• Marco Bonifaccino

How temperature and humidity affect the car's AC performance

Updated: May 10





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 chack the Air Conditioning system.


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 AC performance of your system. One of two things could happen:

  1. You might take your car to a mechanic and pay for a professional service even though it still works properly.

  2. The AC system is not working as efficiently as it could but you don’t know that, which also means you are losing money at the pump because you are filling up more often.

The problem is most drivers don’t have access to an affordable and professional sensor to test their AC system themselves—until now. AC diagnosis offers a quick and easy way to test any car’s AC system so you can always know when it needs attention.


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



AC diagnostics in the modern era


At AC diagnosis, we think that Internet-of-Things (IoT) technology can help automotive mechanics and Auto Repair Shops to take care their customer cars perfectly. The AC diagnosis 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 check the diagnosis. You use your smartphone for many things throughout the day, why not use it to check up on the car’s AC health?