[jblackburn]

Defrost mode has automatically turned on the AC compressor to remove condensation in the air in cars for the last 20 years.

 

[blackbird]

By chance do you have a sunroof or any other windshield or window leaks? Sometimes if there's a small leak allowing water into the car it can work down and gets absorbed into the carpet padding which will cause a lot bigger problem with moisture buildup on the interior windows. Unless it gets really bad for a long time you might not get a moldy smell right away. Or it might just be due to Florida humidity given your location.

Like mentioned the A/C should come on automatically with the climate control system in either windshield defrost mode or in the split floor and windshield setting but the A/C light won't be illuminated. If you let it run for a while and it doesn't defrost the windows but you manually engage the A/C and it immediately clears up you might have a problem with the HVAC (heating, ventilation, A/C) controller or something not properly engaging the air conditioning compressor.

 

[ProDigit]

It is pretty moist over here, however, I used to live on colder climates, and the cars there, had no ac. Just hot air does a better job at removing moisture.pure ac will cause fog.

 

[LiveTrash]

It is pretty moist over here, however, I used to live on colder climates, and the cars there, had no ac. Just hot air does a better job at removing moisture.pure ac will cause fog.

As @jblackburn stated, all cars have a compressor. It's the compressor's job to remove the moisture from the air to run the defrost option. When you put it in defrost mode, the compressor kicks in and will in turn spit dry air (no matter the temperature) on to your windshield. If it's cold outside, of course you'll want to use warm or hot air as that will defrost/defog the quickest. When a car lacks A/C, it still had a compressor. The only thing it doesn't have is the condenser which runs the coolant system to cool the air.

 

[Blasirl]

Can you smell coolant in the cabin?

 

[blackbird]

When a car lacks A/C, it still had a compressor. The only thing it doesn't have is the condenser which runs the coolant system to cool the air.

Cars that don't have air conditioning don't have a compressor. Vehicles with heating-only climate control have regular ventilation which can bring in air at outside temperatures and have a heater core (small radiator) using hot engine coolant circulated by the engine's water pump that can raise temperatures.

The compressor on cars with air conditioning literally compresses the refrigerant required for air conditioning gas phase change process to "move" heat from one place to another (i.e. inside the car to the outside of the car). When you compress a gas it heats up. The hot refrigerant passes through the condenser at the front of the car, which is essentially a radiator that uses air passing through the front end of the car to remove heat. The cooler, still compressed refrigerant than passes through an expansion valve and into the evaporator core in the air box assembly inside the passenger compartment. This air box has the blower motor and flapper valves to direct air between the different vents.

As the refrigerant rapidly expands it cools off and condensate (moisture) collects on the evaporator core, which is essentially another radiator. This gets drained out of the car which is why on a hot day you can see a puddle of water under a car when the A/C is used.

Even on a cold winter day, when the A/C is used it still causes condensation to build up on the evaporator core which dries out air. If you left the defroster on cold you'd get really cold, dry air. Normally you use heat with the defrost mode, which sends the cold, dry air through the heater core in the air box which raises the temperature before coming out the defrost vents.

 

[ChevyGuy]

Cooling air make the air wetter (higher humidity). Anything over 100% becomes condensate. So, yes, A/C can both remove the water AND make the air coming out the outlet. Once the air warms up to the rest of the car, it becomes dryer than when it got sucked into the system. Running A/C and heater does a double-whammy. It pulls the moisture out of the air and then warms it to dry it out. The results will be quite dry.

 

[obermd]

Cooling air make the air wetter (higher humidity). Anything over 100% becomes condensate. So, yes, A/C can both remove the water AND make the air coming out the outlet. Once the air warms up to the rest of the car, it becomes dryer than when it got sucked into the system. Running A/C and heater does a double-whammy. It pulls the moisture out of the air and then warms it to dry it out. The results will be quite dry.

That hot air also has a much higher capacity to hold water vapor, adding further to the ability to defog the windshield without water running down the inside of the glass.

 

[17Hatch6MT]

I myself understand the physics of relative humidity. I'd like to have the option when to run the compressor. The 2001 Sentra had an unmarked detent position on the selector for defog w/o compressor. I appreciated that.

 

[obermd]

The compressor in the Cruze is a relatively efficient variable output design. If it's just being used to dry the air it doesn't take that much power - I wouldn't worry about running the compressor for this purpose. The cabin blower motor takes considerably more power.

 

[Lugnut]

I like this thread and the discussion about thermodynamics. A couple of points to think about. How cold is it when the defogger is turned on? By design, the HVAC system compressor is inhibited at temperatures below 34 degree F. So if the ambient air temperature is near or below freezing, the compressor will not engage to dehydrate the incoming air. The other thing that MAY be happening is the inside humidity sensor could be misreporting the moisture levels inside the vehicle causing the windshield to fog. You will need a scan tool to see what the sensor is reporting. Just a couple thoughts....

 

[jjsimmers]

I actually had the same problem in my 2014 eco, after talking with someone at work he said the issue isn't the air its the plasticizers being released from the dash that stick to the windshield and cause it to fog up quickly. The method he and now I used to remove them is..

1) Wipe glass with lint free microfiber cloth
2) Use a slightly damp Mr Clean magic eraser & clean the entire windshield then dry with lint free cloth
3) Use glass cleaner to re-clean windshield

When I used the glass cleaner I used the home made version
1 cup warm water
1/3 cup alcohol
1/4 cup vinager
3-4 drops dawn dishsoap.
When I cleaned with this I noticed the streaks went away much easier with just re-wiping with new paper towels.

I did this a few weeks ago and it has not fogged up since.
Hope this helps

 

[ProDigit]

Cars that don't have air conditioning don't have a compressor. Vehicles with heating-only climate control have regular ventilation which can bring in air at outside temperatures and have a heater core (small radiator) using hot engine coolant circulated by the engine's water pump that can raise temperatures.

The compressor on cars with air conditioning literally compresses the refrigerant required for air conditioning gas phase change process to "move" heat from one place to another (i.e. inside the car to the outside of the car). When you compress a gas it heats up. The hot refrigerant passes through the condenser at the front of the car, which is essentially a radiator that uses air passing through the front end of the car to remove heat. The cooler, still compressed refrigerant than passes through an expansion valve and into the evaporator core in the air box assembly inside the passenger compartment. This air box has the blower motor and flapper valves to direct air between the different vents.

As the refrigerant rapidly expands it cools off and condensate (moisture) collects on the evaporator core, which is essentially another radiator. This gets drained out of the car which is why on a hot day you can see a puddle of water under a car when the A/C is used.

Even on a cold winter day, when the A/C is used it still causes condensation to build up on the evaporator core which dries out air. If you left the defroster on cold you'd get really cold, dry air. Normally you use heat with the defrost mode, which sends the cold, dry air through the heater core in the air box which raises the temperature before coming out the defrost vents.

I was thinking exactly the same, but didn't dare to say, as I wasn't sure of the claims that cars without AC also have a compressor (which they do not appear to have).


Air here is often over 80% humidity, and often only fogs the windows in the morning where it's like in the seventies or sixties of degrees F.
Using the AC and heater at the same time, also draws more power from the engine.
I have learned that I can't dial in the heater above center point, or over time I'll experience the coolant smell.
But to be honest, when on a longer trip, setting the temperature dial at 11 o'clock is quite warm (close to 80F inside the car).

Just simply turning on the AC (without heater) does consume quite an amount of power.
You'd be surprised, but MPG drops by about 10% compared to not using the AC.

What would have worked well for me, is if the top (windshield) vent, would have been able to be used, in combination with lightly heated air from the heatercore, or just regular fan.
Even if the inside air does not get dehumidified, just running air (of same temperature) over a fogged windshield helps it defog.

 

[dhpnet]

Cooling air make the air wetter (higher humidity). Anything over 100% becomes condensate. So, yes, A/C can both remove the water AND make the air coming out the outlet. Once the air warms up to the rest of the car, it becomes dryer than when it got sucked into the system. Running A/C and heater does a double-whammy. It pulls the moisture out of the air and then warms it to dry it out. The results will be quite dry.

Exactly. Cold air cannot hold as much water as warm air. If you have 90 degree air with 10% relative humidity and cool it to 50 degrees, the amount of water in the air stays the same, but the 50 degree air will now have high relative humidity because it cannot hold as much water as the 90 degree air could. It's called relative humidity because it is relative to the temperature. The dew point is the temperature where air hits 100% humidity (air pressure is also an important factor for dew point, but not necessary for this discussion). At the dew point, dew forms because the air can no longer hold the same amount of water it held at higher temps.

In a car, if you blow hot, wet air at a cool windshield, dew will form on the windshield if it reaches the dew point, or the point where the air can no longer hold the moisture. The dew will be deposited on the glass. The reason most modern cars run the AC compressor with the defrost is to dry the air so that it doesn't do this. With the AC compressor on, you blow hot, dry air at the windshield, and the air doesn't reach the dew point when it hits the glass. The AC compressor pressurizes the air and removes water.

Blowing hot, wet air on a cool windshield can actually make the condensation problem worse. Sometimes blowing cabin air can help because it keeps the air circulating and it doesn't make contact with the glass and have time to cool, but it could make it worse.

The problems above can also happen in reverse. If you have the AC on inside the car and the air outside is hot and humid, you can have condensation form on the outside of the glass.

And, yes, there is some relation to the plasticizers that are outgassed from the plastics and fog on the windows, but it is still related to temperature and humidity. Anything that leaves a residue on the windows can cause the windows to fog up more quickly because it attracts the moisture in the air.

There are also products that you can wipe on the windshield to reduce fogging. I think those work by adding a layer to the glass that prevents the air from coming in contact and cooling, and by not allowing the moisture to stick to the windows.

 

[dhpnet]

Another note, if you frequently get into your car with water or snow on your feet, the carpets will get wet and increase the humidity in the cabin. This will increase fogging as well.

 

[ProDigit]

I do understand that AC and heater extract more water out of the air.
However, it also takes a lot more from the engine.
On average, the AC uses up about 10% of engine power (10% lower MPG in 6th gear).
I think it uses up even more gas in smaller gears, because the AC is more efficient in smaller gears (it's engine driven, not gear driven).

 

[snowwy66]

The compressor system is designed to work in defrost mode. Keeps everything lubricated and functioning to assure it will work come summer time. Otherwise, there's the risk of no ac when summertime approaches. Cuz no one uses the ac in winter. And it's unlikely anyone would even bother turning it on every so often to make sure everything stays in working order. My 67 and 72 olds and 78 buick was that way also.

Supposedly it helps with humidity too. I live in dry country so i'll never know about that aspect. But i would imagine you all in the wet country need all the help you can get.

 

[ProDigit]

So, I swapped out the cabin air filter (as well as the engine air filter).
It's a 2012 car, with 66k miles on, and the first filter swap.
The cabin air filter looked like new.
The engine filter as well, with the exception that the last guy who installed it, left his dirty engine oil paw prints on one side of the filter.
The prints were still there, but for the rest, the filter looked pretty new to me.

But none of the filter swapping did anything to the humidity.
It's very humid outside, but after a day standing outside (we live close to golf courses that fog up in the morning), the car is so humid it takes forever to clear the windows!

So my first question would be if perhaps the AC drain pipe could be clogged?


Second,
The Ac of my 2012 Cruze never has worked well!
Anything above 83, and I ramp that sucker up to the maximum, and anything above 90F outside, and the AC can't even keep the heat out!
Everything checks out well. Freon levels (or what likes) are good, compressor works...
It's just that anywhere above like 86F outside temps, it's always too hot in the cabin.

And here in Florida, it is 3/4th of the year above 86F.

One day the temperature went below 70F, and the AC just shut down on me!
I mean, my windshield was fogged, and remained fogged for most of my journey!


Is there a way to 'trick' the AC system into going a bit cooler?


I also noticed that the needle indicating engine temperatures, isn't exactly at the halfway point, but 1 line cooler. I don't know if this indicates anything, like a malfunction or erroneously configured?

 

[snowwy66]

So, I swapped out the cabin air filter (as well as the engine air filter).
It's a 2012 car, with 66k miles on, and the first filter swap.
The cabin air filter looked like new.
The engine filter as well, with the exception that the last guy who installed it, left his dirty engine oil paw prints on one side of the filter.
The prints were still there, but for the rest, the filter looked pretty new to me.

But none of the filter swapping did anything to the humidity.
It's very humid outside, but after a day standing outside (we live close to golf courses that fog up in the morning), the car is so humid it takes forever to clear the windows!

So my first question would be if perhaps the AC drain pipe could be clogged?


Second,
The Ac of my 2012 Cruze never has worked well!
Anything above 83, and I ramp that sucker up to the maximum, and anything above 90F outside, and the AC can't even keep the heat out!
Everything checks out well. Freon levels (or what likes) are good, compressor works...
It's just that anywhere above like 86F outside temps, it's always too hot in the cabin.

And here in Florida, it is 3/4th of the year above 86F.

One day the temperature went below 70F, and the AC just shut down on me!
I mean, my windshield was fogged, and remained fogged for most of my journey!


Is there a way to 'trick' the AC system into going a bit cooler?


I also noticed that the needle indicating engine temperatures, isn't exactly at the halfway point, but 1 line cooler. I don't know if this indicates anything, like a malfunction or erroneously configured?

What's the freon levels look like?????? I have never been able to see the levels. Since you say it barely works. I'm willing to be you're low on freon.

What's the high side pressure. AT 70 degrees you need around 145 - 160 on the high side. 75 degrees needs 150 - 170. 80 degrees needs 175 - 210.

It's preventive maintanence to have a/c looked at every other year and topped off. If you've never done it. It's low. Freon don't last forever. Just like everything else.

I top off my system as needed. About every 2 1/2 to 3 years.