Swimming Pool Air Handling Units: a humidity problem, not a temperature one

Why a pool hall is not an ordinary ventilation problem

A commercial swimming pool continuously evaporates water into the space above it. That evaporation is driven by the difference between the water surface and the air, and it is increased by pool surface area, water temperature, air movement across the surface, and — significantly — by activity. A busy session with splashing and wave features evaporates far more than a still pool at 6am.

The result is a space with a permanent, large moisture load that has to be removed continuously, in a warm environment, in air carrying chlorine and other disinfection by-products. That combination — warm, wet and chemically aggressive — is why pool ventilation is a specialist discipline rather than an air handling unit with a bigger fan.

The critical relationship is between air temperature and water temperature. Convention is to hold the air roughly 1°C above the water. Do that and evaporation is suppressed and bather comfort is good. Let air temperature drop below water temperature and evaporation accelerates sharply, driving up both the moisture load and the energy needed to deal with it. Raise it much further above and you gain nothing but running cost and uncomfortable, stuffy conditions.

Condensation: the failure that hides inside the walls

The visible symptom of poor pool ventilation is condensation on windows and cold surfaces. That is not the expensive problem. The expensive problem is interstitial condensation — moisture that migrates into the building fabric and condenses within the structure, where nobody can see it.

Left unaddressed, that causes corrosion of structural steelwork, degradation of insulation, and rot in timber elements. It is progressive, invisible, and frequently discovered years later during unrelated work — by which point the remedial cost dwarfs whatever was saved on the original ventilation specification. There is a well-known history of pool building failures attributable to exactly this mechanism.

Preventing it means maintaining humidity within design limits reliably rather than on average, holding the hall at a slight negative pressure relative to adjoining areas so moist air is not pushed into changing rooms, circulation spaces and voids, and directing supply air across the cold surfaces — particularly glazing — where condensation would otherwise form. A pool AHU that satisfies a humidity sensor in the middle of the hall while allowing moist air to migrate into a roof void is not doing its job.

Construction: the specification that gets value-engineered out

Standard air handling plant does not survive a pool environment. Chlorinated, humid air attacks casing, coils, fasteners and dampers, and the failure is not gradual and graceful — it is a coil that starts leaking or a casing that corrodes through.

A properly specified pool AHU is corrosion-resistant throughout: casing and internal panels appropriately treated or manufactured in resistant materials, coils with protective coatings or corrosion-resistant construction, stainless or otherwise resistant fixings and fasteners, and drainage detailed to cope with continuous condensate without pooling.

This is precisely the specification that gets trimmed when a project is over budget, because the saving is immediate and visible while the consequence is five to ten years away and lands on someone else's budget. If you are reviewing a pool AHU quotation, the corrosion specification is the line to examine hardest, and the one to compare most carefully between bidders — two quotes that look similar in duty can differ enormously here.

Heat recovery, and why it is not optional

A pool hall ventilation system moves a large volume of warm, moist air out of the building continuously. Without heat recovery, all of that energy is thrown away and replaced by heating cold outside air — an enormous and permanent running cost.

Air-to-air heat recovery between the exhaust and supply streams is therefore standard on pool AHUs rather than an upgrade. Plate heat exchangers and run-around coils both appear, with the choice partly driven by the need to avoid cross-contamination between the chemically-laden exhaust and the fresh supply. Recovery efficiency is a headline number worth comparing between bidders, because on a load that runs continuously the difference compounds every hour.

Dehumidification by refrigeration adds another opportunity. A heat pump dehumidifier cools the air below its dew point to condense moisture out, then reheats it — and the heat rejected by the refrigeration cycle can be reclaimed into the air or, better, into the pool water. Heating pool water with heat that would otherwise be wasted is one of the genuinely efficient moves available in a leisure building.

The other control lever is demand-based operation. Moisture load varies enormously between a packed session and an empty hall overnight, and running a fixed high ventilation rate around the clock wastes a great deal. Variable-speed fans modulating to actual measured humidity — with pool covers used out of hours, which cuts evaporation dramatically — is where a large share of the achievable saving sits. Our ventilation and MVHR page covers heat recovery principles more generally, and air handling units covers AHU construction and capacity.

Sizing, and what a designer needs from you

Sizing a pool AHU starts from the evaporation rate, which is calculated from pool surface area, water temperature, intended air temperature and humidity, and — crucially — the usage pattern. This is where a designer needs honest information from the operator.

A 25 m competition pool used for lane swimming by adults has a very different evaporation profile from a leisure pool of identical surface area with flumes, wave machines and splash features full of children. Water features increase evaporation dramatically because they expose far more water surface to the air. A system sized on surface area alone, without accounting for features and activity, will be undersized in exactly the conditions where it matters.

Other inputs that matter: whether a pool cover is used outside opening hours, which substantially reduces overnight load and can change plant sizing; the fresh air requirement needed to control disinfection by-products, which sets a minimum ventilation rate independent of the moisture calculation; and the spectator area, if there is one, since spectators are dressed and want a cooler, drier environment than bathers and effectively constitute a separate zone.

Maintenance in an aggressive environment

Pool plant needs more frequent attention than general ventilation, and the reasons are environmental rather than mechanical. Coils and casings should be inspected for early corrosion, because catching it early is the difference between remedial coating and replacement. Filters load faster. Condensate drainage must be kept clear, since standing water in a pool AHU is both a corrosion accelerator and a hygiene issue. Heat exchanger surfaces need cleaning to maintain recovery efficiency, which otherwise degrades invisibly. Humidity sensors need calibration — a drifting sensor will hold the hall at the wrong condition indefinitely while reporting that everything is fine, which is one of the more insidious failure modes in this application.

Ductwork hygiene under TR19 applies here as it does to other ventilation systems. And it is worth reviewing the control setpoints periodically against how the pool is actually being used — programmes change, and a control strategy set for the original usage pattern may be some way off years later. See maintenance and PPM and the SFG20 explainer.

Frequently asked questions

What does a swimming pool air handling unit actually do?

Its primary job is removing moisture and controlling condensation, not cooling. A pool continuously evaporates water into the hall, and the AHU removes that moisture, supplies fresh air to control disinfection by-products, maintains the correct air temperature relative to the water, and directs air across cold surfaces such as glazing where condensation would otherwise form.

What temperature and humidity should a pool hall be?

Humidity is commonly held around 50–60% relative humidity, and air temperature is conventionally kept about 1°C above the pool water temperature. Confirm both against your specific design, since pool type and use affect the right figures. Letting air temperature fall below water temperature is the mistake to avoid, because it sharply accelerates evaporation.

Why do pool air handling units need special construction?

Because warm, humid, chlorinated air is highly corrosive. Casings, coils, fasteners and dampers all need corrosion-resistant construction or protective coatings, and drainage must handle continuous condensate without pooling. Standard air handling plant fails in this environment, typically through coil leaks or casing corrosion, and the corrosion specification is the line most often cut when a project is over budget.

Can I just use a dehumidifier instead of an AHU?

Not for a commercial pool. A dehumidifier addresses moisture but not the fresh air requirement needed to control disinfection by-products, and it does not provide the air distribution needed to keep cold surfaces and the building fabric free of condensation. On a commercial pool the ventilation, dehumidification and air distribution functions have to work together.

How is a pool AHU sized?

From the calculated evaporation rate, which depends on pool surface area, water temperature, the intended air condition, and the usage pattern. Water features and busy leisure use raise evaporation far above a still lane-swimming pool of the same surface area, so a system sized on area alone will be undersized when it matters most. Pool cover use out of hours also affects sizing.

We design and install specialist pool hall air handling and dehumidification across the UK. See air handling units for AHU construction and capacity, ventilation and MVHR for heat recovery principles, and maintenance and PPM for the servicing regime. Or request a quote.