My heater only blows hot air when the car is moving and cool air when I sit at idle. Also the inside window fog up if I try to use the defroster mode 1st thing.

What make an model an year vehicle do you have ? The air blows out of the floor vents an center vents but not defrost ? Possible mode actuator , stuck mode door ! There is a door inside the heater / A/C assembly that directs the air to the desired position . These doors can be moved by a vacuum operated actuator or an electric motor . Sometimes thing's put on the dash end up inside the heater box , fall through defroster vent ! Pens , pencils etc.. Depends on the make vehicle an system , manual , or automatic climate control . Some of the newer vehicles the HVAC system as it is called , the control panel is a electronic control module an can store DTC'S - diagnostic trouble codes !

That's not unusual if you have a lot of people in the car. I will fog up because the glass is colder then the warm moist air hitting it, so the water condenses on the glass. Two ways to solve this issue. First open the windows a small amount to get rid of the warm air and introduce some cold air. OR second, set your defroster from full hot to slightly on the cool side with air circulation from the outside as well (i.e blend air as opposed to only cabin air)..

the windows are fogging up from the water vapour inside the car hitting the glass and freezing. All the heater is doing is making the water vapour evaporate into smaller particles and freezing quicker. The way to overcome this is to dehumidify the air in the car (take the water vapour out). To do this use the AC as this is what it is designed to do . run the heater at the same time and the glass will clear and stay clear. In most cars this is called climate controlled air conditioning.

try running the ac as well as the heater. The ac removes the humidity (water vapour) in the air in the car which is causing the fogging up . Hot moist air on glass creates fog ( same as clouds). You will get a comfortable temperature inside without the fog

Make sure the vent system is set to draw air from outside. If it is set to recirculate the same air that's inside the vehicle without drawing fresh air from outside, the resulting humidity that builds up inside the vehicle can fog the windows even when you run the defrost. You can also try opening the windows a bit to get some more air moving inside.
If it still fogs when the vents are set properly, you may have a leaky heater core. The heater core is a small radiator that is located under the dash that the heater system blows air through to heat the inside of your car and provide heat to defrost the windshield. The fog may be coming from the heater core leaking hot coolant steam that's being circulated through the defrost system. If this is so, you will need to replace the heater core.

leaking heater core. The coolant level may be low also. What it is doing is fogging up the window with the steam coming from the hot engine coolant that runs through your dash board when the heater or defrost is on. This should be under some kind of warranty on an '08.

Hi. The most common cause of the fogging is the selection of the "recirculate" mode, with the defroster on, on your heater control panel..

The Recirculate mode is the selection where the car has an 'arrow doing a u-turn" in it.
Select the other mode (which is the 'Fresh' mode);
the car 'with an arrow running thru it'.

Recirculate mode is ONLY for use with the AC on.
Always use Fresh with the heater and the defroster.

That was a very good question, and one that I receive quite often.
The owner's manual does not explain this very well.

While in defrost mode press and engage a/c button on dash(leave in hot air position) and see if that helps with defrost issue. What this does is with a/c operating it removes moisture from the air in car which is all fogged windows are is humidity build up.Dont worry cold air wont blow through vents even though a/c on due to position of hot/cold adjuster.

If you're getting hot air, it's not the heater core.

Windows fog up because the humidity level inside the vehicle increases while the temperature of the windows decreases to a point where the dew point of the air is higher than the temperature of the glass. When this happens, moisture from the air condenses onto the glass. When you breath out, you increase the humidity of the air in the car because of the moisture in your breath.

The key for defrost to work isn't necessarily heat, it's more important to dry the air. There are two ways of doing this:

1.) Make sure you have the air recirculation turned off. This feature causes your car to recirculate air from inside the car through the air ducts. In the summer when it's hot and humid outside, this is what you want. However in the winter when the air is dry, you want to bringing fresh dry air from outside to replace the humid air inside the car.

2.) Turn on the A/C compressor. This will cool the air before it passes through the heater core. Cooling air removes moisture. This is why your car drips water from underneath in the summertime when you run the A/C. It is literally removing moisture from the air.

However, if your heater isn't working (blowing cold air even when on the warmest setting), then you may have a different problem.

If you can feel the air from the blower motor blowing up on the windshield it is probably one of two things. Either your heater core has a leak and the anti-freeze mist film is all over the windows or your heater control valve is not working.
Does the heater get hot or can you feel hot air on the windshield? If so your heater core is probably leaking and making steam inside your car.