Get pool dehumidifier sizing wrong and you waste money either way.
An indoor pool dumps hundreds of litres of water vapour into surrounding air every day. Standard HVAC handles temperature, but it cannot manage the moisture load a heated pool generates.
This article covers the pool dehumidifier sizing methodology that facility managers and engineers need for commercial indoor pools in Australia. We walk through evaporation calculations, the variables that shift your numbers, and how to pick between desiccant and refrigerant technology.
Wondering whether your facility actually needs a dedicated unit? Read our guide to commercial indoor pool dehumidification first.
What Humidity Range Should Your Pool Room Target?
ASHRAE recommends maintaining indoor pool environments between 50% and 60% relative humidity. This narrow band exists for good reasons.
Below 50% RH, swimmers feel cold when they leave the water. Evaporation from the pool surface accelerates, pushing heating costs up and forcing the dehumidifier to work harder.
Above 60% RH, condensation forms on windows, walls, and structural steel. Corrosion accelerates. In chlorine-treated pools, chloramine gas concentrations rise with humidity, creating respiratory risks for swimmers and staff.
Air temperature in the pool hall should sit 2–4°C above water temperature. This is the single cheapest way to reduce evaporation before the dehumidifier does any work.
- 50–60% RH target range (ASHRAE Applications Handbook)
- Air temp 2–4°C above water temp to limit surface evaporation
- 4–6 air changes per hour for pool rooms
- Below 40% RH: uncomfortable conditions, excessive evaporation
- Above 65% RH: structural damage, corrosion, health risk
The Variables That Drive Pool Dehumidifier Sizing
Pool surface area is the single biggest factor. But it is far from the only one. Here is what a sizing engineer actually accounts for.
- Pool surface area (m²), because a larger pool means more evaporation surface
- Water temperature, where a therapy pool at 33°C evaporates far more than a lap pool at 26°C
- Air temperature, since the gap between water and air temperature controls evaporation rate
- Activity level, with competitive swimming generating 2–3× more evaporation than a calm surface
- Occupancy, as each swimmer adds roughly 55 watts of latent heat to the space
- Room volume and ventilation rate at 4–6 air changes per hour (ASHRAE)
- Fresh air makeup, because outdoor air brings its own moisture load
- Local climate, since a pool in Cairns faces different ambient humidity from one in Adelaide
| Factor | Low impact scenario | High impact scenario |
|---|---|---|
| Pool surface area | 50 m² hotel pool | 1,250 m² Olympic pool |
| Water temperature | 26°C lap pool | 34°C therapy pool |
| Activity | Resting surface | Competitive meets, diving |
| Occupancy | 5 swimmers | 200+ spectators and swimmers |
| Climate zone | Arid inland | Tropical coastal |
Miss any one of these and your dehumidifier will be undersized or oversized. Both cost money.
How to Estimate Pool Evaporation Rate in Metric
The standard approach uses a simplified version of the ASHRAE evaporation equation. For facility managers who need a ballpark before engaging an engineer, here is how it works.
Base evaporation from a still water surface sits at roughly 0.08 kg per hour per square metre of pool area, assuming 27°C water and 29°C air at 60% RH. That baseline shifts dramatically with temperature and activity.
These figures are estimates for planning purposes. Actual specification requires a full engineering assessment that accounts for your enclosure, ventilation design, and local weather data.
Worked Example for a 25-Metre Lap Pool
- Pool dimensions: 25 m × 12.5 m = 312.5 m²
- Water temp: 27°C, air temp: 29°C
- Base evaporation: 312.5 × 0.08 = 25 kg/hr at rest
- Daily base load (24 hours): approximately 600 litres
- Activity multiplier for moderate use: 1.5×
- Occupied load: approximately 37.5 kg/hr, or 900 litres per day during operating hours
- Required removal capacity: minimum 40–50 litres per hour to maintain 55% RH
A therapy pool at 33°C water temperature sees roughly double the evaporation rate of a lap pool at 26°C. Water temperature is the variable most people underestimate.
For aquatic centres with multiple pools, wave machines, or diving platforms, total evaporation can exceed 2,000 litres per day. These facilities typically need multiple commercial dehumidifier units running in parallel.
Why Standard Dehumidifiers Fail in Pool Rooms
Commercial dehumidifiers designed for warehouses or offices do not survive pool environments. Three factors kill them.
- Chloramine and chlorine gas attack standard copper heat exchanger coils within months
- Pool rooms demand 24/7 continuous operation, while standard units run intermittent duty cycles
- High humidity combined with warm, chemically treated air corrodes unprotected electrical components
Pool-rated dehumidifiers use epoxy-coated or stainless steel heat exchangers, sealed electronics, and corrosion-resistant cabinetwork. They cost more upfront but last years longer.
What to Specify for a Pool Environment
- Epoxy-coated or stainless steel heat exchangers
- Corrosion-resistant housing rated for chlorine environments
- 24/7 continuous duty rating
- Integrated controls with humidity setpoint adjustment
- Fresh air intake handling
- Optional heat recovery to pool water or space heating
A unit sized correctly on paper but built for warehouse duty will fail within a year in a pool room. Specification matters as much as capacity.
Desiccant vs Refrigerant for Pool Dehumidification
Two technologies handle pool dehumidification. Choosing the right one depends on your water temperature, climate, and whether you want to recover waste heat.
| Factor | Desiccant | Refrigerant |
|---|---|---|
| Operating temp range | −20°C to +50°C | Above 15°C only |
| Heat recovery | Built in (reheats air, can heat pool water) | Requires add-on |
| Energy source | Gas or electric reactivation | Electric compressor |
| Removal rate per unit | Higher (handles concentrated loads) | Moderate |
| Best for | Therapy pools, cold-climate facilities | Warm-climate pools, smaller facilities |
| Upfront cost | Higher | Lower |
Desiccant units shine in pool applications because the waste heat from regeneration can be redirected to heat pool water or supply air. In a therapy pool running at 33°C, heat recovery from a desiccant unit can offset 30–50% of pool heating costs.
Refrigerant units work well enough in warm climates where the pool hall stays above 20°C year-round and heat recovery is not a priority. For a hotel pool on the Gold Coast, a ducted refrigerant system might be the pragmatic choice.
For anything in Victoria, Tasmania, or inland NSW where winter temperatures drop below 15°C outside, desiccant technology is the safer specification. Refrigerant units lose capacity rapidly as temperatures fall.
How Pool Covers and Climate Zones Affect Sizing
A pool cover over the water surface when the pool is unoccupied reduces evaporation by approximately 90%. That fact alone can change your dehumidifier specification significantly.
- Cover on, pool unoccupied: evaporation drops roughly 90% (manufacturer specifications)
- Night sizing: if the pool is covered overnight, the dehumidifier only needs to handle the occupied-hours load
- Uncovered 24/7: the dehumidifier must handle peak load continuously
- Retractable covers offer the most flexibility for commercial facilities
Australian climate zones add another layer to the calculation.
- Tropical QLD (Cairns, Townsville): High ambient humidity year-round means the dehumidifier fights both pool evaporation and outdoor moisture in makeup air. Size up 20–30%.
- Temperate NSW/VIC (Sydney, Melbourne): Seasonal variation. Winter brings condensation risk on cold surfaces. Summer humidity adds to pool load.
- Arid SA/WA (Adelaide, Perth): Lower ambient humidity means less load from makeup air. Potentially smaller units, but evaporation rate from the pool surface stays the same.
A 25-metre pool in Cairns needs a materially different specification from the same pool in Melbourne. The pool evaporates the same amount of water, but the unit in Cairns must also strip moisture from incoming ventilation air.
Pool Dehumidifier Sizing for Common Facility Types
These figures are indicative. Use them for early budgeting and scoping, not final specification.
| Pool type | Typical area (m²) | Water temp (°C) | Est. daily evaporation (L) | Approx. capacity needed |
|---|---|---|---|---|
| Hotel/resort pool | 60–120 | 28 | 100–250 | 15–30 L/hr |
| School 25m lap pool | 312 | 26 | 400–600 | 50–70 L/hr |
| Therapy/rehab pool | 40–80 | 32–34 | 150–350 | 25–45 L/hr |
| Council aquatic centre (50m) | 1,250 | 27 | 1,500–2,500 | 180–300 L/hr (multiple units) |
These estimates assume moderate occupancy and no pool cover during operating hours. Actual requirements depend on your building envelope, ventilation design, and local conditions.
Therapy and rehabilitation pools punch above their size. The higher water temperature drives evaporation rates that rival pools with three times the surface area.
Getting the Right Specification for Your Facility
Pool dehumidifier sizing is not a catalogue exercise. The interaction between pool evaporation, ventilation rates, building envelope, and heating systems means every facility needs a calculation specific to the site. Requirements from the National Construction Code around ventilation and moisture management apply to every enclosed pool.
Moisture Cure Commercial has 28 years of experience specifying humidity control solutions for Australian commercial and industrial facilities, including indoor pools, aquatic centres, and therapy pool rooms.
Contact Moisture Cure Commercial for a consultation. We will assess your facility, run the numbers, and recommend the right unit with the right capacity for your specific conditions.
Frequently Asked Questions
What is the ideal humidity for an indoor pool area?
ASHRAE recommends 50–60% relative humidity for indoor pool rooms. Below 50%, swimmers feel cold when wet and evaporation accelerates. Above 60%, condensation forms on surfaces and chloramine concentrations can reach unhealthy levels.
How do you calculate evaporation rate for an indoor pool?
The standard method uses pool surface area, water temperature, air temperature, and an activity multiplier. A still pool at 27°C evaporates roughly 0.08 kg per hour per square metre. Multiply that by the pool area and an activity factor of 1.0 (still) to 2.5 (competitive) for an operating estimate.
Can you use a standard dehumidifier for a pool room?
No. Standard commercial dehumidifiers use copper coils and standard electronics that corrode quickly in chlorine-rich pool air. Pool rooms need dehumidifiers with epoxy-coated or stainless steel heat exchangers, corrosion-resistant housings, and 24/7 continuous duty ratings.
Does a pool cover reduce the dehumidifier size needed?
Yes, significantly. A pool cover reduces evaporation by roughly 90% when the pool is unoccupied. If you cover the pool overnight and during closed hours, you can size the dehumidifier for the occupied-hours load rather than the full 24-hour load.
What type of dehumidifier works best for an indoor pool?
It depends on your climate and heating needs. Desiccant dehumidifiers work at any temperature and offer built-in heat recovery that can offset pool heating costs. Refrigerant units are less expensive upfront but lose capacity below 15°C and do not recover heat without additional equipment.
What happens if pool room humidity is too high?
Uncontrolled humidity above 60–65% RH causes condensation on windows, walls, and structural steel. Over time, this leads to corrosion, mould growth, peeling paint, and structural deterioration. Chloramine gas concentrations also rise, creating respiratory health risks for swimmers and staff.
