The Spaholm Glossary of Hot Tub and Swim Spa Terms
Reviewed for 2026, updated September 8, 2026.
Plain-language definitions of the words that appear on specification sheets, in warranty documents, and in dealer conversations, followed by answers to the broad questions every buyer eventually asks.
- Acrylic shell
- A spa shell formed from a sheet of cast acrylic that is heated, vacuum-formed over a mold, and then reinforced from behind with fiberglass, resin, or a structural composite. Acrylic is the dominant shell material in the premium segment because of its glossy finish, resistance to chemicals and ultraviolet light, and repairability.
- Air control (venturi)
- A valve on the spa rim that draws air into the jet plumbing so that air mixes with water at the nozzle. Opening the air control makes the jets feel stronger and more effervescent; closing it produces a smoother, warmer flow and reduces heat loss.
- Alkalinity, total
- A measure of the water's ability to resist swings in pH. Keeping total alkalinity in its recommended range makes pH easier to hold steady and protects the heater and other metal components from corrosion.
- Bather load
- The number of people using the spa and how often, which determines how quickly sanitizer is consumed and how frequently the water should be drained and replaced. A swim spa used daily for exercise carries a much higher bather load than a hot tub used on weekends.
- Bonding, electrical
- The practice of connecting the spa's metal components and nearby conductive surfaces to a common conductor so that no voltage difference can exist between them. Bonding is a standard code requirement around spas in the United States and is separate from grounding.
- Bromine
- A sanitizer commonly used in hot tubs as an alternative to chlorine. It remains effective at high water temperatures and across a wider pH range, has a milder odor, and is usually dispensed as slow-dissolving tablets in a floating or inline feeder.
- Cartridge filter
- A pleated, replaceable element through which spa water is drawn to capture hair, skin cells, and other particulates. Premium spas often use larger or multiple cartridges to increase filter area, which extends the time between cleanings and improves water clarity.
- Circulation pump
- A small, low-wattage pump that runs for many hours a day or continuously to move water through the filter, heater, and any ozone or ultraviolet system. It is distinct from the larger jet pumps that provide hydrotherapy and only run on demand.
- Cover lifter
- A mechanical arm or hydraulic assist that folds and lifts the insulated cover off the spa and holds it beside or behind the unit. A lifter protects the cover from being dragged and makes daily use practical for a single person.
- Crane placement
- The method by which most swim spas and some large hot tubs are set onto their foundation. A crane lifts the unit from the delivery truck over obstacles such as fences and roofs, so overhead lines, tree canopy, and access for the crane itself must be assessed before delivery.
- Diverter valve
- A large rotating valve on the spa rim that redirects the output of a jet pump between two or more groups of seats. Diverters let a single pump deliver full pressure to one area at a time rather than splitting it across every jet.
- Dual-zone swim spa
- A swim spa built with two separate bodies of water, typically a long swim lane and a hot tub compartment, each with its own heater, filtration, and temperature control. The format allows exercise at a moderate temperature and soaking at full heat in the same unit.
- Free chlorine
- The portion of chlorine in the water that is still available to sanitize, as opposed to combined chlorine that has already reacted with contaminants. Test strips and drop kits report free chlorine, and it is the number that matters for safe water.
- Full-foam insulation
- A construction method in which the entire cavity between the shell and the cabinet is filled with sprayed polyurethane foam. It provides excellent thermal performance and supports the plumbing, but leak repairs require removing foam to reach the fault.
- GFCI disconnect
- A ground-fault circuit interrupter housed in a disconnect box, typically installed within sight of the spa and a required distance from the water, that cuts power instantly when it detects current leaking to ground. It is a standard safety requirement for spa electrical installations.
- Hydrotherapy
- The use of heated water and directed jets to relax muscles, ease joint stress through buoyancy, and improve circulation. In a spa, the quality of hydrotherapy depends on seat design, jet placement, pump flow, and the ability to adjust pressure.
- Jet
- A nozzle set into the shell through which pressurized water, often mixed with air, is directed at the bather. Jets vary in size, pattern, and motion, from small pulsing nozzles to large rotating or directional units, and are grouped by seat to target specific muscles.
- Ozonator
- A device that generates ozone gas and injects it into the circulation plumbing, where it oxidizes contaminants and reduces the amount of chemical sanitizer required. Ozone dissipates quickly and does not replace a residual sanitizer such as chlorine or bromine.
- pH
- A measure of how acidic or basic the water is. Spa water is typically kept slightly above neutral so that the sanitizer works efficiently, the water is comfortable on skin and eyes, and metal components and the shell are protected.
- Recessed installation
- Setting a spa partially or fully below the surrounding grade or deck so that its rim sits at or near floor level. Full recess requires a vault with drainage and ventilation and a service access path to the equipment side of the cabinet.
- Salt chlorine generator
- A system that produces chlorine electrolytically from a low concentration of salt dissolved in the spa water. It provides a steady supply of sanitizer with less handling of chemicals, though the cell requires periodic cleaning and eventual replacement.
- Sanitizer
- The chemical or generated agent, most commonly chlorine or bromine, that kills bacteria and other organisms in spa water. Every spa needs a residual sanitizer regardless of any ozone, ultraviolet, or mineral system that reduces the required dose.
- Shock (oxidizer)
- A periodic dose of a chlorine or non-chlorine oxidizer that breaks down organic waste, sweat, and combined chlorine that have accumulated in the water. Shocking restores clarity and sanitizer effectiveness and is usually done weekly or after heavy use.
- Swim current
- The flow of water generated at one end of a swim spa against which the user swims in place. Currents are produced by high-flow jets, by paddlewheels, or by turbine-style propellers, and they differ in width, smoothness, and adjustability.
- Swim spa
- A long, deep, self-contained spa designed for swimming in place, aquatic exercise, and play, usually with a current system at one end and seating at the other. Swim spas are heated and covered like hot tubs but hold far more water and require a more substantial foundation.
- Thermal barrier insulation
- A construction method that insulates the shell and the inside of the cabinet walls separately, leaving an air cavity that captures waste heat from the pumps. It offers easier access for service than full foam while still delivering strong efficiency when well executed.
- Total dissolved solids (TDS)
- The accumulated concentration of minerals, chemicals, and other dissolved matter in spa water. As TDS rises over months of use, the water becomes harder to balance and less pleasant, which is the main reason spas are drained and refilled on a schedule.
- UV sanitizer
- An ultraviolet lamp housed in the circulation plumbing that neutralizes microorganisms as water passes through it. Like ozone, it reduces sanitizer demand but does not provide a residual and must be paired with chlorine, bromine, or a salt system.
- Wet test
- Trying a spa in the showroom with the water heated and the pumps running, in swimwear, before purchase. A wet test is the only reliable way to judge seat fit, jet pressure, noise, and, in a swim spa, the quality of the current.
Questions people ask
What is the real difference between a hot tub and a swim spa?
A hot tub is built for soaking, massage, and social use, with sculpted seats and water held at soaking temperature. A swim spa is a much longer and deeper vessel with a current system for swimming in place and exercise, usually kept at a cooler temperature. Dual-zone swim spas combine both in one unit with separate compartments.
How much space does a swim spa need?
Beyond the unit's own footprint, plan for a clear working margin on all sides for cover handling and service access, and confirm the equipment side of the cabinet can be reached. Crane placement on delivery day also requires open overhead space and a place for the crane to set up, which is often the tighter constraint.
Do hot tubs and swim spas need a concrete slab?
Swim spas almost always do, because the filled weight is enormous and must be supported uniformly. Many hot tubs can sit on a properly engineered deck or a compacted gravel pad instead, but the manufacturer's specification is the final word, and an unlevel or inadequate base can void the warranty.
What electrical service does a luxury spa require?
Full-size hot tubs and swim spas typically run on a dedicated 240 volt circuit with a GFCI disconnect installed within sight of the spa, along with bonding of nearby metal. Dual-zone swim spas may require more than one circuit. A licensed electrician should size and install everything, and a permit is usually required.
How much does it cost to run a hot tub or swim spa each month?
It depends on climate, water volume, temperature setpoint, cover quality, insulation, and local electricity rates, so any single figure is misleading. A well-insulated hot tub in a mild climate is modest to run; a large swim spa in a cold climate held at a high temperature is not. Ask dealers for the assumptions behind any estimate they give.
How often should spa water be changed?
Most hot tubs are drained and refilled a few times a year depending on bather load, while swim spas can often go longer because their larger volume dilutes contaminants. Rising total dissolved solids, persistent cloudiness, or water that resists balancing are the practical signals that a change is due.
Is a saltwater system better than chlorine or bromine?
A salt chlorine generator still produces chlorine; it simply makes it continuously from dissolved salt, which many owners find gentler and more convenient. It adds equipment cost and requires periodic cell cleaning and replacement. Traditional chlorine and bromine remain entirely effective when dosed consistently.
Can a swim spa provide a real workout?
Yes, with the right current system and enough lane length. Premium jetted, paddlewheel, and turbine systems produce a flow wide and smooth enough for sustained swimming, and the lane also suits water running, resistance training, and rehabilitation. Testing the current in person is essential, because systems differ considerably.
Should a spa be installed above ground or recessed?
Above ground is the simplest, least expensive, and easiest to service. Partial recess into a deck offers a built-in look while preserving equipment access. Full recess is the most elegant but requires a drained and ventilated vault plus a service access path, and it makes any future relocation far harder.
What should a luxury spa warranty cover?
Expect layered coverage: a long term for shell structure, a shorter term for the shell surface, and separate periods for plumbing, equipment, and cabinet. Read the exclusions carefully, particularly those concerning water chemistry and foundation, and ask the dealer whether labor and travel are included in a claim.
How long does a premium hot tub or swim spa last?
With a sound foundation, balanced water, and a cover in good condition, a premium acrylic spa is typically expected to serve for well over a decade. Pumps, heaters, covers, and filters are replaced along the way; the shell and frame determine the true working life of the unit.
What is the most common mistake buyers make?
Choosing on the basis of jet count, price, or showroom appearance rather than wet testing, reading the specification sheet, and preparing the site before purchase. The second most common is buying a swim spa as a compromise when the household really wanted an excellent hot tub, or the reverse.