What is a jacuzzi? Hot tub sizes, outdoor installation and buying guide

Two-person models have a footprint of roughly 150x200 cm, three- to four-person models around 200x200 cm, and six- to eight-person models 230x230 cm and above. Depth usually falls in the 80 to 95 cm range.

Whirlpool bathtubs that go inside the bathroom follow standard bathtub dimensions: 70x170, 75x170 and 80x180 cm are common sizes. Corner models come in square sizes such as 140x140 and 150x150 cm.

When choosing a size, look at the seating layout as much as the number of people. On the same footprint, a model with a reclining lounge seat takes fewer people than one with upright seats only, but it is noticeably more comfortable. Look at how many seats there are and how deep they are, not at the number of people printed on the label.

A filled spa-type hot tub, with its water and bathers, creates a serious load, and that load can exceed what a timber deck or ordinary ground can carry. You need a reinforced concrete slab or strengthened deck joists. The base must also be level; on a sloping base the shell twists.

Second, electrics. The power drawn by the heater and pumps is beyond what a standard socket circuit supplies. You need a dedicated circuit from the distribution board, cable of the correct cross-section, residual current protection and earthing. Have this included in the electrical installation design; a hot tub run on an extension lead is dangerous.

Third, service access. One side of the hot tub houses the motor and control board compartment, and at least 60 cm of clearance should be left in front of that panel. With tubs pushed against a wall or sunk into a deck, the shell has to be removed when something breaks down. Planning this before installation is easier than dealing with it at repair time.

They can be moved, they are easy to service and the initial outlay is lower. Visually, they look like something added to the garden afterwards, and you have to accept that.

In a built-in installation the shell is lowered into the ground or to deck level, so the water surface sits flush with the surrounding level. It looks clearly better. In return it needs excavation, drainage, a service pit and an access hatch. A removable deck panel must always be left for access to the service compartment.

The third option is a concrete spa: instead of buying a ready-made shell, the hot tub is built in reinforced concrete together with the pool, with the jets embedded in its walls. In villa projects, the warm section integrated into the pool edge is built this way and shares the pool's plant room.

A spa is built in one corner of the pool, raised above the pool level; the warm water spills over into the pool and creates a waterfall effect.

The technical requirement for this layout is separate circulation. The spa runs at 36 to 38 °C and the pool at 26 to 28 °C, so the two cannot share the same pipework. A separate pump, a separate heater and a separate filter are needed, and the plant room has to be sized accordingly.

The chemical balance is managed separately as well. Warm water quickly reduces the effectiveness of chlorine, so the spa section needs checking more often than the pool. In this layout automatic dosing is close to essential.

The conditions are a well-insulated shell, an insulated cover and pipework protected against frost. On good-quality models the area around the shell is foam-insulated and the cover contains rigid foam several centimetres thick.

In winter, the cover has the biggest effect on the heating bill. A hot tub left uncovered loses its heat overnight and the heater runs non-stop. The cover's hinges and lock are also needed for safety.

If there is a period when you will not use it, you have two options: leave the water in and keep it at a low temperature, or drain it completely and blow the pipework clear with air. In a half-drained hot tub that freezes, the pipes and pump crack, and the repair can cost close to the value of the tub itself.

Together, these mean the chemical balance goes off much faster than in a pool. Chlorine or bromine levels and pH should be checked regularly, and more often when the tub sees heavy use.

The filter cartridge is taken out and rinsed once a month, and soaked in a chemical cleaning solution once a season. A clogged cartridge strains the pump and the water turns cloudy. Keep a spare cartridge, since you need something to fit while the other one is being cleaned.

The water must be replaced completely, every few months depending on how heavily the tub is used. You cannot keep it going on chemicals forever, because dissolved solids build up. When you change the water, clean the shell and the jet nozzles too.

The quality of the acrylic surface and its backing layer determines the life of the shell. Ask about the variety of jets as well as their number; putting different jet types in different zones matters more than the jet count.

Heater output and insulation class are the two figures that set your running costs if you plan to use it in winter. Is the cover included, and how thick is it? How many pumps are there, and does filtration run on a separate pump?

On the service side: where is the authorised service centre, are spare parts available, and what are the warranty period and coverage? A hot tub is a product with an electronic control board; buying a brand whose board has no spare available means throwing the product away on the day it fails.