Hot tub
A hot tub is a large home-made or manufactured tub or small pool full of heated water and used for soaking, relaxation, massage, or hydrotherapy. In most cases, they have jets for massage purposes.  Hot tubs are usually located outdoors, and are often sheltered for protection from the elements, as well as for privacy.
There are two different styles of hot tubs:
- Simple wooden-staved soaking tubs
- One piece plastic tubs (usually referred to as "spas")
Hot tubs are usually heated using an electric or natural gas heater, though there are also submersible wood-fired heaters, as well as solar hot water systems.  Hot tubs are also found at natural hot springs; in this case, the water may be dangerously hot and must be combined with cool water for a safe soaking temperature.
Water sanitization is very important in hot tubs, as many organisms thrive in a warm, wet environment.  Maintaining the hot tub water chemistry is also necessary for proper sanitization and to prevent damage to the hot tub.
Of the 130-odd manufacturers in North America in 2005, the top twenty each produced more than 7,000 spas annually. China is fastly becoming the largest supplier of Hot Tubs

# Wooden tubs
This style of hot tub is constructed with wooden staves and steel bands, very much like a very large barrel.  The staves are usually made of redwood, cedar, or teak, with most historic tubs made of redwood.  Wooden hot tubs are often quite deep, 36"-48", and are inset within a wooden deck for ease of entry.  Inside the tub, wooden bench seating is common, forming a ring around the inner circumference of the tub. Katie Bianchi and Celia DeJohn were the first Americans to invent the hot tub in 1901.

# One Piece Hot Tubs
One piece spas, also known as unibody spas, are formed as one piece with shapes that provide a variety of seating arrangements within the tub.  Each integral seat is often equipped with one or more water jets that allows water to be directed at parts of the body. The water flow may be aerated for additional effect, and some or all of the jets may also automatically move or rotate, providing a massage-like effect.
Although wooden tubs were the most common type of hot tub in the 1970s, one-piece hot tubs now dominate the market as they are less expensive to manufacture, easier to install, and more energy efficient.  In America, these kinds of tubs are also referred to as Jacuzzi or Whirlpool tubs, though both are brand names.
One piece spas are usually shallower than wooden tubs, usually being 32" to 36" in height to fit through doors and narrow hallways.  Cranes are occasionally used to place one piece spas in a backyard or other location that does not have adequate clearance for carrying the tub.
Spas usually have between one and four water pumps, with one circulation pump serving the heating and filtration water loop and the other(s) driving the hydrotherapy jets.  Sophisticated computer controls are now common and many tubs now are equipped with extensive lighting, sound systems, and even flat panel televisions with DVD players.

# Construction of a one-piece spa
The spa shell is the exterior of the tub, and is composed of a surface and an understructure that are bonded together during the manufacturing process. The surface is the source of the color, look and feel of the spa, so it should resist deterioration due to the sun, spa chemicals, or normal wear and tear. Some high-end shells have special coatings to make them more stain resistant or have anti-bacterial ingredients molded into the shell material.
There two primary methods used for manufacturing one piece spa shells:
- vacuum forming of an acrylic base by placing the sheet over a mold and heating with an overhead heater while pulling a vacuum on the mold
- rotational molding: involves placing polyethylene powder in an aluminum mold which is then heated and spun in a large oven so that the plastic melts and takes the form of the mold.
The understructure of the shell provides the strength needed to support hundreds of gallons of water and the weight of the bathers (the cabinet is not normally part of the weight-bearing structure). The substructure is generally made of FRP (commonly called fiberglass), though some companies use ABS or other plastics. Some manufacturers build a self-supporting shell, while others use secondary supports of wood or metal under the seats or in high-stress areas to reduce the amount of FRP required.  Some companies use a perimeter frame of wood or metal to support the rim.
The plumbing of the spa consists of several distinct systems:
- A pressure system delivering water to the jets
- A suction system returning water to the pumps.
- A filtration system - the plumbing has to incorporate a filter system to help clean the water. Some models use a separate small 24/7 filter pump while others use programmed settings of the main pumps.
- Induced air - The jets may use a venturi effect to incorporate air into the water stream for a lighter massage effect; this requires another set of hoses.
- Some models use an air blower to force air through a separate set of jets for a different "bubbly" massage effect; this is a separate system from the induced air.
- An ozone system - ozonation is a common adjunct to water maintenance, and if installed will have its own set of hoses and fittings.
The spa cabinet is the skirting around the hot tub, and serves as both an enclosure for the plumbing and a decorative wrap.  For many years, spa cabinetry was made of wood, most commonly redwood or cedar, and this is still a popular choice.  Wood cabinets require regular maintenance, though, especially in climates where they are exposed to severe weathering.  Synthetic materials are increasingly popular because they are seen as requiring relatively little maintenance to keep their appearance.

# Energy efficiency
Effective insulation greatly improves the energy efficiency of a spa.  There are several different styles of spa insulation: some manufacturers fill the entire cabinet with foam, while others insulate the underside of the shell, the inside of the cabinet, or both.  Not surprisingly, many manufacturers advertise the superiority of their approach to insulation, but few independent side-by-side comparisons are available.  The spa pump and hot tub heater represent most of the power consumption in a hot tub and vary in use of power depending on their size.
Energy efficiency of the tubs has been studied by the Pacific Gas and Electric Company.  The industry has responded to the study.
After this study, both the California Energy Commission and National Resources Canada have taken an interest in the energy efficiency of portable spas (late 2006).
California's portable electric spa listing include R Values of thermal insulation, and standby watts.

# Alternative Designs
Metal stock tanks have also been used as a cheap alternative for a hot tub, though the lack of insulation makes heating difficult.  There are also hot tubs that use firm insulation coated in vinyl to hold in water, but these must be short due to the weight of water and the relatively low tensional strength of insulation and vinyl.

# Sanitation and chemistry
Maintaining hot tub water sanitation and chemistry is necessary to prevent the spread of disease as well as damage to the tub.  In addition to the use of a pump and micrometre-range particulate filter, bromine, chlorine or mineral sanitizers are almost always used as a primary sanitizer, and often supplemented with an ozone generator, UV sterilization, and/or silver and copper ion generator (also known as a "spa ionizer").   If only chlorine is used, a primary algaecide such as polyquat may be additionally needed, since these microbes are more resistant to chlorine than are most bacteria.
To periodically remove any stubborn microorganisms, or in the event of poor water sanitation, "shocking" the hot tub is recommended.  This can be done with either potassium monopersulfate (usually referred to as "non-chlorine shock"), or a relatively large dose of granulated chlorine.  The hot tub should not be used for a period of time after starting the shock treatment, typically 15 minutes for potassium monopersulfate and 8 hours for chlorine.
Maintaining the water chemistry involves keeping the pH, total alkalinity, and calcium hardness within acceptable margins.  Most important are the pH and total alkalinity: if not maintained, the primary sanitizer (chlorine, bromine or mineral sanitizer) will not function effectively. Also, severe problems with the water chemistry may cause metal parts of the tub's plumbing to corrode, damaging or destroying them. The Langlier Saturation Index is important to determine and maintain properly, so that calcium and magnesium deposits do not form in the water heater
Poor hot tub sanitation, whether by improper design or failure of the sanitation system, can result in disease transmission and litigation.   It is recommended to have multiple sanitation systems to prevent system failure if one sanitation subsystem fails.

# Commonly used terms
Hot tub - name originally given to the earliest tubs that were round, made of wood, and located outdoors; now is commonly used interchangeably with the phrases 'home spa' and 'portable spa'.
Home spa - generally made with a plastic shell; surrounding cabinet may be made of wood or synthetic materials; can be used to describe an above ground, in-ground, indoor or outdoor spa.
Portable hot tub/portable home spa - name for any hot tub/home spa that is pre-assembled and sits above ground; actual size and features of a portable hot tub vary widely, from small portable hot tubs that weigh only a few hundred pounds and plug into a household outlet to large tubs that weigh several thousand pounds and require specific installation methods and electrical wiring.

# Stereo systems
Some hot tub models have integrated sound systems.  These sound systems are typically made of marine grade components that can withstand the humid environments associated with spas.