Glossary

Heating and plumbing terms, explained

Plain English definitions of the words on your quote, your boiler and your energy bill. Each one names its sources and the date we last checked them.

Boilers and boiler types

Back boiler: A back boiler is a small, compact boiler fitted at the back of an open fireplace hearth, behind a gas fire. It heats water for central heating and hot water. Back boilers were popular until the Building Regulations changed in 2005, and many old standard efficiency units are still in use.
Best boiler temperature setting: The best boiler temperature setting is the lowest radiator flow temperature that still heats your home comfortably. For a modern combi, Vaillant puts this between 60 and 70°C, because a higher setting stops the boiler condensing. Hot water is set separately, and manufacturers give different figures, so check your own manual.
Biomass boiler: A biomass boiler heats water for radiators and hot water by burning wood fuel such as logs, pellets or chips, instead of gas or oil. In England and Wales the Boiler Upgrade Scheme offers a £5,000 grant towards one, but only for off-gas, rural properties with an emissions certificate.
Boiler flue: A boiler flue is the pipe or duct that carries the exhaust gases from a boiler to the outside air. Those gases include carbon dioxide and water vapour, and can include carbon monoxide if the boiler is faulty. A blocked or damaged flue is dangerous, so only a Gas Safe registered engineer should inspect or work on it.
Boiler size (kW output): Boiler size is the heat output in kilowatts (kW), not the physical dimensions of the unit. The right size depends on how much heat your home loses and how much hot water you use. Rules of thumb based on radiators and bedrooms give a starting point, but a heat loss calculation is the proper method.
Central heating pump: A central heating pump is the electric pump that pushes hot water from the boiler round the pipes and radiators and back again. Without it the water would sit in the boiler. On most modern boilers the pump is built in behind the casing, and it often runs for a short time after the burner stops.
Condensate pipe: A condensate pipe is the plastic drain pipe that carries the acidic waste water made by a condensing boiler away to a drain. In cold weather the water in an outside section can freeze and block it. The boiler then shuts down for safety, often with a fault code and a gurgling noise.
Diverter valve: A diverter valve is a motorised valve inside a combi or system boiler that decides where the boiler's heat goes. It sends hot water either round the central heating circuit or to the hot water side. When you open a hot tap it switches over, which is why the radiators can pause.
Electric boiler: An electric boiler heats the water for radiators, and sometimes a hot water cylinder, using electricity instead of burning a fuel. It has no flue, so it can go where a flue is not possible. Manufacturers quote efficiency near 100%, but electricity costs more per unit than gas.
Expansion vessel: An expansion vessel is a small pressurised container fitted to a sealed central heating system. Water takes up more space as it heats, and the vessel gives that extra volume somewhere to go. Without it, the system pressure would rise too far and the pressure relief valve would keep opening.
Filling loop: A filling loop is the short flexible hose, with a valve at each end, that connects the cold mains supply to a sealed central heating system so the water pressure can be topped up. Under water regulations it should not stay connected, so many are removed or stored detached after use.
Gas boiler: A gas boiler burns natural gas, or LPG in homes without a mains supply, to heat the water that warms your radiators and, depending on the type, your taps. Modern gas boilers are condensing models. By law, only a Gas Safe registered engineer may work on one in a home.
Heat exchanger: A heat exchanger is the part of a boiler that passes heat from the burner's hot flue gases into the water in your heating system without the two touching. Combi boilers usually have two: a primary one for the radiators and a plate one for hot water. Only a Gas Safe registered engineer should work on it.
How a combi boiler works: A combi boiler is one gas appliance that supplies central heating and hot water without a hot water cylinder or cold water tank. It takes water from the mains, heats it with a gas burner and a heat exchanger when you ask for heat, and sends it to the radiators or the taps.
How long do boilers last: A well-maintained gas boiler usually lasts between 10 and 15 years, according to Worcester Bosch, Vaillant and Ideal Heating. Many run for longer, and some fail sooner. Annual servicing, clean system water and local water hardness all affect it. Worcester Bosch says boiler type makes no difference.
Ideal boiler: An Ideal boiler is a gas boiler made by Ideal Heating, a UK company founded in 1906. Its domestic ranges include Logic and Vogue combi, system and heat-only boilers. Warranties run from 2 to 12 years depending on the model, and Ideal says they can drop to 12 months if the boiler is not registered within 30 days.
Magnetic system filter: A magnetic system filter is a small cylinder fitted to the heating pipework near the boiler. A strong magnet inside it catches the black iron oxide sludge that forms in radiators and pipes, and traps catch other debris. It keeps that dirt away from the boiler, pump and valves.
Modulating boiler: A modulating boiler adjusts its heat output up and down to match what the home needs, instead of firing at full power and then switching off. This helps it hold a steady temperature, cut short bursts of firing and run efficiently. Approved Document L expects a gas combi boiler to modulate down to the home's typical heating load.
Pressure relief valve: A pressure relief valve is a safety valve on a sealed central heating system or boiler. If the pressure in the system climbs too high, it opens and lets water escape through a discharge pipe, usually to the outside of the house. It protects the boiler and the pipework from damage.
Regular boiler (heat-only boiler): A regular boiler, also called a heat-only, conventional or open vent boiler, heats the water for radiators and for a separate hot water cylinder. It usually works with a cold water tank and a feed and expansion tank in the loft. It copes well with low mains pressure and with homes that need hot water at several taps.
System boiler: A system boiler is a central heating boiler that heats a separate hot water cylinder and has the main system parts, including the pump and expansion vessel, built into its casing. It needs no loft tanks. It stores hot water ready to use, so several taps and showers can run at once.
System boiler vs regular boiler: The main difference is how each keeps the heating water under pressure. A regular boiler uses a feed and expansion tank in the loft. A system boiler uses a sealed circuit filled from the mains, with the pump and expansion vessel inside the boiler. Both heat a separate hot water cylinder and store hot water for later.
Vaillant boiler: A Vaillant boiler is a gas boiler from Vaillant, a heating company founded in Remscheid, Germany, in 1874 and now sold in more than 60 countries. Its UK range includes the ecoTEC plus. Vaillant gives a 5-year guarantee as standard, and says it can reach 10 years when registered through an approved installer.
Worcester Bosch boiler: A Worcester Bosch boiler is a gas or oil boiler made by Worcester, Bosch Group, a UK company founded in Worcester in 1962 and part of the Bosch Group. Its main domestic gas range is called Greenstar. Guarantees of up to 12 years apply when an accredited installer fits the boiler with a Greenstar system filter.

Faults, servicing and gas safety

Boiler fault codes: A boiler fault code is a letter and number combination shown on the boiler display when it detects a problem and shuts down or limits itself. The code points to the area of the fault, such as low water pressure or failed ignition. The same code can mean different things on different brands, so use the maker's own list.
Boiler leaking water: A boiler leaking water means water is escaping from the boiler or its connections. Common causes are high system pressure forcing the pressure relief valve to discharge, corroded parts, worn seals or joints, and a poor installation. Whatever the cause, finding and repairing the leak is gas work for a Gas Safe registered engineer.
Boiler lockout: Boiler lockout is a safety feature. When the boiler detects a fault, such as low water pressure or ignition failure, it shuts itself down and stays off until the fault is dealt with and the boiler is reset. It protects the boiler and the people in the home from it running in an unsafe condition.
Carbon monoxide: Carbon monoxide (CO) is a poisonous gas with no colour, taste or smell. It is produced when fuels such as gas, oil, wood or coal do not burn properly. It stops the blood carrying oxygen around the body and can kill quickly without warning, which is why faulty gas appliances and flues are dangerous.
Carbon monoxide alarm: A carbon monoxide alarm is a small detector that sounds a loud warning when carbon monoxide builds up in a room. The gas has no smell or colour, so people cannot detect it themselves. An alarm gives early warning of a fault but does not stop one, and it does not replace servicing by a Gas Safe registered engineer.
E133 fault code (Baxi): E133 is a fault code on Baxi boilers that Baxi's own error code list describes as a gas supply fault. The boiler has shut down. Baxi says a frozen condensate pipe can cause it, and gives warming steps. Anything else is for a Gas Safe registered engineer.
EA fault code (Worcester Bosch): EA is a fault code shown on the display of some Worcester Bosch boilers when the boiler repeatedly locks out or stops working. Worcester Bosch links it to a blocked condensate pipe, most often one frozen in very cold weather. Worcester says you can usually thaw the pipe safely yourself with warm water.
F22 fault code: F22 is the fault code a Vaillant or Glow-worm boiler shows for low water pressure in the heating system. Both makers give the normal range as 1 to 1.5 bar. If the gauge is under 1 bar, you can top it up through the filling loop. Above 1 bar, or if it keeps dropping, an engineer is needed.
F28 fault code: F28 is the fault code a Vaillant or Glow-worm boiler shows when it tries to light and no flame signal is detected. The causes include no gas reaching the boiler, a frozen condensate pipe, a faulty electrode or circuit board or a flue problem. Some checks are safe for householders. The rest is for a Gas Safe engineer.
F75 fault code: F75 is the fault code a Vaillant or Glow-worm boiler shows when the pressure change is too low as its pump starts. The makers list low system pressure, air in the system, a faulty pressure sensor, a faulty pump or a wiring break as causes. Under 1 bar you can top up. Otherwise call an engineer.
Gas Safe Register: The Gas Safe Register is the official register of businesses and engineers who are competent and legally allowed to carry out gas work in the UK. Anyone working on gas appliances in a home must be on it. Registered engineers carry an ID card showing their photo, number and the types of gas work they can do.
Gas safety check: A gas safety check is an inspection of every gas appliance and flue in a rented property, carried out by a Gas Safe registered engineer. Landlords must arrange one every year. It produces a written record, often called a CP12 or landlord gas safety certificate, that the landlord must keep and give to tenants.
Glow-worm fault codes: Glow-worm fault codes are the F-numbers, such as F.22 or F.28, that a Glow-worm boiler shows when it detects a problem. Each points to an area such as water pressure, ignition or a sensor. Glow-worm publishes the meanings and says most need an engineer, and that you should not open the boiler casing.
Kettling: Kettling is the term for water reaching boiling point inside a boiler, which makes steam and a banging, popping or whistling noise like a kettle coming to the boil. It usually happens because the flow of water through the boiler has slowed down, most often through limescale or sludge, or a faulty pump or thermostat.
No heating but hot water works: Hot water but no heating means the boiler is heating the taps but the radiators stay cold. The cause is often a setting, such as the timer, thermostat or flow temperature, or low pressure or air in the system. On a combi boiler it can also be a faulty diverter valve, which needs an engineer.

Central heating, radiators and hot water

Central heating inhibitor: Central heating inhibitor is a chemical added to the water in a heating system to slow the corrosion of its metal parts. Less corrosion means less rust and sludge circulating, so water and heat reach the radiators more easily. It is added after the system is cleaned and its strength should be checked regularly.
Central heating sludge: Central heating sludge, properly called magnetite, is a black substance made of dissolved and rusted metal. It forms when the water in a heating system reacts with the steel or iron in radiators and pipes. Being heavier than water, it settles in radiators and pipework and, if left, can block flow and damage the boiler.
Heated towel rail: A heated towel rail is a bathroom radiator with bars built to hold and dry towels. It can run from the central heating, from an electric element on its own, or from both, which is called dual fuel. It warms the room as well as the towels.
Immersion heater: An immersion heater is an electric heating element fitted into a hot water cylinder. It heats the stored water directly from the mains electricity supply, separately from the boiler. Many homes use it as a back-up if the boiler fails, or to heat water on a cheaper overnight electricity tariff.
Leaking radiator valve: A leaking radiator valve is a valve or pipe connection at a radiator that lets heating water escape. Vaillant names radiator valves as one of the most common places for a heating system to leak. The signs are damp patches, stains or a boiler that keeps losing pressure. It needs a competent person to repair.
Lockshield valve: A lockshield valve is the valve at the bottom of a radiator, opposite the thermostatic valve. It balances the system by controlling how much water leaves the radiator. It usually has a plastic cap and is set by the installer during installation, so it rarely needs adjusting and is not for everyday control.
Open vented vs sealed heating system: An open vented heating system has a feed and expansion tank, usually in the loft, that keeps the water topped up and takes up expansion. A sealed heating system is closed off from the air, is filled with mains water through a filling loop and uses an expansion vessel instead.
Power flush vs chemical flush: A power flush uses a machine to pump cleaning chemicals through the system at high pressure to shift sludge, rust and debris. A chemical flush adds a cleaning chemical to the system water, lets it circulate and then drains and rinses it out. Both finish with fresh water and an inhibitor.
Radiator BTU output: Radiator BTU output is the amount of heat a radiator gives off, stated in British Thermal Units per hour. One watt equals 3.412 Btu per hour, so 1 kW is 3,412 Btu/hr. The figure only means something at a stated water temperature, called the delta T.
Radiator cold at the bottom: A radiator that is hot at the top and cold at the bottom usually has sludge blocking its lower flow channels. Hot water cannot reach the bottom section. It is not normally trapped air, so bleeding will not cure it. The usual remedy is to clean the radiator or the whole system and then protect the water.
Radiator not heating up: A radiator that is not heating up is getting too little hot water. Trapped air is the usual reason when it is cold at the top. A valve that is set wrongly or stuck, sludge, low boiler pressure or a boiler fault are the other causes. Which one applies depends on which radiators are cold and where.
Thermostatic radiator valve (TRV): A thermostatic radiator valve, or TRV, is a self-regulating valve that fits directly onto a radiator. It controls the temperature of that radiator by changing how much hot water flows into it. It usually has a numbered dial, so each radiator can be set separately from the boiler's own controls.
Unvented cylinder: An unvented cylinder is a hot water storage vessel fed with cold water straight from the mains supply pipe, with no vent pipe and no loft tank. Because the water is sealed in under pressure, it must carry safety devices that stop the stored water going above 100°C. It is also called a mains pressure hot water cylinder.
Vented vs unvented cylinder: A vented cylinder is fed from a cold water storage cistern, usually in the loft, and has a vent pipe open to the air. An unvented cylinder is fed straight from the mains with no vent pipe, and relies on safety devices instead. The difference decides water pressure, loft space needs and who may install it.
Wet vs electric underfloor heating: Wet underfloor heating circulates warm water through pipes under the floor, fed by a boiler or heat pump. Electric underfloor heating warms the floor with electric cables or mats. Wet systems cost more to fit and suit whole rooms. Electric systems are thinner and quicker to install, and suit a bathroom or kitchen.

Controls, energy efficiency and insulation

Cavity wall insulation: Cavity wall insulation is insulating material put into the gap between the inner and outer leaves of a cavity wall. Filling that gap slows the heat passing out through the walls, so the heating has less heat to replace. Whether a wall is suitable depends on how it is built and its condition.
Central heating programmer: A central heating programmer is the timer that tells your boiler when to run. It lets you set the times the central heating and hot water switch on and off each day. It controls when heat is available. A room thermostat controls how warm the house gets while the heating is on.
EPC (Energy Performance Certificate): An Energy Performance Certificate (EPC) rates how energy efficient a property is on a scale from A (best) to G (worst). It shows typical energy use and costs and lists steps to improve the rating. You need one when you sell, rent out or build a property, and it is valid for 10 years.
Flow temperature: Flow temperature is the temperature of the water leaving a boiler or heat pump on its way to the radiators or underfloor heating. A lower flow temperature usually means the heat source works more efficiently, but it needs larger radiators or underfloor heating to deliver the same warmth to each room.
Heating zones (zone control): A heating zone is an area of a home with its own temperature control, so it can be heated separately from the rest. Zone control is the system of controls that does this. In the building regulations a heating zone is an area on one floor with the same thermal characteristics and control needs throughout.
Hive vs Nest: Hive and the Google Nest Learning Thermostat are both smart thermostats that let you control central heating and hot water from a phone app. Hive Thermostat and Hive Thermostat Mini are made by Hive. Nest is made by Google. They differ in price, controls, learning features and which heating systems each lists.
Loft insulation: Loft insulation is a layer of insulating material in the roof space that slows the heat rising out of a home through the ceiling. With less heat lost, the heating needs to work less hard to keep the rooms warm. It is one of the measures government energy-saving schemes pay for.
Thermostat: A thermostat is a control that senses temperature and switches the heating on or off to hold the temperature you set. A room thermostat measures the air in a room. A programmable thermostat adds a daily timetable, so the heating follows your routine instead of a single fixed setting.
Weather compensation: Weather compensation is a heating control that varies the temperature of the water sent to your radiators according to the weather outside. An outdoor sensor, or online weather data, lets the system run cooler on mild days and hotter on cold ones, instead of using one fixed flow temperature all year.

Heat pumps, grants and regulations

Air to air heat pump: An air to air heat pump takes heat from the outside air using electricity and delivers it indoors through fan units that circulate warm air. It uses no radiators or underfloor pipework and often does not provide hot water. Most models can also cool, so it looks and works much like an air conditioning system.
Air to water heat pump: An air to water heat pump takes heat from the outside air using electricity and passes it to your central heating and hot water. It works a little like a fridge in reverse. MCS says it is the most common type of air source heat pump in the UK, and it can work with radiators or underfloor heating.
Are heat pumps worth it: A heat pump is worth it when the home holds heat well, the heating system can run at lower temperatures and the grant and running costs add up. It is a poor fit for a draughty, poorly insulated house with small radiators. Modern units keep working below freezing, though efficiency falls as it gets colder.
Benchmark commissioning checklist: The Benchmark commissioning checklist is the record an installer completes to show a heating appliance was installed and commissioned to the manufacturer's instructions. It also holds the servicing record. It has moved from a paper form to a digital record on the Benchmark website and app.
Buffer tank: A buffer tank is an insulated water store fitted between a heat pump and the radiators or underfloor heating. It adds system volume so the heat pump does not switch on and off too often, gives it water to draw heat from during defrost and can separate the heat pump circuit from the heating circuits.
Building Regulations Part L: Part L of the Building Regulations is the part of the law in England that sets energy efficiency standards for buildings. Approved Document L explains how to meet it. For a boiler replacement it covers minimum efficiency, controls and commissioning, and the installer must give you a compliance certificate.
COP and SCOP: COP (coefficient of performance) is the heat a heat pump delivers divided by the electricity it uses at one moment. SCOP (seasonal coefficient of performance) is the same ratio averaged over a whole heating season. A SCOP of 4 means four units of heat for each unit of electricity.
Electric heating: Electric heating is any system that uses electricity as the main energy source to heat a home. Energy Saving Trust says that usually means electric storage heaters, electric boilers or electric underfloor heating. Heat pumps use electricity too, but they are not normally counted because they move heat rather than make it.
Green Homes Grant: The Green Homes Grant was a government voucher scheme that paid up to two-thirds of the cost of energy efficiency and low carbon heating work, to a maximum of £5,000, or £10,000 for low-income households. It launched on 30 September 2020 and closed to new applications on 31 March 2021.
Ground source heat pump: A ground source heat pump heats your home and hot water using the steady low-level heat in the ground. A loop of buried pipe carries a water and antifreeze mix that absorbs the heat. The heat pump then raises its temperature for radiators or underfloor heating. It needs outdoor space and runs on electricity.
Heat pump vs gas boiler: A gas boiler burns gas to make heat. A heat pump uses electricity to move heat from outside air, ground or water into your home, so it delivers several units of heat for each unit of electricity. A boiler runs hotter and needs little from the house. A heat pump runs cooler and works best in a well-insulated one.
Hybrid heat pump: A hybrid heat pump is a heating system that pairs an air source heat pump with a gas boiler. A controller chooses which one runs, based on the outside temperature or the cost of electricity against gas. The heat pump covers most day-to-day heating and the boiler steps in when it is very cold.
MCS certification: MCS certification is the UK's quality mark for small-scale renewable energy such as heat pumps, solar panels and battery storage. It sets installation and product standards and gives consumers protection if something goes wrong. A heat pump grant under the Boiler Upgrade Scheme needs an MCS certified installer.
Warm Homes Plan: The Warm Homes Plan is the UK government's policy paper, published in January 2026, for upgrading homes to cut energy bills and fuel poverty. It sets out £15 billion of public investment to upgrade up to 5 million homes by 2030 through grants, loans, a heat pump grant and new rules for landlords.

Air conditioning

Air conditioner BTU sizing: Air conditioner BTU sizing means matching a unit's cooling capacity, quoted in BTU per hour, to the heat a room gains. One kW is 3,412 BTU per hour, so 12,000 BTU is about 3.5 kW. Daikin says typical UK homes need around 0.1 kW per square metre before adjustments.
Air conditioning running costs: Air conditioning running costs are the electricity charges for operating a unit. You work them out by multiplying the unit's power draw in kilowatts by the hours it runs and by your price per kilowatt hour. Inverter models, room size, insulation and the temperature you set all change the result.
Air conditioning servicing: Air conditioning servicing is the routine maintenance of an air conditioner: cleaning filters and coils, checking the drain and electrics, and checking the refrigerant circuit for leaks. Refrigerant work must be done by an engineer with a personal F gas qualification, working for a company certified to handle F gas.
Air conditioning vs heat pump: A reversible air conditioner is an air-to-air heat pump: it moves heat between the outdoor air and the room, cooling in summer and heating in winter. The difference people mean is usually air-to-air (warm air, no hot water) against air-to-water (heated water for radiators, underfloor heating and taps).
Inverter air conditioner: An inverter air conditioner has a compressor whose speed can rise and fall to match the heat the room needs. A fixed-speed unit can only run flat out or switch off. By modulating instead, an inverter unit holds a steadier temperature, avoids repeated start-ups and generally uses less electricity.
Multi-split air conditioning: Multi-split air conditioning connects two or more indoor units to a single outdoor unit. Each indoor unit is controlled on its own, so rooms can be heated or cooled separately without a separate outdoor unit for every room. It suits homes with several rooms that need individual temperature control.
Portable vs split air conditioner: A portable air conditioner is one self-contained unit you can move between rooms and run without installation. A split air conditioner has an indoor unit on the wall and a compressor in an outdoor unit, joined by pipework fitted by specialists. Split units suit continuous use and larger rooms.
Refrigerant: Refrigerant is the fluid sealed inside an air conditioner that carries heat from one place to another. It evaporates indoors, absorbing heat from the room air, then is compressed and releases that heat outdoors. In the UK most home units use an HFC gas, and only certified people may handle it.
SEER: SEER stands for Seasonal Energy Efficiency Ratio. It rates how efficiently an air conditioner or heat pump cools across a whole cooling season, not at one test point. The higher the SEER, the less electricity the unit needs for the same cooling. It is the cooling counterpart of SCOP, which rates heating.
Split system air conditioner: A split system air conditioner has two parts: an indoor unit in the room and an outdoor unit containing the compressor, joined by refrigerant pipes and cabling. It cools the room in summer and, on most models, can reverse to heat it. It suits a single room or one open-plan space.

Plumbing and water

Airlock in pipes: An airlock is a pocket of air trapped in a water pipe that blocks or restricts the normal flow of water. It shows as weak or no flow from a tap, gurgling or banging pipes and, in a heating system, cold spots on radiators. Running the taps often clears it, and bleeding radiators clears one in heating.
Burst pipe: A burst pipe is a pipe that has split or broken and is letting water escape. The most common cause in winter is a frozen pipe that cracks as it thaws. The first step is to turn off the stopcock to stop the water, then limit the damage and call an approved plumber.
Frozen pipes: Frozen pipes are water pipes in which the water has turned to ice, blocking the flow. You may see frost on the pipe and find taps running slowly or not at all. A frozen pipe can crack and burst as it thaws, so turn off the stopcock, thaw it slowly with warmth and check for leaks.
Limescale: Limescale is the hard, chalky deposit that builds up when hard water is heated. It is mainly calcium carbonate, left behind by the dissolved calcium in the water. It coats kettle elements and can narrow pipes and boiler heat exchangers, which slows water flow and can cause noise such as kettling.
Low water pressure: Low water pressure means water comes out of your taps or shower more weakly than it should. The cause may be on the water company's side, such as works or a burst main, or in your own plumbing, such as a partly closed stop tap, trapped air, a frozen pipe or a leak. A few checks usually show which.
Stopcock: A stopcock, also called a stop tap or stop valve, is the main valve that shuts off the mains water supply into your home. It is usually under the kitchen sink. Turning it clockwise stops the water. Knowing where it is lets you limit damage fast when a pipe bursts or leaks.

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