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Air Button
Air buttons are surface mount push button switches that control pumps, heaters & more... Spas that use air buttons are often called pneumatic systems as air is used to switch electrical components. There is a small bladder or bellows inside the switch, when depressed this bellows sends pressure down the air tube (attached to the button) to an air switch on the component. Air buttons are surface mount so one will only see the trim or face not the body underneath. Air Control Valve
Air control valves are used in spas and baths to allow air into the jets to create bubbles. An air control valve can create bubbles with out the presence of an air blower. Certain spa and bath jets can create a suction inside the jet body that can pull air into the water stream when the valve is opened. Draw back is outside air can be cooler than the heater water, allowing cool air into the tub can rapidly decrease the water temperature. Air controls are surface mount so one will only see the trim or face not the body underneath. Air Check Valve
An air check valve (aka non-return/one-way valves) is installed between an air blower and the water. A spring inside the valve often set to a certain pressure, 1/4lb etc... allows air to pass through the valve. It is important to have the valve flow the correct way, as the valve is designed to prevent air but specifically water flowing back through the valve to the air blower. Air Injector
Air injectors aka bubblers/air jets... come in many shapes and sizes. The injectors are small through wall style fittings which allows air (bubbles) into the spa. Air injectors are found on therapy seats. Air injectors are connected to the tubs air blower. Air injectors are typically surface mount so one will only see the trim or face not the body underneath. 
Air Manifold
Air manifolds like water manifolds are PVC fittings used to divert or split the flow of air/water from a central source to many destination fittings. Air manifolds come in many shapes and sizes. Generally air manifolds are 1" (air control valve installation) or 1.5" (air blower) PVC with 3/8" barbs.
Air SwitchAir switches come in many shapes and sizes but all function the same. These switches convert an pneumatic input to an electrical signal to operate a piece of equipment (pump, blower etc...). The air switch which is normally in a control box or on the equipment receives a signal (puff of air) from the air button which expands the internal diaphragm acitvating/deactivating the internal micro switch.
There are many types of air switches to learn more about which one you need see our Air Switch post here.
B
BasketFilter Baskets are used as large debris strainers. Filtering out large particulates prior to entering the spa filter.
Bath Heater Bath heaters are small inline self contained heaters designed for whirlpool/jetted bath tubs. Many new jetted tubs have a factory installed blank allowing quick installation of the heater.
Blower
An air blower is an air pump. Using a fan motor to push air into the spa jet lines to create bubbles or through specialized air injectors for therapeutic needs. Air blowers can reduce the water temperature as they are introducing cooler outside air into the tub. C
Cartridge (Filter)
Filter cartridges are used my many spas and pools to help maintain water cleanliness. The cartridge is contained by a filter canister which allows the water to flow through the paper element removing large particulates. Filter cartridges come in hundreds of sizes, all round. It is important to match all the dimensions and features when replacing a cartridge. Capacitor (Motor)
Pool and spa motors have Start & Run Capacitors. These capacitors assist the motor by altering the current during start up and continuous operation. Run capacitors are usually the black or gray polypropylene cylinders mounted on the back or top of the motor. These are continuously energized when the motor is running. Commonly when they fail the motor can become noisy, lose its electric field or overheat among other symptoms. Start capacitors are the larger silver/Aluminum canisters on top of the motor. The function of the start capacitor is to boost the starting torque. They only run until the motor reaches 3/4 of operating speed. A humming or non-starting motor could the result of a faulty start capacitor. 
Circuit Board
Electromechanical spas use Printed Circuit Boards (PCB) aka motherboards to manage your spa. All the equipment in your spa is attached to the circuit board; control panel, sensors, heater, pumps etc... the board controls all these.
Circuit Breaker
Circuit breakers used in spas come in a wide variety of shape and sizes. They all perform the same function: protecting the spa and its occupants form power overload and short circuits. Unlike a fuse a circuit breaker can be reset once it has 'tripped'. Many mechanical spa control boxes utilize panel mount circuit breakers, all you will see is the face of the switch. 
Circulation Pump Circulation or circ pumps come in many shapes and sizes. Their function in spas and hot tubs is dedicated filtration and heating cycles.
In an average spa the main jet pump will have two speeds - low speed for filtration and heating and high speed for jets. The main draw back to this setup is power consumption, often these pumps use much more electricity and needed for heating and filtration. This is why some spa makers utilize circ pumps for dedicated functions. One will usually find circulation pumps in high end spas with a high jet count & in spas with low flow style heaters.
Clock (Timer)
Older mechanical spa controls use time clocks to operate the spas heating and filtration cycles. Time clocks come in many styles but function largely in the same way, turning equipment on/off based on the cycle selected. One or more pieces of equipment can be connected to the time clock. Spas can use 7 day or 24 hour cycle clocks to operate the spa. The tabs on the rotary dial indicate the on/off times & length of cycles. More modern spas have circuit boards and electronic time clocks to run the spa.Contactor
A contactor is essentially a switch whose function (similar to a relay) is switching power from one electrical circuit to another. In the case of spas a contactor will switch electrical power supply from a pump to a heaters etc... There is an electromagnetic coil inside the contactor which controls the switching operation. The low voltage supplied by your input switch or button moves the coil allowing it to make or break contact with one circuit or another. These moving mechanical parts can wear out and fail. Contactors are more common in older spa controls or simple control boxes for in ground pools & spas. Contactors carry high current so always use caution when dealing with them! D
Diffuser (Jet)
Jet diffusers are the rear portion of adjustable spa jets. The diffuser has multiple functions in your spa. Controls the water flow when you turn the jet face. It has the clips which hold the jet into the jet body. The diffuser typically corrodes and fails at one point in a spas life. Spa owners will replace them or the whole jet at some point. Although many brands of spa jets have diffusers, only Waterway & Pentair (BWG) sell the diffusers as replacement parts.
Diverter (Valve)
Not to be confused with air control valves, diverter valves are used to control/divert water flow. Often setup to control water to flow to one area or another. Waterfalls, therapy seats or high flow jets are usually setup on a diverter valve to allow use on an intermittent basis. Usually these valves are found in larger spas with many functions. One can distinguish a diverter from an air control valve by the size of the handle, 4" or larger. E
Equipment PackAn Equipment pack is a complete spa mechanical assembly. The equipment packs include a control w/ heater, pump and any options you desire beyond the basics such as an air blower 2nd pump etc... If you want to build your own spa an equipment pack is ideal.
Element (Filter)
See Filter Cartridge
Element (Heater)
Spa heater elements are
comprised of a filament encased in and insulating powder covered by
protective skin. The filament of the element is usually a Tungsten coil
which functions similarly to a light bulb. Instead of lighting up when
current is applied the element resists & heats up. This heats up the
coating which is typically Magnesium oxide. The protective skin which
can be incoloy, Teflon or titanium allows the element to be immersed in
water.
EpromCircuit board chip-set. Often containing programing/spa operation functions. Few aftermarket replacement parts available. Circuit board mount, soldered to the PCB. Although replacement are available we recommend trained technicians replace these.
Escutcheon
Technical word for: Trim Ring, face trim, cover plate. Usually circular/decorative.
We're not literary experts, just here to help provide useful information and definitions for spa owners. To learn more visit the following:
Spa Part Glossary of Terms F-M
Spa Part Glossary of Terms N-Z
