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Defrosting windshields -- which way is faster, and why?

L

L00PY

Golden Member
It's wintertime and occassionally frost has formed on the inside of my windshield. Even without actual frost, the humidity from my breathing (or my sweat after a workout) fogs up the windshield pretty rapidly.

Assuming the heat's turned on and is constant, will my windshield defrost / defog faster at the lowest fan speed, highest, or somewhere in between? We'll also assume the air is set to recirculate and the A/C isn't switched on / doesn't turn on automatically.

When the fan's on low, the air coming out of the vents "feels" warmer. Given what I know about heat exchange though, turning it on high probably means more transfer from the heat source into the air.

I'm asking what I think is essentially a heat transfer problem. The one nuance I'm not sure about is whether frost would be enough to create turbulent airflow, thereby increasing heat exchange, and changing the the answer for that situation.

There is the difference I'm not clear about of whether glass clears because the glass is heated (which is via convection), or whether just raising the ambient air temperatures so the relative humidity (and the dew points?) changes would be enough.

So which is better and why?
 
The fan speed doesn't really matter, having the compressor on (def or a/c setting) is what causes fast defrosting because it dehumidifies
 
Turn the AC on. It will remove the moisture faster.

As far as your question is concerned, the logical answer would be: Yes.

 
I know A/C removes moisture faster, but this is for my winter beater. It uses R12(?) instead of RJ-45(?) and the Freon's long since leaked out.
 
Originally posted by: L00PY
I know A/C removes moisture faster, but this is for my winter beater. It uses R12(?) instead of RJ-45(?) and the Freon's long since leaked out.
Click to expand...
I think R-134a is what you're looking for there. 😉
 
the heat melts away (or evaporates or whatever) the frost. So the higher you have the fan on, the faster th eheat is going to spread to the entire window.
 
It's fricking sub zero outside and you want to turn the AC on? Are you insane? I'd just turn the heat on and let it melt away the damn frost.
 
Originally posted by: Baked
It's fricking sub zero outside and you want to turn the AC on? Are you insane? I'd just turn the heat on and let it melt away the damn frost.
Click to expand...

The A/C acts as a dehumidifier.
 
Turning on the AC actually happens automatically on many modern cars when the airflow is set to "defrost". Removing moisture helps a lot -- but it's not helpful in my case.

Originally posted by: FrankyJunior
the heat melts away (or evaporates or whatever) the frost. So the higher you have the fan on, the faster th eheat is going to spread to the entire window.
Click to expand...

Heat doesn't spread across the entire window evenly though. As air flows across the windshield, you have transfer from the air to the window. Thus, air coming out of the vent is significantly warmer than air at the top of the windshield. I suspect it's going to be some function of temperature of air out of the vent, out side temperature, airflow speed, heat transfer speed, and relative humidity. But my physics isn't quite up to snuff to figure it out / get an intuitive answer.
 
And I forgot, in addition to air heating the glass, you do have the effect of heat transfer from the airstream out of the vents to heating the rest of the air in the car. That's likely to be more pronounced at faster airspeeds, no?
 
Most cars should heat up very quickly unless it's very far below zero, unless of course your thermostat is broken.

My car had it's thermostat broken, so it would take ~20 minutes to 'heat up' (with regards to the air being circulated in the interior). I had it fixed, $40 later, it warms up in ~2 minutes (unless its really really cold outside, in which case it's more like ~5 minutes).

 
Originally posted by: JeepinEd
Turn the AC on. It will remove the moisture faster.

As far as your question is concerned, the logical answer would be: Yes.
Click to expand...

Does A/C still work as well if the temp slider is on hot?
 
A/C does not limit you to cold air.

The fastest way to defrost is to have the A/C on.

Most modern cars turn the A/C on automatically in any defrost mode.

This defrosts the windows faster, and it also forces you to run the A/C system in the winter to lube it up and keep it running properly so it will last longer.
 
Originally posted by: Heisenberg
Originally posted by: L00PY
I know A/C removes moisture faster, but this is for my winter beater. It uses R12(?) instead of RJ-45(?) and the Freon's long since leaked out.
Click to expand...
I think R-134a is what you're looking for there. 😉
Click to expand...

Lets hope so...otherwise we all have cheap refridgerants and need to make them work somehow.
 
Speaking of which....where can I buy an ice scraper in SoCal?

It actually does get cold enough some nights where I live (at the foot of the mountains) to form a solid layer of ice over the windshield/windows, like this morning. Tried to find an ice scraper last winter but Home Despot and the local hardware store didn't carry them....
 
And now, for something completely different, here's a serious answer... :shocked:

If your car has air conditioning, and it allows you to turn it on while the temp is set to warm and the air flow to defrost when it's working, do it. The AC dries (desiccates) the air so it will absorb more moisture as it passes over the windshield.

This works even before the engine is warm enough to deliver heat to the system, but if it's cold out, it's more comforable once you've got some temp.

This only on cars that allow you to set the AC this way which exludes most GM cars I've seen.
 
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