Keeping Your Windshield Clear:

Defrost vs. Demist

This page addresses the issue of automotive ventilation systems and how they may be best designed and utilized to maintain visibility through the windshield and other automobile glazing.

It sounds like a simple topic and not worthy of effort; however, my own experiences, most notably with rental cars of recent manufacture, lead me to believe that the engineers working for these automakers simply have no clue what these systems need to do.  A recent drive through Northern Europe in a rented Mercedes A140 with a frustratingly badly designed ventilation system was the final straw.  I am posting this page in the hopes that someone in the automotive industry will come to a better understanding of the needs of automotive ventilation systems and begin to design them better.

Before progressing further, I will mention my own qualifications: as described on my resume page, I am a Mechanical Engineer licensed in the State of Florida.  Now retired, I spent several years designing Heating, Ventilation, and Air Conditioning (HVAC) systems for commercial buildings in the Tallahassee area.  While I have not designed automotive systems, I do understand such things as relative humidity and stochiometric charts.

There are several conditions an automotive ventilation system needs to deal with in order to maintain driver visibility.  I have no idea if anyone has already defined terms for each, so I will take the liberty here.  The first thing that an automotive ventilation system must be able to do is "defrost".


I believe that defrost is the one idea the automakers understand.  Basically, it is cold outside, below the freezing point of water, and ice has formed on the windshield.  It is imperative that this ice be removed from the windshield prior to driving, as it is usually completely impossible to see through.  The typical application of defrost mode, therefore, is upon first starting of the car and before the car moves.

Some cars, such as my '83 Jaguar XJ-S, automatically put the ventilation fans on high speed when defrost mode is selected, as though moving the warm air faster will somehow help.  This is probably a bad idea, since a slower fan speed would result in the air being hotter, and there's no problem with the air reaching the windshield or anything.  The problems are compounded by the fact that the engine is usually being started from cold and therefore there is no heat in the heater core, so the fans are really blowing around a lot of cold air.  All of this adds up to perhaps the most damaging operation an automobile can be subjected to: idling for a long time with a cold engine without moving.  There are products available that allow owners to start their cars remotely so they can sit there and idle as long as it takes for the engine to warm up and effectively defrost the windshield and warm the interior so the owner can get it started as soon as he gets out of bed.

People who live in cold climates are well aware of the wear issues with such operation.  The oil is thick, making the engine hard to start.  The fuel system will pour in lots of fuel to get it going, enough fuel that it will wash all the oil off the cylinder walls.  The cylinder walls are not lubricated by the pressurized oil system; rather, they are only lubricated by oil being slung off the spinning crankshaft -- and the crankshaft often doesn't spin fast enough at idle to sling the oil far enough to lube the cylinders.  So the engine is idling -- for ten minutes, thirty minutes, an hour, whatever it takes to get that windshield clear -- with no lubrication on the cylinders whatsoever.  The wise owner installs an engine heater in his car or stores his car in a heated garage, so when started the fuel system doesn't have to dump all that rinsing fuel into the cylinders and the engine can either defrost the windshield quickly or the windshield never gets icy to begin with.

For the specific application of defrost, it would probably be a far better idea to put the fan on its lowest speed and install a resistance heater in the defrost duct.  It might even work to leave the fans off and merely let the heat from the strips rise naturally out of the ducts onto the windshield.  Either way, resistance heat requires a lot of electrical power and should only be used while the engine is running (because if the engine is off, you might kill the battery and be unable to start it) and once the coolant system in the engine is warm enough, the strips should be turned off and the conventional heater core used.

Alternatively, automakers might offer a household voltage windshield heater system that can be plugged in long enough to clear the windshield, possibly without even starting the car.  Of course, the owner might be able to fake that by simply wadding up an electric blanket on top of the dashboard.

Once the engine is warmed up and the car is underway, defrost is usually not a problem.  The temperature of the outside surface of the glass must be 32ºF (0ºC) or below for ice to form, and since the occupant will typically have the heat on strong enough to maintain at least mid-60's (18ºC or so) it would have to be very cold outside indeed for ice to be a problem.


Another mode the automotive ventilation system needs to address I shall call demist.  This is completely different and totally unrelated to defrost, and it's my impression that the carmakers have no clue in this regard.

When it is very humid out and a bit on the cool side, condensation can form on the inside surface of the windshield.  This is because it is always more humid inside the car than outside, thanks to the fact that human occupants emit humidity via perspiration and respiration (breathing).  In the small confines of an automobile, especially with a lot of passengers, the relative humidity inside can quickly reach 100%.  If it is cooler outdoors -- even only a few degrees -- then the inside surface of the windshield will be cooler than the inside air and condensation will form.  It's not easy to see through.

Heat, as provided by a conventional defroster, won't do a lot for clearing this condensation.  Heat lowers the relative humidity of the air, but as long as the glass remains cool there will still be condensation forming.  To halt the condensation, the heat must be applied long enough to heat the glass itself above the dew point of the interior air.  Unfortunately, by the time the glass has been heated that much, the interior of the car has been heated a lot, and the passengers are sweating like pigs!  Hence, the humidity inside the car is rising rapidly, maintaining a relative humidity of nearly 100% as the temperature rises.  Eventually the occupants are cooked and the windshield is still misting up.

A better solution is to turn the A/C on.  Heat does nothing for the total amount of moisture in the air; it merely lowers the relative humidity by increasing the capacity of the air to hold moisture.  A/C, on the other hand, actually does remove moisture from the air, draining it as condensate out the bottom of the car.  In general, you want to get the evaporator coil of the A/C system (that's the radiator-looking thing inside the ductwork that has freon in it) as cold as possible without ice forming on it so that moisture condenses on it in great quantities.  Then this moisture drains off and out the bottom of the car, leaving the air much drier.  This will help prevent moisture from condensing on the inside of the windshield.

Unfortunately, the air still has a relative humidity near 100%, because getting it drier also made it colder.  To begin with, the windshield started to clear up because the air coming out the vents was now colder than the glass, so the glass appears to be warm and condensate will no longer form and will actually start to dry up.  The occupants, meanwhile, have chattering teeth.  And eventually, another nasty problem comes along: the cold air eventually cools the glass enough that condensation begins to form on the outside.  You have to turn your wipers on to keep it clear.

The solution is obvious to any HVAC engineer: you must turn on both the heat and the A/C at the same time.  It works like this: the air goes through the A/C evaporator first, cooling it to perhaps 45ºF (7ºC) and draining a lot of moisture out the bottom of the car.  Then the air goes through the heater coil, heating it back up -- and lowering the relative humidity greatly.  Anyone who has a car on which this can be done (it can work on some cars, but it usually seems an accidental capability) knows how well this works: the windshield clears instantly (like, within five seconds) and stays clear.  It is a perfectly workable plan to then vary the amount of heat to maintain comfort conditions inside the car; whatever amount of heat you add will work to keep the windshield clear, and you will need to apply some to keep from freezing to death inside the car.

Unfortunately, many automakers seem determined to prevent you from being able to do this.  The Jaguar XJ-S (and XJ6) operates with both heat and A/C running most of the time, because such a system also provides the best comfort conditions for the occupants and these are supposed to be luxury cars in which comfort is all-important.  It doesn't appear that there was any intention to be able to use this for demist capability -- the position on the controls for the windscreen is a typical defrost, high fan speed and lots of heat -- but by staying out of the defrost and merely using the system to provide dry air to the interior will usually keep the windshield clear.  The mid-80's Hondas I own likewise have no specific demist mode, but you can put the system into a mode where the windscreen vents are operating and the A/C is on and then apply a bit of heat.  The Mercedes A140 I rented would not allow operation of the A/C and heat simultaneously; the German engineer involved should have been shot.  The best demist action I could get, while driving many hours through drizzle coming off the North Sea in the summer, was to switch back and forth from A/C to heat every couple of minutes.  Put the control in the middle, and rather than getting A/C plus heat, you get neither -- and the interior gets very muggy very quickly and the windows fog up.

Other Windows

The windshield isn't the only glass you'd like to be able to see out of.  Ideally, you'd like to be able to see out of all the windows in the car; the windshield is merely the highest priority.  Second priority is the rear windshield, which is typically fitted with electric resistive strips embedded in the glass.  These work pretty well for both defrost and demist:  for defrost, they will heat the glass enough to loosen ice so you can scrape it off and then prevent it from forming while driving; for demist, it will heat the glass enough to prevent condensation from forming on the inside.  The only problem is that some moron decided to put a timer on the thing, so that it always shuts itself off a couple of minutes after you turn it on.  I made one memorable trip in a rental car with two other people during which we drove through freezing weather in NW Texas with heavy winds and light snow for several hours, and the front seat passenger was assigned the task of hitting the rear window defroster button every couple of minutes for the entire drive because the rear windshield would become impossible to see through in a matter of minutes with it off -- and then it would take several minutes for the rear defroster to clear it again.  Another engineer that should have been shot.  Just ask anyone in the car that night.

The next priority is the front side windows, most notably the lower front corners of them that the driver must see through to see the side view mirrors.  Many cars now have special little vents directed at this area, a wonderful development.  Unfortunately, they often are blowing air that is too cold for window clearing (like, they are blowing the same cold air used to cool the interior) and cause condensation on the outside of the glass.  And there's no wiper there.  You end up having to roll the windows down, and of course the seal on the window is never good enough to provide a clean wiper action to clear the glass.  And because this area is at the bottom edge of the window, as soon as you roll the window down far enough to reach out with a rag, the section you need to get at is down inside the door!  Please, a little thought here: these side vents should be connected to the windshield vents, not the dash outlets.

Other Concerns

It would go without saying if I were an automotive engineer, but it seems beyond the grasp of the guys actually designing cars: the ventilation system needs to be capable of maintaining a clear windshield without sacrificing the comfort of the occupants!  The idea that you put the system either into defrost mode or into heating or cooling mode is simply incorrect.  The windshield vents should be controlled to provide warm (defrost mode) or dry (demist mode) air for keeping the glass clear, and there need to be separate controls for maintaining comfort conditions for the occupants.  Once this concept is grasped, you will find that it's not all that difficult to accomplish.  The currently-popular idea of minimizing the number of control knobs or buttons will need to be abandoned, however; I'm not sure what the objective is -- knobs and buttons aren't that expensive -- but it generally makes even conventional control difficult to understand and more complicated tasks such as cooling the occupants while demisting the windshield all but impossible.

A good start might be as follows:  One button to turn the A/C compressor on and off.  One button or lever to open or close each vent, including the windshield vent, with all those controls independent -- you can have them all open if you want, opening one does not close another.  One knob to set fan speed.  One lever to control heat to the occupants, and one lever to control heat to the windshield vent.  If you want to get fancy and provide thermostatic control, replace the lever to control heat to the occupants with a thermostatic control and leave the manual control for the windshield.

Behind the scenes, the system could be set up to work as follows: all air goes through the A/C coil; there is no thermostatic control involved (other than the freon circuit controls); the occupants know when they need A/C and when they don't, the button on the dash is the only thing that controls whether or not the compressor is activated.  The fan control is similarly manual control only.  The air leaving the evaporator then can either go through the heater core or bypass it to get to the interior vents, and it can likewise go through the core or bypass it to get to the windshield vents.  The only thing that is subject to thermostatic control is the set of flaps that control how much air goes through the heater core to the interior vs. how much air bypasses it.

Such a system would give the occupants full control over the clearing of the windshield and their own comfort conditions.  I cannot see why all cars -- at least all cars that have A/C -- can't have controls that work this way.