General Clear windscreen

[RobinPJ]

Air con on, heat max, mist gone.

You can then open the windows briefly to suck out all the damp air and replace it with drier air (assuming it is drier outside the car than inside it at that time).

 

[jrkitching]

Remember though that the aircon will be disabled once the outside air temperature falls below about 4C.

 

[dave]

dont 500s have a demist button? on a croma one button sets all the air to go up to screen turns fans up full and puts rear dimister on. still takes ages compared to any non aircon cars ive had though

there used to be stuff you could put on screens that claimed to stop misting, dont know if they work or are still sold though

Edit here is one http://www.halfords.com/webapp/wcs/..._productId_201119_langId_-1_categoryId_255225

 

[bgunn]

Remember though that the aircon will be disabled once the outside air temperature falls below about 4C.

Only if you're drawing fresh air in. If you put recirc on, as the in car air temp raises, the evaporator temp will increase and the a/c will work.

 

[bgunn]

dont 500s have a demist button? on a croma one button sets all the air to go up to screen turns fans up full and puts rear dimister on. still takes ages compared to any non aircon cars ive had though

there used to be stuff you could put on screens that claimed to stop misting, dont know if they work or are still sold though

If you have climate, you get a 'defrost' button - which puts the temp to max, fans to max (when the coolant temp is high enough), a/c on, airflow aimed to screen and the heated rear window (and mirrors) on. After around 10 - 15 mins, it'll cut back to the last settings you used.

 

[Most Easterly Pandas]

Only if you're drawing fresh air in. If you put recirc on, as the in car air temp raises, the evaporator temp will increase and the a/c will work.

No it won't as it used external air temp sensor to cut it off. Not all of the AC components are in the cabin don't forget

dave yes they do, but only those with climate.

 

[bgunn]

No it won't as it used external air temp sensor to cut it off. Not all of the AC components are in the cabin don't forget

Not true. I know for a fact that my a/c compressor clutch has been cutting in and out whilst the indicated outside air temp has been below 4 degrees C.

The a/c system on a 500 pop has no reference to an external temp sensor for example - it's a closed loop system based upon refrigerant pressure, whether the fan is switched on, and whether the air *leaving* the evaporator is above around 4 degrees C. All that really matters is that the condensed water on the evaporator isn't frozen (and thus blocking it).

The climate control is no different in this regard except the control of the air temp flap is automatic to regulate the temperature on a feedback loop with the *internal* air temp sensor.

As for a/c components being in the cabin. Don't forget how an a/c system works - the compressor is drawing in refrigerant that has been cooled from the condenser, but will still be higher than cabin temperature (this is the thermodynamic principle of how a/c works). Prior to reaching the condensor, the refrigerant will be in a superheated state as the evaporation process 'takes' the heat from the air in the cabin (to use a very primitive/laymans term). Just to bear in mind, the boiling point of r134a is -26.3 degrees C - so that is the ambient temperature at which it will fail to function in the way intended. 4 degrees positive is considerably above this.

 

[Most Easterly Pandas]

I stand corrected

 

[KaZetec]

I'm pretty sure the a/c stops working at -4C not +4C.

 

[Buttoncup]

well tried everthing that you have all said, still mists up this morning on the drive to work.
anyone tried that antimist spray? did it work?
cheers

 

[jrkitching]

I stand corrected

Not so fast. The system's operation is more complex than anything that's been posted yet.

I've attached FIAT's own description - I warn you it's one of the more badly written (or translated) bits of eLearn but you'll see that the system gets information from both the temperature sensor on the evaporator and the OAT sensor in the passenger mirror (question for anyone who actually knows for sure: do pop versions with AC have the OAT sensor added as part of the package).

bgunn is correct in that the important thing is to prevent ice from blocking the evaporator and to do that, the sensor must measure the surface temperature of the evaporator (the air leaving the evaporator will always be a little warmer).

Have a look also at this table; it might surprise you to know that air at freezing point can hold over half the water vapour of air at 10C so the potential for ice accumulation is there even on very cold days.

To prevent the screen from misting up, the temperature of the surface of the glass must be higher than the dewpoint of the air in the cabin. So to get rid of it, you either need to raise the temperature of the glass or lower the dewpoint of the air.

On soggy mild winter days, the dewpoint will be close to ambient temperature. The AC is very effective in demisting because it removes moisture from the air, reducing its dewpoint. When the temps drop to freezing, the dewpoint of the ambient air outside will be much lower than the dewpoint of the air in the cabin, which is why opening the windows to clear the screen works better on colder days.

Another way to stop the screen from misting is to raise its surface temperature above the dewpoint of the cabin air, which is why blowing hot air over it or using a heating element is also effective.

The air we breathe out is always 100% saturated, so if you stay in a stationary car with the windows up, the dewpoint of the cabin air rises & the windows steam up.

 

[bgunn]

Not so fast. The system's operation is more complex than anything that's been posted yet.

I've attached FIAT's own description - I warn you it's one of the more badly written (or translated) bits of eLearn but you'll see that the system gets information from both the temperature sensor on the evaporator and the OAT sensor in the passenger mirror (question for anyone who actually knows for sure: do pop versions with AC have .the OAT sensor added as part of the package).

bgunn is correct in that the important thing is to prevent ice from blocking the evaporator and to do that, the sensor must measure the surface temperature of the evaporator (the air leaving the evaporator will always be a little warmer).

Have a look also at this table; it might surprise you to know that air at freezing point can hold over half the water vapour of air at 10C so the potential for ice accumulation is there even on very cold days.

To prevent the screen from misting up, the temperature of the glass must be higher than the dewpoint of the air in the cabin. So to get rid of it, you either need to raise the temperature of the glass or lower the dewpoint of the air.

On soggy mild winter days when temps are around 10C, the AC is very effective in demisting because it removes moisture from the air, reducing the dewpoint. When the temps drop to freezing, the dewpoint of the ambient air outside will be much lower than the dewpoint of the air in the cabin, which is why opening the windows to clear the screen works better on colder days.

Another way to stop the screen from misting is to raise its surface temperature above the dewpoint of the cabin air, which is why blowing hot air over it or using a heating element is also effective.

The air we breathe out is always 100% saturated, so if you stay in a stationary car with the windows up, the dewpoint of the cabin air rises & the windows steam up.

I agree that it's very easy for the evaporator to ice up in most conditions, but you have to remember that the A/C system is operating in hysteresis, so it'll be merrily cutting in and out (and does) all of the time.

 

[bgunn]

And apologies for helping take the thread way off topic!

 

[jrkitching]

And apologies for helping take the thread way off topic!

You haven't.

In layman's terms, what we're saying is:

if the screen is misting and it's warm outside, turn on the AC

if the screen is misting and it's cold outside, open a window.

(In cold weather, opening a window will lower the dewpoint of the cabin air faster than the AC can lower it in recirc mode.)

In either case, turn the heating control to max and the air control to screen. The heater will not have much effect unless the engine coolant has warmed up.

On very cold days when the car has been parked outside, the only way to stop the windscreen from misting when you first drive off may be to drive with the windows open (brrrr!).