Run AC in Winter

If they don't mention it in the manual I'm not going to worry about it.

I probably will end up using the defrost occasionally and that will exercise things.

 

Yes, I have seen this recommendation either in Owner's or Repair Manual.

 

Just leave the a/c on and in the Auto setting. That way it runs it when needed such as on a damp morning. That way your windscreen doesn't mist up, you keep the car dry inside, the seals don't dry out and you also stay warm.

 

^ I do not know about leaving AC on, all the time. With ours, if I depress the AC button, the light is on, and as far as I know: the AC's always running, albeit cycling on/off. I can't abide that; I know it's got an impact on mpg, roughly 10%.

That said, it is a good idea to run AC occasionally, to keep the oil in the system homogenized. With our rainy winters, in particular with more people in the car, wet clothes etc, if it's really starting to fog up I'll give up and turn AC on for duration of the drive. That happens frequently enough to satisfy the need for circulating the oil, I think.

I can see running AC all the time in a cab though, different situation.

 

I imagine that it's a really light duty cycle when it's cold out. Definitely not worth 10%.

 

Ok, I may be exaggerating a bit, lol. But we have a regular run into town, and I keep an eye on my ScanGuage's accumulating mpg for the trip. Without AC, by the time we get home, I'll typically be close to 4.0 liters/100 km. Maybe even under.

With AC: not so good, and it takes more effort to get what I get. OTOH, the times I run AC tend to be nasty wet nights, so headlights, wipers, increased friction from wet roads, all tend to be factors. Plus, I'm less inclined to be paying attention to "hypermiling", more involved in seeing the lane lines in the wet glare.

I guess I treat AC like antibiotics: use only as needed.

 

Enabling the a/c (turning it on) does not mean that the compressor runs. It means that the compressor runs if its needed.

In winter the compressor is generally not needed to cool the car.

In cool humid weather (eg. coastal BC) the compressor is however often needed to dehumidify the car. If you don't enable it, condensation inside the car can be expected and the risk of mold is far higher. Think foggy windows, unpleasant odor, possible adverse health effects.

In really cold weather (eg. Canadian prairies) you get enough dehumidification from heating outdoor air which has almost no moisture in it and the compressor won't start anyway even though it is enabled.

 

As far as I know, there's no humidity sensor in the AC system, only temperature. And just my feeling: whenever the AC button is depressed the AC is cycling on frequently. There might be an XGauge available for ScanGuage to monitor that, I'm gonna look into it. But it's very easy to tell by the feel, AC button depressed, the air has that dried feel. It's not running sporadically, it's running frequently.

Also, nothing empirical, but I'd suspect frequent AC use to promote mold growth: by it's nature it's accumulating moisture on it's fins inside the ventilation system. Anytime I've run AC, then shut the car down and it sits for an hour or two, then try to start up without AC, there's a temporary dump of humidity, everything fogs up for 15 minutes or so.

Sorry, not sold. I'll continue to use it strategically.

Addendum: yeah there is an XGuage, for AC power usage:

Toyota/Lexus/Scion Specific : Linear Logic : Home of the ScanGauge

 

AC use does leave residual moisture in the evaporator fins--I definitely agree with that. It can be difficult to avoid having to use it again and again as a result.
Even if the compressor's running frequently, the current it draws will likely be lower with the condensor so cold.

 

According to the Repair Manual description of how the Climate Control system works there is a humidity sensor.

 

Integrated with the temperature sensor would typically mean that its in the same housing. May have a single part number. Wiring to the temperature sensor and humidity seems be separate in this case. Once that was always the case but now they sometimes use active sensors that transmit digital data rather than passive analog devices.

 

Read the last paragraph in the attached description again. It describes more than just an analog temperature signal. This sensor also sends appropriate signals to the AC amplifier.

 

With a humidity sensor, this would explain AC running frequently in cool (and damp) weather.

Still, I'm in the sparing use camp.

 

As others have said, there is a humidity sensor.

I'm not sure how much our a/c runs but window fogging tells me it's not not enough in cool humid weather. It may run more if we stay out of ECO under those conditions so we will try that.

IIRC the Prius compressor is variable speed and can run continuously at low which is great for economical cooling with good temperature control.

For dehumidifying it's better to cycle it on and off to maximize condensation on the coil so thats what probably what the Prius does when dehumidification is needed.

The way it should work is that moisture condenses on the cooling coil, flows into a drain pan and then through a drain to outside. If the a/c is configured with the coil is upstream of the fan, there needs to be a trap (like under a sink) so that negative air pressure at the drain pan does not prevent drainage.

If the car is stopped with the a/c condensing, some water will remain on the coil and in the pan which can/will lead to conditions favorable to mold growth.

Running the fan for a few minutes after shutting the a/c down should help deal with this issue. I'm not sure why they don't program that into the a/c controls but you can do it manually if you wish. I must admit that I typically don't think of it but I'll give it a try myself.

Good design practice is to provide really good access to the drain pan and coil for inspection and cleaning. Access is now required in building a/c but is rarely if ever provided in car a/c (space and layout issues).

 

One of my previous Ford products had a tube with a Schraeder valve end next to the radiator for blowing out the evaporator drain with low pressure air.
I never needed it but it was good to see.