below further discusses heater control valve operation:

How Do I Know If My Heater Control Valve is Bad?

An inoperative heater is the most common sign of a faulty heater control valve. Heater control valves can also develop coolant leaks. Of course, other faulty components can produce the same symptoms, so you’ll want to test the heater control valve to ensure it’s to blame. 

How Do You Test a Heater Control Valve? 

Do you suspect your car’s heater control valve might be faulty? To be sure, you’ll want to perform a thorough diagnosis before replacing the valve.

Note: The method for testing the valve will vary, depending on the system design. So, it’s a good idea to consult a repair manual or repair database before you begin testing. 

Begin by performing a visual inspection of the valve, looking for leaks and other obvious signs of damage. If everything looks okay, you’ll want to move on to checking the temperature of the valve’s inlet and outlet hoses by doing the following: 

  • Bring the engine up to operating temperature and switch the heater to the hottest setting. 
  • Check the temperature of the valve’s outlet hose (between the valve and the heater core)—it should feel nearly as hot as the valve’s inlet hose.  
  • If the outlet hose is noticeably cooler than the inlet hose, the valve is likely stuck closed, preventing coolant from circulating through the heater core.
Replacement Heater Valve
The method for testing the valve will vary, depending on the system design.

But before condemning the heater control valve, you’ll want to make sure that its control system is operating properly. 

  • If the valve is cable-operated, ensure that the cable operates smoothly without binding. 
  • If the valve is vacuum-controlled, ensure that the vacuum line is properly routed and intact.
  • If the valve is electronically operated, ensure that the valve’s control circuit is working properly. 
Heater Valve Direct Fit
Perform a visual inspection of the valve by looking for leaks and other obvious signs of damage.

Heater Control Valve Location

The heater control valve is typically in line with the heater core’s inlet hose, which is often near the vehicle’s firewall, but the exact location of the valve will vary by vehicle. To determine the location of your car’s valve (if it has one), it’s a good idea to consult a repair manual or repair database. 

Can You Bypass a Heater Control Valve? 

Some people opt to bypass a faulty heater control valve rather than replace it. Unfortunately, bypassing the valve can cause several issues. 

mechanic performs hood inspection
Bypassing the heater control valve is not a good idea because the modification compromises the cooling system’s integrity.

If the valve is removed and the hose is connected directly to the heater core, hot coolant will be running through the core all of the time, potentially resulting in climate control performance problems. On the other hand, if the valve and heater core are both bypassed, the vehicle’s heater will be inoperative. 

In either scenario, bypassing the heater control valve is not a good idea because the modification compromises the cooling system’s integrity. Because heater control valves are generally fairly affordable and easy to replace, your best bet is to replace the valve rather than bypass it.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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