Solving the great automotive mysteries of life: Using the AC / Heater system- Mtbr.com

# Thread: Solving the great automotive mysteries of life: Using the AC / Heater system

1. ## Solving the great automotive mysteries of life: Using the AC / Heater system

I was born into the car repair business, and as I approach AARP membership age, I am still baffled by the inability of the many to be able to correctly operate the AC / heater system of their vehicle.

I've seen people driving down the road with all the windows fogged up to the point that it's actually raining inside the car.

I've seen people inadvertently cultivate cat piss smelling mold that would gag a maggot inside their car.

And I've gotten into people's cars and found the fan on the highest speed, and all the dash vents closed.

WTF.

So maybe this will help:

Vehicles use various buttons, knobs, and switches to control the following functions, but they all control the following items:

Function control: Diverts the air output to where you want it to come out. There are vents by your feet, vents in the dash, and vents at the base of the windshield.

Temperature control: Controls the "mix" of unheated and heated air.

Blower motor control: Controls the speed of the fan that moves the air.

AC control: Controls the activation of the AC system.

Recirc control: Shuts off the fresh air intake, "recycling" air from within the vehicle.

Some of the controls may be integrated into each other, and some of the terminology may vary. For example, "MAX AC" means that in addition to activating the AC system, it also activates the Recirc control.

I'll try out some examples of situations, and recommend the most appropriate use of the AC / heater system:

2. Scenario # 1:

Let's say that it's 110 degrees outside, and it's 160 degrees in your parked car when you get in. You want to get cooled off as quick as possible.

Roll the fricking windows down. That will get you from 160 degrees to 110 way faster than the AC system is capable of. Roll them back up as soon as the temperature inside the car is the same as the temperature outside the car. Probably just a few seconds if start driving immediately.

This is probably the ONLY situation that you want to use "MAX AC" (or "Recirc" and "AC" at the same time). This will recirculate the air through the AC evaporator core again and again, until you reach an acceptable temperature. The important part is that as soon as you can bear to turn it from "MAX AC" to "AC", (or turn the "Recirc" control back to "Fresh"), the better. The more you recirculate, the more that your breath passes through the dark, damp, and usually warm interior of the AC / heater case. It grows mold from your own halitosis. Some of the stinkiest, foulest, cat-piss smelling mold known to man. It's really hard to kill, and incredibly expensive to remove- on most cars, you have to drop the steering column and pull the dash to even reach the AC / heater case. Then you have to disconnect all the lines to the the heater core and evaporator core. Then they split the case and clean it out with bleach water. Then it goes back together, refill and bleed the engine's cooling system, and evac/recharge the AC system. Hopefully the mold doesn't come back, because it's not hard to spend \$1,000 each time you do this.

Turn the temperature control all the way down. Leave it all the way down until AFTER you've gone from "MAX AC" to "AC" or turned off the "Recirc" control. No sense in turning the temperature up and continuing to recirculate. Once you're just using regular "AC" and the temperature reaches a bearable level, go ahead and adjust the temperature control to a comfortable level.

Turn the fan speed all the way up. Leave it all the way up until after you've gone from "MAX AC" to "AC" or shut the "Recirc" control off. More air speed means more cooling at this point. Once you reach a bearable temperature, go ahead and adjust the fan speed to a comfortable level.

Turn the function control to the dash vent setting, open all the dash vents, and point them at the passengers. Once you reach a bearable temperature level, go ahead and adjust the dash vents to a comfortable position.

3. Scenario # 2:

Let's say it's 15 degrees out, and you've got frost on the windows.

Start the engine first. The heat for the heater system comes from the cooling system of the vehicle's engine.

Turn the temperature control all the way up. You need heat to melt frost and to keep it from re-forming on the windows. Once the windows are clear, go ahead and adjust the temperature to a comfortable level.

Turn the function control to the windshield vents. On most vehicles, this is labeled as "Defrost", although newer cars tend to use icons to communicate the same thing.

Turn the AC system on. On many vehicles, the AC system is tied to the defrost setting of the function control, and you'll see the "AC" indicator light come on as soon as you select the "defrost" function.

If you're not in the car turn the fan speed all the way up. The AC system will dehumidify the air inside the car, even if you're not getting any heat out of the heater yet (because the engine is still cold). If you're inside the car, just put the fan speed on the lowest setting until the engine's temperature guage starts to show some heat in the engine, then turn it all the way up. If you turn the fan up while the engine is still cold, you're just blowing cold air on yourself for no reason- you're not going to get any heat until the engine has some to give. Once the windows are clear and you can feel your fingertips, go ahead and adjust the fan speed to a comfortable level.

Your car may also have electrically heated rear window and/or outside rear view mirror defroster elements that you may need to turn on seperately- usually a little button that you push, that lights up with a little toaster-looking icon. They are on a timer- you won't need to shut them off.

4. Scenario # 3:

Let's say it's cold and rainy, and your windows are fogged up, and you need to clear them.

TURN THE AC SYSTEM ON. Don't make the mistake of thinking that the AC system's only function is to make you cold. The AC system dehumidifies the air, and you're trying to get rid of moisture. You can use the temperature control to adjust the temperature to a comfortable level. This is the first mistake that the "terrerium people" make.

DO NOT USE "MAX AC" OR "RECIRC". To do so would be to just keep putting the same moisture back up on the windshield. This is the second mistake that the "terrerium people" make.

Turn the temperature control all the way up. The heat will help to get the moisture off the glass, and into the air, where the AC system can remove it from the vehicle. Once the windows are clear, you can turn the temperature down to a comfortable level.

Turn the fan speed all the way up. Large air movement will work with the heat to get the moisture off the glass. Once the windows are clear, you can turn it down to a comfortable level.

Turn the function control to "defrost" (or to the windshield vent icon). You need to concentrate your efforts on the windshield, for safe driving.

If your car has electrically heated rear window defroster, you can use it to defog also.

5. Some general common sense things:

DO NOT USE "MAX AC" OR "RECIRC" UNLESS YOU HAVE A REALLY, REALLY GOOD REASON TO. And that reason should be more important than the risk that you choose to run, of growing cat-pee mold smell inside your car.

Don't close off the dash vents to try to control the temperature or the air speed. Use the temperature control to control the temperature, and use the fan speed control to control the volume of air movement. It's pointless to have the fan on "high", and then shut the vents off to stop the air flow. You're just going to burn out a blower motor and/or resistor.

There's no point in having the fan off, or on the lowest speed, and then trying to use the temperature control to make large, rapid temperature changes inside the car- there's no freekin' air moving to accomplish your desired change of temperature. If you're trying to majorly cool off or warm up the inside of the car, turn the fricking fan speed up, at least until you've achieved the change that you were shooting for.

There's no point in driving around with the AC / heater system completely shut off, even if you're driving with the windows down. Even if you're completely comfortable in the car, you can set the function control to the windshield/defrost vents, put the fan speed on low, use regular AC (not "MAX AC" or "Recirc"), and use the temperature control to maintain your comfort level. It will keep the AC system in good running order, and it will keep the inside of your car dry.

6. I've heard Click and Clack say Recirc doesn't totally block off outside air. There was a good opportunity to test that yesterday here in Lubbock. We had 30 mph winds gusting to 50 and tons of dust. Sure enough, after 10 minutes of driving with recirc on, there was some noticeable dust on the vent grates.

My experience with mold and smells has always been linked to standing water, wet shoes, (or forgotten produce ) somewhere in the car. I would give the manufacturer some credit and assume they anticipated having breathing humans in the cabin.

I had a '90 Civic with a huge greenhouse and a weak A/C that pretty much required recirculated air to do any cooling. In the three years I owned it nothing grew or smelled besides me. I would agree that keeping the fan on will move air and keep the cabin fresher, but regardless of the dials or buttons you're getting some outside air.

7. Thanks for the thread, helps a lot. The biggest thing I saw in it was that AC isn't just for cold stuff. Didn't know that.

Although I don't understand why people would leave their windows closed when they get in their 160 degree F car...

8. Originally Posted by AWDfreak
Thanks for the thread, helps a lot. The biggest thing I saw in it was that AC isn't just for cold stuff. Didn't know that.
Happy to help. Try it out, I think you'll be pleasantly surprised.

Although I don't understand why people would leave their windows closed when they get in their 160 degree F car...
Me neither, but I've seen it numerous times.

9. CRX Pilot, you're getting hung up on semantics. Every vehicle is different, but If someone shut off the outside air intake to your body, to the same degree that your car's recirc door shuts off the outside air intake to your AC / heater system, you'd die.

I can assure you that me pulling countless dashes out of vehicles is a lot different than you listening to a radio show hosted by a couple of comedians that used to have a garage back in 1973.

10. I live in arizona, and always use recirc when I'm using the A/C, it's more efficient, it costs less, and it never fogs anything.

Recirc doesn't really shut off outside air. Cars are not air-tight, just like the airplanes I fly are not air-tight. It shuts off the majority of the outside air, but due to partial pressure of gasses, you get exchange of gasses still.

It's rather simple, I set up the A/C so it blows the coldest air, so I set the controls appropriately. When the air has reached the temperature I desire, I turn it down, but I keep it on recirc, otherwise I'm just cooling down extra-hot air from outside.

11. ## Doesn't this belong....

In some kind of car talk forum or something?

Where's the MTB content?

12. Originally Posted by pimpbot
In some kind of car talk forum or something?

Where's the MTB content?
After MTBing real hard one day in AZ (in the mountains, but still pretty damn hot) I got in my car with the A/C blasting and it totally screwed me up, just too big of a shock on my body to go from that hot to that cold. Since then, I do NOT put on the A/C after riding in 90+ weather, I wait with the windows down, even if my car is well into the hundreds.

13. Originally Posted by Jayem
I live in arizona, and always use recirc when I'm using the A/C, it's more efficient, it costs less, and it never fogs anything.

Recirc doesn't really shut off outside air. Cars are not air-tight, just like the airplanes I fly are not air-tight. It shuts off the majority of the outside air, but due to partial pressure of gasses, you get exchange of gasses still.

It's rather simple, I set up the A/C so it blows the coldest air, so I set the controls appropriately. When the air has reached the temperature I desire, I turn it down, but I keep it on recirc, otherwise I'm just cooling down extra-hot air from outside.
Well, you're almost doing it right, although I have no idea what you mean by "more efficient" and "costs less".

Arizona is dry enough that your chances of growing funky mold in your AC / heater case may not be as high as in other places on our planet.

Or...

The inside of your car may actually reek like stale cat piss, and you're just used to your own funk.

Most people are not aware of their own odor (bad breath, body odor, whatever), and most of the people in their lives are too afraid of hurt feelings to mention it.

In the auto repair profession, the way that it normally comes up is something like one of the following actual customer stories:

1) I called a guy's house to talk to him about his car. The wife picked up the phone. I explained that my technician was refusing to get into her husband's car to bring it inside to do his 30,000 mile service, because it stank so bad inside. She blurted out that that she had smelled it too. She had gotten into her husband's car, and thought that a cat had gotten trapped inside and sprayed. When she talked to her husband (the primary driver), he told her that he didn't smell anything, and that she was nuts. She thanked me, because she was beginning to doubt her own sanity, as she was beginning to smell the odor on her husband's clothes just from him driving the car.

2) Young guy with a cool car, invites the hot new female co-worker to lunch. At first, she seemed really interested, but when she got in his car, she wouldn't roll the window up, and was leaning so far to the right that her head was almost out the window. He thought that she might be too warm, so he turned the AC on a little more. She glared at him, reached over, and shut it off. They had an awkward, quiet lunch, and then she produced some lame excuse not to ride back to work with him, and called a friend of hers for a ride. Several days later, a male co-worker told him that he had overheard her telling other employees that she thought that he was incontinent and made a habit of pissing himself in the car. He had no idea.

3) Soccer mom with kids, casually mentioned that her kids are always getting sick with upper respiratory infections, while we were both getting a cup of coffee in the waiting room during her oil change visit. I just happened to pick her car at random to do a Quality Control check, and the smell was enough to gag a maggot. I initially thought that there must be a two-week old piss-soaked disposable diaper under one of the rows of seats, but a quick check revealed none. The lube flunky hadn't mentioned anything, but he was a smoker and probably couldn't smell anything anyway, and probably wouldn't care even if he did.

In all three cases, the primary operator of the vehicle used the recirc function pretty much all the time, because they didin't know any better.

Why do you think cars even have a fresh air intake?

14. Originally Posted by RIS
Arizona is dry enough that your chances of growing funky mold in your AC / heater case may not be as high as in other places on our planet.
Correct.

It's better because it produces colder air (by using re-circ). More efficient because I can use a lower setting.

Your entire premise might make sense to idiots, but the idea that you "shouldn't use the recirc" is equally idiodic. Yes, if it's all you use, you leave the windows rolled up all the time, and you live in some place that has a good deal of humidity, you might get fungus and mold, no doubt. If you aren't an idiot, you can use recirc quite a bit and never have these problems.

15. Originally Posted by Jayem
Correct.
Yes, I know. When I post something, it's almost always because it's correct. To do anything else would be kind of pointless.

It's better because it produces colder air (by using re-circ). More efficient because I can use a lower setting.
How did you go from "more efficient" and "costs less" to "better" and "more efficient".?

What do you mean by "a lower setting"?

Is it that you feel you're conserving something? The engine mounted AC compressor takes the same amount of horsepower to run regardless of what fan, function, temperature, or recirc setting that you select.

Your entire premise might make sense to idiots, but the idea that you "shouldn't use the recirc" is equally idiodic. Yes, if it's all you use, you leave the windows rolled up all the time, and you live in some place that has a good deal of humidity, you might get fungus and mold, no doubt. If you aren't an idiot, you can use recirc quite a bit and never have these problems.
Your car doesn't have an idiot sensor, although if you habitually drive around with "Recirc" or "MAX AC" on and the windows down, we can probably make a safe assumption about you in that area.

The way that it works is that If you do something wrong, you run the risk of something bad happening, regardless of whether you're an idiot or not.

16. you can use your max ac and recirc to your hearts content. its perfectly fine, zero problems what so ever in all conditions. if your car stinks, its because you're a dirty foul person. you should take a shower and not leave rotting french fries under your seats.

someone with reasonable hygiene and clean habits isnt going to have a disgusting car simply because they use the ac.

your ac stays colder recirculating cool air from the cabin than it does sucking air off your 120+ degree black cowl. im not talking about economy and fuel mileage and system efficiency, you wont notice that. you'll notice colder air exiting your vent though. its just common sense that its easier to keep 70 degree air cool than it is 130 degree air.

you DO however, need to click it to fresh before you get out, to vent the car. if you have a newer car, chances are it does this automatically for you. if you run your ac in your driveway, you'll notice puddles of water from the ac system, putting it on fresh lets this dry out.

if you spray water in a basement and close the door, it gets funky. if you spray water on your driveway its gone in a few hours. same concept. just needs to get vented out.

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