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AHU maintenance checklist: tasks and frequencies

Updated 23 October 2026 · SEO Dons Editorial

Which AHU maintenance tasks have a published interval?

Very few. The law asks for maintenance of an air handling unit without setting a calendar for it. Regulation 5 of the Workplace (Health, Safety and Welfare) Regulations 1992 says the equipment, devices and systems it covers, which include mechanical ventilation systems, must be maintained, including cleaned as appropriate, in an efficient state, in efficient working order and in good repair. It adds that, where appropriate, they must be subject to a suitable system of maintenance. HSE's advice is to make sure mechanical systems are maintained in line with manufacturers' instructions. The tasks that do carry a published interval are below.

TaskPublished intervalSource
Visual check of the ventilation equipment, including coils, drain pans and humidifier reservoirsAt least once a weekCIBSE
Fire damper test by a competent personAt least annually, and more often in dust-laden or polluted settingsBS 9999:2017, as reported by BESA
Ductwork hygiene inspectionSet by system type, level of risk and frequency of useTR19, as described by BESA
Humidifier cleaning and monitoringRegularly, where appropriate; no fixed figure on HSE's pageHSE
Filter changeBy time or by conditionCIBSE

Everything else comes from the manufacturer's manual and a maintenance specification. SFG20 is the best known. Its schedules detail priority, frequency and skill level for each task, but it is accessed through its own digital platform, so its frequencies are not reproduced here. Ask a contractor which AHU schedule has been applied. The contract checklist and the PPM schedule guide cover that side.

What should an AHU maintenance checklist cover?

ComponentWhat to checkWhere the interval comes from
Filters and gaugesEvery stage fitted and sealed, no bypass, pressure drop against the data sheet's final value, alarm workingCondition or time. See the filter grades guide
Fans and drivesBelt tension and wear, pulleys, bearings, vibration, impeller cleanliness, EC fan fault statusManufacturer's manual
Coils and drain traysFin condition and cleanliness, leaks, pan clean and draining, no standing waterWeekly visual check
Heat recovery wheel or platesSeals, leakage between airstreams, wheel drive, pressure difference between extract and supplyManufacturer's manual
Dampers and actuatorsSmooth travel, linkage, actuator response, fire damper closing and resettingFire dampers at least annually
HumidifierCleanliness, reservoir water, scale, flushing cycles, recordsRisk assessment. See steam versus adiabatic
Sensors and controlsReadings against a reference, setpoints, schedules, fan failure warning, filter alarmManufacturer's manual and commissioning record
Casing and internalsPanel seals, insulation, corrosion, internal cleanliness, safe accessManufacturer's manual

Where the unit has a direct expansion coil, the refrigerant circuit has its own F-gas leak-check rules, set out in the PPM schedule guide.

Why do these checks matter?

Fans. CIBSE Journal says a belt drive carries inefficiencies of 2% to more than 10%, depending on maintenance, and that direct drive is likely to reduce maintenance because there are fewer bearings and cleaning requirements. The EC fan retrofit guide covers that option.

Heat recovery. CIBSE's 2020 ventilation guidance, as reported by CIBSE Journal, recommends inspecting for leaks that might transfer air from discharge to intake. For thermal wheels, a higher pressure on the extract side can cause air leakage to the supply, particularly in poor installations, so the pressure difference should be measured and corrected by a trained technician if necessary.

Failure warning. Regulation 6(2) requires ventilation plant to include an effective device giving a visible or audible warning of failure where necessary for reasons of health or safety. Test it, because an alarm nobody checks is not a safeguard.

Water. HSE lists humidifiers and air washers as systems that could be a source of legionella growth, with stored or re-circulating water and a food source such as scale or biofilm among the factors. A drain pan that holds water is worth treating with the same suspicion. The legionella guide explains the duties.

Ductwork. The internal cleanliness of ducts follows TR19; see duct cleaning to TR19 Air. Damper types are in the dampers guide.

How can condition monitoring stretch intervals safely?

CIBSE says filter maintenance can be based on time intervals or on condition. Condition-based change is practical because new non-residential ventilation units, from 1 January 2018, must have a visual signal or an alarm in the control system that activates when filter pressure drop exceeds the maximum allowable final pressure drop. A sensible regime follows five rules.

  1. Keep the fixed rows fixed: the annual fire damper test, humidifier hygiene and records, and any F-gas checks.
  2. Stretch only tasks with a measurable trigger, such as filter pressure drop, and set the trigger from the data sheet.
  3. Keep a calendar backstop in case a sensor fails.
  4. Record the reading and the reason whenever an interval departs from the manufacturer's baseline.
  5. Revisit the decision after any change in occupancy, outdoor air quality or fan control.

What should each visit record?

Enough to show what was found and what was done. For humidifiers HSE is explicit: there is a duty to keep records of all maintenance carried out, together with monitoring results. Apply the same habit to the whole unit:

  • the unit reference, date and who attended
  • filter stages fitted, their class and the pressure drop readings
  • belt, bearing, drain pan and coil condition
  • damper and fire damper test results
  • humidifier cleaning, water checks and any monitoring results
  • faults found, actions taken and anything deferred, with the reason

Readers who want an AHU surveyed against a checklist can send an enquiry through the site's form. The air handling units page covers the equipment, and the maintenance and PPM page covers contracts.

Sources

Common questions

How often should AHU filters be changed?

There is no single interval. CIBSE says filter maintenance can be by time or by condition, and new non-residential ventilation units must carry a filter alarm that activates at the maximum allowable final pressure drop. Change on that trigger, with a calendar backstop.

Does an AHU need a legionella risk assessment?

If it has a humidifier or air washer, yes, because HSE lists them among systems that could be a source of growth and says you must carry out a risk assessment. A unit with only coils still needs its drain pans kept clean and draining.

Is SFG20 compulsory for air handling units?

No. SFG20 describes itself as the industry standard for building maintenance specification, not law. The legal duty is the Workplace Regulations requirement for a suitable system of maintenance, and SFG20 is one way of building one.

Do heat recovery wheels need special checks?

CIBSE's guidance says to inspect heat recovery equipment for leaks between airstreams. On thermal wheels, extract-side pressure higher than supply can push air into the supply, so the pressure difference is measured and corrected if needed by a trained technician.

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