Dirty evaporator coil

Q What effect does a dirty evaporator coil have on a residential air conditioner? How can the coil be cleaned?

A It's all about the airflow. Airflow is king and when it's reduced, whether due to a dirty filter, poor zone control or dirty evaporator coil, the system will fail. In comfort cooling applications, when the evaporator gets dirty, the volume of air moving across the coil is reduced. One of themost glaring results of low airflow, in an otherwise properly charged system, is too little or no superheat leaving the coil. Liquid refrigerant makes its way back to the compressor, causing a mechanical failure. Depending on the motor type and size, a dirty coil also may cause condensate blowoff due to the higher velocity air going across the coil. As an example, ECMstylemotors that increase rpm's to counteract the effects of the higher resistance of the dirty coil will have very high velocities. Dirty coils also result in colder evaporator temperature, leading to frost and ice buildup, which leads to less airflow and faster compressor failures. There is nothing good or acceptable about a dirty coil.

As for cleaning, in most cases a fin comb will work well, making sure the fins per inch of the comb match the coil. There are also coil cleaners available, but you need to be careful not to use caustic cleaners that would

diminish or destroy any of the antibacterial coatings found on some newer coils. In some extreme cases where the dirt gets deep into the fins, the coil may have to be removed from the system to be cleaned. Service technicians would be well served to instruct homeowners on the benefits of regular filter changes on every call.

Question from Robert Skurski, hvacr student, Chicago. Answer by Jack Bartell, director, service and training,

Virginia Air Distributors.