Hot tubs are large tubs or small pools full of warm water; people use hut tubs for relaxation, hydrotherapy or massage. Many hot tubs have jets for massage; if they are located outdoors, they are protected for privacy and bad weather. There are simple wooden-staved soaking tubs and plastic tubs, known also as spas. Natural gas heaters or electric heaters are the most popular devices to heat hot tubs. There are also solar hot water devices, really cost-effective and ecologic. Natural hot springs can host hot tubs; in this case, the water can be really hot and a cooling system is a must.

Wooden hot tubs and spas
As many small organisms thrive in warm water, sanitization is vital in hot tubs. The hot tubs water composition must be maintained to keep the proper sanitization and prevent any damages to the tub. Wooden tubs are made with wooden staves and steel bands. Cedar, teak and redwood are most commonly used; inside the barrel there is a wooden bench. Spas are made in a large scale of shapes, with a variety of seating arrangements. Some models are equipped with water jets. The water jet can be aerated, with moving and rotating features, to provide a massage effect. Spas are cheaper than wooden hot tubs, easier to install and clean, and less energy consuming. Usually, spas have a circulator pump to heat and filter water and another pump to drive the hydrotherapy jets. The most sophisticated spas have computers for special lights, sound systems, television and DVD systems. There are also cost-effective versions of spas, made in metal but the lack of insulation makes heating more difficult.

The newest spa models are combining the latest achievements in technology and design, to create a beautiful, solid spa. New features, such as the digital color change waterfall and digital controls are allowing the customer to have an enjoyable experience. People can even entertain their guests in large spas, because there are large spas for 8 or 9 persons with 20 hydrotherapy jets and waterfall.

Hot tubs sanitation
It is necessary to maintain hot tub water sanitation to prevent diseases and damage of the tubs. Water filters are used, also chlorine and bromine, ozone generator are the most common sanitations solutions. People can use also UV sterilization, silver or copper ion generators. Periodically, specialists recommend shocking the hot tub to remove microorganisms, using potassium monopersulfate or granulated chlorine. After the shock treatment, the hot tub should not be used for 9 hours. Total alkalinity and pH must be maintained, to maintain the water chemistry within acceptable limits. Metal parts of the tub can be corroded; chlorine and bromine will not function in the calcium hardness is out of limits.