Car Windows Fog Up When Heat Is On (Secret Fix)

One of my first cars was a small Italian, rear-engine, two-seater. Great memories, I loved that little car, but boy would she fog up, and not just on Saturday nights either.

To clear foggy windows quickly, press the windshield demist button. If your car doesn’t have to demist function, follow these five steps:

Set the fan speed to max
Turn temperature to max
Set the air distribution to windshield
Turn air recirculation off
Press the a/c button on (if fitted)

Foggy windows are a nuisance, damn Sat Nav won’t stick, but they are also dangerous, especially at junctions. In this post, I’ll cover the reasons your windows are fogging up when you turn on the heater, how you can fix it, and also the secret tip.

Quickly Clear Foggy Windows

If you have a modern or a car made in the last twenty years, you’ll have an HVAC (Heating Ventilation Air Conditioning) aka climate control system. The HVAC system is a heater that’s controlled by a computer. The computer knows the best settings for various weather conditions but can be overridden.

To quickly clear your foggy windows, you have 2 choices, use the automated demist function (easy) or set the controls manually.

Option 1 :

Find and press the fully automated demist button, and that’s it! (as per pic above).

All modern cars have an automatic demist button, see picture above. Pressing it hands control over to the HVAC controller (computer). It knows the best and quickest blend of settings to clear the windows. It will clear the windshield as a priority. The process takes a minute or so if it’s functioning correctly.

Option 2:

Set the controls manually.

Turn fan to max (doubles as the “ON” button)
Set temperature 1 & 2 to max
Direct the air distribution to windshield
Turn off air recirculation
Turn on air conditioning

It’s very important to have both your air recirculation button set to “Off ” and your air conditioning “On”. A common cause of persistent fog cover windows is a misuse of the air re-circulation button. More on this below.

Detailers Tip: Fog has a harder time sticking to clean windows.

If your windows are constantly fogging up when the heat’s on, there may be an underlying fault. (more on that later). If your vehicle is simply prone to fogging when the cars are fully loaded with passengers, consider fitting vent shades and cracking a window or two.

Clear Foggy Windows On Older Cars

If your car has an older heater system, it means it’s a basic manual type heater where the operator needs to make constant adjustments to maintain comfort.

So how do you know which one you have?

If your car was made more than twenty years ago, it’s likely a manual system. Using a manual system is a little more work, but not difficult.

To quickly clear your foggy windows using manual controls.

Set air distribution knob to windshield. This directs the air to the windshield, which should be cleared first.
Turn heat temperature to max, your car may have dual heating zones, if so turn both.
Turn fan speed to max.
Turn the air re-circulation button off (If fitted).
Turn air conditioning button on (If fitted).

Why Your Windows Are Fogging Up

As you probably know, fog forms on a surface where hot air meets cold, known as Dew point. Turning on your heater, passengers breathing, wet clothing creates heat and moisture, the perfect ingredients for fog.

The outside temperature provides the cold and the glass provides a surface for the moisture to condense. Nice work, you made a cloud inside your car.

The most common reason for fog inside a car is the incorrect heater control setting. And I don’t blame car owners, some HVAC (Heating Ventilation Air Conditioning) systems look like they belong in an aircraft cockpit. I wrote a whole post about using a HVAC system right here. “How do I turn on my heater”.

Fog inside your car occurs naturally, however some settings will make matters worse and the fog may even be caused by a serious fault. This is a list of common issues that will make your car windows fog up like crazy, even with the heat on:

Recirculation button on
Wet dog or clothes inside the car
Water leak
Heating system leak
Heater fault
Air conditioning fault

Air Recirculation

The recirculation button is designed to prevent outside air from coming into the cabin. Useful for keeping fumes out and getting the cabin to the desired temperature quickly. However, the cabin air quickly goes stale and moisture-laden as passengers breathe. The recirculation button wasn’t designed to be used for more than five minutes or so.

Some systems will automatically turn it off after a set time period. Breathing in stale air is obviously not good for the body and it also causes the moisture-laden air to condense like crazy on the windows.

Two symbols are commonly used to identify the recirculation button. The modern symbol in the picture above and the older symbol in the picture below.

So use the recirculation button sparingly and never when trying to clear the fog from the windows.

Wet Dog Or Clothing

My father’s dog Sammy can’t pass a puddle and he’s in it, he’s a messy mutt, but we love him. Anyway, we all know what happens when we’ve wet panting dogs in the car. It’s impossible to keep the windows clear, there’s so much moisture in the air, even the smartest HVAC system struggles.

Wet clothes snowy boots, wet floor mats all add moisture to the cabin, and that will turn to fog. Keeping the moisture within your car to a minimum will help control foggy windows.

Water Leak

A water leak into the car is more common in older cars than newer ones, but new cars aren’t immune. You already know that moisture inside the car will cause foggy windows no matter how good your heater is.

So how does water get into the car?

The most common ways include:

Blocked sunroof drains
Damaged sunroof seal
Blocked A/C drain
Faulty windshield sealer
Blocked scuttle panel drains
Broken bulkhead body sealer
Damaged door window seals
Damaged trunk seals

If you’ve been to the car wash and notice a trickle of water coming in, there’s a good chance rain can get in too. Check your floor mats for dampness, or better, lift your carpets at the kick panel, just enough to get your hand underneath.

If you think you might have a leak, check out this post “Car floor wet under mat”, it covers some of the easy fixes most people don’t know about. A leak will need to be repaired as soon as possible, as it will eventually lead to expensive electrical issues.

Car heater leak
Heating System Leak

A heater system leaking is a nuance but a heating system that leaks inside the cabin is a total pain in the jacksie and a serious health hazard.

The engine heats coolant (liquid anti-freezing formula & water) and pumps it around the engine and into the cabin where it passes through the heater core. The core resembles a mini car radiator and in older cars often develops a small leak.

As the core is positioned inside the car, buried behind the dashboard. The leak pools in the front foot-wells, soak into the insulation and carpets.

The leak will usually go undetected, however, the passengers may notice the following symptoms:

Constant fogging of windows.
Sat Nav won’t stick to the windshield.
Sweet smell inside the car.
Car coolant needs topping up regularly.
Heating temperature temperamental.

Regular users of the car developed a chesty cough that won’t shift. Obviously, your health comes first, so fixing the leak is a priority. A leak that goes undetected can also cause engine frost damage, especially if the user tops up coolant with straight water.

Heater Fault

Modern heaters – HVAC/Climate control systems are very sophisticated, they use lots of temperature sensors positioned inside the car and out. Sun load sensors, fueled and electric pre-heaters, air conditioning system, a battery of blend doors to direct air through a maze of ducting to where it’s needed, fast. And of course, a HVAC controller (computer) to manage it all.

Some top-of-the-range cars have HVAC’s that go a lot further – Pre-heated or cooled from your smartphone before your journey even begins. No scraping ice or cold bum’s, we’re just not living!

Common faults with modern climate systems that will cause fogging inside the car, include:

Blend door fault
Fan motor fault
Sensor faults

Blend doors are motorized flaps positioned inside the heater matrix. They open and close proportionally as directed by the HVAC controller. The blend doors direct air to different zones in the heating system.

They’re very precise in their movement and the controller must know the exact door position. A faulty blend door motor is common and will need replacing. Controller losing blend door position is common too and simply requires calibration.

However, you may need a dealer-level computer to instigate the re-learn procedure. A code reader is useful for reading faults, but a basic handheld fault code reader won’t read a HVAC controller, you’ll need a reader with greater functionality.

I listed the scan tool I use to fault find, actuate motors for testing and calibrating the HVAC system. You can check it out here on the Auto electrical repair tools page. Faulty or loss of blend door position will cause fogging even with the heat on.

A fan motor (aka blower) fault is also common, the motor may fail completely or possibly the fan speed resistor. If you have a fan fault, it should be pretty obvious as the fan is pretty noisy.

It’s also possible for the fan speed control button on the central heater control panel to fail. Proper diagnosis is important, as fitting some HVAC parts can be expensive and very time-consuming. A good fault code reader will also have live data readings, and motor activation functionality.

Air-Conditioning Fault

Your air conditioning is very important in the fight against foggy windows. Seems crazy to turn on the A/C when the heat is on, most would think that counterproductive. Yes and no, your heater’s hot air will easily outgun the cold.

We’re interested in the A/C’s other great strength, its ability to dry air. It removes moisture from the car interior and as you know, that’s what causes the foggy glass in the first place.

Without a functioning air-con system, your windows will be difficult to clear even with the heat on and especially in damp conditions. The A/C system is controlled by the A/C control module (computer) and will register any faults detected.

Common A/C system faults include:

Low refrigerant
Cooling fan fault
Refrigerant leak
Compressor Fault

Low refrigerant will be detected by a pressure sensor and the system will be shut down by the A/C controller (computer). Low refrigerant is likely caused by a very small gas leak. A technician will top up the system, check for leaks, using leak detecting dye and black light.

A cooling fan is potentially a serious fault. Not only will the A/C not work, but your engine will overheat in traffic which could blow the head-gasket. A large refrigerant leak is commonly caused by a hole in the condenser, hose pump seals, or Schrader valves. A dye and black light test will be needed.

A compressor fault may be caused by an electrical fault or a mechanical failure. A compressor clutch is a common failure, the clutch is turned on and off, supplying drive power to the compressor.

The Secret Hack

This is an anti-fog hack that actually works, however, don’t use this on the windshield because it may cause glare when night driving. But it’ll work great on all other glass.

Using some shaving foam (not gel) and a kitchen towel, rub the inside of the glass and buff it off. This applies a detergent coating to the glass and reduces surface tension. That allows the condensation to flatten out, making it easier to see through.

So the moisture is still on the glass, it’s just more transparent. Are you asleep yet? Anyway, it works, but you’ll need to reapply every month or so.