Car Heater Only Works When Driving

Car Heater Only Works When Driving

Cold winters and early mornings mean you want to switch on your car’s heater for some warmth. However, until you’ve been driving for a few minutes, you only get cold air. The heater seems to work after a few minutes. So why does the car heater only work when driving?

Car heaters use the heat dissipation from the engine to heat the cabin. Driving for a few minutes produces enough engine heat to allow the heaters to work. Issues with the engine’s cooling system or faulty ducts cause heater malfunctions.

Understanding why the heater doesn’t work without the engine requires understanding a car’s heating system. We’ll also look at signs that your heating system is having issues which is why your heater doesn’t work.

How Does The Car Heater Work?

Car Heater Only Works When Driving

The heat produced by the engine is the mechanism behind the heating system in a car. The cooling system that keeps the engine temperature at a safe level is used to heat up the air. 

Coolant made hot from the engine is piped through a heater core from where it is transferred to warm up the cabin. In this way, it is not gas that is heating up the vehicle via a phase change like the cool air but using recycled waste heat from the engine.

The heater core takes the coolant where it is exposed to air. The heat transfer raises the temperature of the air. The heater core is a component that looks like a small radiator. From here the air is blown by a fan and carried out via vents to warm the interior up.

The temperature blend door allows the mixture of air by opening and closing to allow more heat in. Normally such heat would be vented outside. The heat is just being redirected from the engine into the cabin. 

This simply means that your car heater will not work from a cold start. When an engine is started from being idle, it will take time to warm the coolant up.

Electric vehicles are able to run the heater immediately. Unlike cars with an engine run on fuel that produces waste heat, electric vehicles don’t put out much heat from their engines. 

However, an electric car’s heater will consume a lot of the battery’s energy.

What To Do If The Heater Doesn’t Work At All

There are some simple tests to perform to try and narrow down what the issue is.

If you turn the fan on, you should be able to feel it blowing through the vents. These fans will be run by the battery so any electrical issues may prevent the fan from blowing.

Certain doors and valves need to be open within the heating system to allow the heat to get to the heater core and cabin. If you can feel a very weak heat at the vent it may be that the temperature blend door or other component is not opening fully to let the heat in.

If the fans have issues, you may still get warm air into the cabin because of air pressure, sometimes referred to as ram air. As you drive the car, pressure is built up in the air conditioning intake of the air conditioning system. 

This air pressure forces air through into the cabin which can also push heat into the cabin. This is particularly the case if the mode is set to intake. You can set it to recirculation mode and test if it still happens.

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.

Air bubbles can form in the system preventing air from traveling to the cabin.

Pop the hood and find the highest spot in the cooling system. Disconnect a hose or find some way to bleed the air out. Bubbles always go to the top, so this is the best place to target.

The overflow tank will be labeled or on top of the system. If this is at least half full it should be a backstop on which air pressure acts. This will push air bubbles along the system so that they get expelled.

Another issue is with the engine coolant itself. If the cooling system for the engine is malfunctioning this may result in the coolant not undergoing the heat exchange process properly. If the coolant doesn’t heat up, it’s got no way to transfer this heat to the cabin.

You must check the coolant levels when the engine is cold. If it is low you may need to perform a full reflush which should remove any air pockets.

There may be leaks in the hoses or faults with the heater core. Check the connections and joins, followed by getting soapy water in a spray bottle. Spraying the mixture on the hoses can help show up bubbles that will be betrayed by the bubbles forming.

How Can I Warm My Car Up Quickly?

Car Heater Only Works When Driving

To get the most out of your heater, you should set it to recirculate mode. Turn the vehicle’s engine on and the heater on. The fan is not strictly necessary but it may help blow the hot air with more force.

Putting more pressure on the engine will result in it getting hotter. The hotter it gets, the more the coolant has to work to maintain the engine temperature. This means that the coolant will get hotter, upon which heat exchange to power the heater will be greater.

Driving for a few minutes should be more than enough to get the heater running at maximum heating capacity.

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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