Safe...and Comfortable Winter Driving..

[wwest]

During wintertime driving the very BEST way to operate a Toyota or Lexus climate control system is to turn the A/C compressor off(***), totally and completely, switch the airflow mode to footwell and windshield combined air flow and then set, maintain, your temperature setpoint to your own personal comfort level, or that of your passenger(s).

Saturday morning I drove home, Portland to Seattle eastside, ~200 miles, temperature hovering around 34-36, overcast and light snow flurries the full way.

The significant level of warm airflow diverted to the windshield surface helped to keep mine and my wife's upper body warm. Otherwise NipponDenso, Denso US' unique dual layer airflow would have left our upper body subject only to the radiant cooling effects of the surrounding landscape amd more likely than not an encounter, an episode, with sudden and unexpected windshield fogging.

*** In earlier times I resorted to unplugging the a/c clutch circuit.

In my '92 LS400 I added a "winter mode" operational switch so the OAT sensor always indicated sub-freezing conditions during the winter months. Lexus disables the A/C entirely if the OAT is below ~35F.

My '01 AWD RX300 has two c-best optional settings that address this issue. one of those allows you to disable the A/C indefinitely by simply switching it off manually one time. The second unlinks the A/C from operating automatically (with no A/C "on" indication) in defrost/defog/demist mode.

Later Lexus model have additional C-best options, one that prevents the system from automatically switching into cooling mode when the cabin temperature rises to within a few degrees of your temperature setpoint. A bit discomforting , that.

I notice that '08 models automatically switch to heating, footwell airflow with slight flow to the windshield, once OAT declines to the point of the A/C being disabled.

Previous models are prone, HIGHLY prone, to instances of sudden windshield fogging when the A/C is automatically disabled as the OAT declines below ~35F.

'90 LS400 to '08 models, 18 years and they still haven't gotten it "right".

That's what they meant by...

"Relentless Pursuit of Perfection"

Relentless = NEVER getting to your goal(s).

Do you suppose that's why they changed it too....??:

"Passionate Pursuit of Perfection"

[LexKid630]

So you're saying it's best to have "A/C" switched off, and the mode on "defrosters and footwell"? That's nice to know! I'll try it when i go home from work today. I do notice that sudden window fog that seems impossible to get rid of...

[SW03ES]

Mine stays on "A/C" and "AUTO" all the time regardless of the circumstances and I stay toasty warm.

[RX in NC]

My wife leaves her RX on A/C auto mode all the time as well. I've found that both of the Lexus vehicles we've owned seem to produce more dashboard outgasses than any other vehicles we've had in our stable. As long as I do a good interior glass cleaning on her windshield every couple of weeks, interior glass fogging hasn't been a problem. But if I fail to do that, then she will complain of fogging upon engine start-up. I just use an old cotton diaper (clean and dry of course) and whatever glass cleaner we have on the laundry room shelf at the time. The key is to use more elbow grease than you do glass cleaner and therefore remove as much residue on the interior glass as is possible. I spend 5 to 6 minutes on the interior windshield whereas most people probably spend no more than a minute or two making a couple of passes with their rag. Then they wonder why their windshield is still fogging up after they've "cleaned" it. They've actually done nothing more than spread the dirt and grime around....

[wwest]

Mine stays on "A/C" and "AUTO" all the time regardless of the circumstances and I stay toasty warm.

Does your climate control automatically switch into "cooling" mode once the cabin temperature almost reaches the setpoint? Or has the dealer set the system otherwise using the c-best option? Or do you simply switch it back into heating, footwell airflow, yourself?

I have found that on a COLD day, especially when the surrounding landscape has already chilled and the radiant heat equation is well into the negative, the automatic switch into cooling mode has a definite chilling effect on my face, neck and entire upper body.

So I have ALWAYS over-ridden the system out of the cooling mode on cool or cold days.

I recently found that it helps even more to have it in combined mode wherein I get some level, however minor, of warm airflow to the upper/forward portions of the cabin to help combat the adverse radiant effects.

And given the extreme propensity of these Denso climate control designs to fog over the interior surface of the windshield, especially suddenly and unexpectedly on/in certain "upset"(***) or climatic conditions, it doesn't hurt at all to keep that windshield interior surface WARMED as far above dewpoint as is possible.

***Introducing a higher moisture load into the cabin, say sweaty/wet snow skiiers or even picking up someone after work that has stood in the rain briefly. Or maybe even having the A/C go "off-line" due to declining OAT or having switched it off manually wherein the previously accumulated condensate will now be free(d) to enter the cabin and maybe raise the cabin Rh significantly.

When you first start the vehicle the climate control system is well prepared, and will provide a significant level of HEAT, to the windshield in defrost/defog/demist mode, but once the cabin temperature rises to the point wherein it will normally switch into cooling mode there is no way to provide HEATED airflow to the windshield absent simultaneously RADICALLY turning ip the temperature setpoint. Once the cabin is warmed to within a few degrees of your setpoint the system relies EXCLUSIVELY on the A/C's ability to dehumidify the incoming FRESH airflow.

That's why the latest firmware revision ('08 MY) involves automatically switching the system into HEATING mode once OAT declines below about 35F and the A/C compressor is DISABLED. Not that the A/C wasn't already useless for this purpose to begin with as result of the Rh of the outside atmosphere.

[SW03ES]

Does your climate control automatically switch into "cooling" mode once the cabin temperature almost reaches the setpoint? Or has the dealer set the system otherwise using the c-best option? Or do you simply switch it back into heating, footwell airflow, yourself?

It switches to circulated air if the temp has reached its setpoint. No c-best settings have been changed.

As I said before I leave it on "AUTO" and "A/C", I don't switch it to anything it all.

[wwest]

Does your climate control automatically switch into "cooling" mode once the cabin temperature almost reaches the setpoint? Or has the dealer set the system otherwise using the c-best option? Or do you simply switch it back into heating, footwell airflow, yourself?

It switches to circulated air if the temp has reached its setpoint. No c-best settings have been changed.

As I said before I leave it on "AUTO" and "A/C", I don't switch it to anything it all.

"...switches to circulated air..."

Do you mean recirculating airflow as opposed to fresh.....?

Or did you mean to say that it switches to dash outlet airflow....??

NOT implying YOU switch, switched, anything, just trying to clarify if the dealer preset some of the c-best option given your area, climate, of operation.

[SW03ES]

I mean it switches to air that is normal temperature, niether hot nor cold.

[wwest]

I mean it switches to air that is normal temperature, niether hot nor cold.

Then I take it you wear a fairly heavy long-sleeved jacket when you drive your Lexus as the owners manual states that switching to normal airflow results in cooler airflow than via the footwell. My measurements indicate "normal" airflow temperatures as much as 20F cooler than in heating mode. And don't forget that with the A/C in operation the CHILL factor of that airflow will sometimes be elevated due to the dehumidification of the incoming airflow.

If I'm going to drive any distance then my perferred mode of dress is tenny-runners, blue jeans and a short sleeve mesh football jersey over a cotton undershirt. So to remain comfortable I probably NEED more heating "upstairs" than a "fully" clothed driver.

[wwest]

I mean it switches to air that is normal temperature, niether hot nor cold.

Sorry, my apologies in advance, but to be more exactingly correct do you mean to say that your climate control system switches from ONLY footwell airflow into dash outlet airflow or combined dash/footwell airflow...??

The system will return to "normal" airflow temperature, a more comfortable airflow temperature, after the cabin is initially heated up even if left, forced, or simply remaining in footwell mode.

[SW03ES]

No, I never drive with a jacket or coat on. The only time I do is if I'm wearing a suit and an overcoat.

The air is cooler than the footwell, but its not cold. Its room temperature.

[wwest]

No, I never drive with a jacket or coat on. The only time I do is if I'm wearing a suit and an overcoat.

The air is cooler than the footwell, but its not cold. Its room temperature.

"...Its room temperature...."

My point, EXACTLY...!!

When the radiant heat equation is in the negative, your body is "giving up" more heat than the surrounding landscape is contributing as "input". Your body will ALWAYS feel COLD even with a HEATED surrounding atmosphere.

The warm, heated, atmosphere surrounding your body, localized to your body, can never bring the Human Comfort equation into balance on a cold winter day, let alone a cold wintery night. It will help, but not make you fully comfortable.

Think of yourself in a chamber built with blocks of ice, you can make the the surrounding atmosphere as warm as your body can tolerate and you will STILL feel cold, absent multiple layers of clothing.

[SW03ES]

As usual I have no idea what you're talking about. All I know is I leave the system on "AUTO" and "A/C" and I stay very comfortable.