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  • Immagine del redattoreMarco Bonifaccino

How temperature and humidity affect the car's AC performance

Aggiornamento: 18 ago 2022

You may have noticed that the AC system doesn’t seem to work as well during a heat wave. More often than not, the outside heat simply makes the AC blow slightly warmer air and there is nothing to worry about.

Outside ambient conditions, such as temperature and relative humidity, play a critical role when we need to evaluate the performance of a car’s AC system.

In an AC system, the condenser allows heat to flow from the hot refrigerant (freon) to the cooler outside air. If the outside air is hot, the refrigerant won’t be able to cool down as much. The end result is slightly warmer air coming at your vents. Check this article for a deep understanding of "How car Air Conditioning works and removes the heat".

Humidity is also an important factor in air conditioning performance. When humidity is very high, the AC system expends most of its effort removing the moisture out from the air. This decreases overall performance, as well.

Refrigerant performance depends on the environment.

Because of the effects of outside ambient conditions, refrigerants come with a performance chart that takes into account the outside parameters and provides the coldest air temperature that should blow from the central vent (Maximum Left Center Discharge Air Temperature). Check out the chart below for refrigerant R-134a (Fig. 1):

There are a lot of numbers in that chart, so lets break it down with some real examples.

Testing AC system in two environments.

We wanted to test the air conditioning system of a car which uses refrigerant R-134a in two different weather environments:​

  • Car 1: Outside air was 95°F (35°C) with 85% humidity

  • Car 2: Outside air was 75°F (24°C) with 38% humidity

We measured the temperature from the central vent with an AC test thermometer and we got the following results:

  • Car 1: Air temperature coming out the vent was 58°F (14°C)

  • Car 2: Air temperature coming out the vent was 49°F (9°C)

According to the test, Car 1’s AC vent was blowing 9 degrees warmer at the max setting. At this point, it's natural to assume that Car 1 has an issue with the air conditioning system while Car 2's air conditioning system seems to work efficiently (Fig. 2).

Let’s take a closer look at the chart for analysis.

We cross-reference the information—outside weather parameters and vent temperature—with the R-134a performance chart.

According to the chart, the coldest vent temperature (Maximum Left Center Discharge Air Temperature) reached by the air conditioning system should be:

  • Car 1: Optimal vent temperature at 58°F (14°C)

  • Car 2: Optimal vent temperature at 43°F (6°C)

This data turns our natural assumptions upside-down.

In fact, Car 1’s AC system is performing in an optimal range because it reached the maximum cool air temperature. On the other hand, Car 2’s AC system was blowing 48°F, while it should be able to get down to 43°F.

That means Car 2 is the one which should have a service, not Car 1 (Fig. 3).

This is why it’s so important to know how the outside environment affects AC performance and take into account when we want to test the Air Conditioning performance.

What the results tell us.

If you are testing a car’s air conditioning performance without taking the environment into account, it can be easy to misread the real Air Conditioning performance of the system. One of two things could happen:

  1. You could skip air conditioning maintenance when it needs it leaving your client in trouble.

  2. You might miss a maintenance service, so a profit, with a chance for your customer to be serviced by someone else.

The problem is car mechanics don't have access to an affordable and Smart Diagnostic Tool to test Air Conditioning systems accurately—until now. AC diagnosis offers a quick and easy way to test any car’s AC performance system so you can always know when it needs attention and prevent A/C breakdowns.

Modern cars contain more technology than ever. When you think about it, it’s silly to rely on a standard thermometer to test the A/C (which has been around since 1714).

Does your old-fashioned thermometer tell you if your car's Air Conditioning system works properly?

Air Conditioning diagnostics in the modern era

At AC diagnosis, we think that Internet-of-Things (IoT) technology can help automotive mechanics and Automotive DIYers to take care their customers' cars perfectly and provide an outstanding AC service. The AC diagnosis Smart Sensor takes into account all outside weather conditions, calculates the optimal cooling performance, and communicates if the air conditioning is working properly (or not). It does all of this in just a few minutes.

In short, AC diagnosis turns your phone into a professional-level AC diagnostic tool with the power of IoT technology built-in.

Simply clip the sensor to your vent, connect it via Bluetooth to your phone, and start the test. AC diagnosis and Smart Technology will do the rest ensuring the most accurate Air Conditioning performance test.

You use your smartphone for many things throughout the day, why not use it to check up on the car’s AC health and track test results?

Want to stay up to date with the latest Automotive Smart Technology?


31.370 visualizzazioni2 commenti


Eng. Mahir
Eng. Mahir
Mar 13, 2022

Hi, I am living in Iraq and the weather here in summer is hot and dry! the temp. outside the car may reach 70 degrees Celsius. I have a question, if you don't mind .... If I used a humidifier inside the car, does it improves the car's AC performance?

Marco Bonifaccino
Marco Bonifaccino
Mar 24, 2022
Replying to

Hi Mahir,

using a humidifier inside the car will just improve the comfort inside the car. Best,


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