Car Heater Blowing Hot Air When Off

Car Heater Blowing Hot Air When Off

While driving on a nice day you may turn your heater off because it’s not needed. However, you notice hot air is still blowing out the vent. After turning the knob to full cool, hot air is still coming out and so you turn off the AC completely. But then why do you still have hot air blowing?

Hot air can flow through the system due to a broken blend door, wrong circulation mode selected, or hot air in the vents. Set the system to recirculation, do further checking for broken parts, or vent the hot air before driving.

I’ll explain how hot air can still blow even though the heater is turned off. We’ll look at how the heater works and some easy tips you can follow to stop this issue from happening to you so that you can get the most out of your vehicle and the air conditioning. 

Why Does Hot Air Still Blow Even Though The Heater Is Off?

Car Heater Blowing Hot Air When Off

One cause of this is what is referred to as ram air. As you drive along with the vehicle, pressure is built up in the intake of the air conditioning system. While some car models close the recirculation door, not all systems do. 

This outside hot air is moving past the blower motor, to the heater core and beyond into the cabin. This can occur even when the system is turned off. It’s less likely if the mode is set to recirculate rather than intake.

You can test what the actual issue is by adjusting the heater system controls. If the air flow is hot regardless of the temperature knob set, then likely the problem is an issue with the blend door actuator.

Warm water is always in the heater core, but most vehicles use a system called a blend door to send air through the core or bypass it. However as some blend doors are controlled by a cable or actuator, if this breaks it can mean hot air still gets into the cabin.

These systems are also controlled electronically and can break in the open position. Further, the sensor or motor system can fail to leave it in the open position. If the blend door actuator keeps the blend door open constantly, hot air will flow into the cabin.

Try changing the circulation mode back and forth and see if you can detect the blend door moving when you adjust the position. You may only be able to hear or feel it from the inside of the vehicle.

With recirculation off, the air is still being forced into the cabin while the vehicle is moving. This is even more common at higher speeds. So if the outside air is very warm it can feel like your heater is pumping out hot air when in fact it is just leaking in from outside.

The heater in your vehicle is getting that heat from the waste heat from the engine. A stuck blend door or other leaks can mean this heat is flowing directly into your vehicle even with the heater turned off.

A similar phenomenon can happen if the car is in direct sunlight which causes the air sitting in the vents to warm up significantly. This can often happen in extremely hot climates or if the vehicle has been sitting in a closed area under the sun’s rays.

One way to avoid this is to set the cabin air system to a full fan with the AC off. Turn off recirculation and set to intake mode while leaving the doors or windows open. Leaving the vehicle like this for a few minutes should vent out the warm air.

Some systems require the system’s temperature set to a full cool setting before turning off the fan. If you do not do this the temperature setting of warm can continue despite the fan being set off and warm air will vent in from the heating system.

The heating system of a car is slightly more passive than the cooling system. This means that even with the heater set to off, heat can still get in without requiring the actual heater to be on.

Where Does Hot Air Come From In A Vehicle?

Car Heater Blowing Hot Air When Off

Car heaters don’t use the car’s fuel during the heating process directly. The cooling process does use gas but the heat coming from the heaters is actually just the heat dissipation from the engine piped into the car’s cabin.

The heater in a vehicle works via the waste heat coming from the cooling system for the engine. It does this via a process called heat exchange. The engine has coolant running through a radiator to cool the engine down. 

After passing through the engine and cooling it down, the coolant becomes heated. This heat is then sent through the heater core and carried out via vents to warm the interior up.

A heater core looks like a mini radiator. The heater core has a hose system inside which carries the hot coolant, with the core acting as a heat exchanger between the coolant and cabin air.

Fins attached to the heater core tubes increase the surface area so that the surrounding air is heated and sent to the cabin.

The car’s interior gets the hot air via a component referred to as the blower motor. The blower pushes the warm air via recycling waste heat from the engine.

The other component which works to finish the cycle is known as the temperature blend door. This controls the mixture of warm and cold air to maintain temperature.

Due to the operation of these various parts, the heater will not work straight away from a cold start. It will take time for the engine to start creating heat once the ignition is turned. 

Gui Hadlich

Hey there! I'm Gui. To be honest, I'm not really that interested in cars. But what I'm really, really not interest in is spending lots of money fixing my car up. Thankfully, I have a father-in-law who's obsessed with cars and a brother getting a PhD in internal combustion engines, so I get to learn about fixing cars. And with Fixing Engines, I hope to help you save a lot of money and take good care of your cars.

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