Why Does My Car Heater Blow Cold Air?
By Victor Ivan Coll
Updated April 23rd, 2023
When it is a humid ninety degrees and sunny, you expect your car vents to blow cold air. When temperatures are closer to freezing, you do not. Malfunctions at either end of the thermometer can present problems. An air conditioner that does not blow cold air is at least inconvenient. But it could also make for a potentially dangerous condition in the extreme heat. Likewise, if your climate control system malfunctions in the bitter cold, safety can be compromised. If your heater is blowing nothing but cold air, not only do you feel like you are driving a refrigerator, but you could also face a lack of visibility if the defroster is not working. And a heater that blows cold air could be a symptom of an engine problem.
How does the heater work?
When you turn on the heater in your car, truck, or SUV, you expect warm air to blow from the vents in the dash, under the dash, or above the dash. It seems obvious that there is some kind of electric fan motor that pushes the air into the passenger compartment (there is), but beyond that, where does the heat come from?
Well, unless you drive an electric vehicle (or possibly a hybrid) that features an electric, battery-powered heater, the heat in your vehicle comes from, oddly enough, the engine cooling system.
For optimal performance, your engine is supposed to run in a narrow temperature range — somewhere around 200 degrees. Coming up with the heat is no problem; the combustion process inside the engine creates a good deal of heat. So the cooling system is in place to prevent overheating.
The cooling system circulates a mixture of water and coolant (aka antifreeze) through chambers in the engine block where the heat is transferred from the engine to the liquid coolant mixture. The coolant passes out of the engine, through the radiator (where the heat dissipates), and back to the engine for another go. But some of the hot coolant is diverted to the passenger compartment where it enters another radiator-like component called a heater core. The fan blows air across the warm tubes of the heater core and the warmed air reaches the passengers through the vents.
What causes the heat to stop working?
Now, if part of the cooling system is not working properly, the result could be a lack of heat from the vents.
For instance, if the water pump that circulates the engine coolant fails, warm coolant will not reach the heater core. Likewise, if there is a leak in the system, sufficient coolant will not be circulated and will not get to the heater core. Will low coolant cause no heat? Sure. Therefore, in both cases, the heater core will not heat up and neither will the passengers.
Another problem that could cause a lack of heat is a stuck thermostat. The thermostat is a heat-sensing valve that opens and closes as the temperature of the engine rises and falls. Before the engine heats up (upon startup), the thermostat is in the closed position, preventing coolant from flowing and allowing the temperature to rise. When the temps get to a certain point, the thermostat opens and coolant begins circulating to cool the engine. If the thermostat gets stuck in the open position, the engine may not get up to a sufficient enough temperature to send hot coolant to the heater core. If, on the other hand, the thermostat sticks closed, the coolant will not circulate at all. Oh, and the car engine may overheat.
Other problems related to the cooling system might include air in the system that prevents the coolant from circulating. Or a clogged or leaking heater core. Any of these problems can prevent the heater from blowing warm air.
There are also a few concerns besides the cooling system, such as a stuck blend door on the heater core (a mechanism that blends warm air and cool air to regulate temperatures in the passenger compartment) or a faulty heater control switch.
So, what do I do about cold air coming from the heater vents?
If your car heater will not blow hot air, make sure you get it fixed right away. Lack of heat in the passenger compartment is not just an uncomfortable drive, it is a potential sign of engine trouble. A bad water pump or stuck thermostat can lead to engine overheating which can lead to thousands of dollars in damage. And a foggy or icy windshield can cause a hazardous driving condition which can end in a crash.
Checking the coolant level is simple. If it is low, you can add coolant. But you need to make sure it is the correct type for your vehicle. Remember: NEVER open the radiator or cooling system when the engine is warm. Wait instead for a half hour until the temperature drops and cover the cap with a rag or towel before opening.
The causes of cold air coming from the heater are sometimes easier to explain than to diagnose. Best to have a qualified technician at a trusted repair shop inspect the cooling system and the heater controls to determine the problem and get you safely on your way.
Victor Ivan Coll | Victor Coll | Author: Victor Ivan Coll | Copyright 2023
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