Turn the heating on

My first suggestion is a thorough reading of the appropriate section of your owner's manual.

The "auto" setting for the climate control works very well. To deviate from its settings, you need to read the manual and follow directions.

When trying to defrost, or eliminate moisture from the inside glass, it is essential to have the AC compressor engaged - this is what is going to pull the moisture out of the air.

 

Not really sure as I am a new owner myself.
But my guess is that you could turn the heat up by hitting the temp up button.

Also every time I use Auto for a/c the blower starts blowing up full speed, can this setting be changes somehow?

 

Set your climate control system to A/C Auto. Make sure the A/C compressor is enabled. To get heat, turn up the temperature setting. To get cooling, turn down the temperature setting. Press the defrost button if you need to clear the windshield.

Just as a note, at 78°F you will not have frost on your windshield. It may fog up, but it certainly won't freeze into frost.

Tom

 

The defroster on the Prius is very effective. If you still have fog after a few seconds, I would suspect one of two things:

1) The A/C compressor is disabled.

or

2) You have a dirty spot on the inside of the windshield that holds moisture.

Tom

 

It's been relatively hot and humid there recently, true? so maybe condensation on the *outside* of the glass? Otherwise a coolant leak in the heater core is an ugly possibility; got any funny odors or weird fluids on the inside of the glass?

There's no way to explicitly turn on the heater. You just have to set the temperature higher.

And yes, closely read through the entire Owner's Manual.

 

because you didn't allow the climate control to fuction properly (AC enabled). AC isn't just about cooling air, it's also about extracting moisture. hitting the "Demist" button without AC enabled will just keep blowing the same cold damp air at the screen.

 

Hitting the demist button turns the AC button on (does on mine) and cranks the fan speed up to max. Works a treat.

 

This sounds like the most likely cause. If the window glass temperature drops to the dew point of the outside air, condensation will form on the outside of the glass. Its the same thing that causes moisture to form on the outside of a cold drink container in the summertime. The eastern US has been having dew points in the 70s the past few weeks so it doesn't take much cooled air blowing on the windshield to cause condensation.

 

In my parts, I turn on the heat by rolling down the windows. j/k

 

The fan and compressor are controlled to respond roughly in proportion to the deviation from temperature setpoint.

If the temperature inside the car is just a little higher than the setpoint, the fan and compressor will only deliver a little cooling air and a little cooling. If, as is usually the case when you first turn a/c on, the temperature inside the car is high and the setpoint is normal, the difference is large so the fan and compressor will run as hard as they can to bring the temperature down quickly.

If if the noise and draft bother you more than the heat, you can raise the temperature setpoint until the fan slows down. You can then progressively lower the temperature setpoint to your preverred setting as the car cools.

Running in Eco mode will also limit compressor operation to save fuel. I'm not sure what effect it has on the fan.