Air Handling Units (AHUs) & Ductwork: capacity, cost and efficiency

What an air handling unit is, and how it works

An air handling unit — an AHU — is the box that conditions and moves a building’s air. Where a chiller makes cold water and VRF moves refrigerant, an AHU does something different: it draws in fresh outside air, filters it, tempers it through heating and cooling coils, recovers heat from the air it is exhausting, and pushes conditioned supply air out through ductwork to the spaces that need it. It is the central lung of a mechanically-ventilated building, and it is where energy, ventilation and indoor air quality all meet in one piece of plant.

A modern AHU is a series of sections in a casing: intake and exhaust dampers, filters, a heat-recovery device, heating and cooling coils, and fans. The fans are increasingly high-efficiency EC (electronically commutated) fans that modulate to match demand rather than running flat out. The coils may be fed with chilled and hot water from central plant, or the AHU may carry its own DX (direct-expansion) refrigerant cooling coil — in which case F-gas rules apply to that circuit. The heat-recovery section, a thermal wheel or a plate exchanger, captures heat from the outgoing exhaust and transfers it to the incoming fresh air, which is what stops ventilation becoming a punishing energy penalty.

Where AHUs fit — the buildings that need central air handling

AHUs are specified wherever a building needs controlled volumes of filtered, conditioned fresh air rather than just local temperature control. The classic applications are:

• Offices — supplying tempered, filtered fresh air to open-plan floors, often alongside VRF or fan-coils that handle the sensible cooling load.
• Hospitals and laboratories — where ventilation rates, filtration grades and pressure regimes are clinical and regulated, and air handling is safety-critical rather than a comfort nicety.
• Leisure and pools — high-humidity, high-fresh-air environments where the AHU controls humidity as well as temperature.
• Retail and public buildings — high-occupancy spaces where CO2 and air quality must be managed across large volumes.

The distinction from VRF and chillers matters: those systems primarily control temperature, while an AHU’s job is to deliver the right volume of clean, correctly-conditioned fresh air. In practice they work together — an AHU handles the fresh-air and filtration duty, while VRF or fan-coils trim the space temperature. That is why air handling is inseparable from ventilation and MVHR and from indoor air quality.

The flagship decision — refurbish or replace?

This is the question that defines AHU work, and where an honest specialist earns their keep. A great many UK commercial buildings run 1990s or 2000s AHUs that are tired but structurally sound, still moving air but doing it inefficiently through old belt-driven fans, worn coils and no meaningful heat recovery. The default sales response is a full replacement. The honest answer is often refurbishment.

An ageing AHU can frequently be brought close to new-build performance for a fraction of the cost and disruption of replacement:

• EC-fan retrofit — swapping old belt-driven AC fans for direct-drive EC fans cuts fan energy hard and gives precise, modulating airflow control. Because fan power runs whenever the building is occupied, this is usually the single biggest running-cost saving available.
• Coil replacement — new coils restore heat-transfer efficiency and let the AHU hit its design duty again.
• Heat-recovery upgrade — adding or improving a thermal wheel or plate exchanger recovers heat from exhaust air that was previously thrown away, cutting the heating and cooling load on the coils.
• Filtration upgrade — moving to F7 or ePM-rated filters lifts indoor air quality without replacing the whole unit.

Replacement is the right answer where the casing is corroded, the internal layout is wrong, access has become impossible, or the building’s duty has genuinely changed. But that should be a survey-led, whole-life-cost decision, not a default. We give you the numbers on each option so it is an engineering call, not a sales one — the same refurbish-versus-replace honesty we bring to chillers.

Sizing and economics

AHUs are sized on airflow and the duties the coils must serve, not floor area. A typical commercial AHU handles between 0.5 and 20 cubic metres of air per second with fan power from 1 to 50 kW, taking a plant-room or roof footprint of roughly 10–100 sqm. Indicative project values run from around £15,000 for a single packaged AHU up to £400,000 for a large built-up unit or a multi-AHU scheme with new ductwork, controls and heat recovery. A refurbishment typically costs a fraction of a like-for-like replacement, which is exactly why the survey comes first.

The running-cost and efficiency case — specific fan power

The efficiency metric that matters for air handling is SFP — specific fan power, the electrical power the fans draw per unit of air moved, measured in watts per litre per second. It is the number that determines what an AHU costs to run, because the fans run whenever the building is ventilated. Old belt-driven fans have poor SFP; modern EC fans, correctly sized ductwork and clean filters bring it right down. The heat-recovery efficiency of the wheel or plate exchanger then determines how much of the ventilation heat penalty you avoid. Together, low SFP and good heat recovery are what separate an efficient air handling system from an expensive one — and both can be retrofitted to existing plant. There are indicative ranges for every system type on our cost guide.

Filtration and air quality — matching the AHU to the building’s duty

The filtration section of an AHU is what makes air handling more than just temperature control, and the grade matters. Filters are rated to the ISO 16890 ePM classes — a general office AHU usually runs an ePM1 or F7-equivalent filter to catch fine particulates, pollen and traffic pollution drawn in from outside, while a hospital, laboratory or clean environment steps up through progressively finer grades, sometimes to HEPA, and adds pressure control so air flows from clean spaces to dirtier ones rather than the reverse. Getting the filtration grade right is a design decision tied to the building’s use, and a common failing in older plant is filters that were adequate when installed but no longer meet the building’s needs or air-quality expectations.

Filters carry a running-cost consequence too: a finer filter creates more resistance, so the fans work harder to push air through it. The trick is to specify the grade the building genuinely needs and to keep filters changed on schedule, because a blocked filter drives fan energy up sharply and starves the space of fresh air at the same time. This is one reason filter management sits inside a PPM contract — it is both an air-quality duty and an energy-efficiency lever.

The compliance and IAQ angle

AHUs sit under a distinctive set of standards. Ventilation provision and rates follow CIBSE Guide B and Approved Document F principles, ensuring the building gets the fresh air it needs. Ductwork carries a TR19 hygiene duty: kitchen extract and general supply/extract ductwork must be cleaned to the TR19 standard for fire safety as well as air quality — grease-laden kitchen ductwork in particular is a genuine fire risk, and TR19 cleaning and documentation is a legal expectation an insurer will ask about.

Where an AHU incorporates a DX cooling coil, that refrigerant circuit falls under F-gas rules — leak checks by an F-Gas registered company, records kept — exactly as a VRF or chiller circuit would. And because the heat-recovery specification directly lifts a building’s modelled energy performance, AHU upgrades support the EPC and MEES position: EPC E to let since 1 April 2023, EPC B proposed by 2031 for larger privately-rented non-domestic buildings, subject to secondary legislation (confirm the current position on gov.uk).

The electrification and IAQ tie-in

On the electrification ladder, air handling is the enabling step that makes low-carbon heat viable. A heat pump delivers heat efficiently only at low flow temperatures, and low flow temperatures work only when the ventilation heat loss is under control. An AHU with good heat recovery and demand-matched EC fans slashes the fresh-air heat penalty, which is precisely what lets a commercial heat pump run at a good SCOP. Get the air handling and ventilation right first, and the electrified heat that follows is far cheaper to run. That is the joined-up sequence — efficient air handling and ventilation, then electrified heat, then solar to power it — that runs through our whole commercial HVAC approach.

Objections we hear, answered honestly

“Our AHUs are old but still working — surely we just replace them?” Often you should not. Many ageing AHUs are strong refurbishment candidates: EC-fan retrofit, new coils, heat-recovery and filtration upgrades cut fan energy and lift air quality for far less than replacement and its disruption. Replacement is honest only where the casing is corroded, the layout is wrong or the duty has changed. We survey and give you the whole-life cost of each.

“How disruptive is AHU work?” Refurbishment is usually far less disruptive than replacement because the casing and ductwork stay in place — EC fans and coils can often be changed section by section, sometimes out of hours, keeping the space ventilated. A full replacement involves craneage and ductwork and is planned around your operating calendar.

“Is ductwork cleaning really necessary?” Yes, and it is a legal and insurance expectation. TR19 hygiene cleaning of extract and supply ductwork is a fire-safety duty as much as an air-quality one, especially on kitchen extract. It is documented and carried out on a schedule, normally inside a PPM contract.

Thermal wheel or plate exchanger — the heat-recovery choice

The heat-recovery device inside an AHU is not a single thing, and the choice between the two common types shapes both efficiency and hygiene. A thermal wheel (rotary heat exchanger) is a slowly-rotating matrix that picks up heat from the exhaust stream and carries it into the supply stream; it recovers a high proportion of the heat, and enthalpy wheels also transfer some moisture, which helps humidity control. A plate exchanger keeps the supply and extract streams fully separated by fixed plates, so there is no risk of cross-contamination between the two airstreams — the reason it is preferred where the extract air is contaminated, such as some hospital, laboratory or kitchen applications. Choosing between them is a balance of recovery efficiency, humidity handling and the cleanliness of the extract air, and it is exactly the sort of design detail that decides whether an AHU delivers on its efficiency and air-quality promise or merely looks good on the schedule.

Frequently asked questions

Should I refurbish or replace my air handling units?

Often refurbish — EC-fan retrofit, coil replacement, heat-recovery and filtration upgrades cut fan energy and improve air quality for a fraction of the cost and disruption of a full replacement. Replace where the casing is corroded, the layout is wrong or the duty has changed. It should be a survey-led, whole-life-cost decision.

What is SFP and why does it matter?

Specific fan power (SFP) is the electrical power the AHU’s fans draw per unit of air moved. Because the fans run whenever the building is ventilated, SFP largely determines what the unit costs to run. EC fans, correctly sized ductwork and clean filters bring SFP down significantly.

What is TR19 and do I have to comply?

TR19 is the ductwork-hygiene standard. Cleaning extract and supply ductwork to TR19 is a fire-safety and air-quality duty — particularly for kitchen extract, which is a genuine fire risk — and insurers expect documented compliance. It is normally handled inside a planned maintenance regime.

Do AHUs come under F-gas rules?

Only where they incorporate a DX (refrigerant) cooling coil. That refrigerant circuit is subject to F-gas leak checks and records like any other. AHUs fed by chilled and hot water from central plant carry no F-gas duty of their own.

How do AHUs affect indoor air quality?

They largely determine it. The AHU filters incoming air (F7/ePM grades), controls the fresh-air rate and, with demand control, supplies it where needed. Good filtration and adequate fresh air are what deliver healthy indoor air — and heat recovery is what delivers them without a heating penalty.

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We install, refurbish and maintain air handling units and ductwork across the UK, including London, Manchester and Bristol. For an AHU survey and an honest refurbish-versus-replace comparison, request a quote, or read the most common commercial HVAC questions.