Sizing an ice bath chiller for Thailand, and what it costs to run

Step one: litres, not people

A chiller does not care how many people sit in the water, only how much water there is and how far it must fall. Cooling is energy: litres x 4.186 x degrees ÷ 3,600 gives kilowatt-hours of heat to remove. A 420 litre tub from 29 °C to 4 °C is about 12.2 kWh; a 1,100 litre plunge pool is about 32 kWh. Our sizing rule turns that into three units:

Pull-down times ignore heat gain and assume a 29 °C start, so read them as the shortest possible first night. A 500 litre tub therefore needs the 2 HP unit; a 1,100 litre pool needs the 4 HP; anything above 1,200 litres needs two units in parallel.

Step two: the air the condenser breathes

A chiller moves heat from the water to the air. The hotter that air, the harder the compressor works and the less heat it can move; a sizing guide from one chiller maker makes the same point, that ambient temperature matters for heat rejection. Our table assumes 35 °C air with a clear path in and out. In Thailand the hottest month's mean afternoon high runs from about 33 °C on Koh Samui to 36.7 °C in Chiang Mai in April (Thai Meteorological Department, 1991-2020 normals), so 35 °C is a design point, not a worst case. Put the unit in the shade and it gains headroom; put it in a closed cupboard and it loses it.

Step three: duty, in hours

• Private tub, one session a day: the unit runs on a timer for several hours in the night or early morning and idles after.
• Villa plunge pool, a few guests a day: an overnight pull-down, then top-ups after sessions; shade decides how often.
• Hotel or club pool, up to 16 hours a day: two units in parallel, one carrying the load and one as margin and backup.

Cost to run: the arithmetic, with an example you can redo

A chiller's electrical draw is roughly 0.75 kW per horsepower while the compressor runs, so a 4 HP unit draws about 3 kW. Cost = kW x hours run x your tariff. The example below uses an assumed tariff of THB 4 per kWh for illustration; your electricity bill shows the real one, and the layout we draw for you includes a monthly kWh estimate for your sun and shade.

Change the hours and the tariff and you have your own number. Heavy sun, long hours or an unshaded pool raise it; a cover, shade and a sensible timer lower it. We do not publish a universal running cost because none exists.

Four ways a chiller ends up too small

• Counting people instead of litres, which is why step one starts with the water.
• Reading a catalogue figure taken at 25 °C air and expecting it to hold at 35 °C.
• Forgetting the water in the filter and the pipes, which has to be cooled too; add it to the pool's volume.
• Leaving the pool in full sun, where the surface gains heat all day and the unit never catches up.

Noise, in plain words

A chiller sounds like a large refrigerator or an outdoor air-conditioning unit while the compressor runs, and goes silent when it cycles off. What matters is where it stands: away from bedrooms and neighbours' windows, with its exhaust pointing at a wall or planted bank, on vibration pads inside a louvred niche. We put a noise estimate in the layout before a unit is ordered.

Where it lives, in three lines

Shaded, ventilated, short pipes. The niche has free air in and a separate exhaust out, half a metre or more of service access and a drain for condensate; the lines to the water are lagged with closed-cell foam. The chiller page has the checklist, and the plunge pool page shows the niche in context.

Five questions to ask any chiller supplier

1. What ambient temperature is the capacity quoted at? If the answer is 25 °C, ask what it delivers at 35 °C.
2. What is the heat exchanger made of, and is it rated for your sanitiser or salt? Titanium resists corrosion in sea water and chlorine; ask for the maker's statement.
3. Is the coil protected for coastal air? Coil coatings against seawater and salty winds exist (the maker Blygold sells them); ask what is fitted.
4. How many hours a day is it designed to run? A hotel pool needs sixteen; a private tub does not.
5. Who services it, and how fast? Our warranty is two years plus a year of assisted service visits; compare it with theirs.