Why Do We Coil Clean?

Regular maintenance of HVAC systems reaps benefits far past efficiency gains. Regularly scheduled maintenance offers the trained technician the ability to identify possible problem areas and build a data set over time. New tools measuring available data points can help see issues coming down the pipeline.

However, necessary cleaning is also a large part of system efficiency and increasing the lifespan of an HVAC system.

Reasons For Coil Cleaning

Heat Transfer
The purpose of all HVAC/R coils is to transfer heat from one medium to another through the walls of the tubing. The most common type of coil is used for the direct transfer of heat from the refrigerant to air through the tubing or channels using fins or spines attached to the tubing. These fins assist in transferring heat by adding additional surface area for the air to contact the metal as it passes by.

Metals like copper or aluminum are often used for coils because they have good thermal conductivity, which means heat moves through them easily. When coils get a coating of dirt on them, it can act as thermal insulation on the metal impeding the movement of heat in and out of the coil.

Therefore, because heat moves from higher temperature to lower temperature through materials via conduction any material on the coil (like dirt) with lower thermal conductivity and higher R-value results in lower heat transfer and therefore some potential performance impact on system performance.

Air Flow
When dirt and other contaminants block coil fins, it causes a more substantial air pressure drop across the coil, which results in either lower airflow over the coil or higher power consumption on the motor depending on the motor and fan type.

Axial fan blades with fixed single speed AC motors are common in air conditioning and refrigeration condensers. This type of motor and fan blade combination is heavily impacted by increases in air resistance known as “static pressure.” As the coil gets dirtier, the motor moves less air, resulting in less heat transfer and lower efficiency.

Broad Impacts of Dirty Coils

Inefficiency
The primary purpose of HVAC/R systems is to move heat from one place to another. Anything that inhibits this movement of energy results in an increased energy input on motors, compressors, and pumps to compensate. Cleaning coils is critical to maintaining peak efficiency with studies showing energy loss before cleaning of 10 - 37% depending on the type of system and the level of fouling.

Performance & Capacity
When coils get dirty, the amount of heat the system can move tends to decrease. Many applications require the system to work at full capacity during times of critically high load, and this is the very moment when dirty coils tend to cause the most significant issue. Dirty evaporator and condenser coils both cause high compression ratio, which results in less refrigerant circulation by mass and fewer BTUs of heat movement per hour.

System Longevity
Dirty coils result in a high compression ratio, which causes the compressor
to run hotter and harder.

The overwork can result in early failure of the compressor as well as increased vibration, which can result in rub-outs and abrasion leaks. Dirty condenser coils also result in high head pressure, which makes leaks on the high side of the system more likely.

Dirty Evaporator Coils
The evaporator coil is responsible for absorbing heat from the space to be cooled in a DX (direct expansion) system. When the coil is dirty, it can impede both airflow and heat transfer resulting in less heat load on the coil. The loss of heat load reduces the amount of heat entering the refrigerant, which causes the pressure and therefore, the temperature of the coil to drop. These actions result in several common symptoms and system issues:
• Low Suction Pressure
• High Compression Ratio
• Coil Freezing
• Tripping Low-Pressure Switch
• Low Capacity
• Poor Efficiency
• Compressor Damage Due to Liquid Flood Back

Dirty Condenser Coils
The condenser performs the reverse function of the evaporator by releasing heat from the system. When the coil is dirty, the movement of heat out of the refrigerant is impeded, causing an increase in pressure and condensing temperature that may negatively impact the system in the following ways:
• High Head Pressure
• High Compression Ratio
• Compressor Overload
• Tripping High-Pressure Switch
• Oil Breakdown and Compressor Wear
• Low Capacity
• Poor Efficiency