Pool climate control in Phuket: warm water in the rainy season, cool water in April

But two times of year bring complaints we hear again and again:

• In the rainy season the water feels cold. After a few grey, windy days a pool can drop to 25 °C. That is fine for a quick dip, but nobody stays in for long.
• In March and April a dark pool in full sun turns into a bath. At 31–33 °C the water no longer refreshes, especially in the afternoon.

One of our clients swam in a heated pool on a cold, rainy day and said: "Wow, I want the same." That is what pool climate control does. The water stays at the temperature you choose, all year round.

Why Phuket pools get cold, even though the air never does

Phuket air almost never goes below 23 °C. So why does the pool feel cold in the rains?

It is not the rain itself. Even 50 mm of rain at 24 °C lowers a 1.4 m deep pool by only about 0.2 °C. What really cools a pool is the weather that comes with the rain:

• No sun. On a bright day the sun puts more heat into the water than anything else. Under heavy cloud most of that heat is gone.
• Wind and evaporation. Evaporation is the biggest heat loss of any outdoor pool. Wind speeds it up.
• Shade. A pool shaded by the house, trees or a hillside misses the sun even on good days. In our estimate a shaded pool runs about 2–3 °C cooler than the same pool in the open.

Infinity pools and pools on a windy cliff lose the most

On an overflow pool the water constantly runs over the edge as a thin sheet, then through a gutter and a balance tank. That sheet is a small cooling tower. Evaporation pulls the water toward the so-called wet-bulb temperature. In Phuket's humid air that is about 2–3 °C below the air temperature. That is why an infinity pool with a long overflow edge can feel colder than the air around it. Cliffside and cantilever pools add wind on the water and on the walls that hang in the air.

And why dark pools overheat

A dark finish absorbs much more sunlight than a white or light blue one. In the hot season our estimate puts a dark pool in full sun about 2 °C warmer on average than a light one. In the afternoons it gets even hotter. Many owners love the deep colour of a dark pool; they just don't love the warm water.

What pool climate control is

At its heart is a reversible pool heat pump. It works like an air conditioner for your water:

• Heating. It takes heat from the warm tropical air and moves it into the pool. In Phuket's climate it gives about 5–7 kWh of heat for each 1 kWh of electricity.
• Cooling. It runs in reverse, takes heat out of the water and releases it into the air. In cooling mode the return is lower, about 3–4 kWh of cooling per 1 kWh of electricity.

The water passes through the unit together with the normal filtration, so nothing changes in how you use your pool.

Smart control. Modern units connect to Wi-Fi. From an app on your phone you can:

• set the temperature and see the actual water temperature;
• set a schedule, for example: no cooling in the heat of the day, cooling in the evening when you want a refreshing swim, or heating only before the weekend;
• switch to a quiet night mode.

Many units also work with smart-home systems.

Which pools need it most

• Pools that feel cold in the rainy season, especially white or very light pools.
• Pools in shade: tucked under the house, trees or a slope.
• Infinity pools with a long overflow edge, and cliffside or cantilever pools open to the wind.
• Dark pools that overheat in March–May.
• Villas for rent, where guests expect warm water every day of the year, and families with small children.

How much electricity does it use?

We estimated typical Phuket villa pools. Assumptions: a light-coloured pool in the open; water held at 30 °C in the rainy season; a "rainy month" with 20 ordinary days and 10 cold, windy, rainy days; electricity at about 4.8 THB per kWh (PEA household rate for use above 400 units a month, including Ft and VAT). Cooling: a dark pool in March–April held at 29 °C.

What this means in practice:

• On a sunny day a light pool in the open needs very little help. Most of the electricity goes on the grey, windy days.
• Shade and long overflow edges can double the cost.
• Cooling costs more per degree than heating. The good news: the hot season is short, and you can cool only in the evening.

The cliffside column assumes the overflow runs along the long side in a windy spot. That is why a long, narrow pool can cost more there than a slightly larger square one.

These are estimates, not a promise. The real figure depends on wind, how much the pool is used, the overflow flow rate, the chosen temperature and your tariff. If your estate resells electricity at a higher rate than PEA, the cost rises in proportion.

How to keep the bill down

• Cover the pool at night, where the design allows it. A cover cuts evaporation by 70–90 %, the biggest heat loss.
• Use the schedule. Heat in the afternoon, when the air is warmest and the unit works best. Cool at night and early morning.
• Time-of-use tariff. With a PEA time-of-use meter, off-peak electricity (22:00–09:00 on weekdays, all day at weekends and on public holidays) costs about 3 THB per kWh instead of about 6.4 THB at peak times. Running mostly off-peak can cut the cost noticeably. Check it against your whole villa's consumption first.
• Slow the overflow when nobody is using the pool. A smaller waterfall means less evaporation.
• Solar panels. A typical villa heat pump draws from under 1 kW to about 6 kW (large units more), much of it in the daytime, which suits solar panels well.

Heating only or cooling only

Not everyone needs both. There are also units that only heat and units that only cool.

• Heating only. Enough for a shaded pool, a pool that is only used in the rainy season, or a hot tub. Depending on the model, such a unit can cost less to buy. It does not save electricity: the same heat needs the same power.
• Cooling only (chillers). Made for cold plunge pools and ice baths. Typical sizes are about 8–30 kW of cooling, and they take water down to single figures. A chiller can also cool a dark pool in the hot season, but it cannot warm it in the rains.
• Electric heaters. The cheapest to buy: a heating element in a box. But for the same heat they use about 5–6 times more electricity than a heat pump. For the 8 × 4 m pool above, a rainy-season month would cost about 10,700 THB instead of about 1,800 THB. A heater big enough for a villa pool usually also needs three-phase power. It makes sense only for a small hot tub that is heated now and then.

Rough running costs, as an estimate: a small home cold plunge of about 3.6 m³, kept at 10–12 °C with a lid, uses about 600–700 THB of electricity a month. Without a lid and insulation it uses several times more. A covered hot tub of about 1.5 m³ at 38 °C uses about 150 THB a month.

For a swimming pool we usually suggest one unit that does both: it covers the whole year, and the price difference is often small.

An honest word

Climate control uses electricity. It is a comfort choice, not an eco solution, and we won't pretend otherwise. If a warm pool in the rainy season or a refreshing pool in April matters to you and the running cost is acceptable, it works well and quietly.

What is on the market

Pool heat pumps are sold by their maximum heating power. An inverter unit rarely runs flat out: a "16 kW" unit, for example, works anywhere between about 4 and 16 kW and draws from about 0.25 to 2.7 kW of electricity.

• Compact units, about 8–10 kW, single-phase 220 V. For plunge pools and small pools up to about 25 m³. They draw at most about 1.3–1.7 kW.
• Mid-size units, about 12–24 kW, single-phase. For typical villa pools of about 40–90 m³. They draw at most about 2–4 kW. The largest single-phase units need up to 30 A, so the villa may need a 30(100) A meter.
• Large units, about 28–40 kW, usually three-phase 380 V. For pools of about 100–150 m³. They draw at most about 4.7–6.7 kW.
• Fixed-speed (on/off) units, mostly three-phase, from about 12 kW to over 200 kW. Simpler and can be cheaper to buy, but louder and less efficient than inverters. They hold the water within about ±2 °C, and many have no app. They suit hotels and large pools better than villas.
• Commercial units, from about 24 up to 240 kW, three-phase. For hotels and large pools. Several units can work together as one system.
• Cold plunge chillers. Compact 1–7 kW units with a built-in pump and filter connect straight to a tub. Larger 8–30 kW units serve big plunge pools and wellness areas. Both take the water down to about 2 °C and give about 3 kWh of cooling for each 1 kWh of electricity.

Features to look for:

• Full DC inverter. The unit adjusts its power smoothly instead of switching on and off. That means a much better efficiency at part load and less noise, about 40–55 dB(A) at 1 m.
• Heating and cooling in one unit, typically for water from 8 to 40 °C.
• Titanium heat exchanger. Suited to salt-chlorinated pools.
• Refrigerant. Newer units use R32 or the natural refrigerant R290, which has a very low climate impact.
• Solar input. Some units can take power directly from solar panels.
• Corrosion-protected coil and cabinet. For sea air on the coast.
• Wi-Fi app with timers. Many units work in common smart-home apps.

What it costs

Approximate prices from us, including VAT:

• a unit with heating and cooling for a small pool of up to about 25 m³: around 75,000 THB;
• for a typical villa pool of about 40–75 m³: around 90,000–120,000 THB;
• for a large pool of about 90 m³ and more: around 190,000–220,000 THB;
• installation on top: usually 30,000–40,000 THB, and 45,000–60,000 THB for large units;
• electrical work, if the villa needs a new power line, a meter upgrade or a three-phase supply, is quoted separately.

We give the exact price after the survey. Running costs are in the table above.

How we install it

• Survey. We look at the pool, the overflow, the shade and the plant room, and measure the volume.
• Sizing. We choose the unit by volume, overflow edge, shade, the temperature you want and whether you want cooling.
• Placement. The unit goes outdoors, with free air around it and away from bedrooms. It must not go into a closed pump pit: the unit would breathe its own cold or hot air and lose efficiency.
• Plumbing. The unit is connected after the filter and before the salt chlorinator, with a bypass and valves for service.
• Electrics. A separate breaker and RCD. Large units need three-phase power, and a standard 15(45) A meter may need an upgrade.
• Set-up. We set the temperature and schedule with you and connect the app.

Then routine care: keep the air free around the unit, rinse the coil now and then, and check the drain and flow every year.

Want a warm pool in the rains or a cool one in April? Send us the size of your pool, a few photos and the location. We will tell you which unit fits, roughly what it costs and what it will cost to run.

How we estimated

• Daily heat balance for each pool: sun, evaporation, air, sky radiation, rain and the overflow edge.
• Weather: Phuket airport averages 1991–2020; wind and humidity for the rainy and hot seasons.
• Evaporation: ASHRAE method, reduced for a quiet residential outdoor pool.
• Heat pump: heating 6 kWh of heat per 1 kWh of electricity, cooling 3.5.
• Electricity: PEA household rate from September 2026, 4.3583 THB per unit above 400 units, plus Ft 0.1623 and 7 % VAT, about 4.84 THB/kWh.
• Not included: the extra running hours of the filtration pump, if any.

All figures are estimates for comparing pools, not a guarantee of running cost.