Air recirculation in the winter--automatically disabled under 32 degrees

???????? HUH????
I don't know what you mean....
I doesn't build up, or it shouldn't, and if it does the drain is at least partially glogged.
Clear the drain and pour some vinegar in where the cabin air filter is...

 

There's plenty of threads on here about mildew smell coming from the vents because when the moisture builds up on the condenser and then eventually melts and drains out the tube under the car, the place where all that occurs can't be easily cleaned and people who aren't fussy about most smells can get extremely fussy of the slightest hint of that particular smell. For me I sometimes notice it, but it doesn't last long and I'm the kind of person who freaks out about other peoples car exhaust that is deadly poison rather than mildew smells which are almost always harmless.

 

That's funny!
According to your statement, seems you are the one having the issues.
Maybe you should seek help?
One of the solutions we did for Mercedes was spray a whole can of lysol in the
air intake for the ac/heater, then put it on recirculate for 15 minutes. It always seems to
kill any smells.

 

I made it clear it's not an issue for me... I was simply explaining to you what those discussions have been about on here over the years.

 

Technical note: setting vent mode to Heat/Defog will run AC, as often as not. I think at lower watts, and dependent on some parameters the car is sensing. At least on our CDN 2010, and even if AC button is NOT depressed, AC indicator light off.

This was monitoring with a scangauge xgauge, monitoring AC wattage: it would cycle on and off, in lowish wattages.

 

I don't know the exact reason for it, I assume that it has to do with reducing moisture build-up/ fogging but have never seen documentation to support that. Just know that some cars have been disabling recirculation while in defrost mode for many decades.
Really, I don't care - either about there being a default setting or about any perceived efficiency loss. I believe that you are that first one I have heard about that has this concern.

Posted via the PriusChat mobile app.

 

If the temp is set low, and the fan speed is low, and in recirc mode, the evaporator will tend
to freeze with cooler and cooler air going over the very cold evaporator.
Going to "outside air" brings hotter air over the evaporator keeping it from freezing, and blocking.
Yes, the ETR, or temperature sensor "should" keep this from happening. But not always.
It's the edges that will start to freeze then more and more.
Also, the system will shut off the compressor at verious time because the air is at the set temp.
This also helps in keep the evap clear of ice.
The only time I've notice any mildow smell from a car's ac system is when the drain is clogging or clogged
or the housing is cracked, and water is getting inside the car and on the rug and it not being dried.
There are certain to be other ways for the mildew smell... but.....
As always: result may vary!
As always: result may vary!
As always: result may vary!

 

As for the why, methinks much of car design and refinement has to do with idiot-proofing the systems so everyone drives the car without issue. But smart people tend to not want to be treated like idiots and want/deserve more control.

As for how to fix it, I suspect you could tap into the CAN bus and gather up all the signals that are sent when you touch the touch screen and then set a rasberryPi or Arduino to send pre-programmed signals to give you more control over certain HVAC scenarios. Most aftermarket app developers for Prius have gone through this process and I'd gladly pay significant money if an app on my phone handled all HVAC presets in my Prius so I didn't have to always remember to revert back to my preferred setting every time I start the car up.

 

Like a lot of other situations, just because you have not seen it does not mean that it doesn't exist.

You got the right answer in more than one post in this thread.
You just have to find it among the smoke.
Like a lot of other threads.