A cold plunge chiller is a small refrigeration unit that draws water from the tub, removes heat from it and releases that heat into the surrounding air. Horsepower (HP) usually describes the size of its compressor motor, while cooling capacity in kilowatts (kW) describes how much heat it can remove. When comparing chillers, cooling capacity and a cool-down time with the conditions stated tell you far more than HP alone.
This article is part of our cold plunge buying guide.
How a cold plunge chiller works
A chiller works on the same principle as a fridge or an air conditioner. A pump circulates water from the tub through a heat exchanger. Inside, a refrigerant evaporates and absorbs heat from the water. A compressor then pushes that refrigerant to a second coil, where a fan releases the heat into the air around the unit. The cooled water returns to the tub, and the cycle repeats until the set temperature is reached.
Two practical points follow from this. First, a chiller moves heat rather than creating cold, so it always gives off warm air and needs space to breathe. Second, once the water reaches its set temperature, the chiller only has to remove the heat that leaks back in, so it cycles on and off rather than running flat out.
HP vs kW: what the numbers mean
Horsepower (HP)
Horsepower is a unit of power equal to roughly 746 watts. On a chiller it normally describes the compressor motor. It gives a rough sense of size, but it does not tell you how much heat the unit removes or how much electricity it uses, and two chillers with the same HP figure can perform quite differently.
Cooling capacity (kW)
Cooling capacity is the rate at which a chiller removes heat from the water, usually quoted in kW (and sometimes in BTU per hour, where 1kW is about 3,412 BTU/h). This is the most useful number for comparing performance, as long as you know the conditions it was measured under.
Electrical input (kW)
Electrical input is the power the chiller draws from the socket. Because a chiller moves heat rather than generating it, its cooling output can be several times its electrical input. For example, the ThermoX Drop is rated at 3.5kW of cooling from a 1.02kW power supply. The ratio between the two is called the coefficient of performance, and it changes with water and air temperature. Do not confuse the two figures when you compare models or estimate running costs.
Why cool-down time is the figure to compare
Water holds a lot of heat. Cooling one litre of water by 1°C means removing about 4.2 kilojoules. Scale that up and cooling 300 litres from 20°C to 3°C means removing roughly 5.9kWh of heat.
On paper, a chiller delivering 3kW of cooling could do that in about two hours. Real cool-down times are longer, because heat keeps flowing back in from the air, the floor and the pump, and because chillers generally deliver less cooling as the water gets colder. The ThermoX Elite, with 3kW of cooling, is quoted at about 3.5 hours to cool 300 litres from 20°C to 3°C at a 25°C ambient temperature. That is the kind of figure worth looking for: volume, start and finish temperatures and ambient temperature all stated.
Cool-down time depends on:
- Volume. Double the water, roughly double the heat to remove.
- Starting temperature. Tap water is usually colder in winter than in summer, so the first cool-down varies through the year.
- Target temperature. The last few degrees are typically the slowest.
- Ambient temperature and sun. Warm air and direct sunlight add heat to the water and make the chiller less efficient.
- Insulation and cover. A good cover is one of the cheapest ways to cut cool-down time and running costs.
How ambient temperature affects a chiller
Ambient temperature is the temperature of the air around the chiller. It matters twice over. Warmer air means the water gains heat faster, and it also means the chiller has to reject its heat into air that is already warm, which reduces its efficiency. A plunge that holds 5°C easily in a cool garage in spring may work much harder on a hot July afternoon.
To help a chiller work well:
- Leave the clearance around the unit recommended in the manual, and never box it into a cupboard where it recirculates its own warm air.
- Keep it out of direct sunlight where possible.
- Keep the insulated cover on whenever you are not in the water.
- Clean filters regularly, as restricted flow reduces performance.
Very cold weather brings different concerns. Water in pipes and pumps can freeze and cause damage, so follow the manufacturer's winter guidance. For example, our Drop guidance is to empty the water if the ambient temperature drops below 2°C.
Heating: when a chiller also warms
Some cold plunge systems can heat the water as well as cool it. This is useful if you want a warm soak, want to alternate hot and cold in one tub, or share the tub with someone who prefers milder water. Heating capacity varies widely: the ThermoX Pro, for instance, has 4kW of heating and a range of 3 to 40°C.
Heating elements draw a lot of power while they run, so check that your supply can handle the unit's rated power and that you will not share the socket with other high-power appliances. If you only ever want cold water, a cooling-only unit is simpler.
What to look for when comparing chillers
- Cooling capacity in kW, not just HP.
- A cool-down time with volume, temperatures and ambient conditions stated.
- The lowest temperature it can hold, which shows headroom for warm weather.
- Electrical input and socket requirements.
- Noise level if it will sit near a bedroom or a neighbour's fence.
- Weather rating if it will live outdoors.
- Warranty, support and filter availability.
For the full checklist, see how to choose a home cold plunge. If you already own a bath, read can you turn your bath into a cold plunge?
A quick safety note
However good the chiller, cold water is a real stress on the body. The RNLI describes cold water shock, a sudden gasp and rapid breathing that can follow immersion in water below 15°C, and the British Heart Foundation notes that cold immersion can raise heart rate and blood pressure. Speak to your GP first if you have a heart condition, high or low blood pressure, are pregnant or have any other medical condition. Never plunge after drinking alcohol, avoid plunging alone, and start warmer and shorter.
Frequently asked questions
Is a 1HP chiller enough for a cold plunge?
It depends on the water volume, the target temperature, the ambient temperature and the insulation. HP describes the motor, so compare cooling capacity in kW and a quoted cool-down time rather than relying on HP alone.
How long does a cold plunge take to cool down?
Usually several hours from a fresh fill. As one example, the ThermoX Elite is quoted at about 3.5 hours to cool 300 litres from 20°C to 3°C at 25°C ambient. Larger volumes, warmer weather and no cover all lengthen it.
Why does my chiller blow out warm air?
That is normal. A chiller removes heat from the water and releases it into the surrounding air, so it needs good airflow and should not be enclosed.
Does a chiller use its cooling capacity in electricity?
No. Cooling capacity is the heat removed, while electrical input is the power drawn. A chiller moves heat rather than creating it, so its cooling output can be several times its electrical input.