Hot tub hire glossary
Plain-English explanations of the terms you will come across.
- Biofilm
- A slimy layer of bacteria that can build up inside pipework and on surfaces, shielding germs from sanitiser. Regular cleaning and correct chemical levels prevent it.
- Bromine
- An alternative sanitiser to chlorine that stays more stable at hot tub temperatures and has less smell. Commonly used in commercial spas.
- Combined chlorine
- Chlorine that has already reacted with sweat and other waste, also called chloramines. It causes the strong chlorine smell and irritated eyes, and is removed by shock dosing.
- Filter cartridge
- A pleated filter inside the tub that traps dirt, hair and debris as water circulates. It needs rinsing every few days and replacing when worn.
- Free chlorine
- The active chlorine in the water that is available to kill germs. It is what you measure with a test strip, and needs topping up as bathers use it up.
- Ground protector
- A padded mat or sheet placed under an inflatable tub to insulate the base and protect it from sharp objects and uneven ground.
- Hard-shell spa
- A hot tub with a rigid moulded acrylic shell set in a cabinet, with seats and water jets. Heavier and more powerful than an inflatable.
- HSG282
- HSE guidance on controlling Legionella and other infections in spa pools. It applies to commercial hot tubs, including those at holiday lets and hire businesses.
- Legionella
- Bacteria that can grow in warm water systems and cause Legionnaires' disease when inhaled in fine water droplets. Poorly maintained hot tubs are a known source.
- pH
- A measure of how acidic or alkaline the water is. In hot tubs it is usually kept around 7.2 to 7.6 so the sanitiser works well and the water is comfortable.
- RCD
- Residual current device. A safety switch that cuts the power very quickly if electricity leaks to earth, reducing the risk of a fatal shock. Hot tubs must be RCD protected.
- Shock dose
- A larger-than-normal dose of sanitiser or a non-chlorine oxidiser used to break down built-up waste, usually after heavy use or when water turns cloudy.
- Test strips
- Paper strips dipped in the water that change colour to show sanitiser, pH and alkalinity levels when compared against the chart on the container.
- Thermal cover
- The insulated, usually lockable lid that keeps heat in and debris out. Keeping it fastened is the biggest single factor in heating cost and safety.
- Total alkalinity
- A measure of the water's ability to resist changes in pH. Keeping it in range stops pH swinging up and down after chemicals are added.
Last updated 6 November 2026