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  • Marco 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).