Defrosters and good gas mileage seem to be at odds. With a week of rain here, the windshield has been fogging up as I drive (and I'm not recirculating air).

It appears that there is no way to get air flow onto the windshield except in the last two climate control "windshield defrost vent" modes which turn OFF economy mode and auto stop and turn ON the AC. The gas consumption penalty is immediate and obvious. (Gliding along a flat it cut mpg from 120 to 80 until I turned off the defroster)

My only solution has been to run in dashboard vent (or dashboard/ floor vent) mode until the windshield starts to fog up. I then turn on defroster mode with high fan for 30 seconds to clear the windshield, and then return to dashboard vent mode (clicking off the AC). This involves a lot of dial twirling during the drive and I've still lost 1 or 2 mpg at a time when I'm also losing mpg with the energy needed to push through all the water. Trying some heat to reduce relative humidity did not help, the warm hair fogged on contact with the windshield; running with no heat was uncomfortable and the windshield still fogged.

Any other solutions?

 

Your observations are correct. Almost all cars these days use AC to dehumidify the air when set to blow air on the front window. Running the AC does kill mileage. There is nothing I know of that you can do any better than you already are. I guess you could try out that de-fog rain-X.

 

We hit this one before a while back. There is a "hack" that you can do that will allow you to turn off the A/C and use ECON all while using the defroster. What I can't remember is if it worked for the 2004/05 owners. I got it to work on my '03. That's about all I can tell you. You may want to move this one over to the HCH section to get more visibility (haha, a pun) on the subject. Maybe someone has been able to perform the defroster hack on a '05.

 

I'm mulling this over too. The temps here in Abq, NM are not that cold in the mornings yet, but fogged up windows are a nuisance.

The outside humidity is rarely over 30% -- seems like I should be able to take advantage of the fact. Would AC at a low temp, with high flow air inititally non-recirc then recirc be a good approach ?

Perhaps someone can educate me what AC conditions are most efficient at removing moisture, without necessarily requiring heat exchange ?

Any advice for the Prius much appreciated

 

just a thought..

Here's what I do; if temps are below 32F then the AC will not engage and you can use heat to defrost. For temps above 32F, you can set the switch to defrost but turn the fan switch to off and add dial in a little heat plus open a couple of windows just a bit. As you drive, heated air will flow across the windshield and defog. Also, buy some vent shades for the windows so the rain doesn't get in. (This was taken from a previous post by another member and I find that it works quite well).

 

The heater should work at any temp, but will take slightly longer to clear the window when it's warm outside too. Only downside is you just might get a little toasty in the summer...

You need drier air hitting the glass to evaporate the water. That can be drier from the AC dehumidifying it or drier by virtue of the fact that hotter air can hold more moisture than cooler air.

 

According to my reading of the manual (and experiments in the car), the heater does work because you cannot cause air to flow through the windshield vents except when using the defroster mode. In my older cars that was what I always did, turn up heat and direct air to the windshield.

 

It won't help a whole lot, but cracking the front windows ever so slightly will do its part. Certainly won't solve the problem but you may just end up running the AC a few seconds less each time as the air flow from outside brings the interior temp closer to equilibrium with exterior.