Natatorium and indoor pool dehumidification field guide for HVAC

What natatorium HVAC is, and why an indoor pool destroys a building that ignores it

A natatorium is an indoor swimming pool and the room that holds it, and from the day it fills it works against the building around it. Warm chlorinated water evaporates into the air every hour the pool sits there, idle or in use, which loads the room with humidity and with the chloramine vapors that come off the water surface. The HVAC has one job that splits four ways: pull that moisture back out of the air, hold the humidity below the point where cold surfaces sweat, keep the room under negative pressure so the corrosive air stays inside it, and capture the chloramines down at the deck where they settle. Miss any one of those and the structure corrodes, the windows run with condensation, and the air begins to sting the eyes of the people swimming in it.

This guide is the air side of an indoor pool. The water side, the pump, filter, chemistry, and anti-entrapment, lives in the swimming pool and spa mechanical guide. The general dew-point and dehumidification-method theory lives in the humidification and dehumidification control guide. Here the subject is the pool room itself.

Everything that follows comes back to one design driver: the moisture the pool throws into the air. Size for that load, hold the dew point below the coldest surface, keep the room negative, and use materials that survive the environment you just built. The rest is detail around those four calls.

The evaporation load: the latent load that drives the whole design

The number that sizes a natatorium system is not the heating load or the cooling load. It is the evaporation rate, the pounds of water per hour that leave the pool surface and the wet deck and have to be removed from the air. This is a latent load, the energy tied up in turning liquid water into vapor, and in most indoor pools it dwarfs everything else the HVAC is asked to do.

Evaporation climbs with water temperature, with how dry the air above the surface is, and with activity. A still therapy pool gives off far less than a wave pool full of swimmers, because splashing, spray features, and bodies moving through the water expose more surface and stir the air. ASHRAE's HVAC Applications Handbook, in the natatorium chapter, gives an evaporation equation and an activity-factor table built from field and test data, where the activity factor scales the calculated rate up for a public or recreational pool well above a quiet residential one.

Get this wrong on the low side and nothing downstream can save you. A dehumidifier sized to a quiet pool cannot keep up with a busy one, the humidity climbs past setpoint, and the room goes wet. Calculate the evaporation rate for the real activity the pool will see, confirm it against the equipment manufacturer's selection software, and treat that load as the spine of the whole design.

Why an indoor pool wrecks a building that was not designed for it

The damage is rarely sudden. It happens inside the walls and above the ceiling where nobody looks, and by the time it shows on the surface the repair is structural.

Warm humid air finds its way into the building envelope, and when it reaches a surface colder than its dew point it condenses there. Inside an insulated wall or roof that is the cold sheathing, the steel deck, the joist, the fastener. The water condenses on metal and the metal rusts. It soaks insulation and the insulation stops working, which makes the surface colder, which makes more water. Wood framing and decking rot. The chloramine vapors riding in that air accelerate the corrosion, attacking galvanized steel, fasteners, and connectors faster than humidity alone would.

On the visible side the failure shows as condensation running off the glass and dripping from the ceiling, stains spreading on the structure, and corrosion blooming on any exposed metal. None of it is the equipment failing. It is moisture going where the design let it go. That is why the pool room is treated as a contained, controlled box: hold the moisture in the air until the dehumidifier removes it, keep it out of the structure with pressure and a vapor barrier, and keep the corrosive air off the rest of the building. Get those wrong and the building pays for it for the rest of its life.

How do you dehumidify an indoor pool?

Three methods remove moisture from a pool room, and the right one depends on the climate and the load. Most facilities use a mechanical dehumidifier, some use outdoor air, and a few special cases use desiccant. They are not mutually exclusive; many systems blend mechanical dehumidification with an outdoor-air economizer that takes over when the outside air is dry and cold enough to do the work for free.

The table below is the starting point for the selection, not the end of it. The actual choice gets confirmed against the evaporation load, the local design weather, the energy cost, and the dehumidifier manufacturer's selection software. Run the load first, then pick the method that carries it at the lowest lifetime cost for that climate.

The mechanical dehumidifier: the packaged DX unit that runs the room

The workhorse for most indoor pools is a packaged mechanical dehumidifier built on a refrigeration circuit, the same direct-expansion (DX) cycle as an air conditioner, configured to wring water out of the air rather than just cool it. Room air passes over a cold evaporator coil, the moisture condenses out, and the now-dry air is reheated and sent back to the room.

What sets a pool dehumidifier apart is where the heat goes. The latent heat pulled out of the air is not wasted to the outdoors. The unit recovers it, sending it to the pool water through a refrigerant-to-water condenser and to the supply air as reheat. The water that the pool keeps losing to evaporation gets reheated by the same machine that is drying the air, so the load that costs you on one side pays you back on the other. Reclaiming that heat to the pool can cut the pool's separate heating bill substantially over a year.

These machines run hard and nearly continuously, because the pool never stops evaporating. Size the unit to the calculated evaporation load with margin for the real activity, confirm the selection and the heat-recovery capacity against the manufacturer's software, and remember that a dehumidifier sitting in this air needs the same corrosion protection as everything else in the room. The coil, the cabinet, and the internals live in warm chloramine-laden air all day.

The outdoor-air method: ventilation dehumidification

Outdoor-air dehumidification works on a simple trade: when the air outside holds less moisture than the air inside, bring the dry air in and push the wet air out. No refrigeration cycle, just fans and dampers moving air. In a cold or dry climate this is close to free dehumidification for much of the year, which is why it shows up as an economizer mode on most pool units even when a mechanical dehumidifier carries the design load.

It has two limits worth respecting. First, it only helps when the outdoor air is actually drier than the room target. On a hot, humid summer day the outside air carries more moisture than the pool room, and bringing it in makes the load worse, not better, so the system has to fall back to mechanical dehumidification. Second is the freeze risk. Pulling in large volumes of cold winter air means the heating side has to temper it, and any coil, drain, or wet surface in the path can freeze if the controls and the freeze protection are not set up for it.

Treat outdoor-air dehumidification as a seasonal tool that the controls bring on when the psychrometrics favor it, not as the whole strategy. The crossover point, where outdoor air stops helping and starts hurting, is a control decision the design and the manufacturer set for the local weather.

Desiccant dehumidification: the low-dew-point special case

Desiccant dehumidification uses a material, often a slowly rotating wheel, that adsorbs water vapor directly out of the air and then releases it to a regeneration airstream that is exhausted. It is the tool for reaching a low dew point, lower than a refrigeration coil reaches comfortably, and it keeps drying even when the air is cool.

In a typical pool that runs warm and wet, a refrigeration-based dehumidifier is the natural fit and desiccant is overkill. Desiccant earns its place in special cases: spaces that demand an unusually low dew point, rooms with cold surfaces that force the dew point down further than DX can reach, or hybrid designs where a desiccant stage handles part of the load. If the design calls for it, size and select it with the manufacturer; it is not the default for an ordinary natatorium.

What humidity should an indoor pool be kept at?

Indoor pools are commonly held around 50 to 60 percent relative humidity. ASHRAE's natatorium guidance points to that band for swimmer comfort, and to keeping RH in roughly the 40 to 60 percent range to discourage mold and other organism growth. The exact setpoint is a design call, hedged to the equipment and the building, not a fixed law.

There is a tension built into the number. Run the room humid, toward the top of the band, and the dehumidifier works less and the energy bill drops, because higher room humidity means a smaller difference for the machine to pull against. Run it dry, toward the bottom, and you have more margin against condensation but you pay for it in dehumidification energy and you push evaporation up, which the swimmers feel as a colder, more drafty room.

The setpoint is not chosen for comfort alone. It is chosen so the room's dew point stays below the coldest surface in the space, which is the next section. A 50 to 60 percent target at a typical pool air temperature lands at a dew point in the mid to high 60s Fahrenheit, and every surface in the room has to stay warmer than that. The RH setpoint, the dew point it implies, and the surface temperatures are one coupled decision. Set them with the design and confirm against the dehumidifier manufacturer's range.

Dew point and condensation: the physics that protects the building

Dew point is the temperature at which the air is full and water begins to come out of it. It is the number that matters for the building, more than relative humidity, because it tells you exactly which surfaces are at risk. Any surface colder than the room's dew point will collect condensation. It is not a maybe. It is physics, and it happens every time.

A pool held at roughly 82 degrees F and 50 to 60 percent RH sits at a dew point around 67 degrees F. So every surface a swimmer or the structure touches, the window glass, the frames, the wall behind a thermal bridge, the steel above the ceiling, has to stay above about 67 degrees F or it sweats. Single-pane glass on a cold night is the classic loser. It runs with water, the water drips, and over time the sill and the wall below it rot.

This is where the air side and the building meet. The HVAC holds the dew point down by removing moisture, and the architect keeps the surfaces warm with insulation, warm glazing, and supply air washed across the cold surfaces. Both have to hold. Drop the dew point with a drier setpoint and you buy margin against a cold surface; warm the surface with better glazing and you can run the room a little wetter. The design balances the two. Confirm the dew-point target against the coldest design surface temperature in the room, hedged to ASHRAE and the project.

Why are pool rooms kept under negative pressure?

The single most important rule for protecting the rest of the building is to keep the natatorium under negative pressure relative to every space around it. The pool room is full of warm, moist, chloramine-laden air. Negative pressure means air leaks into the pool room from the corridors and offices, never out of it, so that corrosive, humid air stays trapped where the corrosion-resistant materials and the dehumidifier can deal with it.

Let the room go positive, even slightly, and you have built a problem with a long reach. The pool air pushes out through every door crack and penetration into the lobby, the locker rooms, the ceiling plenums, and the offices, carrying moisture into walls that have no vapor barrier and corrosion into metal that was never specified for it. A musty lobby and rusting door hardware two rooms away from a pool usually trace back to a room that went positive.

Practice commonly holds the natatorium between about 0.05 and 0.15 in. w.c. negative to the adjacent spaces, often by exhausting more air than is supplied, with a variable-speed exhaust fan trimming to a space pressure sensor. The exact offset and control method are design and equipment calls. The non-negotiable is the sign: the room stays negative, always, and the commissioning proves it across the operating range with the doors in their normal positions.

What are chloramines, and why they drive the air quality design

Chloramines are the combined-chlorine compounds that form when the pool's chlorine reacts with the nitrogen that swimmers bring in, the sweat, body oils, and urine. They off-gas from the water into the air, and they are what people are smelling when they say a pool smells strongly of chlorine. That smell is not clean water. It is a sign of combined chlorine and a room that is not exhausting it.

The vapors do real harm. They sting eyes and irritate airways, they trigger the cough and asthma symptoms that coaches and lifeguards who work pool decks know well, and they corrode metal in the room and anything the air reaches. The water-chemistry side of keeping combined chlorine low lives in the pool mechanical guide. The air side is what this guide owns: dilute the chloramines with outdoor air and remove them at the source before they build up.

The reason the air side cannot fix this alone with ceiling exhaust is the next section. The vapors are heavier than air and they sit low, right in the breathing zone of swimmers and right over the deck. The air-quality design is built around getting at them where they actually are, which is down at the water and the deck, not up at the roof.

Deck-level exhaust: pulling the chloramines off where they sit

Airborne chloramines and the other disinfection byproducts are heavier than the surrounding air, so they do not rise with the heat and humidity. They pool low, in a layer over the water and across the wet deck, exactly where swimmers breathe at the surface and where deck staff stand all day. Exhaust grilles up at the ceiling pull off the warm, light, relatively clean air and leave the heavy bad air right where it does harm.

Source-capture exhaust fixes this by pulling air from low in the room, at or near deck and water level, so the exhaust grabs the chloramine layer at a high concentration before it spreads. Designs range from low wall returns and exhaust grilles set near the deck to gutter evacuators and deck plenums that draw the polluted air out through channels buried at the pool edge. The detail varies, but the principle is fixed: get the exhaust intake close to the source, down low, not at the ceiling.

This is becoming the expected approach for competition and heavily used pools, and the Model Aquatic Health Code's air-quality guidance pushes toward source capture and deck-level exhaust for the same reason. Coordinate the low exhaust with the supply-air pattern so the room sweeps the bad air toward the deck intakes instead of stirring it back up into the breathing zone.

Air distribution: sweep the glass, leave no dead air

Where the supply air goes matters as much as how much of it there is. The first duty of the air pattern is to wash the cold surfaces, the windows and exterior walls, with dry supply air. That warms the surface above the room dew point and carries away the moisture trying to condense on it, which is what stops the glass from fogging and dripping. A natatorium with great dehumidification and poor air distribution still gets condensation on the glass, because the dry air never reached the cold surface.

The second duty is to leave no dead air. Still corners and stagnant zones let humidity stratify and chloramines accumulate, so the design moves air across the whole room, from the supply at the cold surfaces, over the water, and toward the low source-capture exhaust at the deck. The supply, the room, and the exhaust have to work as one path, not three independent systems.

Get the diffuser layout from the design and confirm it in commissioning by checking that the glass stays clear on a cold day and that there are no stagnant pockets. The classic mistake is aiming all the supply at the occupied zone for comfort and starving the glass, then chasing the resulting window condensation as if it were a dehumidifier problem.

Pool water temperature versus air temperature

Evaporation is driven by the difference between the water and the air above it, so the relationship between the two setpoints is a lever on the whole load. Keep the air a few degrees warmer than the water and evaporation slows; let the air go cooler than the water and evaporation accelerates and swimmers feel chilled when they step out.

ASHRAE's natatorium guidance commonly puts the space air temperature about 2 to 4 degrees F above the pool water temperature, while keeping the air below roughly 86 degrees F so the room does not become uncomfortable. So a pool kept at 82 degrees F water runs the air around 84 to 86 degrees F. The warmer air also helps keep surfaces above the dew point and reduces the chill on wet skin.

The trap is letting the air setpoint drift below the water setpoint, which spikes evaporation and the dehumidification load at the same time. Set the air above the water, hold the difference, and confirm the exact targets against the design and the type of pool, since a competition pool, a leisure pool, and a therapy pool each carry their own water temperature.

Outdoor air for the swimmers and for chloramine dilution

Even with mechanical dehumidification doing the moisture work, the room needs a minimum of outdoor air for the people in it and to dilute the chloramines that source capture does not catch. ASHRAE Standard 62.1 sets ventilation rates for pool spaces based on the water and deck area, on the order of a fraction of a CFM per square foot of wet surface, and that minimum holds regardless of how the dehumidification is done.

The outdoor air does double duty. It dilutes the chloramine concentration in the breathing zone, and it is the supply side of the negative-pressure balance, where the room exhausts more than it brings in. The amount has to satisfy both the code ventilation minimum and the pressure offset the design needs, and the larger of the two requirements governs.

Confirm the outdoor-air rate against the adopted edition of ASHRAE 62.1, any local health-code requirement, and the Model Aquatic Health Code air guidance, since aquatic ventilation rates have been revised over recent cycles and the jurisdiction's adopted version controls. The general dehumidification trade-offs around outdoor air sit in the humidity control guide; here it is a fixed minimum the pool design has to meet.

Corrosion-resistant materials: duct, equipment, structure, and fasteners

Everything exposed to natatorium air corrodes unless it was chosen not to. Warm humidity plus chloramine vapor is one of the harshest indoor environments a building holds, and ordinary galvanized steel, plain carbon-steel fasteners, and standard equipment finishes do not survive it. The material specification is part of the mechanical design, not an afterthought for the contractor to value-engineer.

Ductwork in a pool room is commonly aluminum, painted or coated galvanized, a suitable stainless grade, or corrosion-resistant fabric, with the selection made by the design for the exposure. Worth knowing: chlorides attack many stainless grades through pitting and stress-corrosion cracking, so stainless is not automatically safe in this air and the grade has to be chosen for chloride exposure. Below-grade duct, the kind feeding a deck plenum, is often a coated or plastic material rather than bare metal because it sits wet. Coils get a protective coating, often fully dipped so the whole coil is covered, and fasteners and hangers are specified in corrosion-resistant materials or they become the first thing to fail.

The rule is to assume the air will find any unprotected metal and ruin it. Specify the duct, the equipment cabinet and coil, the structural connectors, the grilles, and the fasteners for the environment, and confirm the materials against the design and the manufacturer. A single run of bare galvanized or a box of plain steel screws becomes the corrosion that stains the ceiling in a few seasons.

Heat recovery: the energy these systems can give back

A natatorium dehumidifier runs almost constantly, so the energy it moves is large, and recovering it is where the operating cost is won or lost. The latent heat pulled out of the room air is real energy, and a well-designed system puts it back to work instead of dumping it outside.

The two places it goes are the pool water and the supply air. Recovering heat to the pool water offsets the separate pool heater, which can cut the pool's heating energy meaningfully over a year, since the same evaporation that loads the dehumidifier is also cooling the pool that the recovered heat reheats. Recovering it as supply-air reheat avoids burning new energy to temper the dehumidified air back to room temperature. On the ventilation side, an air-to-air heat exchanger can recover heat from the exhaust to temper the incoming outdoor air, which matters most in cold climates where that outdoor air would otherwise be a heating penalty.

Size and confirm the recovery capacity with the manufacturer's selection, since how much heat the pool can actually absorb depends on the water temperature and the pool's own load. Heat recovery is not a luxury feature on these systems. It is most of the reason the energy numbers work.

The building envelope and the warm-side vapor barrier

The HVAC cannot protect a building whose envelope lets moist air into the structure. The pool room is a warm, high-dew-point box wrapped in walls and a roof that are cold on the outside in winter, and the envelope has to stop the room's moist air from reaching the cold layers inside the assembly. That is the job of the vapor barrier, and on a pool it goes on the warm, humid side of the construction.

This is the opposite habit from a normal heated building in some climates, and it is where pool envelopes go wrong. The vapor retarder belongs on the inside, the pool side, of the walls, ceiling, and where applicable the floor, so warm moist air never gets past it to a surface cold enough to condense. Behind that barrier the assembly needs enough insulation to keep the interior surfaces above the room dew point, and the glazing has to be warm enough, often high-performance insulated units, to stay above dew point on a design-cold night. A gap in the vapor barrier, a penetration that was not sealed, a thermal bridge through a steel member, becomes the cold spot where moisture condenses inside the wall and the rot starts.

This is a design that the mechanical engineer and the architect have to do together. The HVAC holds the dew point down and the room negative; the envelope keeps the moist air out of the structure and the surfaces warm. Neither one covers for the other. Confirm the vapor-barrier location and the insulation and glazing performance with the design team early, because it cannot be fixed after the walls are closed.

Waterslides, spray features, and spas raise the load

Anything that moves the water, breaks its surface, or adds more warm water raises the evaporation rate, and a leisure pool is full of those features. Waterslides, spray pads, fountains, vortex channels, and bubblers all expose far more surface area and agitate the air over the water, which pushes the evaporation well above what a flat, calm lap pool of the same footprint would give.

A spa is its own load. It runs hotter than the main pool and it churns, so a small spa can throw off moisture out of proportion to its size, and a busy spa adds to the bather load that drives the activity factor up. Size the system for the features running and the pool busy, not for a still pool on a quiet morning, because the design load is the worst realistic case, not the average one.

When the program includes features, get the real evaporation contribution of each one into the load calculation, and confirm the assumptions with the manufacturer's selection software. The classic miss is sizing the dehumidifier off the pool surface area alone and ignoring the slide and the spray pad that were added later, then wondering why the room runs humid when the features are on.

Controls: humidity, temperature, and pressure together

A natatorium control system juggles three loops at once, and they interact. It holds the space humidity at setpoint by running the dehumidifier and bringing in outdoor air when the air outside is dry enough to help. It holds the air temperature at its target above the water. And it holds the room negative by trimming the exhaust against a space pressure sensor. Move one and the others shift, so the controls have to coordinate rather than fight.

The integration is where these systems succeed or fail in service. The dehumidifier, the outdoor-air economizer, the heat recovery, the exhaust fan, and the pool-water heating all answer to the same control sequence, and a sequence that was never properly commissioned will hold one variable while letting another drift. Monitoring matters too: space humidity, dew point, temperature, and pressure trended over time tell you the room is staying in band, and they tell you early when a coil is fouling or a damper is stuck before the windows start dripping.

Tie the readings and the service history to a field record so the trend is not trapped in a controller nobody opens. A field tool such as FieldOS keeps the setpoints, the readings, and the service log in one place, so the next technician sees what the room has actually been doing rather than starting from the panel cold.

Commissioning the room before anyone swims

A natatorium is not finished when the equipment starts. It is finished when commissioning proves the room holds humidity, holds pressure, keeps its surfaces dry, and keeps the air breathable, all at once and across the operating range. Skip the commissioning and the building finds the problems for you, slowly, from the inside.

The checks that matter are specific. Confirm the dehumidification holds the humidity setpoint under load, not just on a mild empty-room afternoon. Verify the room is negative to every adjacent space, with the doors in their normal positions, across the range the exhaust will operate. Run a condensation check on a design-representative cold condition and confirm the glass and the cold surfaces stay clear, which tells you the dew point is below the surfaces and the air is sweeping them. Check the air quality and the source-capture exhaust are actually pulling the chloramine layer off the deck. And balance the supply, return, and exhaust so the pattern and the pressure are what the design intended.

Document every result against the design target. Most natatorium callbacks are not failed equipment; they are commissioning that nobody finished, a pressure that was never measured negative, or a condensation check that nobody ran on a cold day. The room that was proven out before opening is the room that does not rot.

Maintenance over the life of the room

A pool room ages faster than any other space in a building, so maintenance is partly ordinary HVAC service and partly a standing corrosion watch. The environment is working on the equipment every hour, and small problems compound.

Inspect for corrosion on a schedule: the coil, the cabinet, the duct, the fasteners, the hangers, and the structural connections that live in the air. Keep the coil clean, because a fouled coil loses dehumidification capacity and the room creeps wet. Change the filters and confirm the airflow, since the air pattern that keeps the glass dry depends on it. Check the controls and the sensors against a reference, because a humidity or pressure sensor that has drifted lets the room drift with it. Verify the heat recovery is still moving heat to the pool and the air. And watch the chloramine side from the air end: rising combined-chlorine smell or stinging air means the exhaust, the outdoor-air rate, or the water chemistry has slipped.

The room degrades quietly. A dehumidifier losing capacity, a sensor drifting, a damper sticking, none of it announces itself until the windows drip or the metal stains. Hedge the service intervals to the manufacturer's requirements, and keep the corrosion inspection on the calendar whether or not anything looks wrong.

What to document

The natatorium record is what lets the next person understand a room that is actively trying to corrode itself. Capture the design evaporation load and the activity factor it assumed, the dehumidifier selection and its heat-recovery capacity, the humidity and dew-point setpoints with the surface temperatures they were sized against, the measured space pressure, the materials specified for corrosion, and the commissioning results for humidity, pressure, condensation, air quality, and balance.

Keep that record live, not buried in a closeout binder nobody opens. Tie the setpoints, the readings, the corrosion inspections, and the service history to a field tool such as FieldOS so the trend and the history travel with the room. When a window starts dripping three winters from now, the record is what tells you whether the dew point crept up, a sensor drifted, or a coil fouled, instead of starting the diagnosis from zero.

Common mistakes

• Sizing the dehumidification below the real evaporation load, so the room runs wet whenever the pool is busy.
• Letting the room go positive, which pushes corrosive, humid air into the rest of the building.
• Holding a dew point above the window and wall surface temperatures, so the surfaces condense and corrode.
• Exhausting only at the ceiling and skipping deck-level source capture, leaving the heavy chloramine layer over the deck.
• Specifying ordinary galvanized duct, plain-steel fasteners, or an unprotected coil that the chloramine air destroys.
• Putting the vapor barrier on the wrong side, or leaving gaps, so moist air condenses inside the walls and the structure rots.
• Aiming all the supply air at the occupied zone and starving the glass, then chasing window condensation as a dehumidifier fault.
• Letting the air setpoint drift below the water temperature, which spikes evaporation and the dehumidification load.

Field checklist

Standards and references

ASHRAE is the framework for natatorium air. The HVAC Applications Handbook has a chapter on natatoriums that carries the evaporation equation, the activity-factor table, and the design guidance for humidity, the air-above-water temperature relationship, and the negative-pressure intent. ASHRAE Standard 62.1 sets the outdoor-air ventilation rate for pool spaces based on the wet area. Treat the relative humidity, the dew-point target, the pressure offset, and the ventilation rate as values to confirm against ASHRAE, the dehumidifier manufacturer, and the project design, not as fixed numbers, since they depend on the building and the equipment.

The dehumidifier manufacturer's selection software and published data govern the equipment: the evaporation load it is sized to, the heat-recovery capacity, the airflow, and the operating envelope. The Model Aquatic Health Code gives air-quality guidance that pushes toward source-capture and deck-level exhaust for chloramine control, and local health codes adopt and amend aquatic requirements, so the jurisdiction's version controls. The design team, mechanical engineer and architect together, owns the dew-point-to-surface coordination and the vapor-barrier location.

Three things carry the whole design and are worth repeating. Remove the latent load and hold the dew point below the surfaces. Keep the space negative and exhaust the chloramines at the deck. Use corrosion-resistant materials behind a warm-side vapor barrier. The water-chemistry and anti-entrapment side lives in the swimming pool and spa mechanical guide, and the general dew-point and dehumidification-method theory lives in the humidification and dehumidification control guide.

Units and terms

A natatorium design set mixes terms from HVAC, the pool world, and building science, so the same idea shows up under several names across the documents.

Relative humidity is read in percent and dew point in degrees Fahrenheit. Evaporation and the latent load are in pounds of water per hour, sometimes converted to a latent heat load. Space pressure is in inches of water column (in. w.c. or in. wg). Outdoor-air ventilation is in CFM, often per square foot of wet surface. Keep the dew point and the surface temperature in the same units when you compare them, because that comparison is the one that decides whether a surface sweats.

Natatorium

An indoor swimming pool and the room enclosing it, treated as a contained, controlled environment

Latent / evaporation load

The moisture leaving the pool and deck, in pounds of water per hour, that the HVAC must remove; the design driver

Mechanical vs ventilation dehumidification

Mechanical uses a refrigeration coil to condense moisture out; ventilation brings in drier outdoor air and exhausts the moist air

Relative humidity vs dew point

RH is how full the air is as a percent; dew point is the temperature at which water condenses, and it sets which surfaces sweat

Negative space pressurization

Keeping the pool room at lower pressure than adjacent spaces so its humid, corrosive air stays in rather than spreading

Chloramines

Combined-chlorine compounds that off-gas from the water, sting eyes and airways, corrode metal, and are heavier than air

Deck-level (source-capture) exhaust

Exhaust drawn from low at the water and deck, where the heavy chloramine layer sits, instead of from the ceiling

Vapor barrier

A retarder on the warm, pool side of walls, roof, and floor that stops moist air from condensing inside the assembly

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