How to find a dirty evaporator coil (without really looking)

September 19, 2000
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It may take the form of grass clippings, mud, pet hair, tobacco smoke, rust, or even bacteria. Whatever its form, this so-called “dirt” generally has the same effect on air conditioner coils — it blocks the airflow and degrades heat transfer.

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.

Checking the condenser coil

With the condenser coil, checking for dirt is relatively easy. Since it is outside the house, in plain daylight, typically all you have to do is look.

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.

Checking the evaporator coil

Evaporator coils, however, are a different story. On split systems, evaporator coils are inside the house. They are usually hidden away inside a blower cabinet, furnace, or plenum, which may be tucked back in a closet, attic, or crawl space.

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.

Checking external static pressure

External static pressure is the difference in pressure between the intake and discharge sides of the blower. It is created by the blower as a result of the resistance to airflow in the air distribution system.

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:

  •  Set up to measure external static pressure at the supply and return duct connections.

  •  Drill holes in the ducts for pressure taps, pitot tubes, or other accurate pressure-sensing devices immediately adjacent to the air handler (between the furnace and “A” coil when the coil is external to the air handler).

  •  Connect these taps to a level inclined manometer or magnehelic gauge.

  •  All registers must be open and the filter must be clean. This is essential in order to get an accurate reading.

  •  Turn on the blower and read the external static pressure.

  •  Compare your actual readings to the manufacturer’s airflow data for your particular indoor section and selected speed tap.

  •  If the system was properly set up when installed, then the external static pressure on the present speed tap should indicate the required airflow for the system you are servicing.

  •  If the static pressure on the present speed tap indicates other than the required airflow, then either it was never set up properly to begin with, or there is a good chance that the coil is dirty.

  •  Get out your flashlight and take a look. Clean the coil if necessary, then recheck airflow.

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).

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