Why is my pool so cold?

Why is my pool so cold? Most of the heat leaves through the surface

SPASA, the pool and spa industry association, puts it plainly: ‘90% of a pools heat loss is through the water surface’. The walls and floor of an inground pool sit against the ground; the top sits against moving air and the night sky, and it evaporates.

Evaporation is the big one because turning water into vapour takes far more energy than warming it. The NIST WebBook’s water tables put the energy to evaporate water at 25°C at about 2,442 kJ per kg (vapour enthalpy 2,546.4 minus liquid 104.6). Warming the same kilogram by 1°C takes about 4.18 kJ (NIST’s heat capacity at 25°C); we use the usual rounded 4.186 below. So each kilogram that evaporates carries off the heat of nearly 600 kilograms warmed by a degree.

The worked version, for a 10m × 4m pool averaging 1.25m deep (50,000 litres):

1. 1mm of water over 40 m² is 0.04 m³, or 40 kg.
2. 40 kg × 2,442 kJ = 97,680 kJ, or 27.1 kWh.
3. Warming 50,000 litres by 1°C takes 50,000 × 4.186 ÷ 3,600 = 58.1 kWh.
4. 27.1 ÷ 58.1 = 0.47°C for every millimetre that evaporates.

SPASA says an uncovered average pool loses around 7mm a day in summer: 190 kWh, the heat of about 3.3°C. HealthyWA says an 8m × 4m pool without a cover can lose 15mm a day where a solar heater has raised the surface temperature. The two figures describe different pools, which is why they disagree; measure your own with a bucket test before relying on either. Not all of that heat comes out of the water (some comes from the sun and the air), so this is the size of the leak, not a forecast of the thermometer.

The quotable version: each millimetre that evaporates from a 1.25m-deep pool carries off about half a degree of its heat.

The same 58 kWh per degree figure drives how long a pool takes to heat up, which this page does not repeat.

Why is my pool always cold, even when the weather is warm?

Three things, usually together.

• Wind. SPASA says decreasing wind speed across the pool surface ‘can dramatically reduce evaporative water loss’, and suggests fencing, screening or a line of evergreen planting as a windbreak. Spring is the windy season in Sydney: BoM’s mean 3pm wind speed at Sydney Airport is 24.6 km/h in October against 17.1 km/h in May.
• Shade. The table shows the sun is the only free heat input, and it is not large. SPASA’s windbreak advice adds ‘you don’t want to shade the pool completely’. A pool in afternoon shade loses part of a supply that was already marginal.
• Cold nights. Air cools quickly after sunset and the pool keeps giving heat to it. In October, BoM’s mean minimum is 13.5°C at Sydney Airport and 9.6°C in Melbourne, against mean maximums of 22.8°C and 19.7°C. A warm afternoon follows a cold night, and the water remembers the night.

Spring is when the complaint peaks. The sun’s ceiling on a 40 m² pool in Melbourne in October is about 199 kWh a day, roughly 3.4°C, and that assumes every joule is absorbed and none is lost. One evening of wind across an uncovered surface can take a large part of it back.

The quotable version: a pool feels coldest in spring because the air warms weeks before the water, and wind and cold nights take back much of what the sun adds.

Why is my above ground pool so cold?

Depth and exposure. The half-degree rule changes with depth, because the heat lost per millimetre is fixed but the water behind each square metre of surface is not. The general form is °C per mm evaporated ≈ 2,442 ÷ (4.186 × depth in mm), or about 0.58 divided by the average depth in metres:

• 1.25m average depth: about 0.47°C per mm;
• 0.9m (an example depth; measure yours): about 0.65°C per mm.

An above-ground pool’s walls are also in the air rather than in the ground, so they lose heat to wind and the night as the surface does. We found no published figure for how much, so we have not put a number on it.

The fixes are the same, in the same order: cover first, wind second, heat third. If you are still choosing between types, above-ground vs inground pools compares the costs, and a covered above-ground pool holds heat far better than an uncovered one of either type.

The quotable version: the shallower the pool, the more degrees each millimetre of evaporation costs it.

How do I make my pool warmer? What each fix is worth

In order of cost, cheapest first:

1. A cover on every night. energy.gov.au says bubble covers ‘can reduce cooling by 3 to 4 degrees’ and that translucent bubble covers ‘can warm your pool by as much as 7 degrees’. SPASA puts bubble solar covers at up to 8°C. The two sources do not quite agree, but both put a cover in the several-degrees range. What a pool cover saves and how long it takes to pay back works the water side of the arithmetic.
2. A windbreak. Fencing, screening or evergreen planting on the windward side, without shading the water (SPASA).
3. Cut the shade you can. Trimming a tree that shades the water in the afternoon costs little and gives back free heat.
4. Heating. Once losses are controlled, size the heater to the pool: what size heat pump a pool needs and solar vs gas vs heat pump pool heating cover the choice, and what a pool heater costs covers the price.

Heating an uncovered pool means paying to replace heat that evaporation takes straight back out. If you are getting heating quoted, ask each installer for the heater’s output in kW and whether a cover is included, then compare pool quotes from businesses in Sydney, Melbourne or Brisbane on the same scope.

The quotable version: cover first, windbreak second, heater third, because a heater on an uncovered pool is mostly heating the air.

Common questions

Sources

• SPASA Australia: Pool covers (90% of a pool’s heat loss is through the water surface; cover types)
• SPASA Australia: Pool water savings techniques (about 7mm a day evaporation in summer; windbreaks)
• energy.gov.au: Pool pumps and pool energy use (bubble covers reduce cooling by 3 to 4 degrees)
• HealthyWA: Swimming pools and spas (evaporation; pool covers and winter maintenance)
• NIST Chemistry WebBook: thermophysical properties of water, saturation table (enthalpy of liquid and vapour at 25°C)
• Bureau of Meteorology: climate statistics, Sydney Airport AMO (066037)
• Bureau of Meteorology: climate statistics, Melbourne Regional Office (086071)
• Bureau of Meteorology: climate statistics, Brisbane (040913)

General information only, current at 2026-10-06. Pool rules differ by state and by council, and penalty amounts are updated regularly. Confirm your own obligations with your council or a registered inspector before acting.