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:
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 quickly becoming the largest supplier of 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 incorrectly referred to as Jacuzzi or Whirlpool tubs, though both are brand names.
One piece spas are usually shallower than wooden tubs, usually being 80-95cm (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.
There two primary methods used for manufacturing one piece spa shells:
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:
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.
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 has been tested in the US and Canada by independent labs and universities and the results are that a "thermally closed" or "Thermally Sealed" type of insultaion that uses a warm air barrier is far superior to simply filling the cabinet with foam. If you create a warm air chamber, using a layer method, then heat the air from the water source or from the heat of the equipment it creates a warm room filled with air. If the air is warmer than the water then there is zero heat loss from the vessel into the cabinet. Air is 1341 times less expensive to heat than water. It simply costs less to heat air than water, so if you use a cheap to heat medium outside the shell, AIR and warm it to above the water temp the heat actually is stopped from exiting the shell inside the cabinet. This is also the most expensive way to insulate so many spas just fill the cabinet with foam to save materials and labor. This also leaves the equipment vulnerable to freezing if there is a power failure.
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).
Spa covers have been shown to reduce most if not all of the evaporative losses from the pool when in use. With this component of heat loss being 70% a cover with even a small R-value is able to achieve as much as a 75% reduction in heating costs when used as opposed to leaving the water surface exposed.
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. If the pH is not maintained in the correct range, chlorine will not be effective at eliminating bacteria. 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.
Hot tubs can be dangerous to the health under some circumstances. Studies show that usage may increase miscarriage risk in pregnant women. Also, studies show that usage may hurt male fertility.
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.