The AHU that nobody maintains properly is the one that fails in August. Not a dramatic failure — usually something slower and more frustrating. The supply air temperature starts creeping up. Occupants complain the building feels stuffy. The engineer checks the chiller, the pumps, the BMS setpoints. Everything looks fine on paper. Then someone finally pulls the filter panel and finds a filter that hasn’t been changed in fourteen months, sitting behind a coil so fouled with dust it can barely transfer heat.
Air handling units are the lungs of a building. When they work well, nobody notices. When they don’t, everyone does. And the maintenance gaps that cause most AHU problems are not complicated — they’re just tasks that got skipped because nobody had a structured routine to follow.
This AHU preventive maintenance checklist gives your team that structure. It covers every major component — filters, coils, fans, drains, dampers, and controls — with the task frequencies and acceptance standards that actually matter on site, not the generic manufacturer guidance that most PPM programmes never move beyond.
Figure 1: AHU System — Components and PPM Focus Points
| AIR HANDLING UNIT (AHU) — COMPONENT OVERVIEW & PPM MAINTENANCE FOCUS | |
| 🔲 Filters (Pre-filter G4 + Fine filter F7/F9) | Check ΔP across filter bank. Replace at 250 Pa or per schedule. Never run an AHU with a missing filter — the coil fouls in weeks. |
| ❄️ Cooling Coil (CHW or DX) | Inspect fins for fouling and damage. Check condensate drain pan and trap. Low ΔT across coil = fouling or low water flow. |
| 🔥 Heating Coil (LTHW or Electric) | Check coil condition, fin straightness, and valve operation. Ensure no water carryover from cooling coil to heating coil. |
| 💨 Fan Assembly (Supply & Return) | Check belt tension and condition (belt-drive), bearing temperature and noise, motor current draw vs nameplate. |
| 🌡️ Controls & Dampers | Test fresh air, return air, and exhaust dampers for full travel. Check actuators for binding. Verify BMS setpoints. |
| 💧 Condensate Drain & Trap | Prime the trap before each cooling season. A dry or blocked trap causes condensate to flood the unit or blow back. |
| ⚠️ PPM frequency rule: Monthly = filters and condensate. Quarterly = coils, fans, dampers, controls. Annual = full strip-down, chemical clean, performance test. | |
Understanding What You’re Maintaining: AHU Components and Why Each One Matters
Before your team can maintain an AHU properly, they need to understand what each component does and what failure looks like before it becomes a breakdown. An AHU that’s developing a problem almost always shows warning signs — you just need to know what to look for.
The airflow path through an AHU is straightforward: return air from the building enters the unit, passes through filters, across the cooling and heating coils, through the fan, and is supplied back to the occupied spaces. Every component in that path depends on the one before it working correctly.
The most important relationship to understand: the filter protects the coil. The coil depends on the drain trap to remove condensate. The fan depends on the belt or direct drive being in good condition. The controls depend on dampers actually moving when they’re told to. When one component is neglected, the next one suffers.
| 📌 Compare AHU maintenance with FCU maintenance — see the FCU Maintenance Checklist for PPM. FCU Maintenance Checklist. |
AHU Preventive Maintenance Checklist by Frequency
The most effective AHU maintenance programmes are frequency-banded — different tasks at different intervals based on how quickly they deteriorate and how serious the consequences of missing them are. Here’s the complete breakdown:
Monthly AHU Maintenance Tasks
Monthly tasks are quick — a competent technician can complete them in 20–30 minutes per AHU. The emphasis is on filter condition and condensate management, the two items that cause the most reactive calls if neglected.
| ✓ | Task | What to Check / Accept Standard |
| ☐ | Inspect pre-filter condition (G4) | Hold to light — if less than 30% of light passes through, replace immediately. Don’t wait for scheduled replacement date. |
| ☐ | Check filter differential pressure (ΔP) | Most AHU filter banks have ΔP gauges or BMS alarms. Acceptable range: 50–200 Pa clean. Replace at manufacturer’s max (typically 200–250 Pa). |
| ☐ | Inspect condensate drain pan | Tray should be clean with no standing water. Any water level = partially blocked drain — flush and treat immediately. |
| ☐ | Flush condensate drain line | Pour water into tray and confirm it drains within 30 seconds. Check the trap is primed — a dry trap lets conditioned air bypass and causes pressure issues. |
| ☐ | Listen for unusual fan noise | Bearing noise (grinding/squealing), belt slap, loose access panels — log any abnormal noise and raise a work order before it becomes a breakdown. |
| ☐ | Check BMS status and active alarms | Review AHU alarms on BMS. Any persistent alarm — high ΔP, low flow, temperature deviation — should be investigated, not acknowledged and ignored. |
Quarterly AHU Maintenance Tasks
Quarterly tasks go deeper into the unit — coils, fans, belts, dampers, and controls. Expect 45–90 minutes per AHU depending on size and access.
| ✓ | Task | What to Check / Accept Standard |
| ☐ | Inspect and clean cooling coil fins | Use a soft brush or low-pressure air. Fins should be clean and straight. Bent fins restrict airflow — use a fin comb to straighten. Never use high-pressure water on coil fins. |
| ☐ | Inspect heating coil condition | Check for fin damage, corrosion, or water carryover staining from cooling coil. Any carryover = check drain trap and cooling coil condition immediately. |
| ☐ | Check fan belt tension and condition | Belt should deflect 10–15mm under firm thumb pressure at the midpoint. Cracked, glazed, or frayed belts should be replaced regardless of deflection. |
| ☐ | Check fan bearing temperature | Use a contact thermometer or thermal camera. Bearing temperature should not exceed 70°C ambient + 40°C rise = max ~90°C. Elevated temps indicate failing lubrication or alignment. |
| ☐ | Measure fan motor current draw | Clamp meter on all three phases. Compare against nameplate FLA. More than 10% above = investigate. More than 20% above = schedule motor replacement. |
| ☐ | Test all damper actuators — full travel | Fresh air, return air, and exhaust dampers should stroke fully open and fully closed on command. Stuck dampers cause outside air control failure — significant energy waste and IAQ problems. |
| ☐ | Calibrate supply and return air temperature sensors | Use a calibrated reference thermometer. If sensor reads more than 1°C off, recalibrate or replace. Inaccurate sensors cause the AHU controls to fight the wrong setpoint. |
| ☐ | Check vibration isolators and AHU mounting | Isolators should be in good condition, not bottomed out. A bottomed-out isolator transmits vibration to the structure — you’ll hear it as a hum throughout the building. |
Bi-Annual AHU Tasks — Pre-Summer and Pre-Winter
Two service visits per year should align with seasonal changeover — before the cooling season starts (April/May) and before the heating season starts (September/October). These are the visits where you confirm the AHU is ready for peak demand in each mode.
| ✓ | Task | What to Check / Accept Standard |
| ☐ | Replace fine filter (F7/F9 grade) | Replace regardless of visual condition at 6-month mark. Fine filters are the last defence before the coil — a filter that looks 60% clean can still be restricting airflow significantly. |
| ☐ | Prime condensate trap for cooling season | Before first operation in cooling mode, verify the trap is primed with water. A dry trap at the start of the cooling season is a guaranteed flooded drain pan within days. |
| ☐ | Full coil chemical clean (if required) | If quarterly brush cleaning hasn’t maintained coil cleanliness, apply approved foaming coil cleaner. Rinse thoroughly. Log chemical used and dilution rate. |
| ☐ | Measure and record supply air temperature | Confirm supply air temp against design spec. Typical: 12–14°C in cooling mode, 18–22°C in heating mode. Deviation from design indicates coil, valve, or controls issue. |
| ☐ | Check chilled water valve — full stroke and shutoff | Modulating valve should travel from 0–100% smoothly. Confirm tight shutoff at 0% — a valve that leaks through in full closed = overcooling. |
| ☐ | Inspect ductwork connections at AHU | Check flexible connections for cracking or collapse. Collapsed flexible duct significantly reduces airflow and can be confused with a fan fault. |
Annual AHU Full Service
Once a year, the AHU gets a comprehensive service that goes beyond what the quarterly and bi-annual visits cover. This is when you find the slow deterioration — corroding drain trays, motor insulation starting to break down, actuators sticking at certain positions.
| ✓ | Task | What to Check / Accept Standard |
| ☐ | Full internal inspection — clean all internal surfaces | Remove all access panels. Clean unit casing, fan housing, and coil section. Biological growth in a poorly maintained AHU is a significant IAQ risk. |
| ☐ | Motor insulation resistance test (Megger) | Test winding insulation to earth. Minimum acceptable: 1 MΩ. Below 0.5 MΩ = replace motor. Log and trend annually. |
| ☐ | Lubricate fan bearings (if non-sealed type) | Open bearings: apply correct grease per manufacturer spec — typically 2–3 pumps maximum. Over-greasing causes bearing failure. Sealed bearings need no greasing. |
| ☐ | Full controls calibration — all sensors and actuators | Commission the AHU controls against the design sequence of operation. Verify heating and cooling changeover, frost protection, and CO2/IAQ control if fitted. |
| ☐ | Measure and log airflow rates | Use a pitot tube or anemometer at the AHU discharge. Compare against design airflow. More than 15% below design = investigate — could be filter ΔP, duct blockage, or fan belt slip. |
| ☐ | Inspect drain tray for corrosion — treat or replace | Steel drain trays corrode from the inside out. Check for pin holes and staining. A corroded tray that fails releases water into the unit casing and potentially into the ceiling below. |
| ☐ | Complete service record and update asset register | Log all readings, parts replaced, faults found, and corrective actions. This record is your evidence base for any future warranty or performance claim. |
AHU Fault Diagnosis: Quick Reference for Site Teams
When a fault is called in, this table gives your team a structured starting point. Most AHU problems follow a recognisable pattern once you know what to look for:
| Symptom / Fault | Diagnosis & Corrective Action |
| High supply air temperature in cooling | Check CHW valve is opening — feel the chilled water pipes. If pipes are cold and valve open: check coil for fouling. If pipes are warm: check CHW pump and system pressure. |
| Low airflow — BMS alarm or occupant complaint | Check filter ΔP first — blocked filter is the most common cause. If filters are clean: check fan belt condition and tension. If belt is fine: check for duct blockage or damper stuck closed. |
| Water in AHU drain pan / unit flooding | Blocked condensate drain or dry trap. Flush drain immediately. Re-prime trap. If flooding persists after drain is clear: check coil condition for ice formation (low CHW flow). |
| Noise from fan section | Belt slap = loose or worn belt — check tension and condition. Bearing noise (grinding/squealing) = lubricate or replace bearing. Rattling = loose panel or foreign object in fan — inspect immediately. |
| BMS showing supply air temp below setpoint | CHW valve stuck open or leaking through at full closed. Feel coil pipes when valve commanded shut — if still cold, valve is not seating. Replace valve actuator or valve. |
| High motor current alarm | Check fan belt — a slipping belt increases motor load. Check motor phase balance on all 3 phases. If belt and electrical are fine: check for blocked inlet or outlet duct restricting airflow. |
| Damper not responding to BMS command | Check actuator — it should move when commanded. If actuator moves but damper doesn’t: check the linkage between actuator and damper blade. Replace actuator if it hums but doesn’t move. |
| Fresh air quality complaints / musty smell | Inspect internal AHU surfaces for biological growth. Check drain pan — standing water grows biofilm within days. Arrange chemical clean and UV-C treatment if growth is confirmed. |
What Good AHU Maintenance Records Look Like
Your AHU maintenance programme is only as strong as the records behind it. If an AHU fails and you can’t show what was done, when, and by whom — you have no defence against a warranty claim, a client complaint, or a performance dispute.
For each AHU service visit, your records should capture at minimum:
1. Date, time, AHU tag number, and technician name
2. Filter condition found and action taken — changed, cleaned, or left with reason
3. Condensate drain — clear, flushed, or blocked and action taken
4. Any abnormal readings — motor current, bearing temperature, airflow, ΔP
5. Damper positions confirmed and actuator operation tested
6. Any faults found, work orders raised, and work order reference numbers
7. Supply and return air temperatures measured and logged (quarterly and above)
8. Supervisor sign-off for annual service
If you’re using a CMMS, these readings should be entered against the AHU asset record — not left on a paper form. Trending motor current and airflow over 24 months gives you early warning of developing problems that a single inspection would miss.
| 📌 Use the PPM Planner on mepmasterguide.com to schedule your full AHU maintenance programme. PPM Planner → |
The One AHU Maintenance Task Most Teams Get Wrong
It’s the condensate trap. Specifically, failing to prime the condensate trap at the start of the cooling season.
Here’s what happens: the AHU sits idle through winter in heating mode. The condensate trap dries out completely. When cooling starts in spring, condensate forms on the cooling coil, runs into the drain pan, hits a dry trap — and doesn’t drain. Within a few days the drain pan floods, water finds its way into the fan section or out through the access panel seals, and you have a water damage event that could have been prevented by pouring a cup of water into the trap before the season started.
A dry trap also acts as an air bypass — conditioned supply air blows straight down the drain line instead of going to the occupied space. This wastes energy and makes the AHU work harder to maintain setpoint. It’s one of those faults that doesn’t throw an alarm, doesn’t show up on the BMS, and can run undetected for an entire cooling season.
Add trap priming to your pre-cooling season checklist. It takes thirty seconds and costs nothing.
Frequently Asked Questions
How often should AHU filters be replaced?
Pre-filters (G4 grade) should be inspected monthly and replaced when differential pressure reaches the manufacturer’s maximum — typically 200–250 Pa, or when the filter is visibly more than 30% blocked. Fine filters (F7/F9 grade) should be replaced every 6 months regardless of visual condition. In high-dust environments — construction-adjacent buildings, food processing, dusty industrial areas — replace pre-filters every 4–6 weeks and fine filters every 3 months. Never run an AHU without a filter installed — the coil will foul within weeks.
What is the correct supply air temperature from an AHU?
In cooling mode, a typical central AHU supplies air at 12–14°C off a chilled water cooling coil (with chilled water at 6–7°C supply). In heating mode, supply air is typically 18–22°C depending on the space heating demand and outside air temperature. If your supply air temperature is outside these ranges despite the system appearing to operate, check the cooling or heating coil valve for full travel, check coil fouling, and compare against the original design commissioning data.
How do I know if an AHU fan belt needs replacing?
Check belt tension first: the belt should deflect approximately 10–15mm under firm thumb pressure at the midpoint between the two pulleys. A belt with correct tension but showing cracks, glazing (shiny surface), fraying, or uneven wear should be replaced immediately — not at the next scheduled service. A slipping belt wastes energy, causes the motor to draw higher current, and eventually snaps without warning. Always replace belts in matched sets on multi-belt drives — replacing one belt in a set means the new belt carries unequal load and fails prematurely.
What causes an AHU to smell musty or produce poor air quality?
A musty smell from an AHU is almost always biological growth — mould or bacteria in the drain pan, on the cooling coil face, or on internal AHU surfaces. This happens when condensate sits in the drain pan (blocked drain), when filters are bypassed (damaged filter or missing filter), or when the AHU is shut down for extended periods in humid conditions. The fix is a full internal chemical clean, drain treatment with a biocide, and investigation of how the contamination developed. It should not be ignored — biological contamination in an AHU circulates throughout the occupied building.
Can I service an AHU while it’s running?
For visual inspections and readings — yes, with appropriate PPE (hearing protection near large fans, awareness of moving parts). For any task that requires opening the fan section, removing or replacing filters while the unit is running at full speed, or working near rotating components — the AHU must be isolated and locked out following your site Lockout/Tagout procedure. Filter changes on units with external filter access slides can often be done running at reduced speed, but check your site safety rules and the manufacturer’s guidance first.
Conclusion
A well-maintained AHU runs quietly, delivers the right air temperature and volume to occupied spaces, and rarely causes reactive calls. A neglected one costs you in energy waste, occupant complaints, and eventually a breakdown at the worst possible time.
The checklist in this guide covers everything your team needs to maintain AHUs from monthly filter checks through to annual full service. For the supporting maintenance schedule, use the PPM Checklist Generator → on mepmasterguide.com — or read the FCU Maintenance Checklist → to cover the terminal units that the AHU serves.
