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To help you navigate your way around your whirlpool bath, spa bath or hot tub, we have created this handy glossary of the different terms you may encounter.


IMPORTANT: remember, water and electricity do not mix and can be deadly! Any work you attempt yourself is AT YOUR OWN RISK. Please exercise caution. We cannot be held responsible for any injuries or damage that may arise from advice or installation of electrical components in your spa unit.

  • Stabilised Chlorine Granules

    Chlorine is a widely recognised chemical that is used used to help keep swimming pools and Spas free of bacteria & pathogens that can be hazardous to humans.
    Spa Chlroine granules often contain a stabiliser that helps to keep it active in the water much longer than unstabilised chlorine.  
    Stabilised chlorine can reduce the amount of chlorine you have to use in your pool. Over time you will save money as you will not have to use so much.

    Pure-Spa Chlorine Granules are rapid dissolving and are specifically formulated to disinfect spa and hot tub water, they have a minimal effect on the pH value of the water.
    Spa Stabilised Chlorine Granules are ideal for daily disinfection of the water.

    You cannot used stabilised spa chlorine granules to shock dose your spa - for this you would need Unstabilalised chlorine or an Oxy Shock/Non Chlorine Shock.

  • Air Control

    This is a valve, usually located on the top side of the spa bath. There is a knob to allow you to control the amount of air reaching the jet system – turn either counter-clockwise to increase, or clockwise to decrease or shut off the air flow.

    If you find there is water coming out of the air control, there is a jet blocked somewhere. Remove your bath’s or hot tub’s jet inserts (turn counter-clockwise), remove any debris and also check for any possible missing small parts, such as o-rings. You may find there is a round gasket when you remove the jet insert – ensure that this is out back on.

  • Blower

    The blowers push air piped from the air injectors into the water, through holes in the seat or the floor of your hot tub or spa bath. They can waste energy because they reduce the water temperature, so are less common nowadays.

    Sometimes they are installed with a check valve or a hartford loop and these can fail over time. You may also find that you need to replace the blower if it is making loud noises (this is simpler than trying to replace the motor, which would the usually break the housing anyway).

  • Contactor Relay/Softub Relay

    In simple terms this is a switch that has 2 independent circuits - the lower power circuit has an electromagnet to operate the contacts which open or close the higher power circuit, e.g. a device such as the heater or pump. A spring pulls the contact armature back when the circuit is not energised, which means the circuit is not complete and this is what shuts off the device.

    It’s generally faster and ultimately more economical to replace the whole PCB (printed circuit board) rather than the relay itself.

  • Control System

    The control system or box has a PCB (printed circuit board) inside and this is what controls the running of your spa or hot tub system. It has controllers which run the system in a logical sequence, e.g. to ensure the motor or heater are not overloaded. It’s the most expensive part to replace on your spa or hot tub.

  • Diverter Valve

    Most spas and hot tubs are supplied with diverter valves. These allow you to specifically control the zone of action for the jets, for example to divert the flow of water from one group of jets to another.

  • Filter Canister

    The filter cartridge may be housed in the skimmer, or separately in a filter canister within the equipment enclosure. The canisters are available with bottom plumbing or top plumbing. Pressure/suction/universal types are available too.

  • Filter Cartridge

    The filter cartridge is found either inside a compartment within the spa (often in a skimmer basket), or in its own filter canister. It keeps the water clear by removing particles and dirt as it flows through the filter. It can be either suction type or pressure type and the pump either sucks or pushes the water through it.

    Always check your filter if you find that pump performance or jet massage pressure is reduced. Filters need regular cleaning, generally around once a week. To do this simply remove the filter from the canister, open up the separate pleats and aim a hose nozzle at them to remove any dirt embedded within. Don’t pressure-wash your filter or put it in the dishwasher.

    With care and attention a filter should last between 6-12 months. By having 2 filters you can rotate them – this will allow the pleats on one filter to dry out thoroughly while you replace it with another.

  • Flow Switch

    The flow switches (grey or white) are usually in your hot tub’s plumbing and have a paddle type device which activates a little switch, using the flow of water. The switch should be isolated from the water and may need replacing if it is not active or if water is present when you open it.

  • Fuse

    Hot tub & spa control systems use high current fuses (10 to 60 amps). They are specialist fuses, which is why they can be expensive.

  • RCD

    An RCD is an electrical safety device which will immediately shut off the electricity if it detects electricity that is "leaking" to earth which could harm someone using electrical equipment. They offer a high level of protection from electric shock and they also reduce the risk of fire by detecting electrical leakage in wiring and accessories (especially relevant in older installations).

    They work by comparing the current flow in the Active (supply) and Neutral (return) conductors within an electrical circuit. If the current flow becomes unbalanced, e.g. from a bad heater element, some is leaking to earth. The RCD is designed to disconnect (trip) the electricity supply within 10 to 50 milliseconds if it senses a harmful leakage in the circuit, typically at 30 milliamps.

    Be aware of where all your system’s reset buttons are and what they do. They are rectangular in shape with a ‘test’ & ‘reset’ button. If the RCD is tripped it interrupts the power to everything. If it won’t reset, or if it immediately trips, it is doing what it is designed to do: protecting you from a faulty component somewhere.

    Heaters, blowers and ozone generators can be troublesome as they all have parts which can wear or fail, allowing water in; this should make the RCD trip immediately. RCDs are sensitive and can also be affected by lightning, heavy humidity or condensation. Try pushing the reset button to restore your spa it its usual working order. If you find that the RCD is tripping frequently or when a particular component is being used, it’s time to have a thorough check to try to determine the cause.

  • Heater Element

    Spa heater elements contain special heat-creating nichrome wire, surrounded by a powdered ceramic and encapsulated within a very strong metal casing. They can fail due to corrosion from improper water chemistry. If water gets through the outer casing, the ‘short circuit’ will normally cause the RCD to trip, shutting off all power to the spa.

    They are ‘resistive type heaters’, designed to heat the water as it flows over the element, and the water flow is paramount. Without the water flow the element would burn up extremely quickly and could also damage pipes. Almost all UK spa systems have various controls to help stop this happening.

    To help prolong the life of your heater always turn the temperature all the way down before draining, so that it doesn’t come on straightaway when you first restart your spa. Turn the pump on first, then adjust the temperature and the blowers.

  • Heater Manifold

    The manifold is a stainless steel tube with plumbing couplings on each end. It surrounds the heater element and allows water to flow through it and exchange heat. It may also contain the high limit sensor & pressure switch. Newer spas may have manifolds with sensors which provide accurate data to the PCB board about the water flow & the temperature.

    The tubes don’t usually leak but you may find that the bolts get rusty – check these and replace or restore as necessary.

  • High Limit Sensor

    This is a safety switch, usually closed but designed to open if an excessive temperature is reached, preventing the heater from meltdown if there is an equipment failure somewhere.

    If the high limit sensor is tripping within a few minutes of heater use, then check the flow, as little or no water will cause this to happen. If it is tripping at the end of the heating cycle this may be because the pump is off but the element and the water is still hot. The element temperature may rise briefly, sometimes tripping the sensor. If this keeps happening it’s best to replace the sensor.

  • Impeller

    This is located in the wet end of the pump and its purpose is to spin with the pump motor, which centrifugally pulls water in at the suction side and forces it out at the pump discharge end. There are many brands of impeller available and they cannot be interchanged – you will also need the correct wear ring, as they work together.

  • Jets/Hot Tub Jets

    Jets force the water from the pump and introduce air to the mixture (venturi ). They are available in many shapes and sizes and from many different manufacturers, all with their own way of making the venturi principle work. You can add air into your jets using the air control on the side of the spa.

    Over time the jets will start to pop out. This is inevitable no matter how carefully you look after your spa as it is due to slightly poor water chemistry and a breakdown of the locking clips over time. The only solution is to purchase new jets.

  • Light Assembly/Light Lens Small

    The light assembly illuminates the spa interior. The lights themselves comprise a coloured lens over a clear bulb, or multi coloured LEDs, set by using the topside control.

    As part of your routine maintenance, check both the internal & external parts of the lights. Even a small drip over time can ruin the pump and if they leak they can short the electrical system.

  • Ozonator

    An ozonator produces ozone (natural oxygen, O3) for water purification. In the earth’s atmosphere it occurs naturally, protecting us from harmful rays from the sun. Ozone quickly and efficiently purifies air and water (3,000 times faster than chlorine) and leaves only pure oxygen as a by-product.

    Ozone can be produced by ultraviolet (UV) light which uses a special lamp, or by corona discharge which uses air passed through a high voltage electrical discharge or corona. Modern corona discharge ozonators are more efficient - and typically produce more ozone – than the older style UV ozonators.

    The ozone is introduced to the jets by the use of an ozone injector or mazzei. The ozonator is connected to the spa controller and it operates with the low speed pump.

    Both types of ozonator run for around 18 months before needing replacing. A chip is available for the corona system which can help to reduce the cost of replacement.

  • OzoneInjector/Mazzei Injector

    The ozone or mazzei injector is a device which draws ozone into the water flow via a flexible tube or bypass manfold, to purify the water.

  • Pillows/Headrests

    These add comfort to the edge of your hot tub and with care they can be kept in good condition.

    Rinse the headrests regularly in tap water to help remove any chemical residues, and store away from direct sunlight. Removing your headrests when the hot tub is not in use will help to extend their life.

    Always remove headrests when you add sanitiser or shock treatment so that they are not exposed to gas, which can make them delaminate.

    Excess sanitiser or high/low PH level can affect the headrests, so test your hot tub water every day to ensure it is evenly balanced.

  • Pressure Switch/Flow Switch

    This is a safety device which shuts off the heater circuit if the flow of water is interrupted. During normal spa operation it is closed. It pops open if a pump fails, or if an air pocket forms, preventing the heater from meltdown.

    Correctly set up pressure is vital, as the switch becomes more sensitive over time and may need to be recalibrated to stop FLO errors happening.

  • Printed Circuit Board (PCB)

    Electronic control systems consist of a printed circuit board inside the spa control box.

    Sometimes the wires going into the terminals can work loose due to the pump’s vibration, shorting and damaging the board. To remedy this, try silicoting the screws in the terminal block.

    If the relays are malfunctioning, or the board is burnt on the front or the back, you will need a replacement PCB.

  • Pump/Hot Tub Pump/Spa Pump

    The jet pump moves the water and is the ‘engine’ of your spa or hot tub system. It is generally either a one-speed or two-speed design, with unions at the ends.

    There are 2 parts to a pump: the wet end (plastic) which contains the impeller (to push the water) and a motor (which turns the impeller).

    The water is sucked into the wet end and is forced out of the discharge point to the jets, also passing through your skimmer, main drain or filter.    

    The pump also supplies the flow of water passing through the heater, as the heater will be damaged if it is run without the water flowing. Likewise, for the filter to work it must also have water passing through it. Spas with electric heaters will often have two-speed pumps to cope with the demands of high current, water flow, and heating.

  • Pump Seals

    Seals are needed to prevent the water in the wet end (impeller) from getting into the dry end (motor).

    Poor seals can often cause leaks in spas and hot tubs. Always change the pump seal assembly if the motor is serviced or replaced. A leak that reaches the shaft of the motor will corrode it and ruin the whole motor mechanism. Check regularly around the pump for signs of leaks – this can indicate pump seal failure and if caught early you can replace the seals before motor damage occurs.

  • Skimmer

    Located at the spa’s waterline, the skimmer traps any large debris before it reaches the filter. The skimmer may also have a removable skimmer basket.

  • Spa Side Control (topside control)

    This located on the top side of your spa or hot tub and it connects directly to the control box. The decal (sticker on the front) is the keypad, controlling the heater, jet pumps, lights, blower and other spa functions.  Typically it features a digital temperature display (LCD or LED) and a temperature control.

    Check for any corrosion where their connectors join to the circuit board. Intermittent faults may be caused by moisture at the PCB connection – dry it out and this should fix the problem. The interior of the unit has non-serviceable parts, so if damaged you will need a replacement.

  • Suction

    The suction device is usually found in the footwell, with a cover to prevent any debris getting in, and its job is to return water to the pump.  Some spas have more than one suction feature.

  • Thermowell/Sensor Mount

    This is a housing which protects thermostat or high limit bulbs, which also sense the water temperature. There are various types: thermowell; self-contained PVC fitting; and a sensor mount.

    A sensor mount can be used for adding to a system which does not have a thermowell fitted already.

  • Pump Collar/Union

    Pump unions are typically made of PVC and make it easier to remove the pump when servicing the hot tub. They connect the pump suction and discharge to the spa’s plumbing and heater.

    The union fittings make for quicker & easier connection and disconnection of components. They comprise a male threaded part and a female threaded part sealed together (hand tight)with an o-ring.   

    If you discover the joint is leaking try lubricating or replacing the o-ring (don’t use silicone sealers or tape).

    2” is the most common pump union size. Other sizes include 2 and a ½ or 1 and a ½.

    Measure the outside diameter of the union to ascertain its size:

    1 and a ½ “ union (outside diameter) = 2 and 7/8 ”
    2” union (outside diameter) = 3 and a ½ “
    2 and a ½ union (outside diameter) = 4 and ¼ “

  • Valves

    Valves on hot tubs are usually found at the suction side of the pump and at the heater output and they may be the gate/knife type or the ball style valve. They allow you to remove spa equipment without needing to drain the tub, by shutting off water.

    The hose bib type of valve allows for draining.
    T-handled valves allow for water to be shut off. Up = open, down = closed.
    Ball valves also allow for water to be shut off. Parallel to pipe = open, perpendicular = closed.
    Check valves stop water from flowing in the wrong direction and can be either the spring-loaded or flapper type.

  • Wet End

    The wet end forms part of the pump and contains the impeller and the pump seals. It is available in either 48 or 56 frames and with either a side discharge or centre discharge.

    The wet end measurement where it connects to the motor is as follows:
    1 & a ½ " wet end (outside diameter) screw thread = 2 & 3/8 “
    2" wet end (outside diameter )screw thread = 3"
    2 and a ½ “ wet end (outside diameter) screw thread = 3 and a ½ "

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