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SOP for Operation, Maintenance & Cleaning Manual of AHU


Pre start-up checks:

Verify that the:
  • Equipment was received as ordered.
  • AHU was installed on flat and level surface and with proper clearances.
  • Condensate drains are properly trapped.
  • All wiring and tubing connections are completed.
  • All field piping connections are completed.
  • All shipped loose parts are installed.
  • All ductwork is complete and available for full airflow.
  • Installer removed all dirt, debris, hardware, mould, etc. from interior of the AHU and its ducts.
  • Filter gauge is set to zero.
  • All shipping bolts and other materials were removed.
  • Ensure clean air filters are installed and secured properly.
  • Inspect all field provided wiring completely.
  • Cooling coil condensate drains are properly fitted to allow draining and avoid air suction and water carry-over.
  • Dampers and linkages do not have any obstruction in the operation and are locked at the proper position.
  • Meg the motor to verify that the resistance is at a satisfactory level, compared to the value recorded prior to storage.
  • Verify that the fan wheel is properly aligned, is tight on the shaft, and moves freely.
  • Make sure the fan bearings are properly lubricated.
  • Verify the correct voltage, phase and cycles.
  • Energize the fan motor(s), briefly (bump) and check for correct fan rotation. If the fan rotation is incorrect:
  1. On three-phase equipment, reverse any two motor leads at the load side of the last starter component.
  2. On single-phase equipment, follow the wiring diagram on the motor housing or on the inside of the motor terminal box.
  3. Recheck for correct fan rotation.

Delayed Startup:

If installation and start-up do not occur immediately after delivery, take the following precautions:
  • Protect the equipment in an area not exposed to excessive humidity.
  • Take special care to protect electrical components.
  • Periodically, manually turn all moving parts to avoid any possibility of blocking.
  • For long stocking periods, remove electrical equipment and store in a dry atmosphere.
  • Verify the motor lubrication before start-up and follow the motor manufacturers
  • It should be noted that the bearings are pre lubricated. It is however possible that start-ups will be noisy until the grease is evenly distributed.
  • Check that electrical connections comply with the wiring diagram and that the thermal protection is operational.
  • Isolators: Check that the vibration isolators are free to operate and that no other contact exists between the fan/motor chassis and the supports.

  • After completing the checks as stated above, the unit may be started and the following checks and adjustments performed:
  • Measure motor voltage and amps to ensure proper protections, compare readings with the motor name plate.
  • Check fan air volume and pressure. If the volume is incorrect it may be modified (within the limits of the fan curve) by balancing dampers.
Airflow varies with fan speed, the pressure varying as a square function. The absorbed horsepower varies as a cube of the above relationship. If the system pressure requirements are overestimated the airflow will increase, thus increasing the absorbed power and creating a risk of overloading, the motor. This factor is much greater important for forward curved type fans. For backward inclined fans the absorbed power curves are practically parallel to the speed curves.


The maintenance of air conditioning equipment should be carried out bearing in mind the basic rules of personal security.

Ensure that:
  • The engineer is electrically isolated from the unit.
  • The engineer does not enter the unit before the fan has stopped rotating.
  • The electrical equipment is correctly isolated from the main supply before entering the unit (fan, electric coils, etc.)

The majority of air handling units requires the same type of maintenance.
The maintenance frequency will vary depending upon the cleanliness of the operating environment and the operating schedule of the unit.

Before carrying out equipment maintenance, it is worth remembering that in an air conditioning installation, the fan is selected to give a particular airflow against a certain pressure. Any element, which tends to change the operating point, such as dirty filters, dirty coils or blocked air grilles, will tend to cause poor operation of the installation since it will work at other than the design conditions.


1. General service and checks
  • Lubrication frequency. As per manufacturer’s instructions (for fan and motor bearings).
  • Filter replacement frequency. As per filter loading as specified.

2. Daily service and checks
  • Check and record temperatures: return air, outside air, mixed air, leaving heating coils, leaving cooling coil and after discharge of fan.
  • Check and record steam pressure to heating coil.
  • Check and record water temperatures entering and leaving cooling coil.
  • Inspect filter sections for air bypassing filters.
  • Check that drain pans and drains are clean.
  • Check bearing temperatures.
  • Check motor rotation.
  • Check motor temperatures.
  • Check motor starters.
  • Check controls for proper operation.

3. Monthly service and checks
  • Repeat all daily checks, recordings, and inspections.
  • Check, adjust, and clean outside air intake, outside air intake louvers, screens, outside. Air-motorized dampers and re-circulating air-motorized dampers.
  • Check damper blades for tight closing (outdoor air intake)
  • Check piping appurtenances: air vents, drains and strainers, check piping leaks, and check for damaged insulation.
  • Clean all heating and cooling coils.
  • Check fan bearing alignments.
  • Check fan bearing for wear and tear.
  • Clean unit casings, plenums, and connecting ducts.
  • Check that motor electrical connections are tight, check for arcing of brushes, and check that motor shaft/sheave is secure.
  • Check that all motor started connections are tight, and clean all contacts.
  • Automatic temperature control systems: Inspect, check, clean, adjust, repair and Calibrate all control devices, interlocks, electric-pneumatic (EP) and pneumatic.
  • (PE) switches, compressors, and refrigerated air dryers.

4. Yearly service and checks
  • Repeat all daily and monthly checks, recordings, and inspections.
  • Check louvers, dampers, filter frames, coils, casings, plenums, and fans for corrosion. Clean and treat as recommended by the manufacturer.
  • Check coil fans for corrosion, wear and tear.
  • Check fan shaft as per manufacturer’s recommendations.
  • Check fan shaft for straightness that the fan wheels are tightly fastened to the shaft and the drive sheaves are secured.
  • Clean motor frame and air passages.
  • Check motor acceleration time, voltage, current and speed.
  • Check motor anchor bolts and vibration isolation
  • Motor starter, clean and check contacts, clean housing, check for proper operation and overload protection.
  • Ductwork: Check for dust, moisture, defective linings, defective insulation and leaks. Mechanical insulation: Check and repair all damaged pipe insulation, fitting insulation, valve insulation, duct insulation, casing and housing insulation.


Inspect coil & rotating parts and other internals before start up.

Check for any change in running condition or unusual noise and correct abnormality if found.

  • Check air filters. Clean or replace if clogged or dirty.
  • Lubricate fan and motor bearings as per manufacturers’ recommendation if operating on high speeds, moist or dirty air, or high temperatures.

  • Check fan bearing grease cup connections. Cup should be tight to the bearings.
  • Tighten electrical and control connections.
  • Inspect coils for dirt build up or coil freeze up.
  • Check whether all controls work as adjusted.

  • Inspect the unit casing for corrosion. If damage is found, clean and repaint the surface with a rust-resistant primer and polyurethane painting.
  • Clean the fan wheels and fan shaft. Remove rust with an emery cloth and apply a coat of anticorrosive lacquer.
  • Inspect the condensate drain pan and drain line, remove foreign materials that might obstruct proper drainage.
  • Check damper linkages, set screws and blade adjustment. Clean, but do not lubricate, the nylon damper rod bushings.
  • Clean damper operators.
  • Inspect the control and power box wiring for secure connections and insulation.
  • Rotate the fan wheel and check for obstructions in the fan housing. The wheel should not rub on the fan housing. Adjust the center if necessary and tighten wheel set screws.
  • Check condition of gasket and insulation around unit, door and dampers.
  • Examine flex connections for cracks or leaks. Repair or replace damaged material.
  • Check joints if leaks are found. Clean the surface, re-apply sealants.
  • Check vibration isolators, replace fatigue ones.


It is recommend to keep in stock the following spares which could be need to be replaced during two years operation due to wear depending upon number of hours used in the duration.

  • Cleaning of dampers should be performed before operation is hindered in any way. Maintenance personnel must determine when cleaning is necessary based on observation and operation.

Coil Segment cleaning Tools, equipment and materials
  • Pressure washer that does not exceed 2000 psi. Sprayer (utility garden, etc.) applicator
  • Plastic sheeting and duct tape
  • Screening
  • Coil cleaner (safe, commercial, disinfecting)
  • Garden hose and spray nozzle

Cleaning procedure
  • Clean dehumidification coils annually or when air pressure drop exceeds 125% of design.
  • Cover electrical components such as fan motors, damper motors, compressors, thermostats, etc. with plastic. Care should be taken on interior coil cleaning. Remove filters; cover fan bearings and any insulation to keep these items free of water damage. Condensate drain piping should be screened to allow coil-cleaning water to flow freely. Screening keeps traps and drain lines from clogging with debris washed from the coils.
  • Prior to any application of wet cleaning materials, use a wide soft bristle paint brush to dust off any heavy dust, leaves, bugs or other foreign matter that may be on the coil fin surface.
  • Safety glasses should be worn when cleaning coils.
  • When possible, remove dirt lodged in the depth of the coil by using clean oil-free air under pressure. Caution should be taken not to use extreme high- pressure air as this may cause fin surface damage. Direct the air straight at the openings between the fins and never at an angle, which may bend the fins against one another. Always apply the air from the air leaving side of the coil.
  • On heavily soiled coils, use a safe commercial grade coil cleaner.
  • Follow the safety and mixing instructions as noted on the cleaning agent.
  • Spray the cleaning agent on both sides of the coil to be cleaned. Allow the cleaning agent to remain in contact with the dirty surface for about 5 minutes or as recommended by the agent instructions. Then flush the coil with clean water from a hose (with spray nozzle or from pressure washer). Flush from the air leaving side of the coil. Caution should be taken, as extreme water pressure may result in fin surface damage. Direct the water straight at the openings between the fins and never at an angle, which may bend the fins against one another. This process will wash away surface dirt on the air entering side of the coil, and prevent it from loading within the depth of the coil.
  • Most cleaners are concentrated detergents and can be diluted with up to 10 parts water. Dilute as per cleaning agent instructions and coil condition. Re- spray both sides of the coil with cleaner. Allow to stand five minutes and flush as described previously. Finish flushing from both sides of the coil.
  • Some extreme oil and dirt conditions may require steam cleaning. Most steam equipment can be adjusted to provide a mixture of water and steam at a moderate pressure. Steam alone without the presence of water does not work well with most cleaning agents. Cleaning the coils with steam should be done as described previously.
  • Comb out any bent or flattened areas of the fin surface.
  • Restore the equipment to its operational state.

Condensate drain pan, trap and line cleaning Tools, equipment and materials
  • Toilet bowl brush or similar utility cleaning brush
  • Cleaning agent (safe, commercial, disinfecting)
  • Rags and trash bags
  • Garden hose with spray nozzle or power washer
  • Scraper
  • Screening

Schedule and conditions
  • Clean condensate drain pan, trap, drain line and adjacent wetted surfaces at least annually or as often as required to retard growth of microbial substances.
  • Test Drain Pans - To minimize conditions of water stagnation that may result in microbial growth, field test drain pans under normal operating conditions to ensure proper drainage.
  • Exception - Field testing drain pans are not required if units with factory-installed drain pans have been certified by the manufacturer for proper drainage when installed as recommended.

Cleaning procedure for coil, motors, devices & wiring
  • Cover any nearby components such as motors, control devices or wiring.
  • Sweep, gather and remove debris from drain pan, auxiliary pans and splash guards.
  • Scrape loose and remove any clinging substances.
  • Cover drain pan outlet with screening to prevent drain clogging.
  • Prepare cleaning agent per manufacturer’s instructions.
  • Apply cleaning agent with spray applicator or brush.
  • Apply cleaner to all surfaces including: underside of coil, header and return bends if in air stream, coil supports, coil wall or bulkhead, auxiliary drain pans, splash guards, any other surfaces subject to wetting by condensation dripping or carried by normal airflow, drain pan and outlet.
  • Add ample amount of cleaning agent to drain line and trap.
  • Allow cleaner to stand for some time.
  • Flush with clean water from pressure washer or garden hose with spray nozzle.
  • Apply as much water under pressure as possible to drain outlet to clean trap and drain line.
  • Remove water from any puddle areas with a wet vacuum.
  • Wipe down if necessary to remove any stubborn material.
  • Restore equipment to operational state.

Pre-Filter and Fine filter cleaning Tools, equipment and materials
  • Filter Cleaning Booth
  • Compressed air and water

Schedule and conditions
  • Before starting the Filter Cleaning Booth, please check the water in the re-circulation tank, greasing being applied to the bearing inside the tank and oil to bearing on four wheels, air pressure for pneumatic cylinder lifting system.
  • Ensure that you clean and wash the filter cleaning chamber before placing the filter of different products /different areas.
  • Ensure that you keep the filters on the round tray and that the filter surface face near the nozzle at a distance of 2”.
  • Ensure that compressed air and water is available with a minimum 3-4 kg pressure.

Cleaning procedure for pre and fine filters
  • Place the filters in the cleaning chamber. Preferably run the air cycle first, then water cycle, then again air cycle and finally the drying cycle.
  • Ensure that you check physically that the filter has dislodged the dust particles after cleaning by compressed air.
  • After inspection, turn on the first switch, i.e. air cycle which will start air spraying. Once complete, all the dust / powder will be sucked by the dust collector / scrubber attached to the filter cleaning booth.
  • The dust / powder will be scrubbed by water in the venturi and scrubber which will dissolve the particles with water and will be drained down in the scrubber and will be re-circulated till it is drained off to the ETP / collected and blocked by the filters in DCU.
  • Now put the second switch on, i.e. water cycle if required. After water cleaning is complete, run the air cycle again. By running the air cycle, the water inside the filters will dislodge from the filters to the maximum extent possible.
  • Then, turn on the third switch i.e. heater cycle, by which the blower will start / scrubber blower will turn on and the heater will dry the filters.
  • Put the filter in a plastic bag and place it on the rack and maintain the record.

Coil Freeze protection
All chilled water, hot water and steam coils can be damaged during freezing weather. Take the following re-cautionary measures to prevent freezing:
  • For year round operation, glycol and other anti- freeze solution must be circulated.
  • During winter operation and shutdowns such as power failure, night shutdown and weekend shut- down, the controls must be installed so the valves will go to the full heat position, and all fresh air dampers go to the full closed position. The water circulation pumps must keep circulating water through the coils and/or auxiliary heat must be maintained inside the AHU cabinet.
  • Draining each coil and related piping such as traps and making sure that all low areas also drain.
  • After draining, flush the coils with an antifreeze solution such as propylene glycol. A solution of 50% glycol and 50% water will protect from freezing to approximately 35°F below zero at sea level. Use adequate mixture for the geographic area of the installation.
  • Winterizing Drain Traps - During the winter months when the cooling system is turned off and the unit is exposed to freezing conditions, an anti-freeze solution, which is environmentally friendly and safe for the roof can be poured in the condensate drain trap to prevent freezing and possible damage. The condensate drain trap may also be removed as well as heat traced and insulated.

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