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Defrost/demist without AC?

Years ago, directing blower air onto the windshield didn’t automatically activate the air conditioner. Every car I’ve owned or driven recently turns the AC on in any air-flow position that includes the windshield.

For demisting inside the car, this is sort of OK. But it does force fuel-wasting AC use when a gentle stream of slightly warm air will keep the glass clear, which is most of the time.

For deicing, it’s just silly. I want max heat to melt external ice; pre-chilling the hot air - especially on startup, when the heater isn’t very hot - is a real problem. And in my last three cars, at least, AC does come on even when it’s below zero outside.

Any idea why this design choice was made? Are people really incapable of pressing two buttons when needed? And is there any way to decouple the two functions without doing something that would void the warranty? I’m guessing that it’s a hard-wired connection, or that one would have to take the dashboard apart to unplug a wire someplace.

Because even for deicing it’s necessary to dehumidify the air to prevent fog/frost forming on the inside. If you weren’t inside the car breathing out lots of moisture you’d be correct. Overall, a good idea to have the a/c on.

That has not been my experience over many years’ driving in foul weather. Since the dew point is low in cold weather - often far below ambient temperature - even moderate warming of the internal airflow has always been enough to prevent misting for me. The only exception in my experience was when five people crammed into my tiny Corolla FX on a -3F night before I’d warmed up the car. There was not only mist, but heavy ice inside the windshield before it got clear. All other times, no AC needed.

Not my experience in Anchorage, where I needed the a/c to keep windows from fogging/icing. Some cars have an a/c buttton that lets you turn off the a/c. I guess some don’t.

Now I’m curious. Does Anchorage tend to have high RH, so that dew point is close to ambient temperature? That might explain the difference.

Of course, what I really need is a persistent surfactant to keep the inside of the windshield wettable. Then it would’t be such an issue. Something like the opposite of Rain-X. So far, none of the soaps, detergents and industrial surfactants I’ve tried have been able to keep up with outgassed PVC plasticizers or whatever it is that greases up the glass.

I fundamentally agree that I would rather select the AC on my own terms. The Ac is not needed in some cases.

But in freezing weather, if you have outside air selected, the AC may come on for a few seconds, but the orifice (expansion valve) should close to prevent the AC from actually working. The orifice should close to prevent icing of the evaporator coils and AC is not needed below 40°F.

If you are using recirculating air, you do need to keep the AC on as your breath and perspiration are rapidly increasing the RH of the air inside the vehicle and that is aggravating the fogging conditions.

The current design is fine with me. From what I remember from my last car with separate controls, the air conditioner was needed most of the time anyway. In addition, the air conditioner needs to be run regularly to prevent damage, so that eliminates having to worry about that.

As Keith mentioned, once the temperature drops below 40-45 degrees, the A/C compressor doesn’t come on. If the tempearature is below freezing and you turn on the defroster, your A/C compressor isn’t running.

Since the dew point is low in cold weather - often far below ambient temperature

Dew point is low UNTIL the interior starts warming up from the heat.

While I agree it’s not always needed. I’ve NEVER owned a vehicle that turned the AC on when you turn the defrost on…although I’ve seen it.

And there are many times I turn the AC on during the winter…it does a far better job clearing the windows then heat alone.

Well, Mike, I haven’t owned a car in 50 years that Didn’t run the A/C with the defrost on. I do think it is silly to run A/C when it is very cold and you are trying to melt the ice on the winesheild. I would rather have control of the A/C.

I drove for 35+/- years without any AC and the last 10+/- with the autpmatically-operating AC. I far, far prefer to make my own decision. In warm rain, the AC is a huge advantage, but in cold weather, warm air being pushed up the inside of the windscreen is going to remove moisture on the inside of the windshield anyway.

I too let the defrost vent push hot air to remove ice, and modern systems simply do not do this as well.

Our Elantra defogs beautifully her in Northern California on just the windshield button, without the A/C defroster being used… I haven’t tried to de-ice yet.

With all due respect to OP, for defogging, deicing and not using AC in cars that are designed with venting to take place through the AC vents only…is wishful thinking. Saving that gas is another futile attempt to no good end. Forgive the rant and read the manual.
I hear what @same has to say and agree in principle. Except, that cars are designed a lot differently then years ago. For one thing, they are much tighter and have more restricted airflow through the cabin filter. Even in a dry winter day, the moisture generated by driver and passengers in an enlosed car needs to be dealt with in today’s tighter cars… The sooner the better and AC does it best. Vent the car and you deal with cold air intrusion. Nothing like hot, dry air.

I agree with @ellyEllis and if you are just heating the window up for deicing and are out side an idling car, there may be benefit to not running AC.

Well, Mike, I haven't owned a car in 50 years that Didn't run the A/C with the defrost on

Maybe more common then not. But I guess I keep buying the cars that it’s NOT common on.

Vehicles we’ve owned with Factory AC.

87 Honda Accord
96 Accord,
90 Pathfinder
98 Pathfinder
05 4Runner
07 Lexus ES350.

None of those cars automatically turn the AC on when the defrost is on. I had this debate with my brother-in-law who owns a Civic. His car does do it…He thought ALL Honda’s did.

Other vehicles I had with Factory AC and NOT automatic when on defrost.

66 Fleetwood
72 Ford F150.

Well, MikeInNH, I may be wrong, I don’t know everything, but I doubt that you are right about all those cars.

but I doubt that you are right about all those cars.

Every single car mentioned I’m 100% accurate on. None of those vehicles turn the AC on when it’s on defrost.

Years ago, we didn’t have air conditioning in cars. I had an auxilliary fan that was clamped to the steering column and was pointed at the windshield on my 1950 Chevrolet pickup truck and also on my 1961 Corvair. These fans did the trick. Years later, my 1978 Oldsmobile did not have a rear window defroster. I mounted two fans on the rear package shelf and wired them through a switch on the dashboard. These fans would demist the rear window. In the house, when I steam up the mirror in the bathroom after a shower, I just point the hair dryer at the mirror and in a few seconds I have cleared off a hole in the condensation so that I can see to shave. I don’t turn on the air conditioning in the house to defog the bathroom mirror.

@Triedaq “These fans did the trick. Years later, my 1978 Oldsmobile did not have a rear window defroster. I mounted two fans on the rear package shelf and wired them through a switch on the dashboard. These fans would demist the rear window.”

They worked fine, I agree. But they all had air leaks aplenty in them. The defroster on my tractor works fine without AC too. But when I look down between my feet, I can see the road go by.

My dads 69 Bug barely had heat.

Gee, I’ve owned a (reasonably) modern 98 Contour that came from the factory sans a/c, and never had an issue defogging or antifogging. Same goes for cars of same vintage with no freon equivalent left to do any air conditioning.

So, to those who feel “a/c on with defogger” is a safety issue, two follow-up questions:

  1. Is a manufacturer negligent, today, if they even offer a car for sale without a/c?

  2. Should “a/c inop” be cause to fail a safety inspection?