Assuming that the system is working properly, has enough refrigerant, fins are not bent, cabin air filter clean (or not on this model), how can I get better results in a very sunny and hot climate?

Note: consider that this would be for the time I am driving the car, on trips longer than very brief ones. So, keeping the car cooler while under way.

  • Come on, people, what is really wrong with painting the roof white? Vote early and often. – user13683 May 3 '16 at 23:39

Use windshield shades. Starting with a cooler car reduces the amount of cooling required to get to a comfortable range. You can purchase them from a local automotive retail store or make your own.

If you make your own, be sure to paint or cover with a light colored cloth to reflect the amount of sunlight being absorbed into the shade.

You will also want to fit the shade so that the entire windshield is covered.

Making the shade fit closer to the glass will also prevent "heat leakage" from around the sides of the shade.

  • A shade will only help right at the start. After a few minutes, it won't have mattered. What if I drive for an hour or more? I actually have a silvered mylar bubblepack sort of thing that I bought. It helps, but not while in motion : ) – user13683 Apr 29 '16 at 0:57
  • Are you driving with your windows open? If so, there's not much that can be done to overcome ambient temperatures... – Adam Zuckerman Apr 29 '16 at 0:59
  • I would not use the A/C with the windows open. With them closed, the car gets hotter than the exterior temperature, and only the A/C changes that. If I did not have A/C, yes I would be forced to have windows open. I drove 1000 miles in this car over three days in early September when the A/C was not working. It is very tiring, even in shorts and sandals with no shirt on. But, that was before painting the roof white, too... – user13683 Apr 29 '16 at 1:02
  • If your car air conditioner was hotter than outside, there is a problem with the system. It doesn't work as well when the temperatures are higher than 100F because of the way mechanical refrigeration works. You might have a heater core that is expending heat into the cabin. – Adam Zuckerman Apr 29 '16 at 1:17
  • Possibilities. I guess the time I really need A/C is when it is hotter than 100F outside, which is almost daily for about 2 months here. The A/C actually works properly, but I have a large brown wagon, and the sun is extremely hot, so I was only trying to help things along in an easy and inexpensive way. – user13683 Apr 29 '16 at 1:19

If you have an old car and it is not already a light color (silver, white, tan) you can paint most of the roof white. This reflects away a large amount of sun-heat: about 100 Watts equivalent per square foot. If you have a wagon or other large roof, this could be 15 square feet / 1.5 Sq Meters and so would be as much as a small electric room heater. Imagine running one of those inside your car on a blazing day? Now, throw it out the window. This is the level of improvement I am talking about.

I did this. Touch test: I put my hand on the white part when it had been sitting in the sun for hours: warm but I could leave my hand there indefinitely. Moved my hand one foot over to the "bronze" colored part of the roof: could not leave it there more than two seconds! Big difference. It can be yours, for the cost of a can of spray paint. Doesn't really matter if it is auto paint or some other durable exterior type (it will look rough anyway).

You can do a neat enough job with masking tape and newspaper. Remove the easily removable parts of a luggage rack. Avoid other nearby objects (expensive cars especially) whilst painting. Mostly people cannot see the roof of the car, so it is not a big appearance issue. And if they notice, just say that you drive kids to school (school bus roofs are often painted white, hmm).

  • You could have the entire vehicle painted to a very light color. That would also help. – Adam Zuckerman Apr 29 '16 at 1:20
  • Yes, I thought about that. But it was too much money to spend on a 20 year old car that I didn't know how long it would last. The roof is the most important region to have light-colored, and is easy and cheap to paint yourself. The rest of the car doesn't have very much effect, because it is either not enclosing the passenger area, or is vertical. Can't do a lot about the windows, unfortunately. too bad. – user13683 Apr 29 '16 at 1:24
  • You can paint the windows, but check with your local law enforcement groups about which ones are legal. – Adam Zuckerman Apr 29 '16 at 1:33
  • @AdamZuckerman "...using always Reason and Common Sense." One thing I love about my old car is that there is wildly better visibility than most cars, especially newer ones with thicker pillars / columns, when I look around to make turns and change lanes or back up. So much so, that I would not want to block one square inch of glass. Just my preference. – user13683 Apr 29 '16 at 17:42

If you are driving in an area with relatively low humidity (less than 80%), you could use a misting sprayer as an evaporative cooler. It works by having water moisture being reabsorbed into the atmosphere.

Evaporative coolers lower the temperature of air using the principle of evaporative cooling, unlike typical air conditioning systems which use vapor-compression refrigeration or absorption refrigerator. Evaporative cooling is the addition of water vapor into air, which causes a lowering of the temperature of the air. The energy needed to evaporate the water is taken from the air in the form of sensible heat, which affects the temperature of the air, and converted into latent heat, the energy present in the water vapor component of the air, whilst the air remains at a constant enthalpy value. This conversion of sensible heat to latent heat is known as an adiabatic process because it occurs at a constant enthalpy value. Evaporative cooling therefore causes a drop in the temperature of air proportional to the sensible heat drop and an increase in humidity proportional to the latent heat gain. Evaporative cooling can be visualized using a psychrometric chart by finding the initial air condition and moving along a line of constant enthalpy toward a state of higher humidity.

  • Yes. Unfortunately, I live fairly near the ocean, in a city that a Pastor described as being "built on top of Hell". (No it does not ever freeze over either. : ) – user13683 Apr 29 '16 at 1:13

Use the max a/c function with the interior air recirculating function which cools the interior cabin air. When it begins to feel cooler then change back to normal A/c function. If you have an ECO button disable it for max cooling effect. The ECO button adjusts A/c , automatic transmissions and throttle sensitivity (on vehicles with drive by wire systems) for maximum fuel efficiency. When disengaged A/c operated much better. The more humid the better.

Open the rear windows down a inch while driving and the suction created by the slipstream of your moving vehicle would suck out the hot air and increase the effect of the A/c. Opening the rear windows fully would waste too fuel because your window acts like a parachute. Once you approach your comfort zone roll up the rear windows and chill on.

The 12,000 Btu compressor uses only 4.7162 Hp and since available horsepower is dependent on RPM . When driving in city air conditioning uses a higher percentage of available power. On the highway the reverse is true.


Once the interior is comfortably cool, there's a limit to how many degrees temperature differential a refrigeration system (like your air conditioning) can maintain between the hot exterior and the cooled interior. My experience with older cars puts this at around 30 degrees F for a well functioning system. When the car is traveling at highway speed, you should have adequate cooling airflow, so as not to overheat the engine, so the only accessible factor is how fast your car accepts heat from outside. This depends mainly on two factors: how much solar heating gets in, and how much heat conducts through the body work from the outside air.

Applying a reflective window film can help reduce solar heat uptake, but may be restricted by law (North Carolina, for instance, requires that after market window tints must transmit some minimum amount of light, and tests this for an extra fee at annual safety inspections). A low-absorption roof finish of some kind (white paint absorbs less heat than silver, and causes less glare for nearby high vehicles) can greatly reduce solar heating through the roof.

Another improvement would be to insulate the roof by applying foam between the headliner and roof. Similar insulation could be applied inside the door panels. Combine this with a mirror-finished roof and windows as reflective as your local law allows, and you might increase your maximum differential cooling by ten degrees or more -- which could be very significant on a long drive in high ambient temperatures (like summer in the South or Southwest of the United States).

If you don't need the extra cooling for comfort, you can save fuel by running the A/C at "Max" or "Recirculate" setting, and turning the compressor off and one to regulate cabin temperature -- the power draw of the compressor is the same at "Max" as at any other temperature setting, so turning it off for a while now and then will save fuel compared to running it constantly and adding engine heat back into the chilled air for comfort.

  • Yeah, I do that. My car has a "Recirculate" Button, which works in all modes, rather than Max A/C which was surprising and unintuitive when I first saw it. (And where did the High Beam switch go, anyway? : ) I don't recirculate all the time because the air gets stale-feeling, which would promote sleepiness eventually. Driving around in a sarcophagus is annoying, but it beats the alternatives. – user13683 Apr 29 '16 at 17:23
  • I see that you edited to add a mirror-finished roof as a suggestion. Imagine the effect of this on anyone whose eyeline ever goes above your roof. Truckers, for example, or people slightly uphill from you. White is perfectly adequate, reflecting about 90% as well as a mirror finish. If you cut the insOlation by 90%, there will be no need to add insUlation. If there was no sunlight, the car would heat to the exterior temp, perhaps 110F. In the presence of sunlight (roughly 6000F) the car will heat to... Blazing hot, enough to raise welts on your skin if you touch the exterior too long. – user13683 May 3 '16 at 23:33
  • I will shine a mirror at you while you are driving. No problem, right? – user13683 May 4 '16 at 13:32
  • If it is capable of disrupting someone's vision while in motion, yes it should be banned. I can't tell you how many times I have been blinded by glare from flat or nearly flat vehicle windows. A truck would only be a few feet from your roof, so the nearly flat mirror-finished roof is too much trouble for a 10% gain in efficiency. Try to be reasonable instead of bull-headed. – user13683 May 4 '16 at 13:56

Check to see if all the R-134a leaks out (meaning there's nothing in the loop to carry away heat).

Make sure the compressor is turning.

The other sorts of problems that the AC might be experiencing include: bad switches, bad fuses, broken wires, broken fan belt (preventing the pump from turning), or seal failure inside the compressor. If the system cools, but not much, it could just be low pressure and you can top up the refrigerant.

  • Yep. It's cool. – user13683 Apr 29 '16 at 17:24

Wear fewer clothes while driving so you get the benefit of what cooled air there is. Push socks down, roll sleeves/trouser cuffs up to maximise skin area.

Change your carseat covers for something cooler, and lighter shaded.

Drive with lighter-weight shoes on, or no shoes at all (aside, this is a great way to drive more "gently" too)

Park in the shade, means less work for the AC to do. Consider removing window tints, and putting something lighter coloured on dark dashboards to reduce the sun effect (but don't block vents)

A pale coloured car cover can help too.

When you start to drive, open the windows and let the hotter air blow out for 2-3 minutes, before you start the AC. This helps fuel economy too.

Finally, fet your AC professionally serviced. The receiver-dryer is possibly EOL if the car is 20 years old. It could also have the older refrigerant which should be removed before it leaks, and the newer safer R134 inserted instead. Remembering to do regular maintenance is a life hack.

  • 1
    I've seen dashboards and rear window shelves covered with white shag carpet. In some sunny areas, all exposed plastic and rubber parts will actually be destroyed by sunlight after a few years. – user13683 Apr 29 '16 at 17:30

I know this might sound odd but the shape of the vehicle plays a big role in cooling. If the vehicle were designed as say a moving cylinder, it would solve the cooling problems.

  • 1
    Can you elaborate? I am not getting the picture. And, are there better and worse shaped cars you can point to, like Prius vs Miata or something? – user13683 Apr 29 '16 at 17:32

As was stated earlier, window tinting is a highly effective way to reduce heat entering the cabin. What most people are unaware of is that you can also apply a film that only blocks IR/UV transmission. This is almost optically invisible for those who find normal window tinting to be too dark at night. Additionally, if your car is fitted with privacy glass, it doesn't filter UV or IR which means a large part of heat transmission will be entering via these windows.

Also, do a google search to find what type of coolant is installed in your vehicle. This can be found from manufacturer documentation, or via the type of fittings used to connect to the recharge port. There may be a higher performing coolant that is compatible with your system. This has the added benefit of ensuring that there is no flammable refrigerant in the system, which can be quite a hazard in the event of a frontal collision.

If your vehicle was intended for a different climate to where you live, the condenser may be undersized and not dissipating enough heat. Change it over to something that has more effective surface area to ensure correct operation.

  • Flammable coolant? WTF? It was bad enough when it was merely poisonous to animals and children. Now we have to drive around with a food ingredient in the radiator (propylene glycol), and antifreeze in our food. What is next? Filling the tires with laughing gas? Refrigerant that destroys the ozone layer? Putting lead in the gasoline? Geez! – user13683 May 3 '16 at 23:44

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