[youknowimwhoa]

It is evident that if I have the heat on, my gas mileage will decrease.

But...

When I have the temp on 85, the fan on off, and I still feel hot air coming out of the vent, does this mess with my gas mileage?

I would like to think this air is hot air off the engine, but when I switch the temp to 65, cold air comes out.

Can anyone explain this to me.

[shockey`]

you have it on outside vent, and when your moving the wind pushes air into your cabin without the fan on.

[suprsuby]

Originally Posted by shockey`

you have it on outside vent, and when your moving the wind pushes air into your cabin without the fan on.

Yep. And the vent switches from the A/C thing to the heat from the engine, so it'll slightly feed that air in but the fans not on.

[youknowimwhoa]

Originally Posted by suprsuby

Yep. And the vent switches from the A/C thing to the heat from the engine, so it'll slightly feed that air in but the fans not on.

Exactly. Now that you guys clarified it, does this use any gasoline or is it like a passive source of air?

[Peleg]

The heater uses the heat from the engine block? I remember seeing this option in some cars "engine block heater"............

[Tortfeasor STi]

If the car's interior ambient temperature is higher then the setting the car will vent cold air into the cabin. If the car's interior temperature is lower then the setting it turns on the heater. All the fan controls is the fan. The heating and cooling systems are controlled by the temperature setting.

By the way, a block heater is not using the heat off the engine to heat the interior. A block heater is a device used to warm up the block in extremely cold temperatures.

[youknowimwhoa]

Originally Posted by Tortfeasor STi

If the car's interior ambient temperature is higher then the setting the car will vent cold air into the cabin. If the car's interior temperature is lower then the setting it turns on the heater. All the fan controls is the fan. The heating and cooling systems are controlled by the temperature setting.

By the way, a block heater is not using the heat off the engine to heat the interior. A block heater is a device used to warm up the block in extremely cold temperatures.

Just what I was going to say about the engine block heater to Peleg.

And your description of the heat makes the most sense. Your answer to my question is 100% probable, and I believe that is most plausible. Thanks.

[donmei]

turning on or off the heat does not affect fuel economy to any real extent.

You could argue that the electrical load from the heater/ac fan causes the alternator to put more drag on the engine (which it does, there is no free lunch). But the difference is negligable. I remember reading that daytime running lights which draw 110 watts affect fuel economy by .3 mpg or so. This should be half of that.

So practically speaking no effect.

Now, if you are asking if fuel economy suffers when it is cold out, the answer is YES. But it has nothing to do with the heat.

And contrary to what many people think, it has nothing to do with denser air being ingested by the engine.

It is entirely due to the increased aerodynamic drag on the car.

Don

[donmei]

just to fill you all in on how a heater works.

Your car makes its coolant hot. Obviously.
Your car has something called a heater core. think of it as another radiator, except this one is inside the car, behind the dash, but on the passenger side of the firewall.

Water flows through the heater core and makes it warm. The heater fan blows air over this core, then out the vents to make heat.

Heat is regulated 2 ways. On some cars there is something called a heater control valve. It regulates the flow of water to the core and therefore adjusts the temp of the air coming out of your vents. This isnt used much anymore because of the time lag it creates. If you turn the air to cool, it takes time for the water in teh core to cool down, delaying the cooling of the air.

Most cars use a diverter flap that routes a percent of the air around the core depending on the temperature requested. This method also eliminates the heater control valve, which can be a cause of problems. When one goes bad, it leaks antifreeze all inside the car. not fun.

Air comes out even if the fan is off because the air intake is positioned in a place where there is high pressure, so even if the fan is off, there is some flow.

If you want to stop the flow alltogether, push the recirculate buton with the heat off.

Our car, with a semi-automatic climate control can adjust the heater control flap to maintain temp, as well as adjust fan speeds.

A fully automatic system would be able to change the vents being used as well as switch on or off the A/C compressor as needed. Two things our system can not do.

One other thing our car, and any car with a climate control does. It does not just measure air temp. It also measures solar effects. On the dash is a dot. It measures solar heating. It works this way. If you are driving and the sun is shining very brightly on you, you will feel warmer than the actual temp of the air in the car. So the system takes this into account. If you want to try it, cover the dot with your hand on a cold but sunny day. You will feel the heat and/or the fan speed rachet up. Btw, the air intake for the air temp sensor is on the center stack of the dash.

Don