Coil cleaning and its energy impact

Coils are where an HVAC system actually moves heat. Air passes over thin fins bonded to tubes carrying chilled water, hot water, steam or refrigerant, and heat transfers through the metal. Anything that coats those fins or blocks the gaps between them makes the coil work less well. Dust, lint, pollen, cottonwood seeds, grease and biological growth all accumulate over time, even with good filtration.

A fouled coil quietly costs energy and capacity. The fans work harder to push air through it, and the chiller, boiler or compressor works harder to deliver the same heat transfer. This guide explains how fouling affects performance, how to measure it, and how to clean coils without damaging them.

How fouling affects performance

Higher airside pressure drop

Debris on the coil face and between fins reduces the open area for airflow. Pressure drop across the coil rises. In a constant volume system, the fan moves less air against the higher resistance, which reduces capacity and can upset air balance. In a variable air volume system, the fan speeds up to maintain duct static pressure, using more energy. Because fan power rises steeply with airflow and pressure, even a modest increase in pressure drop can be noticeable. See reducing fan energy for the underlying relationships.

Reduced heat transfer

Dirt and biofilm act as insulation on the fin surface. For a given entering air and fluid condition, a fouled coil transfers less heat. For a chilled water coil, the result is a warmer leaving air temperature or a need for colder water or more water flow to compensate. This pushes chillers and pumps harder and can contribute to low delta T syndrome in chilled water plants.

Condenser coils and compressor efficiency

Air-cooled condenser coils on rooftop units, condensing units and air-cooled chillers are especially sensitive. When a condenser coil is fouled, it cannot reject heat as easily, so the refrigerant condensing temperature and pressure rise. The compressor then works against a higher pressure ratio, drawing more power and delivering less cooling. In severe cases the unit trips on high head pressure on hot days, exactly when cooling is most needed.

Indoor air quality

Cooling coils are wet during dehumidification. Organic debris on a wet coil supports microbial growth, which can cause odours and contribute to IAQ complaints. Fouled coils can also carry water off the coil face and into downstream ductwork and insulation, creating mould risk. Our guide to condensation and mould prevention covers this further.

Measuring coil condition

Visual inspection is useful but can mislead. The upstream face may look clean while the deeper rows are packed with debris, or a light surface layer may look worse than it is. Measurements give a better picture.

Pressure drop comparisons only work if airflow is known, because pressure drop varies roughly with the square of velocity. A baseline taken at commissioning or after a thorough cleaning, with the airflow recorded, makes later comparisons meaningful. This is a good reason to keep air balancing reports on file.

When to clean

There is no universal interval. Coil fouling depends on filtration quality, filter maintenance, outdoor environment, hours of operation and the type of building. Some practical guidance:

• Inspect cooling coils and condensate pans at least as often as ASHRAE Standard 180 and the manufacturer require, and clean when inspection or measurement shows fouling.
• Inspect air-cooled condenser coils before each cooling season, and more often near cottonwood trees, highways, construction sites or industrial areas.
• Clean when pressure drop or approach temperatures have drifted noticeably from baseline.
• Clean after any event that bypassed filtration, such as a failed filter frame, a construction project without protection, or a wildfire smoke episode with heavy loading.

Good filtration is the best way to reduce coil cleaning frequency. Filters that fit well and seal tightly in their frames keep most debris off the coil. Bypass around poorly sealed filter racks is a common reason coils foul quickly. See filter change strategies.

Cleaning methods

The right method depends on the coil type, the depth of the coil and the kind of debris.

1. Dry methods first. Vacuuming with a soft brush and low-pressure compressed air remove loose surface dust. Blow air opposite to the normal airflow direction to push debris out the way it came in.
2. Low-pressure water rinse. Rinsing with water, again opposite to airflow, removes deeper debris. Keep pressure low enough that fins do not bend.
3. Coil cleaning chemicals. Foaming or non-acid alkaline cleaners loosen grease and biofilm. Choose products compatible with the coil materials and approved for the application, and rinse thoroughly.
4. Deep cleaning. Thick or deep coils with packed debris may need specialized equipment that delivers cleaning solution through the full coil depth, or removal of the coil for cleaning.

Precautions

• Lock out and tag out fans and equipment before working on coils.
• Protect motors, electrical components and insulation from water and chemicals.
• Avoid high-pressure washers that flatten fins. Bent fins can be straightened with a fin comb.
• Avoid acid cleaners on aluminum fins unless the manufacturer approves them, as they can corrode fins and coatings.
• Make sure the condensate drain is clear before rinsing, so cleaning water drains rather than flooding the unit or ceiling.
• Follow occupational health and safety rules for chemical handling and ventilation, which are set provincially.

Verifying the result

After cleaning, measure the same indicators used to assess fouling, at a similar airflow and load. Record the new baseline in the CMMS. If pressure drop does not return close to the original value, the coil may have deeper fouling, damaged fins or internal fluid-side fouling that airside cleaning cannot address.

Frequently asked questions

Will cleaning a coil lower my energy bill?

Cleaning a significantly fouled coil reduces fan and compressor or plant energy, and restores capacity. The size of the saving depends on how fouled the coil was and how many hours the system runs. A coil that is already reasonably clean will show little change.

Can coils be cleaned while the system runs?

Some light dry cleaning of condenser coils may be possible with care, but most coil cleaning requires the fan to be off and locked out for safety, and to prevent spraying water and chemicals downstream.

How do I know if the fluid side is fouled?

Fluid-side fouling shows up as poor heat transfer even with a clean airside surface. It is more common in open systems or poorly treated closed loops. Water treatment records and fluid temperature measurements help diagnose it.

Key takeaways

• Fouled coils raise airside pressure drop, reduce heat transfer, and increase fan, pump and compressor energy.
• Air-cooled condenser coils are especially sensitive, with fouling raising head pressure and compressor power.
• Measure pressure drop, approach temperatures and fan speed against a known baseline rather than relying only on appearance.
• Clean from the leaving side, start with dry methods, use low pressure, and choose cleaners compatible with coil materials.
• Good filters with tight seals are the most effective way to keep coils clean longer.