When your car heater blows cold air

october 12, 2020

there are many different ways a car heater can fail, but the problem you describe is usually caused by one of two basic problems. Either no coolant flows through their heater core or the air from the blower motor is not passed through their heater core. There are a number of different underlying causes that can lead to these situations where a car heater suddenly stops working, but they usually deal with either one or the other.

Tuomo vainamo / folio images / getty images

Quick crash course in car heater operation

First of all, everything here applies to vehicles with water-cooled engines. If you drive an old volkswagen with an air-cooled engine or a brand new electric car, you have some kind of electric heater that is either not powered or just broken.

However, most cars on the road still have water-cooled engines, and their heating systems all work on the same basic principle. Hot coolant from the engine flows through a heater core that looks and works a lot like a small radiator, and a blower motor pushes air through it. The air is then heated by the coolant and then in turn heats the interior of the vehicle.

This is the reason why it takes a while for the heaters to blow warm air. Until the motor warms up, the heater core cannot dissipate heat. This is also why a clogged heater core, a stuck thermostat, or air in the cooling system can cause a car heater to blow cold.

Car heater blows cold due to a problem with the cooling system

There are four main problems with the cooling system that can cause a heater to blow cold:

  1. Thermostat stuck.
  2. Air in the cooling system.
  3. Heater core clogged.
  4. Coolant is not flowing through the heater core.

In practice, it’s a little more complicated, but these are the most common problems with the heater that you will encounter.

Thermostats are essentially valves that open and close depending on the temperature of the coolant. To allow the engine to warm up, they remain closed until the coolant in the engine reaches a certain temperature range. And if they do not open at this time, the coolant will not circulate properly, the engine may overheat, and problems may occur when the heater blows cold.

If a thermostat is left open, it can prevent the engine from warming up properly or at least increase the warm-up time. If your heater was blowing lukewarm instead of cold, an open thermostat would have been a possible cause.

Another common problem is when air gets into the cooling system. Since the heater core is often the high point in a cooling system, air can enter and become trapped in the system. If this is the case, the air bubbles will need to be purged to fix the problem.

Clogged heater cores can also cause a car heater to blow cold air. The best way to check this is with a non-contact thermometer, which you can use to check whether or not coolant is flowing through the heater core. If it doesn’t, flushing the heater core will often fix the problem.

Some vehicles have a valve installed in the heater core inlet line that is operated by vacuum or a mechanical cable. If this valve remains closed, this is another reason why a car heater blows cold.

Finally, a heater core can be plugged in several ways. When you hear about a clogged heater core, it usually means that corrosion and other debris have clogged the inner tubes and flushing often clears it up. However, the fins of a heater core can also become clogged with lint, pine needles, and other debris that enters the heater box. The solution to this, of course, is to break open or remove the heater box and clean out the fins.

other reasons why a car heater may blow cold

Most reasons why a car heater blows cold have to do with the heater core, but they can also have a mechanical, electrical, or vacuum problem. The specifics vary significantly from vehicle to vehicle, but most systems have some sort of mixing damper that changes how air does or does not flow through the heater core.

If a mixing valve is stuck, it doesn’t matter if the heater core is working properly. Since the mixing damper is stuck, the heater core is essentially bypassed and you only get cold air.

Of course, there are a number of reasons why a mixing door may stick, and they don’t always stick the same way. A mixer door may remain open, which can cause heat, or remain partially closed, so you only get lukewarm heat.

A mixing valve can also get stuck because a mechanical connection or vacuum line comes loose, a switch is defective, or any number of other reasons. If you suspect a problem with a mixing valve, the specific diagnostic procedure depends on how your vehicle’s heating system is set up.

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