How Does Defrost Work in a Car?

When driving in the cold, it’s not uncommon for condensation and fog to form on the windshield. Once the warm air inside hits the cold glass on your windows, it turns into small water droplets that make it hard to see through. You can always wipe them off using a microfiber towel, but that’s too much of a hustle. This is why cars have defrosters. So, how does defrost work in a car?

A car defroster is also referred to as a demister or defogger. Most cars have two types – a primary defroster for the front windshield and car windows and a secondary defroster for the rear windshield. The former uses the HVAC system while the latter uses a mechanism called resistive heating to clear a foggy or iced windshield. Here’s more on how these two operate.

How does defrost work in a car (Primary defrost and secondary defrost)?

How does defrost work in a car

1.     Primary defroster

The primary defroster uses the HVAC’s blower motor and fan to draw in air from outside the car. The air is passed through the heater core where it comes out as hot dry air.

It is then passed through the vents located at the front edge of the dashboard from where it hits the windshield.

It also comes out through the car’s ventilation system to hit the windows. This melts any ice that forms on the glass and removes condensation from the surface.

2.     Secondary defroster

The secondary defroster works differently to defrost the rear windshield and side mirrors. If you take a look at your rear windshield, you’ll see wire grids that run from left to right. These are wired through the roofline and under the headliner.

They pass current through the windshield, heating the glass to displace condensation on the inside. The heat isn’t too much and you’ll barely notice it. It’s just enough to remove the moisture that accumulates on the window.

If you were wondering how does defrost work in a car, that’s the summary. The next part is knowing how to use the feature.

How to use windshield defroster

The windshield defroster setting in your car has two buttons – one for the front windshield and the other for the rear windshield. The front windshield defroster symbol looks like a curved window with three arrows pointing inside. The rear defrost button has a rectangular symbol with three curved arrows pointing to the side.

Here’s how to use your windshield defroster:

  1. Turn the temperature setting to high or to the right. This allows warm air inside.
  2. Switch on the AC.
  3. Turn on the defrost buttons for both the front and rear windshields.
  4. Set the fun to high. This brings in more warm dry air inside.
  5. Make sure the recirculation setting is off. This prevents the damp air inside from recirculating.

When should you use defrost in a car?

You should defrost your car when your windshield and windows start to become foggy. This is typically when it’s raining outside and you’re driving down the highway. The warm humid air inside the car gets into contact with the cold glass on the window.

It condenses to form small droplets of water that inhibit your vision. The best time to defrost your car is when you notice the fog forming on your windscreen. Simply use the car defrost feature to clear the condensation.

How long should you defrost your car?

Turn on the defrost setting on your car and wait for about 5 – 15 minutes for the windshield and windows to warm up and clear the fog on them. You will see your vision become better and clearer as the fogginess disappears. Once you answer the question how does defrost work in a car, using it is quite easy.

Do you defrost your car on hot or cold?

You should start by defrosting your car on hot. Set the temperature and fan to high then turn on the front and rear windshield defrost settings.

Wait until all the condensation has cleared on your windshield and windows then turn the defrost setting to cold. This prevents the warm air that is sent into the cabin from forming the fog again.

How to test defrost in your car

If you find that the defrost button in your car is not working, turn the key to the ON position and use a test light to test the rear windshield defroster grid.

Connect the test light to a good ground and test each of the wires crossing the rear windshield. The test light should light up when you touch the wires. No light shows that there is an open in the grid.

Locate the rear windshield defroster relay in your car. Check online to know where it’s located. Give it a visual inspection once you remove it from the fuse box.

You want to see if it has any physical damage. Test it using a digital multimeter for resistance and continuity. This will tell you if the relay needs to be replaced or not.

Does windshield defrost use AC?

Yes. The defroster activates the compressor which is part of the AC. The compressor is connected to the AC through a series of belts that link the alternator, water pump, and other parts to the engine.

Your car’s defrost feature also utilizes the same blower motor that is used to suck in air and the same heater core that heats up the air in the AC system.

Read more: How to Replace Ambient Air Temperature Sensor

Does the defrost use more gas?

Yes. When the defrost feature is turned on, it causes additional drag on the engine. This makes the engine demand more fuel to compensate.

If you’re driving down the highway and decide to use the defroster, the engine will need to run more so that it can provide power for driving while keeping the defroster functional.

Some vehicle owners experience a slight change in fuel economy when using the defroster in cold weather.

Does compressor run on defrost?

Yes. The compressor runs on defrost to compress the refrigerant so that the temperature of the outdoor coil can increase. Even though it will make fewer cycles, it still runs to draw in air.

The air is sent to the AC evaporator where moisture is removed. It then flows through the heater core and the vents to heat the windshield.

Read more: How Long Do Car AC Compressors Last?

How do I defrost my car quickly?

Mix two-thirds of isopropyl alcohol or rubbing alcohol with one-third of water. Put the mixture in a spray bottle and spray it on the windshield in the morning. The fogginess and ice will disappear immediately.

You can also get a 12V portable car defroster on Amazon or from the nearest auto parts store. This is a small device that produces heat inside the cabin to help defrost and defog windows.

It isn’t as efficient as the car’s defroster as it doesn’t provide full-blown heat. It only works where it’s placed. It’s a great alternative for cars with broken heater cores.

Is deicer bad for your car?

A deicer is not bad for your car as long as it doesn’t get into contact with the engine oil, coolant, and car interior. Ideally, you should not spray deicer near the fluids that help your car to run. It does not mix well and can cause severe damage to your car.

The deicer spray melts ice and prevents its formation. It does so by inhibiting a bond between the surface and ice and by lowering the freezing point of water. You do not want it to lower the freezing point of your fluids.

Can defrost crack your windshield?

No, it can’t. Defrost warms the windshield and windows up gradually to clear the ice and condensation on them. You should never pour hot water directly onto a windshield that is covered with ice.

The thermal shock can cause it to crack. This would be a very expensive replacement that you can’t drive without.

Read more: How to Fix Scratches On Your Car Windshield

On That Note

If you were wondering how does defrost work in a car, I hope this article has helped answer your question. As always, remember to take your car for its scheduled maintenance so that it can perform at its best. If you have any problems with the defrost feature, reach out to a certified mechanic and have them take a look at it. Stay tuned for more informational guides.