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Air handling unit maintenance strategies

 

The Air-Handling Unit (AHU) filters and treats air, and circulates it to where it is needed. The unit is typically made up of a fan, heating and cooling coils, humidification equipment, filters and dampers. The treated air is circulated through the building space and returned to the air handling unit by ductwork. An effective maintenance strategy relies heavily on information supplied by the manufacturer and installer of the equipment. This will indicate when parts should be replaced and provide advice on appropriate maintenance procedures and schedules.

 

Description of task

Regular maintenance will help to keep air handling units (AHUs) running at optimum efficiency.

 

How to deliver

Carry out a survey of all air handling units (AHUs) and note and prioritise areas for corrective maintenance.

To keep AHUs at maximum efficiency, undertake the following maintenance activities:

  • Clean heating and cooling coils. A dirty coil can reduce the heat transfer efficiency and cause the boiler and chiller to run longer than necessary;
  • Fix leaks in cabinet and supply ducts. Losing only 6 cubic metres per minute from an AHU cuts heating, cooling and airflow capacity by about 5% and can waste more than €100 per year in energy costs;
  • Clean and adjust dampers and actuators. Operating properly, economiser dampers keep the refrigerant compressor from running when outside air temperature is below 15.5°C;
  • Fans, bearings and belts:
    • Clean and check fans for proper direction of rotation. Running backwards, a fan will supply   less than 50% of the rated airflow;
    • Replace worn or noisy fans or motor bearings. Fans in older AHUs have sleeve bearings  that should be lightly oiled two or three times a year with the recommended lubricant;
    • Do not over or under lubricate conventional greased ball bearings;
    • Maintain proper belt tension and alignment. Loose belts slip on the pulley wheels, causing torque loss and rapid wear. Belts that are too tight put an excessive load on motor and fan bearings, causing early failure of the bearings and/or belts.
  •  Use cogged V-belts. These belts run on conventional smooth pulleys, but notches on the inside of the belt reduce internal bending losses and improve grip;
  • Replace filters. Filter changing intervals should be every 1–6 months, depending on particulate loading from indoor and outdoor air. Ideally they should be based on pressure drop across the filter;
  • Direct expansion (DX) coils. Check the defrost function;
  • Economisers. Check that:
    • dampers are operating freely;
    • linkages are properly lubricated;
    • actuator motors and pistons are working and responding to control signals;
    • temperature and humidity sensors are working and properly calibrated.
  •  Balance distribution of ventilation air. Some existing systems may not be balanced correctly (not delivering the intended amount of air to each space served) or may have faulty controls within a damper system. Rebalancing of flows is usually required to establish and maintain proper ventilation and comfort levels in all areas served by the modified system;
  • Tune-up AHU controls. Control setting and sensor calibration may drift over time causing the heating and cooling system to work harder than necessary. Economiser controls are particularly susceptible to problems and should be checked frequently for proper operation.

Implement an ongoing preventative and predictive maintenance programme for your AHUs designed to maintain maximum heating and cooling efficiencies.

 

Points to note/troubleshooting

When deciding your maintenance strategy, consider the following:

  • Age of the equipment. Older equipment costs more to maintain because it is out of warranty and needs more attention;
  • Service history of equipment. Equipment that has not been properly maintained for a long period may require extensive work. Service contractors may charge a premium for such work;
  • Accessibility. If it is difficult to gain access to the equipment or to work around it, maintenance costs will be higher;
  • Changes to operating systems should only be made by technically qualified personnel.

 

Typical payback time

The payback time on a typical AHU refurbishment is 6 months to a year.

For units with malfunctioning economisers, payback time can be <6 months following repairs.

 

Savings

An average unit that has not been serviced for over a year can be expected to perform 5–10% more efficiently following a comprehensive refurbishment.

Units with malfunctioning economisers can save even more following repairs.

 

Further information sources

The Carbon Trust
www.carbontrust.co.uk
Ventilation Fact Sheet (GIL130)
Energy Efficient Mechanical Ventilation Systems (GPG257)
Focus on Energy: A Practical Introduction to Reducing Energy Bills (FOCUS)

Chartered Institution of Building Services Engineers
http://www.cibse.org
Heating, air conditioning and refrigeration publications

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