ok, call me dumb or ignorant, or both, but you're cautioning against high RH for bio reason or just driving condition?!?

Wouldnt most people roll down the windows when things fogged up, regardless of whether they think the ac is on or not?...I'm not living in cold region, so I must not know some common sense or something..?

 

Driving condition.

Driving safety, safely...

The propensity of interior windshield fogging possibly resulting in obscured forward vision is much greater during coolish, or cold weather, say 47F and below.

You must be as old as the hills....

Yes, long ago, or really not THAT long ago, the most common practice for preventing or removing windshield fogging was to lower the rear windows slightly. A 1962 early morning drive up to the ski area in a Renault Dauphine with 4 aboard comes immediately to mind.

In car heaters were hardly ever "up to snuff" in those days, not even the one in our "alternate" ski vehicle, an early 50's Ford.

But since those earlier days we have slowly learned to rely on the increasing technology of our climate control systems as a first resource. When A/C began to be common in our cars that was fine, turn on the defrost/defog/demist mode and the blower speed rises as does the heating level of the system outflow. The RH of the system airflow is dramatically reduced via HEATING, the blower speed is automatically raised "FORCING" the passenger cabin atmosphere out whatever exits, exhauster ports, are available, all the while the interior windshield surface's temperature is being raised to help dispense the condensation and also raise the interior windshield surface above the dewpoint.

For how many years did that techique, manual, automatic, or combined, work perfectly FINE?

Oh, did I forget?

Once A/C became more common it was almost always turned on in defrost/defog/demist mode since RARELY and randomly (not really randowm, it has to do with overall climatic conditions, conditions for which we humans have relaible no sensory perception) it could be an aid to lowering the RH.

The thing to realize is that absent a large hysterisis "bang-bang" servo loop, refrigeration systems cannot be readily moderated, modulated. So climate control system designs quite quickly evolved to the point wherein the refrigernant system, automotive air conditioning, always ran FULL-OUT, full bore, as it were.

So it came to pass that ALL system airflow was first COOLED via flowing through the A/C cooling evaporator, and the A/C servo feedback loop consisted entirely of a single sensor that disabled the A/C compressor if the cooling evaporator tended to decline toward, or too close, to freezing, say below 35F. The A/C system was designed to keep the cooling evaporator as close to freezing as capacity allowed.

The big advantage to this was it increased the probability of dehumidification of the incoming airflow quite dramatically.

The big disadvantage was that this COLD system airflow would be quite seriously discomforting to the passengers. So, now that ALL of the system airflow has been CHILLED and thereby HOPEFULLY dehumidifed it must somehow be warmed to provide a reasonable passenger airflow comfort level.

Passenger comfort was accomplished by passing the CHILLED airflow, or a selected portion thereof, through the heater core.

Pretty wasteful, RIGHT..?

And all to ONLY help prevent or aid in the removal of window and/or windshield condensation, MAYBE, if climatic conditions are such that the temperature and RH of the incoming outside atmosphere is high enough that a "dip" in temperature to 34F (if the blower speed is low enough to allow that) will result in condensation onto the evaporator vanes.

So, what happened to the design aspect of the climate control's automatically switching to high heat and higher blower speeds in defrost/defog/demist modes?

It was deemed too discomforting to the front seat passengers, having all that hot airflow reflected off the interior surface of the windshield..

And after all, wasn't that what using the A/C in the wintertime to aid in dehumidifiction was all about?

Yes, if you like drawing Aces and Eights on occassion.

And then how, just how, do you provide decent FE with the A/C running all year around?

You SEAL the **** car...!!

If no "conditioned" airflow can easily or readily escape the passenger compartment then the "load" on teh A/C is reduced dramatically.

Recipe for disaster..??

ABSOLUTELY..!

So, you are correct, PERFECTLY correct.

There are basically only two things to do if you begin to see condensation beginning to form on the interior surface of the windshield, WARM the windshield and lower the RH of the passenger cabin atmosphere.

Turn the temperature setpoint to MAXimum heating (the blower speed should increase automatically), and then switch your climate control system to defrost/defog/demist mode. In many modern day vehicles with smallish exhaster ports, or ports that are placed where there will be, predominantly, positive air pressure, it is clearly adviseable to lower a rear window or two slightly.

I do not allow the operation of the A/C compressor during the winter months in any vehicle, in years past I must often simply disconnected the compressor clutch circuit. Even during the summer when/if I need A/C to attain the best FE I always adjust the temperature setpoint to MAXimum COLD and then manually adjust the blower speed to maintain my comfort level.

 

Willard, interesting and informative...Thank you! I'll try to observe and remember this the next time we go skiing.

 

Willard... I KNOW you have had a bad experience with fogging windows, and by the sounds of it, and your passion, you had a nearly life-threatening result.

HOWEVER... not everyone lives in a climate as damp as yours, and I for one have never EVER had my windows suddenly fog over. It has always been a gradual process, with plently of warning and time to react.

So in brief, I feel your condition is the exception, not the rule, and your advice may be on the mark for people in similar climates, but certainly not the general population. I for one live in a place with RH of 40% in summer and 10% in winter... no matter the temperature. And no matter where you live, the colder it is outside, the drier it is outside.

 

Tes, I had several life threatening events, ALL involving my then new 1992 LS400.

"HOWEVER..not everyone lives in a climate as damdp as yours,..."

While the events I endured and survived did occur in a more damp, consistently damp, region of the country, I don't think that factoid is related directly to my experiences, indirectly yes. Additionally over the intervening years I have had an opportunity to communicate with many others having this same type of problem and it does seem to be quite widespread geograhically.

The sudden windshield fogging events seem to have to do with some sort of "upset", anomalous operational environment, rather than any geographical "placement. Many of these "upsets", upon investigation and armed with FULL operational knowledge of these newer Denso or NipponDenso climate control designs.

My earliest experiences with the Lexus was its propensity to suddenly begin to fog over the windshield about 2-3 miles down the road on an early coolish morning, Nov-Dec or 91. It was at least three years of serious study and research before I was able to pin this down to a problem with the design aspects of the Lexus climate control.

Let's say it Thursday, late afternoon or early evening, and on my drive home from work the sun is shining brightly, and having just purchased the car I have yet to learn how, when and which of the climate control aspects to control manually. So the system is in fully automatic mode.

Park it in the attached garage, go in to a nice dinner my wife has prepared, NON-EVENT...

The next morning it is coolish, 40-45F, as I start the Lexus , pull out of the garage and head off to work, about 15 miles distant. About three miles down the road I notice that the windshield is beginning to fog over slightly near the bottom..

So, instinctively, just like any one of you, I reach over and activate
the defrost/defog/demist function.

BLAM....!!

****, now my entire interior windshield surface is fogged over so severely I must pull over to the side of the road to "recover". Soon, using standard "manual" procedures, turn up the heat, the blower and maybe lower a rear window, my windshield has cleared up enough that I can proceed safely.

By mid-December these have become such daily and regular events that I started the practice of increasing the setpoint and lowering a rear window as I drive out of the garage each day. This, of course, is accompanied by so many visits to Lexus service that they are clearly growing tired of my constantly complaining.

No, Mr. West, the condensate drain is not clogged and your climate control is operating as designed.

Mr. West, are you certain that the fresh/recirculate function was in "fresh?

Then...say late January.

Mr. West, be sure and always press the "fresh" mode button even when the "fresh" mode indicator is already "on".

I will willingly admit that the above statement seemed so ridiculous to me that, basically, I ignored it. I was WRONG.

But by mid-January I have had about three life threatening windshield fogging instances "on the road" at least one in severely adverse roadbed conditions.

Bellevue Lexus' service manager, Chuck Smiley, is still feeding me the "normal" operation line. But by pure happenstance I overhear an a more elderly coupon talking to Tod Choteau (service writer) in the Bellevue Lexus showroom. Basically the gentleman is explaining to Tod, describing the exact same early morning windshield fogging instances I have been encountering, explaining the need to often pull over out o ftraffic as a result of the fogging.

Tod, basically is giving this man the same old song and dance, are you sure you didn't have the system in recirculate, do you really understand how the system operates, the system is operating normally, as designed, etc.

The wife becomes so exasperated with Tod's seeming ill treatment, consideration, of her husband's knowledge, ability and experience, that she joins in the discussion which now escalates into a rather LOUD argument right there on/near the Lexus showroom floor.

Tod, will later, in court, sworn, will deny any memory of this event.

But in the meantime they now have no choice but to accept the validity of my complaints.

 

I haven't finished reading this thread yet, but I live in the Pacific NW too. With the amount of constant rain we've been getting lately, driving has been a big challenge and fuel economy has been affected as much as 20-25%.

I will study the suggestions here later, for safety and fuel saving reasons.

 

Today after driving 15 miles my FEH was at 186'F water temperature and went into EV at the first stoplight.

I had the heater on low ( fan speed 1 ).

The car went from 186'F to 145'F during this one, 120 second stoplight.

The ambient temperature was 17'F.

Also, the FS ( Fake Shift ) does not work at 17'F.
- John

 

The defrost knocked the mpg on my last tank down about 2mpg. We had some cold rain on Thanksgiving so the windows were constantly fogging with 4 people on board. As soon as you start it up the ICE comes right on. Looks like the MPG will start to suffer once the real cold weather gets here.

 

Jason,

MaxAC does close the outside air vent, but as you noted will force the compressor on. In many cars, including my Prius, you can close the outside air vent and just recirculate inside air without running the compressor to retain heat (it also retains built up moisture). I haven't checked out the Escape/Mariner HVAC controls so you'll have to determine their flexibility yourself.

JeffD

 

Below 32'F ( maybe 33'F ) the A/C compressor does not run at all. Ever. No matter which setting you have it in... even the orange settings.
So this will not put any extra horsepower drain or MPG drain on your engine.

The orange ( defrost or MAX A/C ) will however prevent EV mode at all temperatures. But if it is below freezing, you are not going to be able to use much EV anyway, and you probably want the heat... thus, you probably NEED the ICE on in 99% of cases.

S0... after 18 months of ownership, the "orange" condition of defrost has not been that big of a deal. It irked me at first, but in real life, not that bad!!!

-John

P.S. If you shift to N at a stoplight you can run either orange defrost setting WITHOUT the engine starting. Just FYI.