A-Z Glossary

Our A-Z Glossary is an index of common terminology, acronyms and abbreviations relevant to Heatrae Sadia; the heating and hot water industry; our people and our customers.

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Accreditation - An accreditation is formal recognition of an agreed standard that an organisation is certified as competent to perform specific processes, activities or tasks.

Annual service - An annual service is a physical check by a trained and suitably qualified engineer to make sure your product is working safely, efficiently and reliably.

Air gap - Our Megaflo unvented cylinders were the first hot water cylinders to feature an innovative integral floating baffle, which takes up the expansion as water is heated.  Over time, air may be absorbed in to the water in which case the air gap at the top of the cylinder that becomes depleted may need recharging, which is usually carried out during an annual service of the cylinder by a trained and suitably qualified installer.


Baxi - We're part of Baxi UK.  Baxi Heating UK Limited has some of the best known and most respected heating brands serving the UK and Ireland markets and together we share an award winning aftersales Customer Support team.

BEAB - The BEAB Approved Mark from Intertek is a European Safety Mark used by leading electrical manufacturers to support CE Marking and to demonstrate conformity with the Low Voltage Directive (LVD).  A BEAB Approved Mark on a product demonstrates that Intertek (an independent third-party) has verified its safety.

Benchmark - Benchmark places responsibilities on both manufacturers and installers. The purpose is to ensure that customers are provided with the correct equipment for their needs, that it is installed, commissioned and serviced in accordance with the manufacturer's instructions by competent persons and that it meets the requirements of the appropriate Building Regulations and relevant electrical qualifications. The Benchmark Checklist can be used to demonstrate compliance with Building Regulations and should be provided to the customer for future reference. Installers are required to carry out installation, commissioning and servicing work in accordance with the Benchmark Code of Practice which is available from the Heating and Hotwater Industry Council who manage and promote the Scheme.

BiM - BiM stands for Building information Modelling.  It is a process that involves creating and using an intelligent 3D computer generated model to inform and communicate project decisions.  BiM provides greater clarity through visualisation and collaboration for all stakeholders across the project lifecycle.  See our BiM Library to learn more.

BDR Thermea - We're part of Baxi UK and in turn, we're part of BDR Thermea, too.  BDR Thermea is a world leading manufacturer and distributor of sustainable and smart climate and sanitary hot water solutions and services with many of the leading European brands, with operations in over 70 countries worldwide and employing over 6,000 people.

Bar (pressure) - A bar (b) is a metric measurement unit of pressure, relating to water, gas or air. One bar is equivalent to ten newtons (N) per square centimeter (cm²) and is used in the hot water industry as water is supplied to dwellings with mains pressure.

Building regulations - Building regulations are statutory instruments that seek to ensure that the policies set out in the relevant legislation are carried out. Building regulations approval is required for most building work in the UK.

Cylinder - There are two types of cylinders - either the open 'vented' type or an 'unvented' type.  Vented cylinders have a vent pipe which subjects them to atmospheric pressure, whereas an unvented water cylinder is plumbed directly from the pressurised mains water supply.  Unvented cylinders have many safety features to help prevent excessive pressure build up and hot water expansion.

Commissioning - The process of checking that the product has been correctly installed and is safe for use is known as 'commissioning'.  Commissioning instructions can usually be found in the installation manuals for each product which details the procedures a trained and suitably qualified installer should legally complete a checklist for to comply with the appropriate Building Regulations and may be required in the event of any work being carried out under warranty.

Centralised boiler scheme - A centralised boiler scheme is the type of heating system typically found in an apartment block where there is a central boiler plant which serves heating and hot water to all the individual dwellings, rather than each flat or apartment having its own boiler.  This type of heating system makes maintenance easier and improves efficiency, whilst avoiding the issues for boiler flues for high rise buildings and apartments.

CPD - Continuing Professional Development (CPD) is the term used to describe the commitment to lifelong learning and is the means by which people maintain their knowledge and skills related to their professional lives.  Some institutions and organisations require the completion of a minimum number of CPD hours to be completed annually through training, seminars, workshops, events and conferences to demonstrate learning and keep qualifications current.

CE marking - CE marking is a mandatory conformity marking for certain products sold within the European Economic Area (EEA) since 1985. The CE marking is also found on products sold outside the EEA that are manufactured in, or designed to be sold in, the EEA.

Coil in coil - Our unvented cylinders such as Megaflo Eco have a high performance coil-in-coil heat exchanger which transfers heat from the sealed primary coil for the central heating system to the coil for water heating.

Duplex - At Heatrae Sadia, many of our products are manufactured using marine grade 2205 Duplex stainless steel, which is highly corrosion resistant and is twice as strong as an austenitic or ferritic stainless steel, to handle the high pressures.

Direct - A direct unvented cylinder is one that is heated 'directly' from an immersion heater which is connected to the mains electricity supply.  See also, indirect.

District heating - A district heating scheme is a heat network of insulated pipes which carry hot water or steam, from the point of generation to an end user and provide a means to transport heat efficiently. Networks vary in size and length, carrying heat just a few hundred metres between homes and flats, to several kilometres supplying entire communities and industrial areas. The distance a network can reach is also easily extended by simply adding more providers of heat, or 'heat sources', along the way.

Draw off - The capacity output of useful hot water that is possible to extract from a hot water cylinder before the cylinder supply is depleted.

DHW (domestic hot water) - A common acronym in the hot water industry, DHW refers to domestic hot water.  Domestic hot water is the supply of wholesome water from an outlet for domestic applications; the term is used irrespective of the type of building in which the hot water system is installed, including for commercial premises.

Expansion - When water is heated to 100°C it expands by up to 4 percent of its volume at room temperature.  Water that is heated above 100°C turns to steam and can be extremely dangerous, which is why products such as hot water cylinders and electric water heaters are fitted with many safety devices such as expansion vessels, temperature and pressure relief valves.

Expansion vessel - An expansion vessel is a container that is installed with a hot water cylinder.  When water is heated to 100°C it expands by up to 4 percent of its volume at room temperature, so the cylinder needs additional space for the water to expand in to; this additional space is provided by an expansion vessel which temporarily accommodates the expansion of water from the cylinder as it is heated.  Our market-leading Megaflo unvented cylinders were the first of its kind, featuring an innovative internal floating baffle expansion system, replacing the air pocket that forms at the top of a traditional cylinder with the hot water expansion and counteracting the need for an expansion vessel to be fitted.

ErP directive - ErP stands for Energy-related Products.  ErP is a European Union (EU) directive to improve energy efficiency and performance standards of products.

Efficiency class - The ErP directive is EU legislation which requires space and water heaters to be labelled with an efficiency class, which is a scale ranging from A+++ being the most efficient to G which is the least efficient.  From September 2015, only products which are rated with an efficiency class band of E and above can continue to be sold and any product with an efficiency class lower than this will essentially be banned from sale.

Flow rate - The flow rate is the amount of water that is supplied to an outlet such as a shower or a tap and is measured in Litres per minute.  It is important to know the flow rate of a property when sizing for a cylinder or water heater to ensure the specified cylinder will be able to supply an adequate volume of water for the time required and for the number of outlets which require the supply of hot water from the cylinder or water heater.

Flue - A duct, pipe, or chimney for conveying exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors. Combustion products contain carbon monoxide and other dangerous compounds, so flues are designed to release noxious gases to the atmosphere.

Fiche - A fiche is a technical table which contains data about the performance of products.  The ErP directive requires the completion of a technical fiche for each product which is in scope of the ErP requirements.


G3 - This refers to section G3 of the Building Regulations for England and Wales which details the requirements for sanitation, hot water safety and water efficiency.  The equivalent for Scotland is P3 and for Northern Ireland it is P5.  It is a legal requirement for installers of domestic hot water systems to hold a G3 (or equivalent) registration to demonstrate competence in installation, commissioning, inspection and servicing.  G3 (or equivalent) is recognised as a means of joining a competent person scheme to allow self-certification by the installer of the installation, which avoids the need to notify the local building control department prior to commencing work.

Genuine parts - Heatrae Sadia only recommends the use of genuine parts in its products.  Genuine parts are those supplied by the original component manufacturers; those which are Heatrae Sadia's preferred supply chain partners.  There are over 17,000 parts in stock for current and discontinued product ranges often with same day availability for the vast majority of our spare parts.  Our partner company, Baxi Genuine Parts, are our specialist genuine parts division.


Heat loss - Often used as a comparison between manufacturers, the heat loss figures are a measure of the amount of stored heat that escapes from an appliance such as a water heater or hot water cylinder over a period of time.  The heat loss figures are a measure of the kilowatt hours lost over a 24 hour period.

Heat network - A 'heat network' is a district heating scheme which comprises a network of insulated pipes used to deliver heat, in the form of hot water or steam, from the point of generation to an end user and provide a means to transport heat efficiently. Networks vary in size and length, carrying heat just a few hundred metres between homes and flats, to several kilometres supplying entire communities and industrial areas. The distance a network can reach is also easily extended by simply adding more providers of heat, or 'heat sources', along the way.

Heat recovery - Heat recovery is the transfer and re-use of heat that would otherwise be lost to the atmosphere, such as heat discharged from waste water or recycling the heat expelled from warm air through ventilation to outdoors.  Heatrae Sadia has a portfolio of efficient heat recovery products such as Megaflo SHRU and Advance Plus.

HIU - Heat Interface Units (HIU) are the control that transfers the heat energy from the district heating scheme into a dwelling via plate heat exchangers, providing the heat and hot water without the risks associated with gas boilers and flue issues in apartment buildings.

HWA - The Hot Water Association (HWA) is the leading trade body for domestic hot water storage.  HWA Charter members are independently audited, which supports the Charter principle of being clear and honest.  Members of the HWA Charter have to comply with the Charter standards and they also have to show an external accreditor how they do it.  As a responsible manufacturer, Heatrae Sadia has committed to adhere to the standards set out in the HWA Charter.

Heat exchanger - A heat exchanger is a device used to transfer heat between one or more fluids.  There are different types of heat exchangers including plate heat exchangers such as those found in the Heatrae Sadia Hi-Max Instant ID or the coil-in-coil heat exchangers like those used in our market-leading Megaflo Eco unvented cylinders.


Installer - A heating installer should be a person who is suitably qualified, experienced and assessed as being a competent person to carry out the fitting, commissioning, maintenance and servicing of a heating system.

Immersion - An immersion heater is an electric heating element found in a hot water cylinder.  The immersion heater acts like a kettle to heat the surrounding water in a cylinder.

Indirect - An indirect unvented cylinder is one that is heated 'indirectly' via pipework running from a boiler. It is a very efficient method of heating water and can be used to heat the home at the same time, meaning that the boiler is carrying out two tasks at once.  See also, Direct.

ISO 50001 - An internationally recognised standard, ISO 50001 sets out a formal energy management system.  Heatrae Sadia is one of a select few UK businesses to have achieved the ISO50001 worldwide standard for managing energy use across our business operations and product portfolio.  The highly prestigious accreditation means Heatrae Sadia must regularly demonstrate the ability to measure and monitor energy consumption and use action plans to actively improve energy efficiency with a continual culture of improvement.

Intelliboil™ Plus - Developed by Heatrae Sadia, Intelliboil™ Plus is our patented smart technology for drinking water products, which manages the boiling cycle for improved efficiency.  Intelliboil™ Plus brings the water to boil on demand and if there is no use for an hour it reduces the temperature by 10°C,  saving energy until the appliance is next in use.

Isolation valve - An isolating valve is part of a heating system that restricts or completely stops the flow of water through the system.  The valve is designed to improve safety and allows for easier maintenance of an appliance.

Instantaneous - An instantaneous water heater is an appliance that brings water to boil rapidly on demand.


Joule - A unit of energy, one joule (1J) is equal to the energy transferred (or work done) to an object when a force of one Newton acts on that object in the direction of its motion through a distance of one metre - 1 Newton metre (Nm). It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.

Kiwa - Kiwa in the UK performs testing and certification for appliances using water, electricity, solid fuel, gas, oil, biomass and other renewables. Many Heatrae Sadia and Megaflo products have been tested and certified by kiwa.

Part L - Introduced in October 2010 as a part of the Building Regulations managed by the Department for Communities and Local Government (DCLG), 'Part L' of the regulations is in two parts; Part L1A which applies to newbuild homes and Part L1B which sets the standards for work carried out in existing homes, in order to achieve carbon reduction targets.

Load profile - The declared load profile is a sequence pattern of draw-offs, which are tests carried out to confirm the given combination of useful water flow rate, useful water temperature, useful energy content and peak temperature.


Megaflo - Our market-leading unvented cylinder and calorifier products.  Megaflo has become so successful it is often now seen as the generic term for unvented water heating.

MVHR - MVHR is an acronym for Mechanical Ventilation and Heat Recovery.  MVHR is a whole dwelling ventilation system that supplies and extracts air continuously at a low rate and can be boosted as required.  MVHR product can help to meet the requirements for Building Regulations Part F.


NEMKO - An industry accreditation, Norges Elektriske Materiellkontroll (NEMKO) is a Norwegian organisation that supervises safety testing for electrical equipment manufacturing. The Nemko Group offers testing, inspection and certification services concerning products, machinery, installations and systems worldwide.

Nominal capacity - The nominal capacity is the maximum amount of water that can be stored in a cylinder or water heater at room temperature.


Overheat - All of our cylinders and water heaters have safety controls to prevent overheat such as safety cut-outs to avoid the risks of scalding or serious injury from the appliance exploding under high temperature and pressure.  The signs of an overheating cylinder or water heater are normally quite obvious due to the excessive loud noises which may occur in the instance of a thermostat failure.

Pressure (water) - Static water pressure is the pressure shown anywhere on the water supply piping system when no plumbing fixtures are running.  Dynamic water pressure is the pressure shown anywhere on the water supply piping system when one or more plumbing fixtures is drawing water.  The more fixtures that are running, the lower the dynamic water pressure will be.

Part P - Introduced in January 2005, as a part of the building regulations managed by the Department for Communities and Local Government (DCLG). The Part P requirement is that "Reasonable provision shall be made in the design and installation of electrical installations in order to protect persons operating, maintaining or altering the installations from fire or injury." All work on fixed electrical installations in dwellings and associated buildings must comply with relevant standards. Work affected by Part P must be notified to and inspected by building control bodies. This can be both timely and expensive, which is why many choose to join a Part P Competent Person Scheme (CPS), meaning they are sufficiently competent and qualified to self-certify that their own work complies with the relevant building regulations.

Primary circuit - The primary circuit is an assembly of pipework and water fittings in which water circulates between a boiler or other source of heat and a primary heat exchange inside a hot water cylinder.


Qatar - The city of Doha in Qatar is the location of the Heatrae Sadia representative office for the Middle East region.  Our Export & International teams provide localised support for Gulf construction projects, whilst our products are still precision engineered and manufactured in the UK to exacting standards of quality, durability and reliability.

Recovery (heat) - Heat recovery is the transfer and re-use of heat that would otherwise be lost to the atmosphere, such as heat discharged from waste water or recycling the heat expelled from warm air through ventilation to outdoors.  Heatrae Sadia has a portfolio of efficient heat recovery products such as Megaflo SHRU and Advance Plus.

Renewables - A natural resource or source of energy that is not depleted by use, such as water, wind, or solar power.


Solar thermal - A clean and highly efficient means of using renewable energy from the sun, solar thermal is used to provide a cleaner and more environmentally friendly alternative for providing heating and hot water used in the home. Solar collectors will be connected to one of the coils of a twin-coil cylinder using a sealed circuit containing a special glycol / water solution. This fluid not only withstands the high temperatures in excess of 200ºC but also will not freeze in temperatures down to -25ºC. The pump in the system circulates the heated fluid from the panel to the cylinder where the heat is transferred to the stored water through the lower coil. Solar water heating depends on radiation, not direct sunlight, and so works on dull days.

SpecPro - Product selection and specification software for architects, M&E contractors and engineers.  SpecPro has been developed by Heatrae Sadia as a helpful and easy-to-use resource to calculate the hot water requirement for projects based on some basic input data.  SpecPro provides recommendations on the most suitable products and the capacity required to meet the hot water demand.

Standing loss - A measurement of the heat that escapes from a hot water cylinder through the high radiating areas of its casing such as the immersion heater boss and the T&P valve, the heat loss is commonly referred to as 'standing loss' and is measured in kilowatt hours (kWh) lost over a 24 hour time period.

Secondary return - Secondary returns are a type of plumbing system used in both commercial and residential water heating applications to ensure that hot water is always available at every point of use. When a particular area of the plumbing system calls for hot water that is not available from the primary water heater, the hot water is drawn from the secondary return system instead.

Swept tee - A swept tee is a pipework connection for a central heating system.   For heating systems where secondary circulation is required, a swept tee joint in to the cold feed to the hot water cylinder is recommended.

Strainer - The strainer is a pipework connection placed on the incoming cold water main pipe to filter out any particles that might come from the cold water system, because of the sensitive nature of some of the components (a small piece of grit could cause them to malfunction), the strainer keeps them clear.

System (boiler) - System boilers are perfect for properties with more than one bathroom or for properties which require more hot water than can be provided by a combi boiler.

Santon - A well-known and respected brand name of quality water heaters; Santon was acquired by Heatrae Sadia in 1998.  In 2015, One of the best selling Santon products; the Santon PremierPlus unvented cylinder, moved to the Heatrae Sadia brand.  The Heatrae Sadia PremierPlus is available in direct, indirect, solar and pre-plumbed systemfit.


Tundish - The tundish is a safety device which is connected to the discharge pipework of a hot water cylinder which provides an air gap to allow for a visual inspection.  When water is being discharged through the tundish it is a visual aid to show that the system requires attention by a qualified and competent person such as a heating engineer.  The tundish also provides an alternative escape route for the water if the discharge pipe freezes or becomes blocked.

T&P valve - The Temperature and Pressure Relief Valve (T&P valve) is a safety device to relieve pressure from a hot water cylinder when they reach extremes.  As water expands when it is heated, the T&P valve protects the cylinder by discharging water through the waste pipework when the cylinder temperature and pressure reaches excessive levels. There are several reasons your system could be discharging the water, including but not limited to the following: thermal expansion, excess system pressure, low temperature relief, setting the water heater too high, or some other factor within the heater itself.


Unvented - An unvented cylinder is a sealed system and its water not 'open' to atmosphere which enables it to deliver high pressure hot water as it is not limited to the 'head' of water created by the cold water tank of an open vented system.  Installers of unvented cylinders must carry a qualification to install them and work is notifiable under Building Regulations.

Vented - A vented cylinder has a vent pipe which allows the cylinder to be subjected to atmospheric pressure and it is gravity fed by a cold water storage tank (usually in the loft).  Because the vented cylinder is gravity fed rather than being directly fed by the mains supply like an unvented cylinder, the vented system typically has a lower pressure and sometimes requires an additional pump to ensure a strong flow of water both for the domestic hot water and heating.

Vent pipe - The vent pipe is the main safety feature for an open vented hot water system and it helps the system cope with the expansion of water.  The vent pipe feeds back from the top of the cylinder to the cold water storage cistern to allow for the expansion of the hot water and equalise the pressure within the system.

Ventilation - The process of supplying a house or room continuously with fresh air; our mechanical ventilation and heat recovery products (MVHR) can help to meet the requirements of Building Regulations Part F.  In fact, the Heatrae Sadia Advance Plus is so efficient that its specific fan power (SFP) led to the creation of a lower rating category within the framework used to measure the energy performance of dwellings.

Vessel (expansion) - An expansion tank or expansion vessel is a small tank used to protect closed (not open to atmospheric pressure) water heating systems and domestic hot water systems from excessive pressure. The tank is partially filled with air, whose compressibility cushions shock caused by water hammer and absorbs excess water pressure caused by thermal expansion.


WRAS - The Water Regulations Advisory Scheme (WRAS) is a subscription membership company limited by guarantee. Its purpose is to contribute to the protection of public health by preventing contamination of public water supplies and encouraging the efficient use of water by promoting and facilitating compliance with the Water Supply (Water Fittings) Regulations and Scottish Water Byelaws.

Warranty - A written guarantee; the warranty is a period of protection issued to the purchaser of a product by the manufacturer, promising to repair or replace it if necessary within a specified period of time.  The warranty covers the cost of repairs and maintenance and sometimes includes replacement parts and labour costs.  Our Megaflo cylinders and calorifiers have an industry-leading lifetime* warranty in domestic installations and all Heatrae Sadia products have a minimum warranty of two years (sometimes more), providing products have been correctly installed and maintained.

Water Hammer - The shock wave or series of waves caused by the resistance of inertia to an abrupt change of water flow through a water piping system. Water hammer may produce an instantaneous pressure many times greater than the normal pressure.  For this reason, many building codes now require the installation of a 'water hammer arrestor' to absorb shock waves and prevent damage to appliances.


XL - Our Megaflo Commercial and Megaflo Eco Plus unvented calorifiers are manufactured on our 'XL' production line as they require different manufacturing techniques to our unvented cylinders due to the larger capacities and pressures.

Y plan - The schematic layout for the wiring of a central heating system; 'Y Plan' refers to the wiring of motorised valves, to operate the heating and hot water independently.  Other types of layout include the 'S Plan' where priority is given for hot water.

Zone - A zone is an area, room or floor of a building where the heating or ventilation is under control of one time and one temperature controller, i.e. one timer and one thermostat, or one programmable thermostat. For example heating in a large house may be divided into one zone comprising living rooms and another zone comprising bedrooms, with a timer and room thermostat (or programmable thermostat) for each zone.

Zero carbon - According to the Code for Sustainable Homes, a zero carbon home achieves zero net emissions of CO2 from all energy used in the home, including televisions, kitchen appliances, stereos, mobile phones and computers.  The aim is to reduce carbon emissions, to build houses that are better able to cope with climate change and have a reduced impact on the environment.  Any carbon taken from the national grid would have to be replaced by low carbon or zero carbon generated energy so that over a year the net emissions are zero.


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