SOLAS Plumbing key terms

The national rules governing how water is supplied, used and fitted in premises. They cover backflow prevention, pipe materials, notification of certain work and the duty to avoid waste, misuse, undue consumption, contamination and erroneous measurement.

The unwanted reversal of water flow in a pipework system, so that water travels from a downstream point back towards the supply. It can be caused by back-siphonage from negative pressure or by back-pressure from a downstream source.

Backflow driven by negative or reduced pressure in the supply pipe, which draws contaminated water back into the system. A common cause is a mains interruption while a hose or appliance is submerged in a contaminated source.

The vertical clear distance between the lowest point of a discharge outlet and the spillover level of the receiving vessel. It is the highest-grade backflow protection because no mechanical part can fail, and its minimum size depends on the outlet diameter.

The most severe fluid risk classification, covering water that presents a serious health hazard from pathogenic organisms or highly toxic substances. Category 5 protection requires an air gap or an equivalent physical disconnection such as a break tank.

The level at which water would begin to overflow from the rim of a fitting, cistern or appliance. It is the reference point used when measuring the air gap or the free-standing height of a tap above its receiving vessel.

A storage tank fed from the supply through a float-operated valve, with the outlet arranged so that water cannot return to the mains. It provides physical separation and is commonly used to protect category 5 systems such as some industrial or hosepipe-fed processes.

A backflow prevention device containing two independently acting check valves in series, with a means of testing each one. It suits lower-risk categories and is often fitted at the incoming supply or on hose union taps as a basic safeguard.

A bacterium that causes Legionnaires' disease, a potentially fatal pneumonia. It thrives in warm, stagnant water and in biofilm, and infection follows inhalation of contaminated aerosol from showers, sprays and cooling systems rather than from drinking water.

A documented survey of a water system that identifies sources of aerosol, areas of stagnation, temperature weaknesses and susceptible occupants. It records control measures, monitoring points and review dates, and must be kept under periodic review.

A designated tap or shower chosen as the least favourable point on a system, typically the nearest and furthest from the calorifier on each branch. Its temperature is checked to show that the whole distribution is being controlled.

Routine measurement of stored and distributed water temperatures against control targets, usually hot water kept hot and cold water kept cold. Results are logged, trended and acted upon when they drift outside the permitted range.

The danger of skin burns from hot water, greatest for young children, older people and those with reduced sensation. It is managed by limiting delivery temperature at outlets, thermostatic mixing and warning signage, while still keeping storage hot enough for bacterial control.

A valve that blends hot and cold water to deliver a stable, limited outlet temperature despite changes in supply pressure or temperature. It is used to reduce scalding risk at baths, basins and showers and requires planned maintenance and testing.

A measure of dissolved calcium and magnesium salts in water. Hard water forms scale in heat exchangers, cylinders and fittings, reducing efficiency and flow, and it affects limescale treatment and the choice of materials and controls.

The hard deposit of calcium carbonate left when hard water is heated or evaporates. It insulates heat transfer surfaces, narrows pipe bores and can damage pumps and valves, so it is controlled by softening, dosing or careful temperature design.

Any process applied to water to make it safe or suitable for its use, including filtration, softening, disinfection, pH correction and dosing. Treatment must be selected for the specific risk and maintained with records to remain effective.

Adding chlorine or a chlorine-releasing compound to water to kill microorganisms. It is used for disinfecting new or repaired pipework and tanks before commissioning, with specified concentrations, contact times and a subsequent flush and test.

The pressure exerted by water at rest, determined by the height of the water column above the point considered. Roughly one bar is produced by ten metres of head, which governs pipe ratings and pressure-reducing valve settings.

The height of a column of water above a reference point, used to express pressure in a way that is independent of pipe size. It is the basis for sizing cold water storage, gravity-fed supplies and pump duties.

A device that lowers incoming mains pressure to a stable, safe level for downstream pipework and appliances. It protects fittings from excess pressure, reduces noise and water hammer, and is often set around three bar in domestic installations.

A banging or shock in pipework caused by a sudden change in flow velocity, such as rapid valve closure, which produces a pressure surge. It is reduced by clipping, correct pipe sizing, slow-closing valves and arrestors.

A storage vessel fed from the supply and fitted with a float-operated valve to maintain a set water level. In domestic systems it supplies cold water to outlets or feeds a hot water cylinder, and it must be correctly covered and insulated.

A valve opened and closed by a floating arm or float to keep a cistern or tank at a constant level. Correct setting of the water level and regular inspection prevent overflow, waste and backflow risk.

A vessel in which water is heated and stored, typically indirectly through a coil or heat exchanger. Stored temperature and stratification matter for both available hot water and bacterial control, so stratification and return temperatures are managed.

A hot water storage cylinder fed directly from the mains and sealed against the atmosphere, so it delivers mains-pressure hot water. It requires a safety group with expansion vessel, pressure relief valves and a temperature relief device.

A sealed vessel containing a flexible diaphragm and compressed gas that absorbs the increase in water volume as it is heated. It limits pressure rise in unvented and sealed systems and must be correctly sized and pre-charged.

A safety device on stored hot water that opens at a set high temperature to discharge water and prevent overheating. It is a final safeguard, distinct from the pressure relief valve, and must not be used for routine control.

A renewable heating appliance that moves heat from a lower-temperature source such as air or ground into a building. Efficiency falls as flow temperature rises, so it needs larger emitters and good insulation to perform well.

The ratio of heat delivered by a heat pump to the electrical energy it consumes, expressed as a number. A value of four means four units of heat per unit of electricity, and it varies with source and flow temperatures.

The temperature of the water leaving a heat generator and entering the heating circuit. It is a key design setting, since lower flow temperatures improve heat pump efficiency but require larger radiators or underfloor heating.

A floor drainage fitting with a water seal that prevents foul air from the drain entering the building. It must retain its seal under normal use, be accessible for cleaning and be of a type suited to the expected flow.

The depth of water held in a trap that blocks foul air from passing into the building. It can be lost by evaporation, self-siphonage, induced siphonage or compression, so trap depth and ventilation must be designed together.

A vertical discharge pipe that carries soil from sanitary appliances to the drain and continues above roof level to ventilate the system. It protects trap seals by limiting pressure fluctuations and must be adequately supported and sealed.

A one-way valve that admits air into a drainage system when pressure falls but closes to prevent foul air escaping. It can replace a ventilated stack in some positions, subject to rules on location and access.

A check that pipework or a system holds pressure without leaking, carried out before concealment and before commissioning. The test medium, pressure and duration are specified by the relevant standard and results are recorded.

The process of bringing an installed system into safe, correct operation, including flushing, testing, balancing, setting controls and recording results. It ends with handover of information, labels and certificates to the user.

Cleaning new or altered pipework by flushing to remove debris, then disinfecting with a chlorine solution and flushing again before use. Samples may be taken to confirm the water meets the required bacteriological standard.

The formal transfer of a completed installation to the client, including operating instructions, maintenance requirements, test results and warranties. Good handover records protect both the installer and the future user.

The operation and maintenance document issued at handover. It describes how to run and service the installed systems, lists equipment, schedules and spare parts, and gives contact details and safety information for the user.

Fixing pipework at specified intervals with clips or brackets to support its weight, resist thermal movement and limit noise and vibration. Spacing depends on pipe material, diameter and whether the run is horizontal or vertical.

The expansion and contraction of pipework as its temperature changes. It is accommodated by correct clip spacing, bends, offsets or expansion loops, and ignoring it causes noise, joint strain and eventual leaks.

Two different metals joined in a wet system that can form a galvanic couple, causing accelerated corrosion of the less noble one. The risk is controlled by isolation, dielectric couplings, suitable materials and careful jointing.

Corrosion driven by an electrochemical cell between two dissimilar metals in contact with an electrolyte such as water. The less noble metal corrodes, often rapidly at the joint, producing leaks and contamination.

A fitting made from insulating material that separates two dissimilar metals in a pipe run, breaking the electrical path needed for galvanic corrosion. It is used where, for example, copper meets steel.

A pipe connection made by inserting the pipe into a fitting that grips and seals it with an internal ring and O-ring, without solder or solvent. It suits many materials but requires correct pipe preparation, insertion depth and support.

The service pipe and associated fittings that deliver water from the external main into the building, ending at the internal stop valve. Its size, pressure and protection determine the whole downstream system's performance.

A device that measures the volume of water passing into a property for billing and leakage detection. Its location and type must allow reading, maintenance and correct protection against backflow and frost.

A fixture such as a WC, basin, bath, shower or sink that receives and discharges water for personal or domestic use. It must be correctly connected, trapped, ventilated and accessible for cleaning and maintenance.

An outdoor or garden tap designed to accept a hose connector, which creates a high backflow risk if the hose is left in a contaminated source. It must be protected by a suitable backflow device such as a double check valve.

A structured look at a task or site to identify hazards, decide who might be harmed and how, and set out control measures. It must be recorded where significant, communicated to workers and reviewed when things change.

A written description of how a particular job will be carried out safely and in the correct sequence. It names the responsible people, the plant and materials, the controls and the checks needed before work starts.

A formal written authorisation needed before certain high-risk activities, such as hot work or work on live systems. It records the precautions taken, the isolation applied and the sign-off that the area is safe.

An enclosed or partly enclosed space where the atmosphere may be hazardous or where entry and exit are difficult, such as a tank or deep pit. Entry requires assessment, testing, a rescue plan and often a permit.

The set of legal duties covering substances hazardous to health, including solvents, fluxes, sealants and dusts. It requires assessment, safe storage, suitable controls, health surveillance where needed and information for workers.

The procedure for making electrical equipment dead and proving it, by identifying the supply, switching off, locking off, proving the tester and testing for dead. It protects anyone working on or near electrical parts.

The systematic process of finding the cause of a malfunction by gathering symptoms, forming and testing hypotheses, and confirming the repair. It relies on knowledge of how the system should behave and on measured evidence.

To release trapped air from a heating emitter through its bleed valve until water runs steadily. It restores even heating, reduces noise and should be followed by checking system pressure and topping up if needed.

A measured quantity of water taken in a sterile container and analysed for bacteria or chemicals to confirm system cleanliness. Correct sampling technique, timing and transport are essential for a valid result.