Efficient ventilation mode?

Yup. I can confirm that's exactly what it does on my car.
Today it was the first warm day of spring (+15C) and my car was parked in the sun (inside it was propably 25C or so).
A/C button on, Auto/Fast/Eco setting was eco and drive mode was eco, temp setting at 21C.
It turned recirc on automatically when car started and then it turned off automatically after a couple of kilometers.
It only does this to cool the car and I bet only when A/C button is on.

Many thanks, now I know.

 

What does the ECO

I think you're both right. A/C on does remove moisture of course (although not sure how much if it's really cold, if at all.).

But let me paint a picture from here where it's really cold.
You go to your car in the morning and it's -20C.
Your hair is wet from the shower you took and a couple of kids get to the car through the snow (dragging their feet) and you put them in the back.
There always a little water on the floor mats from the snow on your/kids shoes.
I think you could drive around for 1 day with recirc on if you wanted, no problem (although I've never tried that).

But the next day?
Your car looks like the inside of a refrigerator that has not been melted in a decade.
And not only do you have to scrape the outside of the windows but the inside too.

Trust me, recirc not an option in the wintertime.
If you drive only short trips you want as much of the fresh/cold dry air in as possible all winter in order to try to keep the car dry and push the moisture out.

But still I usually keep A/C on unless there's a cold spell (-4C or colder all day/night for a few days) only then I shut A/C off completely for longer periods.
If it's something like +4C and a slushy wet snowy day IMO A/C helps to keep the car dry. But all that changes around 0 degrees celsius.

This is just my opinion though, I bet some scientist here knows exactly how this works.

 

Yes. I would imagine in mountains or in a rainstorm in Florida recirc + A/C might be a very good option.
At least if you want cooling and getting rid of moisture at the same time.

It's a tricky thing, I tried looking it up.
"The relative humidity or of an air-water mixture is defined as the ratio of the "partial pressure" of water vapor."

I guess during high pressure (sunshine) relative humidity is lower and in low pressure (rain) higher , not hard to believe that.
And cold air can hold less water than warm. The air is most dry during a sunny day somewhere in Siberia when it's -50C, I guess.

The pressure thing does go over my head though.

I have a heat pump in my house too, but I use it mainly to warm the house (Toshiba Arctic 35 RAS-35G2APV-ND is the model, ND=nordic). These winter models can both cool and heat and I guess the nordic ones have some added melting cycles and heaters on the outside radiator in order to get rid of the ice.

I use it to remove moisture during summer thunderstorms too.

In winter the air is so dry people do the opposite, here they use humidifiers.
I have a humidity sensor inside my house and during a cold spell in winter (-15C to -25C outside) the humidity inside is below 30% at normal room temperature 21-22C.
Your piano might do the opposite here, crack its finish due to drying up too much. .

Doesn't the Prius Prime have a 2-mode heat pump like this? That can also heat not just cool air. The system can be reversed in order to heat or cool?

 

Humidity by Jeppefinn posted Apr 15, 2018 at 3:29 PM

Hey I found this that explains a lot (sorry it's in Finnish).
On the left vertical axis there is amount of water vapour in air grams/cubic meter.
Bottom horizontal axis temperature in celsius.
Right vertical axis is realative humidity as a percentage.

At +30C air can hold 32 grams of water vapour per cubic meter at 100% humidity.
At -10C air can hold only 2,5 grams of water vapour per cubic meter at 100% humidity.

 

Hey, I think got it now (I'm 47 but still can think) why fogging happens so easily in the cold.
It takes very little added water vapour to reach the dew point when it's cold.

Let's say my car is -10C inside in the morning and the relative humidity is at let's say 60% same as outside.
When I get in it only takes a few grams of water vapour inside the car to reach the dew point.

At -10C it takes only a few breaths (or grams of water vapour) to reach the dew point.
At -10C it takes 1 gram of water vapour and at +10C it takes 4 grams of added water vapour in order for the air to reach the dew point.

That's why when you breath out in the cold the there's a vapour cloud, the air reaches dew point so fast that a small cloud forms.

No? Yes?