How to clean an air handling unit

Over time and through use, an air handling unit (AHU) gathers dust and debris that can block filters, airflow, fans, and other parts of the AHU. It is vital that an AHU is cleaned and maintained on a regular basis, usually a couple of times a year.

If an air handling unit isn’t cleaned and maintained regularly, the indoor air quality is likely to decrease. Cleaning the AHU’s components, including parts such as the coils, panel filters, and condensate pans, will ensure the unit works at its full capacity. It will also extend the unit’s service life, improve efficiency, and ensure that the air quality is maintained at optimum levels.

Defining the process

AHUs can differ in size, but according to the NADCA (National Air Duct Cleaners Association), a full clean will take around three hours to complete. For the best results, it is recommended to use a certified air systems cleaning specialist who will ensure that the ductwork is also properly cleaned and there is no damage.

The cleaning regime involves:

• Every two months, remove all the AHU’s filters, clean the filter racks, and replace the filters (as recommended by the US Environmental Protection Agency), checking for contamination, damage, and odours.
• Every six months:
  • Check the outdoor air inlets for damage and contamination.
  • Check the air ducts for damage.
  • Check the inner air duct surface for condensation and contamination.
• Annually: measure the levels of dust contamination, bacteria, and mould in the air supply unit.
• Bi-annually: carry out a hygiene inspection and clean ducts as well as the air system as required.

If your HVAC system is located outside, make sure that the area around the HVAC unit is clear from any debris, such as branches, plants, long grass, and general dirt. Your AHU should also have at least two feet of clear space around it.

Define the method of cleaning

Using a cleaning plan, the right cleaning method needs to be selected, i.e., wet or dry cleaning, to ensure no damage occurs to the components or surfaces of the AHU. A cleaning plan will include:

• The ventilation system that needs to be cleaned.
• The selected cleaning method(s).
• The equipment required to carry out the cleaning process.
• Filter efficiency.
• Location of the air exhaust
• Health and safety requirements.
• Verification of cleaning including any notifications.

Wet cleaning, for example, would be used to clean terminal devices (once removed from the ductwork), and heat recovery units if they are installed in a place where the water from wet cleaning can be drained off. This cleaning method is more suited to removing deposits that are stuck to the surfaces, like grease or oil from installation. However, wet cleaning is not usually used to clean air ducts, as they are not watertight.

Dry cleaning, usually using compressed air and hand vacuum cleaners or mechanical brushing, can be used to clean between heat exchanger fins, filter banks, silencer surfaces and any other areas where any water from wet cleaning would damage the AHU. This method removes most of the accumulated dust and dirt that has loosely settled on the components.

Cleaning robots are also useful to clean rectangular air ducts, particularly those that have a camera attached to monitor cleaning progress.

For cleaning areas that potentially contain microbial colonies or living cells, such as AHUs located in outdoor spaces where moisture is at a greater level, different cleaning methods will need to be undertaken.

When you’re ready to clean your air handling unit, make sure it is switched off before you start.

• Wipe down the air vents and floor vents and clear any debris that has collected. If the vents are not clear, airflow will be restricted.
• Remove the dirty air filters, located in the air handler cabinet or furnace close to the air inlet, and replace with new Eurovent certified air filters that comply with Eurovent recommendations and are the correct class of filter for the environment in which the AHU is being used.
• Gently wash away any dirt that has collected on the outside of the unit. Use a mild detergent and warm water to clean the grill, making sure that the air gaps are thoroughly clean. A damp cloth is useful to clean the small spaces and corners.
• Dust gathers over time in the air conditioning unit, even when it is not in use. Using a damp cloth, remove all the dust that has accumulated inside the tray.
• Check the drain lines for any leakages. Water damage can occur within the building if there are any holes or gaps. A damp cloth can be used to clean where the drain lines open.
• Take off the top and side panels, or protective grills, of the air conditioning unit. Clean the coils with a soft brush, a refrigerator coil brush, or a specialist fin comb to clean the aluminium fins in the AHU but be careful as they can easily bend. If you do bend them, use the fin comb to straighten them again. Gently remove any dust and dirt with a damp cloth. If the cooling coils are dirty, the air conditioning unit won’t work as well. If you are struggling to remove the dirt and debris, you can use a commercial air conditioning cleaner on the inside of the coils, but make sure you avoid spraying any electrical parts or the fan. You can also use a hose with a trigger nozzle but be careful not to flood the unit or bend the fins. And again, avoid electrical parts. To be on the safe side, cover these parts with plastic sheeting during cleaning.
• It’s worth checking any nuts and bolts, making sure they are secure and not getting rusty. If they are, spray a little bit of WD-40 and wipe away. Also, check the fan motor for any oil ports that need attention.
• Investigate the refrigerant lines that run from the evaporator coil on the air handler to the condenser unit to make sure that the foam insulation around them is not damaged. If it is, replace it with a foam insulation sleeve specially designed for use on refrigerant lines. You may also want to wrap insulation tape around the sleeve to make sure it stays in place.

Ensure new filters have been installed and then you can test your AHU.

Turn the power to the condenser unit back on, switch your thermostat back on inside and adjust it to the cooling or heating mode you want in order to activate your AHU. If the unit doesn’t reactivate and start working almost immediately, contact an engineer to investigate the problem.

It is of vital importance to regularly maintain your AHU and air conditioning units to make sure they are in full working order and doing their job of effectively cleaning the air in buildings or rooms. This will also increase efficiency and save you money in the long term.

When the unit is not in use, cover it with plastic and tie it down with a bungee or a rope.

If you don’t feel confident about cleaning your air conditioning unit yourself, it is a good idea to hire a certified heating and ventilation engineer who will be able to clean the unit for you. At the same time, they will be able to service your AHU, making sure all parts are working correctly and replacing any parts that are damaged or need replacing.