When troubleshooting a system problem, if you measure a lower than normal suction pressure you might jump to an early conclusion that the system is either low on refrigerant or has an issue with the metering device not feeding enough refrigerant into the evaporator. You may be right, but you could also be wrong. 

Without looking at some other system parameters you cannot tell the true cause of the low suction pressure. One important item to check is the airflow across the evaporator. If the airflow is low, the heat load placed on the evaporator will be low and this will cause the coil temperature to drop which will also result in a drop in the low side pressure. 

Checking the evaporator’s superheat value will help you determine the difference. An evaporator that is starved for refrigerant will have a higher than normal superheat value. An evaporator that is starved for air will either maintain its superheat value or have a lower value depending on the type of metering device used. If you think it’s an airflow issue, there are some additional items to check. 

First and foremost, check the air filter. A dirty air filter will certainly restrict the airflow across the evaporator. Every system inspection, regardless of the system issue, should include checking the air filter and changing it if required. 

Next, check the static pressure drop either across the system’s furnace/air handler or evaporator. When checking the static pressure drop across the furnace/air handler, measure the static pressure drop across the device as it was shipped from the factory. 

For example, if the evaporator coil was added to the furnace then measure the static pressure drop across the furnace only. If it’s an air handler, measure across the blower and evaporator coil. After measuring the static pressure drop, compare it to the external static pressure (ESP) value stamped on the unit’s data plate. 

A measured static pressure drop higher than the name plate rating is an indication of less air moving across the evaporator. As the ESP goes higher, the less air flow is moving across the evaporator. For example, many residential furnaces will have an ESP of 0.5 in w.c. If you measure an ESP of 1.0 in. w.c. less air is moving across the evaporator than designed and this is an issue that needs to be investigated.

The static pressure drop across the evaporator can also be measured. Coil manufacturers publish performance charts for their coils. At a measured static pressure drop, the chart will indicate the amount of air flowing across the coil at that specific static pressure drop. This information can be used to indicate if the system is low on air. For example, you are servicing a 5-ton system and should have approximately 2,000 CFM of air flowing across the coil. 

By referencing the chart, you determine a static pressure drop of 0.22 in. w.c. equates to 2,000 CFM. Next, measure the actual pressure drop and if close to 0.22 in. w.c. there is adequate air flow. If your measured value is much lower, there will be insufficient air flow. The lower the static pressure drop the less air is flowing across the coil.

So the next time you discover a system with a lower than normal operating suction pressure, make sure it is not airflow related.