It isn’t blowing cold enough

By Richard Hawkins, MACS contributor

“It isn’t blowing cold enough.” Most any auto repair shop that does A/C work will often hear customers say this phrase. It can be the result of any number of things that can be wrong with an A/C system.  Often it is a case of common things such as, a low refrigerant charge, a refrigerant overcharge, contaminated refrigerant,  a blend door issue, a cooling fan problem, or maybe a weak compressor.

Other times it can be something more complex such as a sensor issue or controller problem. These are things that can be diagnosed and fixed.

However sometimes when this is heard from a customer, it is a case of the customer expecting too much of their A/C system.  It can be extremely challenging to convince them of that and it is not something that be fixed because there is not anything wrong with the system.

An example of this is when a heat wave hits an area.  For illustration purposes, let’s say you are in an area when the high usually averages around 88° to 90° F.  A heat wave sets in and the high goes up about 10° F.  Now the high is in the upper 90s and hits “triple digits” some days.  Along with the increased temperature, the humidity may increase which makes it feel even hotter. 

Where someone has a system that might generally blow air from the vents at a temperature of 44° or 45, it might now be blowing out in the upper 40s or low 50s. The air coming out of the vents just does not have that ” cold bite” that it did when the ambient temperature was 10 degrees lower and the humidity was lower.

To make matters more challenging, a vehicle might come in that you worked on previously. The customer is trying to blame this “lack of performance” on your work even though it was “working fine” until the temperature hit 100° F.

Below is a chart which may help some in educating customers when these situations are encountered.  This is a performance chart that was included in a MACS Update Clinic a number of years ago.  It was originally developed by GM for a specific vehicle platform.  However in that clinic and here it is used in a generic manner to illustrate the effect that increased temperature and humidity can have on a system. Please see figure # 1.

Figure 1: This is the performance chart that was referenced above.

To illustrate the example outlined above, first please look at the column titled “RELATIVE HUMIDITY”. We will select 40%.  Next, in the “AMBIENT AIR TEMP”  column we will select 90° F.  Now moving over to the “MAXIMUM RIGHT CENTER AIR OUTLET TEMPERATURE” column, that indicates that the maximum right center air outlet temperature should be no more than 51° F (at 2000 RPM).  Please see figure # 2.

Figure #2: The items referenced above are circled.

Next we will go to the “AMBIENT AIR TEMP”  column  and increase the ambient temperature to 100° F.  Looking over at the “MAXIMUM RIGHT CENTER AIR OUTLET TEMPERATURE” column that indicates that the maximum vent temperature increases by 8°, up to 59° F. Please see figure # 3.

Figure #3: The items referenced above are circled.

Next we will go back to the “RELATIVE HUMIDITY” column and increase the humidity level to 50%.  At 90° F, that indicates a maximum right center air outlet temperature of 54° F (which is a 3° F increase).  Please see figure # 4

Figure #4: The items referenced above are circled.

Finally, we will raise the relative humidity to 60% and the ambient temperature to 100° F. That results in a maximum right center air outlet temperature of 68° F. That is a whooping 17° F increase from where we started at a 40% relative humidity and 90° F. Please see figure # 5.

Figure #5: The items referenced above are circled.

An A/C system is doing double duty when cooling the air in the cab of a vehicle.  Not only is it removing heat from inside of the cab, it is also removing moisture.  As indicated in the charts, a significant increase in humidity can result is a significant increase in vent temperature.

NOTE:  Actual results can vary depending on system type and the right center air outlet temperature may be quite a bit lower that what is indicated on the chart.

Keep in touch with all technical aspects of A/C repair Join MACS as a member today!