A chiller is the unit beside a cold plunge tub that keeps the water at the temperature you set, all year round. This guide explains what the numbers on a chiller's specification mean, how it keeps the water clean, where to put it and what it costs to run.
1. What a chiller does
- A pump draws water from the tub, through the chiller and back again.
- Inside, a refrigeration circuit, like the one in a fridge, takes heat out of the water. A fan releases that heat into the air around the unit.
- Most chillers also filter the water as it passes through.
- It runs hard at first to pull the water down to temperature, then switches on and off to hold it there.
- Some units can also heat the water, which helps if you want a year-round tub.
2. Reading the specification
| Spec | What it tells you | What to check |
|---|---|---|
| Cooling capacity (kW, sometimes horsepower) | How much heat it can remove. More capacity pulls a large tub down faster | That it matches the tub size the manufacturer pairs it with |
| Power input (kW or watts) | The electricity it draws. This sets the running cost | Use this figure, not the cooling capacity, in the cost formula below |
| Lowest water temperature | The coldest the manufacturer says it can reach | At what air temperature? Chillers work harder in hot weather |
| Operating air temperature | The outdoor temperatures it's designed to run in | How to protect it from frost in winter |
| Power supply | Voltage, and plug or hard-wired | A 13A plug, or a circuit from an electrician (see our electrics guide) |
| Noise (dB(A)) | How loud it is, measured at a stated distance | Compare models at the same distance |
| Heating | Whether it can warm the water | Heating uses electricity too |
Cooling capacity and power input are often very different numbers, so make sure you use the power input when you work out costs.
3. How cold, and how fast
The lowest temperature a chiller reaches, and how quickly, depends on the model, the size of the tub and the air temperature around it. Each product page gives the manufacturer's figures. The first pull-down, from tap temperature, takes the longest. After that, the chiller only has to remove the heat that creeps in, so:
- keep the lid on whenever the tub isn't in use;
- put the tub and chiller in shade if you can;
- choose a well-insulated tub.
4. Keeping the water clean
Chillers use one or more of these. Follow the manufacturer's routine for each.
- Filters trap hair, skin flakes and debris. Clean or replace the cartridge as the manufacturer says. A clogged filter also slows the flow through the chiller.
- UV (ultraviolet) lamps treat the water as it flows past the lamp. The HSE's guidance on spa pools notes that UV only works at the point of use, so additional disinfection is needed.
- Ozone generators add ozone to the water in the unit. The same HSE guidance says ozone may be used, but only together with a residual disinfectant.
The HSE guidance is written for spa pools and hot tubs, including those used by businesses, not for cold plunges specifically. The principle is still a sensible one: treat UV and ozone as extras alongside a sanitiser the manufacturer approves, not as a replacement. Keep the lid on, rinse off before you get in, and change the water from time to time.
5. Noise and placement
- Noise: chillers make a compressor and fan hum. Check the dB(A) figure and the distance it was measured at. Keep the unit away from bedroom windows, yours and your neighbours'. In England, councils can act on noise that unreasonably and substantially interferes with the use or enjoyment of a neighbour's home.
- Airflow: leave the clearance the manufacturer specifies. The fan pushes warm air out, so don't box the unit in unless the manufacturer supplies or approves an enclosure.
- Shade: out of direct sun, the chiller works less hard.
- Surface: a firm, level spot next to the tub, within reach of the hoses (see our base guide).
- Weather: check whether the unit is rated for outdoor use, and protect it and the pipework from frost as the manufacturer instructs.
- Power: a weatherproof, RCD-protected socket close enough that you don't need an extension lead.
6. Power draw and running cost
We use the same price and method as our running-cost guide:
Cost per day = chiller power input (kW) × hours running per day × your unit rate (£ per kWh)
The unit rate is the Ofgem price cap for 1 October to 31 December 2026: 26.32p per kWh, paying by direct debit. The government has removed VAT from electricity bills from 1 October 2026 to 31 March 2027, and this rate reflects that.
As an illustration, we assume a chiller drawing 0.8 kW that runs 4–8 hours a day. How long it actually runs depends on the weather, your target temperature, the tub's insulation and the lid.
| Use | Hours a day | Energy per day | Cost per day | Cost per 30 days |
|---|---|---|---|---|
| Lower (mild weather, lid on, insulated tub) | 4 | 3.2 kWh | £0.84 | £25.27 |
| Middle | 6 | 4.8 kWh | £1.26 | £37.90 |
| Higher (warm weather, colder target, lid often off) | 8 | 6.4 kWh | £1.68 | £50.53 |
For your own chiller, take the power input from the product page. For example, a 1.2 kW chiller running 6 hours a day uses 7.2 kWh, which costs about £1.90 a day, or £56.85 per 30 days.
These figures leave out the daily standing charge, which you pay whether or not the chiller runs. Warm weather, a colder target and leaving the lid off push the cost up. A good insulated lid, shade and a sensible target bring it down.
FAQs
Do I need a chiller?
Not if you're happy with the temperature of your tap water, plus ice. For regular, year-round use at a steady temperature, a chiller is usually the practical choice. Our cold plunge guide compares the two.
Can a chiller heat the water as well?
Some can. The product page says whether a model heats as well as cools.
Can I put the chiller in a shed or a box?
Only if the manufacturer allows it. Chillers need airflow to shed the heat they take out of the water.
For using a cold plunge safely, see the safety section of our cold plunge guide. General guidance, not medical advice.
Running costs are estimates for guidance, using the Ofgem price cap average for England, Scotland and Wales. Your costs depend on your tariff, region, model, weather and how you use it.
Sources
- Ofgem, price cap from 1 October to 31 December 2026: https://www.ofgem.gov.uk/news/changes-energy-price-cap-between-1-october-and-31-december-2026
- HSE, HSG282 Control of Legionella and other infectious agents in spa-pool systems (information box 5, disinfectants): https://www.hse.gov.uk/pubns/books/hsg282.htm
- UK Government, noise nuisances: how councils deal with complaints: https://www.gov.uk/guidance/noise-nuisances-how-councils-deal-with-complaints