The result is usually diminished performance, lower energy efficiency, and reduced cooling capacity. In some cases, dirty coils cause evaporator coil freeze-up or bring about premature compressor failure.
At ICP, we recommend that homeowners have a qualified service technician perform a complete system check-up once a year. That check-up should include checking the coils for dirt and cleaning them as needed.
In most equipment today, air flows across the coil from outside the condensing unit and is discharged out the top. Because of this, dirt will most likely be visible from the outside of the coil.
However, even if the coil appears to be clean, it is a good idea to pay attention to the system (suction and discharge) pressures. Higher-than-normal pressure readings may indicate a dirty coil.
A visual check of the evaporator coil often requires you to work in a tight space as you remove panels from the furnace or blower cabinet, and use a flashlight to see inside the coil assembly. Even then, you may not be able to make a thorough and accurate examination of the coil.
Here’s an alternate approach. Check system (suction and discharge) pressures. Lower-than-normal pressure readings may indicate a dirty coil. Then check the airflow. This should be a routine part of the annual check-up anyway, and will give you a good idea if dirt is reducing the airflow through the evaporator coil.
When the evaporator is external to the air handler (i.e., gas furnace with an “A” coil), reduced airflow across the evaporator coil is indicated by higher-than-expected external static pressure.
When the evaporator is internal to the air handler (i.e., blower coil), reduced airflow across the evaporator coil is indicated by lower-than-expected external static pressure.
Two factors influence external static pressure:
1. Resistance to airflow. Coils, ductwork, filters, humidifiers, and strip heaters restrict airflow.
2. Blower speed. Changing to a higher or lower blower speed tap will raise or lower the external static pressure accordingly.
To check external static pressure, follow these steps:
If the system still does not achieve proper static pressure with a clean coil, you may need to make changes to the air distribution system.
Note: If the manufacturer’s airflow data is not available, approximate airflow may be calculated by operating the system in heating (on the cooling speed tap).