Hot tub

A hot tub is a large tub full of water used for hydrotherapy, relaxation or pleasure. Some have powerful jets for massage purposes. Hot tubs are sometimes also known as "spas" or by the trade name Jacuzzi.{{cite web|title=Has jacuzzi become a generic trademark?|url=https://www.genericides.org/trademark/jacuzzi|access-date=March 2, 2021|website=genericides.org|archive-date=March 6, 2021|archive-url=https://web.archive.org/web/20210306111422/https://genericides.org/trademark/jacuzzi|url-status=dead}} Hot tubs may be located outdoors or indoors.

In contrast to a typical bathtub, a hot tub is designed to be used by more than one person at a time, with many models accommodating four or more people. Unlike baths, soaps and shampoos are not used in wet-jetted hot tubs (although they can be used in air-jetted hot tubs). Home hot tubs are often closer in construction to standard bathtubs, while the construction of a public hot tub often has more in common with a swimming pool, of which it can be considered a type.

History

The earliest hot tubs were calderas in which hot stones were placed to heat the water.{{citation needed|date=August 2019}} Therma in Ikaria has been a very popular place particularly for hydrotherapy ever since the 4th century B.C.[http://www.greeka.com/eastern_aegean/ikaria/ikaria-excursions/ikaria-ancient-spa.htm Ikaria: The Ancient Spa of Therma in Ikaria Greece, Eastern Aegean], Greeka.com. The remains of wrecked marble bathtubs along with a prehistoric aqueduct that have been unearthed from this area bear ample testimony of the place's popularity in the ancient times.

In 737 A.D., Japan's first onsen opened near Izumo, Shimane, and centuries later, the first ryokan (inns) were built, offering food, accommodations, and soaking tubs called ofuro.

In ancient Rome, there were three types of baths: Baths at home (balnea), private baths (balnea privata), and public baths (balnea publica). The practice of bathing was so engrained that the Roman legions, during their long occupations in foreign lands, built their own baths at mineral and thermal springs in the newly conquered lands. Examples are found all over Europe.Schadewalt H. The history of Roman bathing culture [in Dutch]. Integraal1989;4:25–35

In the 1940s, hot tubs began to appear in the US, inspired by the Japanese ofuro. Hydrotherapy pumps were introduced by Jacuzzi. Fiberglass shell hot tubs appeared around 1970 and were soon superseded by cast acrylic shells.

Water treatment

=Plumbing=

The plumbing of the hot tub consists of:

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.

=Heating and energy use=

Hot tubs are usually heated using an electric or natural gas heater, though there are also submersible wood fire hot tub 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.

Effective insulation greatly improves the energy efficiency of a hot tub. There are several different styles of hot tub insulation: some manufacturers fill the entire cabinet with foam, while others insulate the underside of the shell, the inside of the cabinet, or both. Many manufacturers advertise the superiority of their approach to insulation, but few independent side-by-side comparisons are available. The hot tub 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.{{cite web|title=Tips to Improve Hot Tub Energy Use|url=https://cdn.shopify.com/s/files/1/0332/1441/files/10_Hot_Tub_Energy_Saving_Tips_1.pdf|access-date=18 October 2014}}

Energy efficiency of portable hot tubs has been studied by the Pacific Gas and Electric Company (PGEC),{{citation|url=http://www.energy.ca.gov/appliances/2003rulemaking/documents/case_studies/CASE_Portable_Spa.pdf|title=Analysis of Standards Options For Portable Electric Spas|date=May 12, 2004|author=Pacific Gas and Electric Company|publisher=California Energy Commission|access-date=2008-09-09|archive-url=https://web.archive.org/web/20061006153904/http://www.energy.ca.gov/appliances/2003rulemaking/documents/case_studies/CASE_Portable_Spa.pdf|archive-date=2006-10-06}} (archived from [http://www.energy.ca.gov/appliances/2003rulemaking/documents/case_studies/CASE_Portable_Spa.pdf the original] on 2006-10-06). leading to industry responsesKatherine Wang and Joshua Keim (September 2007). [http://www.poolspanews.com/how-to/turning-up-the-heat_o Turning Up the Heat]. Poolspanews.com. Retrieved on 2012-08-10. and interest from both the California Energy Commission and Natural Resources Canada.{{citation needed|date=November 2012}} California's portable electric hot tub listing include R values of thermal insulation, and standby watts.{{Cite web|url=http://www.energy.ca.gov/appliances/database/excel_based_files/Pool_Products/Portable_Electric_Spas.zip|archiveurl=https://web.archive.org/web/20081027065011/http://www.energy.ca.gov/appliances/database/excel_based_files/Pool_Products/Portable_Electric_Spas.zip|url-status=dead|title=List of portable electric hot tubs approved for sale in California|archivedate=Oct 27, 2008}}[https://web.archive.org/web/20080509063543/http://www.energy.ca.gov/appliances/2006regulations/index.html California's current appliance efficiency regulations]. energy.ca.gov (2006).

In 2019 an update to the ANSI standard for energy efficiency was approved. For the first time, this new standard increases the minimum energy efficiency level for portable spas and inflatable spas.{{citation|url=https://appliance-standards.org/blog/updated-hot-tub-standard-could-bring-surprisingly-big-energy-savings-states|title=Updated hot tub standard could bring surprisingly big energy savings to states|date=January 13, 2020|author=Megan Geuss|publisher=Appliance Standards Awareness Project|access-date=2020-05-14}}{{citation|url=https://www.iapmo.org/media/5369/summary-of-changes-apsp-3-1999-2014.pdf|title=ANSI/APSP/ICC-3 "Permanently Installed Residential Spas"|date=January 8, 2018|publisher=American National Standards Institute }} Hot tub covers have been shown to reduce most of the evaporative losses from the pool when not in use. With this component of heat loss being 70%,{{cite web|date=2023-01-28|title=Shutting down your hot tub to save electricity?|url=https://sashottubs.co.uk/shutting-down-your-hot-tub-to-save-electricity}} 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.{{cite web|date=2023-01-28|title=Shutting down your hot tub to save electricity?|url=https://sashottubs.co.uk/shutting-down-your-hot-tub-to-save-electricity}}

There are several different types of spa covers. Some covers are better for insulation and therefore are lighter on internal parts and energy efficiency. Some examples of covers are insulated, aluminum, rolling, or a tonneau.{{citation needed|date=August 2019}}

=Sanitation and water quality=

Since some hot tubs are not drained after each use it is necessary to treat the water to keep it attractive and safe. It must be neither too alkaline nor too acidic, and must be sanitised to stay free of harmful microorganisms. Partly due to their high water temperatures, hot tubs can pose particular health risks if not regularly maintained: outbreaks of Legionnaires' Disease have been traced to poorly sanitized hot tubs.{{cite web|title=Stoke-on-Trent Legionnaires' deaths: Report calls for hot tub review|url=https://www.bbc.co.uk/news/uk-england-stoke-staffordshire-21755913|author=BBC|date=12 March 2013|publisher=BBC News|access-date=April 20, 2013}} Typically chlorine or bromine are used as sanitizers, but salt water chlorination is starting to become more common.

Sanitation can also be aided by a non-chemical ozonator.

For aesthetic reasons, and for the sanitizer to work properly, water should be neither too alkaline nor too acidic (low pH). The hardness level of the water, measured as the amount of dissolved calcium, is also important. Insufficient hardness can lead to corrosion and water foaming. The ideal range of calcium hardness levels in a hot tub or spa water should be between 150 and 250 ppm (parts per million){{citation needed|date=August 2019}}

Types

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Portable vinyl-liner hot tub: these are cheaper and smaller, and require less power, e.g. 110–120 V 15 A household power circuits in North America.{{citation needed|date=August 2019}}
Roto-molded hot tubs are constructed with the shell and surrounding cabinet as a single piece. They have molded seating.{{Cite web|last=Chahine|first=Aline|date=2020-08-13|title=9 Types of Hot Tubs to Consider For Your Future Home|url=https://www.architecturelab.net/types-of-hot-tubs/|access-date=2021-04-20|website=Architecture Lab|language=en-US}}
Acrylic: these hot tubs have a cast acrylic shell, supported by a frame structure, and are usually surrounded by a skirt of either cedar or simulated wood-grain plastic. They typically can seat two to seven persons or more.{{citation needed|date=August 2019}}{{Cite web|last=Samaan|first=Payal|date=9 July 2021|title=What are Acrylic Hot Tubs|url=https://homeleisure.com/blog/what-are-acrylic-hot-tubs/|archive-date=|access-date=|website=Allstate Home Leisure}}
Cement: hot tubs can also be built of cement, above-ground, or in-ground like a small swimming pool.{{citation needed|date=August 2019}}
Wooden hot tubs: round wooden hot tubs peaked in popularity in the 1970s, having now been replaced by modern construction methods. Wooden hot tubs are often made of redwood or cedar and assembled from vertical staves and the structure is held together like wooden barrels, with metal bands.
Stainless steel hot tubs can be made in any shape and size and are extremely durable.{{Cite web|date=2020-10-28|title=The Best Hot Tubs for the Home|url=https://www.bobvila.com/articles/best-hot-tub/|access-date=2021-04-20|website=Bob Vila|language=en}}
Fiberglass hot tub: glass-reinforced polyester (GRP) hot tubs are made of a plastic matrix reinforced by fine fibers of glass.{{citation needed|date=August 2019}}
Inflatable PVC hot tubs. An inflatable hot tub is an air-filled spa structure. Air is pumped into a vinyl skin that expands and creates a solid form. Most commonly round in shape.{{citation|title=Recent Work on the Design and Construction of Air Inflated Structures|date=August 2016|author=Bob Barton|journal=Procedia Engineering|volume=155|pages=47–60|publisher=Science Direct|doi=10.1016/j.proeng.2016.08.006|doi-access=free}}{{citation|url=https://www.portablehottubfinder.com/articles/what-makes-an-inflatable-spa-different-from-a-regular-spa/|title=What Makes An Inflatable Spa Different From A Regular Spa?|date=24 May 2020}}

Safety

Sitting in water above normal body temperatures can cause drowsiness which may lead to unconsciousness and subsequently result in drowning. The U.S. Consumer Product Safety Commission (CPSC) recommends that water temperatures never exceed {{convert|40|C|abbr=out}}. A temperature of {{convert|37|C|abbr=out|-1}} is considered safe for a healthy adult. Soaking in water above {{convert|39|C|abbr=out}} can cause fetal damage during the first three months of pregnancy.{{cite web|title=CPSC Warns Of Hot Tub Temperatures|url=http://www.cpsc.gov/CPSCPUB/PREREL/prhtml79/79071.html|publisher=U.S. Consumer Product Safety Commission|access-date=2 December 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111011180133/http://www.cpsc.gov/cpscpub/prerel/prhtml79/79071.html|archive-date=11 October 2011}}

It is also recommended to install residual-current devices for protection against electrocution. The greater danger associated with electrical shock in the water is that the person may be rendered immobile and unable to rescue themselves or to call for help and then drown.{{cite web |title=Install Ground-Fault Circuit-Interrupter Protection for Pools, Spas and Hot Tubs |url=http://www.cpsc.gov/CPSCPUB/PUBS/5039.pdf|publisher=U.S. Consumer Product Safety Commission |access-date=2 December 2012 |url-status=dead |archive-url=https://web.archive.org/web/20121115030857/http://www.cpsc.gov/CPSCPUB/PUBS/5039.pdf|archive-date=15 November 2012}}

Hot tubs and spas are equipped with drains that can create powerful suction and between 1980 and 1996, the CPSC had reports of more than 700 deaths in spas and hot tubs, about one-third of which were drownings to children under age five. In the same period 18 incidents were reported to the CPSC involving body part entrapment. To reduce the risk of entrapment, US safety standards require that each spa have two drains for each pump, reducing the amount of suction.{{cite web |title=CPSC Issues Warning for Pools, Spas, and Hot Tubs |url=http://www.cpsc.gov/CPSCPUB/PREREL/PRHTML96/96139.html|publisher=U.S. Consumer Product Safety Commission|access-date=3 December 2012|url-status=dead|archive-url=https://web.archive.org/web/20121202234946/http://www.cpsc.gov/CPSCPUB/PREREL/PRHTML96/96139.html|archive-date=2 December 2012}} From 1999 to 2007 there were 26 reports to the CPSC concerning circulation entrapments hot tubs and spas, including three deaths.{{cite web |title=1999–2007 Reported Circulation/Suction Entrapments |url=http://www.cpsc.gov/LIBRARY/entrap08.pdf |publisher=U.S. Consumer Product Safety Commission |access-date=3 December 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130108034540/http://www.cpsc.gov/library/entrap08.pdf |archive-date=8 January 2013}}

In 2001{{cite web|title=2001 Reported Fires Caused by Hot Tubs|url=http://www.cpsc.gov/en/Recalls/2002/CPSC-and-TruHeat-Corp-Announce-Recall-of-Spa-Heaters/|publisher=U.S. Consumer Product Safety Commission|access-date=3 December 2012}} and in 2012{{cite web|title=2012 Reported Fires Caused by Hot Tubs|url=http://www.cpsc.gov/en/Recalls/2012/Watkins-Manufacturing-and-Therm-Products-Recall-Spas-Due-to-Fire-Hazard//|publisher=U.S. Consumer Product Safety Commission|access-date=3 December 2012}} the CPSC issued recalls for spa heaters which overheated and caused fires.

The Uniform Swimming Pool, Spa and Hot Tub Code is a model code developed by the International Association of Plumbing and Mechanical Officials (IAPMO) to govern the installation and inspection of plumbing systems associated with swimming pools, spas and hot tubs as a means of promoting the public's health, safety and welfare.{{Cite book|url=http://epubs.iapmo.org/2021/USPSHTC/|title=2021 Uniform Swimming Pool, Spa and Hot Tub Code|publisher=International Association of Plumbing and Mechanical Officials (IAPMO)|date=February 2021|edition=18}}

Disease risk

Poorly sanitized hot tubs have been linked to a number of diseases, principally caused by facultative anaerobic bacteria. Such incidents include hot tub folliculitis and legionellosis.{{Cite web|date=2020-07-01|title=Hot Tub Rash (Pseudomonas/ Folliculitis) {{!}} Healthy Swimming {{!}} Healthy Water {{!}} CDC|url=https://www.cdc.gov/healthywater/swimming/swimmers/rwi/rashes.html|access-date=2021-04-20|website=www.cdc.gov|language=en-us}}{{Cite web|date=2021-03-30|title=Respiratory Infections {{!}} Healthy Swimming {{!}} Healthy Water {{!}} CDC|url=https://www.cdc.gov/healthywater/swimming/swimmers/rwi/respiratory-infections.html|access-date=2021-04-20|website=www.cdc.gov|language=en-us}}