Signs of a Bad Heater Control Valve

The heater control valve (or hot water valve) opens to allow hot coolant from your engine to flow through the heater core. Air from either inside or outside the vehicle then passes through the hot heater core, which warms the air before it enters the passenger compartment of your vehicle.

By adjusting the heater control valve, you can allow more or less heat into your car’s interior. The heat can be used to keep you and your passengers warm during cold weather, to clear your windows of ice, snow, or condensation, or both.

Not all cars have a heater control valve

Some vehicles use a blend door, which is a motorized partition between the heater and the fresh air/AC systems. The motion of the blend door balances the amount of hot and cool air coming through the interior vents, based on the climate control system settings chosen. In a blend door system, the heater is always on, so a heater control valve is not necessary to regulate its output.

Symptoms of a bad heater control valve

A problem with your heater control valve can be very a big issue, if it happens during cold or rainy weather.

Because your heater control valve is part of your car’s climate control system, your ability to keep your windows clear and yourself warm can be affected. If you can’t see out or are too cold to drive properly, a bad heater control valve could potentially cause a loss of control and a crash. Many factors can cause a bad heater control valve. Here are some of the warning signs of a bad heater control valve:

• No heat comes out
• Heat is always on and you can’t turn it down
• Heater operates erratically, putting out more or less heat without any control changes
• Coolant leaks
• Low coolant level
• Higher-than-normal temperature gauge readings (from loss of coolant)

Repairing a bad heater control valve

While a lack of interior heat from a bad heater control valve is a serious enough problem, the real danger from a bad heater control valve is the possibility that enough coolant could leak out to cause your engine to overheat.

Your mechanic will thoroughly inspect the cooling and heating system of your car, to verify that your problem is limited to a bad heater control valve. If it is, the bad one can be removed from your car, and then replaced with a new heater control valve. There could also be an issue with the control system that activates the heater control valve. This would be a cable, a vacuum valve or hose, or an electronic device. Repair or replacement of the affected parts should restore the control system’s functionality.

Your mechanic may also recommend that you replace some of the coolant hoses that run to and from the heater control valve, if they look worn, cracked, or clogged. Any other issues uncovered during the check of your cooling system should be dealt with now, before they can cause you problems down the road.

Repairing a bad valve can be tricky

Unless you have had extensive experience in working with automotive cooling and heating systems, leave this to the professionals! The proper repair of a bad heater control valve involves a thorough understanding of both cooling system plumbing issues and the controls that make the heater control valve work.

These control systems are pretty simple if they happen to be cable-controlled. But if you have to troubleshoot vacuum-powered systems, or electronic computer controls, it will get very complicated, very fast! Your mechanic has the experience, the proper tools, and the diagnostic equipment to replace your heater control valve safely and cost-effectively.