Can anyone tell me if running the climate control during winter on auto still runs the AC?...or should I turn the AC off altogether?

I was once told by a car dealer that climate control wasted fuel because the AC and the heater are constantly fighting each other to maintain the temperature selected.

Thanks

 

This applies to the TCH (Camry Hybrid) so YPMV (Your Prius May Vary ), but I doubt it:

In winter, I would suggest keeping Climate Control in AUTO, with AC lit. Compressor will not run, UNLESS it needs to to defrost windshield / windscreen.

As far as heater and AC playing dueling banjos: It is possibly, but normally won't happen.
AC & heater both ON at same time? Yes, IF it is humid: AC compressor would engage to dehumidify interior air & heater on to warm it up.
Heater heat up car, then AC cools down car? NO. Heater warms up the air to desired temp, then lets air cool down a bit, after which it will warm it up again (sounds like the thermostat in your house?).
Only way for heater & AC to "duke it out" is if you manually set temp to let's say 80, heater comes on, then once it gets to set temp, you realize it's too hot, so you manually drop temp to 65, AC cuts in.

I hope this answers you questions.

 

Yep. So in winter I can safley leave the climate control on "auto", without having to worry too much about my fuel economy being negatively affected.

Good explanation. Thanks

 

The automatic climate control as designed/concieved by NipponDenso, or Denso US, the supplier of almost all asian automotive manufacturer's and MOST certainly of Toyota and Lexus is as follows.

The primary thing to keep in mind is that these IDIOT Denso engineers have for some unknown, unknowable, reason decided that an air conditioner's dehumdification capability can be used, EXCLUSIVELY used for windshield & window defogging and/or prevention of same.

NOT...!!!

NEVER...!!

But to that end all of their automatic climate control designs, absent driver intervention or OAT declining below ~35F, will operate the A/C compressor throughout the entire year, four seasons. In the summer for cooling and dehumidification, and in the winter only for dehumidification. Humidification "limiting", really.

For my Lexus LS400, '92, I manually tie-wrap the heater core coolant flow control valve into the CLOSED position during the summer months and simply unplug the A/C compressor clutch circuit when cold weatehr sets in. I also have a low-profile switch mounted under the dash near the driver's left knee that I actuate just prior to switching the climate control to defrost/defog/demist mode. The switch adds a fixed resistance to the IAT thermistor making the climate control ECU "think" the cabin has just suddenly gotten VERY cold.

The result...??

HIGH blower speed and HOT airflow to the interior surface of the windshield the INSTANT I switch system airflow modes. The very same thing that results, automatically results, in my '01 Porsche 911 with a simple switch, ONLY, to defrost/defog/demist mode.

This, EVEN, on the hottest, brightest day of summer. Obviously the Bosch engineers know better than to try and second guess why the driver has activated the defrost/defog/demist mode, or to not even second guess how serious, or not, the windshield fogging condition might be.

By the time my '01 AWD RX300 was available Lexus had implemented two of the corrective suggestions (a matter of court records) I had made back in '93 regarding the flawed climate control design of my '92 LS. I had the dealer change the c-best options from the factory default. One of these allows me to turn off the A/C once, one time, and it will remain off INDEFINITELY (multiple engine restarts, activating "auto" mode, etc.) until I turn it on again. The second c-best option "unlinks" the A/C from operating automatically in any deforst/defog/demist mode partial or full.

So I no longer have to open the A/C compressor clutch in my RX300 during the winter months. But I did have to go to Home Depot and purchase/install a manual coolant shutoff valve to prevent the climate control's reheat/remix cycle from eating FUEL.

I could not use the '92's IAT modification technique in my RX300 because, apparently because, some bright young engineer had added some DSP to prevent the system from reacting to sudden changes in the IAT signal output. But the ability to give the temperature set-point **** a quick twist to a HIGHER setpoint just as I switch the system to defrost/defog/demist mode overcomes that requirement anyway.

So yes, you might be able to improve FE substantially all year around by using these two methods. Disable A/C except when needed for cooling or initial cabin cool down. There may be c-best options for this. Prevent the flow of HOT engine coolant to the heater core with the A/C working.

 

I do not, will not, dispute that the use of the A/C as an aid to defogging or preventing formation of condensation on the windshield and windows is a quite reasonable aspect of automatic climate control systems.

BUT..

The functionality of the use of the A/C for this purpose is at the whim of mother nature.

And frankly, mother nature isn't on our "side" on this issue in cold climates, during cold weather, far from it. And obviously those are the majority, the clear majority, of times our windshields or most subject to interior fogging.

On the other hand moving HEATED airflow to the interior surface of the windshield will always, ALWAYS result in QUICK evaporation of any condensation and warming the windshield, and keeping it warmed, will always be effective for quickly removing interior windshield fogging and prevention thereof.

So, bottom line, A/C may help, but is a real wild card and should only be used, relied upon, for humidification "limiting" as an aid, or backup.

 

Unusually SHARP car dealer you have found, keep him/her.

 

The purpose of using the AC when the defrost is on is somewhat based on US government rules. The government doesn't specifically demand that the AC be used... rather they demand that the fog be cleared from the windshield interior in a certain amount of time. Tests conclude that very "dry" air can be used to get rid of the fog.

Hot air will ALSO clear the fog and very hot air will clear the windshield a good deal faster.

But the problem is when the overnight low was 5ºF outside and you're first starting your car at 7am to go to work you don't have any hot air... and you wont have it for at least 5-10 minutes or more, but you'll probably drive your car anyway.

Hence the use of the AC.

 

"low was 5F outside.."

There is NO automotive A/C that will operate below freezing, they are usually disabled automatically at about 35F and below. Some are disabled as high as 45-50F due to refrigerant "high side" pressure restrictions as a result of the condensor being TOO cold.

Plus: On the type of cold morning startup you describe it is always EXTERIOR frosting of the windshield that is a problem, not interior windshield condensation. And it is the latter circumstance that the government rules are about.

No one especially cares how long it takes to remove exterior frost since the assumption is make that driver's are not stupid enough, and there are LAWS, to drive away with no forward vision.

 

Some of the statements in the posts above are rather presumptuous.

First, the car will be used in extremely different climates. This means some system had to be developed that would work well enough in all of them.

I can't speak for desert climes, but here in Alberta the car A/C Auto works very well, and ditto for "west coast marine".
It defrosts the windshield very well and very fast, even from a cold start. The car will be blowing hot air after driving two to three blocks, but never if left to idle when it's -20C outside - the coolant only reaches 40C at idle from a cold start and the fans will not run at this low coolant temp. But drive the two to three blocks and you get wonderful heat and defrosting.

Here we do not have problems with frost on the outside of the windows (it's dry when it's winter here - hard to have humidity at -20C and below). The defrost is needed here to remove humidity added to the interior air by people, and worst case, when it's snowing by the snow tracked into the vehicle and melting/evaporating inside. I find the defrost works very well, and it starts working in as short as two minutes if I'm driving. Longer if I have to wait at a light. It even gets the side windows completely clear in 5 min or so.

In my experience, the "cooling/drying" part of the system does work very well, even when it's -30C outside. Whether or not it is shut down by the cold outside it defrosts the interior of the windows very quickly.

Note also that there are two 400 Watt electric heaters in the air plenum. I don't see any effect from them here, but those in warmer climes report they work. Something you'd expect, as even a 1500W plug-in car interior heater will only raise the temp to around freezing here in the "dead of winter".

 

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