# How long does an inflatable hot tub take to heat?

Source: https://bubbleandsoak.co.uk/blog/how-long-to-heat-inflatable-hot-tub
Published: 2026-10-12
Publisher: Bubble and Soak

**In short:** Most inflatable tubs take from about 10 to over 24 hours to heat from a cold fill. The minimum time is litres times 4.186 times the temperature rise, divided by 3,600, then divided by the heater kW. For 800 litres from 10 to 38 °C on a 2.2 kW heater that is about 12 hours, and real times are longer in cold or windy weather.

The first heat after filling is the moment many new owners discover that an inflatable hot tub is not instant. A 13 A plug limits the heater to around 2 kW, and a tub holds hundreds of litres of water, so it takes many hours to warm up. The good news is that you can work out the minimum time before you start, and plan around it. This guide shows the sum, gives typical times for common tub sizes and explains what slows heating down and what speeds it up.

## The sum in one line

Heating water needs about 4.186 kJ per litre for each degree. So the minimum energy in kWh is litres times 4.186 times the temperature rise, divided by 3,600. Divide that by the heater's kW and you get the minimum hours, assuming no heat is lost.

Example: 800 litres from 10 to 38 °C. That is 800 times 4.186 times 28, divided by 3,600, which is about 26 kWh. At 2.2 kW, 26 divided by 2.2 is about 11.8 hours. Use our [heat-up calculator](https://bubbleandsoak.co.uk/tools/heat-up-calculator) to do this for your own tub.

## Typical minimum times

The table shows minimum hours at two heater ratings, before heat loss. Real times will be longer, especially from very cold tap water or in cold air.

| Water | Minimum kWh | At 2.0 kW | At 2.2 kW |
| --- | --- | --- | --- |
| 669 L | About 21.8 | About 10.9 h | About 9.9 h |
| 800 L | About 26.0 | About 13.0 h | About 11.8 h |
| 916 L | About 29.8 | About 14.9 h | About 13.6 h |
| 1,000 L | About 32.6 | About 16.3 h | About 14.8 h |
| 1,190 L | About 38.7 | About 19.4 h | About 17.6 h |

Minimum heat-up hours from 10 to 38 °C

## Why real times are longer

While the water warms, heat escapes through the walls, floor and cover. The warmer the water and the colder the air, the faster it escapes. On a cold, windy day an inflatable may gain only a fraction of a degree an hour, and in very cold weather some heaters cannot keep up at all. Makers often give a lowest air temperature for heating; check yours in the manual.

Tap water in the UK is colder in winter than in summer, so the temperature rise is bigger. Some heaters also pause when the bubbles run, and the filter must be clean for the heater to work efficiently.

## How to speed up the first heat

Do not try to speed things up by adding kettle or boiler water through hoses not meant for it, or by running heaters from extension leads. Follow the manual's filling and heating steps.

1. Put an insulating mat under the tub before filling.
2. Fill on a mild day, ideally in the morning so the tub heats through the day.
3. Fit the cover as soon as the tub is filled, and strap it down.
4. Check the filter is clean and fitted, so water flows freely through the heater.
5. Keep the bubbles off while heating.
6. Shelter the tub from wind where you safely can.

Note: Plug the pump into a weatherproof outdoor socket protected by an RCD. Avoid extension leads unless the manual allows them, and have new outdoor sockets fitted by a qualified electrician.

## Keeping it warm after the first heat

Once the water is hot, the heater only needs to replace lost heat. With a good cover in mild weather, that may take short spells each day. If you use the tub every few days, holding a lower temperature and raising it before use can be a sensible middle way. See [temperature settings](https://bubbleandsoak.co.uk/blog/hot-tub-temperature-settings) for how timers and lower holding temperatures work.

Tubs with better insulation hold heat longer. Our [winter guide](https://bubbleandsoak.co.uk/best/best-inflatable-hot-tubs-for-winter) compares tubs with insulated walls, foam walls and freeze protection.

## When heating seems far too slow

If the water barely warms after many hours in mild weather, check that the heater is switched on, the set temperature is above the water temperature, the filter is clean and the cover is on. A heater light that never comes on or an error code may point to a fault. See [heater not working](https://bubbleandsoak.co.uk/blog/inflatable-hot-tub-heater-not-working) for checks in order.

## Frequently asked questions

**How long does a Lay-Z-Spa take to heat up?** It depends on litres, start temperature and weather. Use the sum above with your model's litres and heater rating; expect many hours from a cold fill.

**How many degrees an hour does an inflatable hot tub heat?** For a 2.2 kW heater and 800 litres, the maximum is about 2.4 °C an hour with no heat loss. In practice it is often less.

**Can I use the tub while it heats?** You can once it is warm enough for you, but keeping the cover on while heating is faster.

**Does a bigger heater help?** Yes, but on a 13 A plug heaters are limited to around 2 kW.

## Buying guides on this topic

- https://bubbleandsoak.co.uk/best/best-inflatable-hot-tubs-for-winter
- https://bubbleandsoak.co.uk/best/best-2-to-4-person-inflatable-hot-tubs
- https://bubbleandsoak.co.uk/best/best-inflatable-hot-tubs