Glossary
Glossary
Plain-English definitions of the terms used across Gas Pathway. They're written to help learners understand, not to replace the wording of regulations or standards.
A
- Accredited Certification SchemeACS
- The national scheme for assessing and certifying the competence of individual gas engineers. Engineers who are, or have been, registered prove their competence through ACS assessments in the areas of work they do, and are reassessed every five years. New entrants first complete recognised training followed by assessment, such as a managed learning programme, a qualification or an apprenticeship.
- Taught in M01 Gas safety legislation & the industrySee also CCN1, Gas Safety (Installation and Use) Regulations 1998
- Additional emergency control valveAECV
- An extra emergency control valve fitted downstream of the ECV, for example where the meter is remote from the dwelling, such as in some blocks of flats, so the supply can be shut off nearer to where gas is used. It may not isolate the whole installation, and it never replaces the ECV, which is always required.
- Taught in M04 Gas supply, meters & regulatorsSee also Emergency control valve
- Appliance isolation valveAIV
- A valve that shuts off the gas to a single appliance. On some appliances it takes the form of a self-sealing connection on a flexible hose. In a tightness test to IGEM/UP/1B Edition 4 these valves are left open at the start, and closed only to retest the meter and pipework on their own.
- Taught in M06 Tightness testing & purgingSee also Tightness test
- Approved Code of PracticeACoP
- Practical advice on how to comply with the law, approved by HSE with the consent of the Secretary of State. For gas work it is in HSE's L56. It has a special legal status. If you are prosecuted and didn't follow it, you have to show you complied with the law in some other way. HSE guidance, printed alongside it, isn't compulsory, but following it is normally enough to comply.
- Taught in M01 Gas safety legislation & the industrySee also Gas Safety (Installation and Use) Regulations 1998, Normative document
- At RiskAR
- An unsafe situation where one or more faults could become a danger to life or property in the future, such as inadequate ventilation. Usually it is turned off with the responsible person's permission and labelled. In a few cases turning off wouldn't remove the risk, so the situation is referred to the responsible person or organisation instead.
- Taught in M14 Unsafe situations & emergenciesSee also Immediately Dangerous, Not to Current Standards
- Atmosphere-sensing deviceODSalso oxygen depletion system, atmosphere sensing pilot
- A safety device, usually a special pilot, that shuts an appliance down if the room air becomes vitiated, before carbon monoxide can build up.
- Taught in M11 Gas safety devices & controlsSee also Vitiation, Flame supervision device
B
- Breather ventalso breather hole
- The small hole in a regulator's spring housing that lets air in and out of the space above the diaphragm as it moves. That space normally holds only air, so gas escaping from the breather is a sign that the diaphragm has failed.
- Taught in M04 Gas supply, meters & regulatorsSee also Diaphragm, Meter regulator
- Burner pressure
- The gas pressure at an appliance's burner test point while it is running, checked and set against the figure on the data plate.
- Taught in M12 Commissioning, servicing & relightingSee also Gas rate, Test point
C
- Calorific valueCV
- The heat released when a set volume of gas is burnt, given in MJ/m³. The gross value includes the heat recovered by condensing the water vapour in the products; the net value does not.
- Taught in M02 Properties of gas & combustionSee also Wobbe index, Gas rate
- Carbon monoxideCO
- A toxic gas produced by incomplete combustion. It has no colour, smell or taste, and it stops the blood carrying oxygen by binding to haemoglobin.
- Taught in M02 Properties of gas & combustionSee also Incomplete combustion, CO/CO₂ ratio
- CCN1also core domestic natural gas safety
- The core ACS assessment in domestic natural gas safety. It covers the knowledge every domestic engineer needs, such as tightness testing, ventilation, flues and unsafe situations. Appliance assessments (for example CENWAT or CKR1) are taken on top of it.
- Taught in M01 Gas safety legislation & the industrySee also Accredited Certification Scheme
- CO/CO₂ ratioalso combustion ratio
- The carbon monoxide reading divided by the carbon dioxide reading in the flue products, measured with a flue gas analyser. It is compared with action levels to judge whether an appliance is burning correctly.
- Taught in M10 Combustion performance & COSee also Flue gas analyser, Carbon monoxide
- Complete combustion
- Burning with enough air, so the products are carbon dioxide and water vapour, with nitrogen and excess air carried through.
- Taught in M02 Properties of gas & combustionSee also Incomplete combustion
D
- Diaphragm
- A flexible membrane that moves when the pressures on its two sides differ. In a regulator it balances the force of the loading spring against the outlet pressure to set how far the valve opens.
- Taught in M04 Gas supply, meters & regulatorsSee also Meter regulator, Breather vent
- Draught diverter
- A device on an open-flued appliance that stops conditions in the flue, such as down-draughts, from upsetting the appliance's combustion.
- Taught in M09 Chimneys & fluesSee also Spillage, Open-flued appliance
- Droop
- The small fall in a regulator's outlet pressure as the flow through it increases. A spring-loaded regulator needs a slight change in pressure to open its valve further.
- Taught in M04 Gas supply, meters & regulatorsSee also Meter regulator, Operating pressure
E
- Emergency control valveECValso emergency control
- The valve a gas user turns to shut off the supply in an emergency, usually at the meter. Its outlet marks the end of the gas network. It must be easy to reach, with its open and shut positions clearly marked, and where it isn't next to the meter a notice must say what to do if gas escapes.
- Taught in M04 Gas supply, meters & regulatorsSee also Additional emergency control valve, Meter regulator
F
- Flame rectification
- A way of proving a flame electronically. An alternating voltage is applied across the flame, which lets a small current pass mainly in one direction; the control board only keeps the gas on while it detects that current.
- Taught in M11 Gas safety devices & controlsSee also Flame supervision device
- Flame supervision deviceFSD
- A device that shuts off gas to a burner if the flame goes out or fails to light, for example a thermocouple and magnetic valve, or a flame rectification system.
- Taught in M11 Gas safety devices & controlsSee also Thermocouple, Flame rectification
- Flammability limitsalso limits of flammability, explosive limits
- The range of concentrations of gas in air that will burn. For natural gas it is 5% to 15% gas in air. Below the lower limit the mixture is too weak to burn; above the upper limit it is too rich.
- Taught in M02 Properties of gas & combustionSee also Lower explosive limit
- Flue flow test
- A smoke test on an open flue or chimney, without the appliance running. After warming the flue if needed, a smoke pellet is lit at its base, and the smoke must come out of the correct terminal only, without getting into the room, any other room, the roof space or anywhere else outside the flue.
- Taught in M09 Chimneys & fluesSee also Spillage
- Flue gas analyseralso combustion analyser
- An electronic instrument that samples flue products and measures gases such as oxygen, carbon monoxide and carbon dioxide, to check combustion performance and test room air for CO.
- Taught in M10 Combustion performance & COSee also CO/CO₂ ratio
- Flueless appliancealso Type A appliance
- An appliance that releases its products of combustion into the room, such as a cooker. It relies on the room's ventilation to dilute them.
- Taught in M08 VentilationSee also Open-flued appliance, Room-sealed appliance
G
- Gas incident
- A fire, explosion or exposure to carbon monoxide that has caused a death, unconsciousness or someone being taken to hospital, or property damage that makes part or all of the property uninhabitable. The scene must be preserved, and no work beyond making safe is done until HSE and the gas supplier have been consulted.
- Taught in M14 Unsafe situations & emergenciesSee also Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013, Carbon monoxide
- Gas rate
- The volume of gas an appliance burns, in m³/h. It is measured by reading a metric meter over exactly two minutes while only that appliance runs, then multiplying by 30. Multiplying by the calorific value and dividing by 3.6 gives the gross heat input in kW.
- Taught in M03 Pressure, flow & measurementSee also Calorific value, Burner pressure
- Gas Safety (Installation and Use) Regulations 1998GSIUR
- The main regulations governing gas work in Great Britain. They place duties on the people who install, maintain and use gas fittings, and on landlords. Anyone doing gas work must be competent, and businesses that do it, including self-employed engineers, must belong to the class of persons approved by HSE, which is currently Gas Safe Register.
- Taught in M01 Gas safety legislation & the industrySee also Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013, Normative document
- Gas transporterGTalso transporter
- A company licensed by Ofgem to carry gas through its network of mains and service pipes. The gas transporter is responsible for the network up to the outlet of the emergency control valve, and runs the gas emergency service.
- Taught in M04 Gas supply, meters & regulatorsSee also Service pipe, Emergency control valve
I
- Immediately DangerousID
- An unsafe situation where the installation or appliance, if left connected, is an immediate danger to life or property, such as a gas escape or combustion products entering the room. With the responsible person's permission it is disconnected, sealed and labelled. If they refuse, the engineer turns it off if possible and contacts the Gas Emergency Contact Centre.
- Taught in M14 Unsafe situations & emergenciesSee also At Risk, Not to Current Standards
- Impulse passage
- A small passage that feeds the outlet pressure of a regulator to the underside of its diaphragm, so the regulator can sense and respond to the pressure it is delivering.
- Taught in M04 Gas supply, meters & regulatorsSee also Meter regulator, Diaphragm
- Incomplete combustion
- Burning without enough air, or with the flame chilled or disturbed, so carbon monoxide is produced. Common causes include poor ventilation, blocked flues and dirty or damaged burners.
- Taught in M02 Properties of gas & combustionSee also Carbon monoxide, Complete combustion, Vitiation
- Installation volumeIV
- The total internal volume of the section of installation being tested or purged, made up of the meter, the pipework and the fittings. It decides whether IGEM/UP/1B applies (no more than 0.035 m³), the permitted pressure drop on an existing installation, and how it is purged.
- Taught in M06 Tightness testing & purgingSee also Purge volume, Tightness test
L
- Landlord's gas safety checkalso annual gas safety check, landlord's gas safety record, LGSR
- The safety check a landlord must have carried out on each gas appliance and flue in a rented home, by a Gas Safe registered business, within 12 months of installation and then at least every 12 months. It includes at least the regulation 26(9) checks. The landlord keeps the record until two further checks have been done, and gives tenants a copy.
- Taught in M01 Gas safety legislation & the industrySee also Gas Safety (Installation and Use) Regulations 1998, Responsible person
- Leak detection fluidLDF
- A proprietary fluid applied to joints and valves to check them for gas escapes. It must comply with BS EN 14291 and suit the materials it is used on. Fluids containing ammonia must not be used on copper or brass, or ones with more than 30 parts per million of halogens on stainless steel. Washing-up liquid and soap solutions aren't suitable. It is removed after use so it doesn't cause corrosion.
- Taught in M06 Tightness testing & purgingSee also Tightness test
- Let-by
- Gas passing a valve that is meant to be closed, because the valve isn't sealing. On natural gas it is checked for at the start of a tightness test. The gauge is watched for a minute with the valve closed, and any perceptible rise means the valve may be letting by.
- Taught in M06 Tightness testing & purgingSee also Tightness test, Emergency control valve
- Lock-up
- When no gas is being used, a regulator's outlet pressure rises slightly until the valve closes fully. The pressure at which it closes is the lock-up pressure. On a domestic natural gas meter regulator it should be no more than 30 mbar.
- Taught in M04 Gas supply, meters & regulatorsSee also Meter regulator, Standing pressure
- Lower explosive limitLELalso lower flammability limit
- The lowest concentration of gas in air that can burn. For natural gas, 100% LEL is 5% gas in air.
- Taught in M02 Properties of gas & combustionSee also Flammability limits
M
- Main protective bondingalso main equipotential bonding, earth bonding
- A conductor connecting metal gas pipework to the building's main earthing terminal, so that no dangerous voltage can build up between the pipework and other earthed metalwork. It is connected to the rigid pipework after the meter, as near as practicable to where the pipe enters the premises and before any branch.
- Taught in M07 Equipotential bonding & electrical safetySee also Temporary continuity bond
- Manometeralso U-gauge, pressure gauge
- An instrument for measuring gas pressure, either a water-filled U-gauge or an electronic gauge. It is connected to the installation or appliance at a test point.
- Taught in M03 Pressure, flow & measurementSee also Test point, Millibar
- Meter asset managerMAM
- An organisation working for the gas supplier or meter owner. It makes sure meter installations are designed, installed, commissioned, maintained and removed by suitably qualified people, to industry standards and the law.
- Taught in M04 Gas supply, meters & regulatorsSee also Primary meter installation
- Meter inlet valveMIV
- A valve at the inlet of the meter on installations with a supply pressure above 75 mbar, after the regulator. It is the supply control valve for tightness testing those installations. If it is letting by, the meter asset manager or gas supplier is told, and only someone working on their behalf may repair it. Some older medium pressure installations were fitted without one.
- Taught in M06 Tightness testing & purgingSee also Supply control valve, Emergency control valve
- Meter regulatoralso governor
- The regulator just upstream of the primary meter. It reduces the varying pressure in the service pipe to a steady pressure for the installation. Once installed it is sealed, and only the gas transporter, or someone authorised to act for them, may break the seal to adjust it.
- Taught in M04 Gas supply, meters & regulatorsSee also Operating pressure, Lock-up, Droop, Diaphragm
- Millibarmbar
- The unit used for gas pressures in domestic installations. One millibar is a thousandth of a bar, or 100 pascals.
- Taught in M03 Pressure, flow & measurementSee also Manometer
N
- Normative document
- A standard or code of practice that sets out an accepted way of meeting the law, such as a British Standard, an IGEM standard or the manufacturer's instructions. Following the current normative documents is how an engineer shows the work is safe.
- Taught in M01 Gas safety legislation & the industrySee also Gas Safety (Installation and Use) Regulations 1998
- Not to Current StandardsNCS
- A description for an installation or appliance that doesn't meet current standards but isn't unsafe. The current Gas Industry Unsafe Situations Procedure classifies unsafe situations only as Immediately Dangerous or At Risk, so this isn't one of its categories. CCN1 still expects you to explain how you would deal with such situations.
- Taught in M14 Unsafe situations & emergenciesSee also Immediately Dangerous, At Risk
O
- Open-flued appliancealso Type B appliance
- An appliance that takes its combustion air from the room and removes its products through a flue. It depends on the room's ventilation and on the flue drawing properly.
- Taught in M08 VentilationSee also Room-sealed appliance, Draught diverter, Spillage
- Operating pressure
- The pressure in the system under normal operating conditions, with gas flowing (IGEM/G/13), also called working pressure. At the meter outlet of a domestic natural gas installation it is nominally 21 mbar. With all the appliances at high load it should be between 18.5 and 23 mbar after a minute to settle; outside that range it is reported to the National Gas Emergency Service.
- Taught in M03 Pressure, flow & measurementSee also Standing pressure, Working pressure, Meter regulator
P
- Perceptible movementalso no perceptible movement, NPM
- The pass test for a let-by test and for a new installation. On a water gauge, a movement of 0.25 mbar or less can't be seen, so any visible movement fails. On an electronic gauge, the limit is 0.25 mbar, or 0.2 mbar on a gauge that reads to one decimal place.
- Taught in M06 Tightness testing & purgingSee also Tightness test, Let-by, Manometer
- Pliable corrugated stainless-steel tubingCSSTalso PCSST, pliable tubing, PLT
- Gas pipe made of corrugated stainless steel that can be bent by hand a limited number of times, with a protective cover fitted by the manufacturer. It is made to BS EN 15266, and its tubing and fittings must come from the same manufacturer. Its fittings are mechanical joints, so they must stay accessible, and its cover doesn't count as a sleeve.
- Taught in M05 Installation pipework & fittingsSee also Sleeve
- Pressure lossalso pressure drop
- The fall in pressure as gas flows along pipework and through fittings. For natural gas, pipework from the outlet of the meter installation to each appliance is designed to lose no more than 1 mbar at the design flow rate, so appliances receive enough gas (the limit for LPG is 2 mbar).
- Taught in M03 Pressure, flow & measurementSee also Working pressure
- Primary meter
- The meter nearest to, and downstream of, the service pipe. It measures the gas the supplier provides, and its reading is the basis of the gas bill. It can't be a prepayment meter if gas passes through it to a secondary meter.
- Taught in M04 Gas supply, meters & regulatorsSee also Secondary meter, U6 meter
- Primary meter installationalso meter installation
- The primary meter and the parts that go with it, such as the meter regulator, flexible connector, valves, pipework, fittings and support. In a home it runs from the ECV outlet to the meter outlet connection or outlet adaptor, and is the meter owner's responsibility.
- Taught in M04 Gas supply, meters & regulatorsSee also Primary meter, Meter regulator, Meter asset manager
- Protected area
- A ventilated, fire-resisting part of a building, such as a protected stairway, corridor or lobby used as an escape route, or a service shaft. Gas pipework in one must be steel with screwed or welded joints, a continuous length of copper, or continuous CSST that has passed the required fire test.
- Taught in M05 Installation pipework & fittingsSee also Pliable corrugated stainless-steel tubing
- Purge volumePV
- The volume of gas that must be passed through an installation to make the mixture in the pipe safe. It is one and a half times the installation volume. When gas is being purged out of an installation, it is the volume of air to pass through instead.
- Taught in M06 Tightness testing & purgingSee also Installation volume, Purging
- Purging
- Passing gas through pipework to drive out the air in a controlled, safe way before appliances are lit (or, when taking an installation out of use, passing air through to drive out the gas). The purge volume is one and a half times the installation volume.
- Taught in M06 Tightness testing & purgingSee also Tightness test
R
- Relative densityRDalso specific gravity
- The density of a gas compared with air, which is taken as 1. Natural gas has a relative density of about 0.6, so escaping gas rises.
- Taught in M02 Properties of gas & combustionSee also Wobbe index
- Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013RIDDOR
- The regulations that require certain incidents to be reported to HSE. For gas, gas transporters and suppliers report incidents where someone died, lost consciousness or was taken to hospital because of their gas. Gas Safe registered businesses report, within 14 days, gas fittings whose design, installation, modification or servicing caused or could have caused such harm through a gas leak, incomplete combustion or poor removal of combustion products.
- Taught in M01 Gas safety legislation & the industrySee also Gas Safety (Installation and Use) Regulations 1998
- Responsible person
- In relation to any premises, the occupier, or anyone with authority at the time to take action about the gas fittings there. In rented property the landlord, and their managing agent, are responsible persons too, and must also be told about any unsafe situation. Under GSIUR the responsible person must not use, or allow the use of, an appliance they know or suspect is dangerous.
- Taught in M14 Unsafe situations & emergenciesSee also Warning notice, Immediately Dangerous, At Risk
- Room-sealed appliancealso Type C appliance, balanced flue
- An appliance whose combustion system is sealed from the room it's in. It takes its combustion air from outside (or from a ventilated space that isn't lived in) and discharges its products directly outside.
- Taught in M09 Chimneys & fluesSee also Open-flued appliance
S
- Secondary meter
- A meter other than the primary meter, used to measure gas that one person provides for another, for example a landlord recharging tenants. It needs an AECV close to its inlet, a label, and a line diagram at the primary meter showing how the meters are connected.
- Taught in M04 Gas supply, meters & regulatorsSee also Primary meter, Additional emergency control valve
- Service pipealso service
- The pipe that brings gas from the main in the street to a property. In law it runs from the main to the outlet of the first emergency control valve, and it is the gas transporter's responsibility.
- Taught in M04 Gas supply, meters & regulatorsSee also Gas transporter, Emergency control valve
- Sleeve
- A gas-tight, corrosion-resistant tube built into a wall or floor, through which a gas pipe passes and can be withdrawn. It runs the full thickness of the wall or floor with no joints, is sealed to the building at both ends, and the gap around the pipe is sealed at one end with a flexible fire-resistant compound. A factory coating on a pipe doesn't count as a sleeve.
- Taught in M05 Installation pipework & fittingsSee also Pliable corrugated stainless-steel tubing
- Spillage
- Products of combustion coming back into the room from an open-flued appliance instead of going up the flue. It can be caused by a blocked flue, poor ventilation or extract fans. A spillage test checks for it with the appliance running, doors and windows closed and any fans switched on, using a smoke match as the manufacturer's instructions describe.
- Taught in M09 Chimneys & fluesSee also Draught diverter, Flue flow test
- Standing pressure
- The pressure in the installation when no gas is flowing, with all appliances off, so the regulator has locked up.
- Taught in M03 Pressure, flow & measurementSee also Operating pressure, Working pressure, Lock-up
- Supply control valve
- In a tightness test, the valve immediately upstream of the section being tested, which is turned on and off during the test. On a natural gas supply of up to 75 mbar it is the ECV or AECV. Above 75 mbar it is the meter inlet valve, with the ECV left open.
- Taught in M06 Tightness testing & purgingSee also Emergency control valve, Additional emergency control valve, Meter inlet valve, Let-by
T
- Temporary continuity bond
- A cable with clamps fitted across a section of metal pipework before it is cut or a meter is removed, to keep the electrical path continuous while the work is done. The law requires one wherever it's needed to prevent danger, and BS 6891 says to use one every time, whatever permanent bonding is fitted.
- Taught in M07 Equipotential bonding & electrical safetySee also Main protective bonding
- Test pointalso pressure test point
- A small sealed nipple on the installation or an appliance where a manometer can be connected. Its screw must be closed and the joint checked for tightness after use.
- Taught in M03 Pressure, flow & measurementSee also Manometer
- Thermocouple
- Two different metals joined at a tip. Heated by the flame, it produces a small voltage that holds a magnetic gas valve open; if the flame goes out, the valve closes.
- Taught in M11 Gas safety devices & controlsSee also Flame supervision device
- Tightness test
- A pressure test that shows whether an installation is gas-tight. The installation is brought to test pressure, allowed to stabilise, then watched for a set time for any pressure drop. A new installation must show no perceptible drop. An existing one with appliances connected may show a small drop within a limit set by its installation volume, but it is then retested with the appliances isolated and must show no perceptible drop. A smell of gas is always a fail.
- Taught in M06 Tightness testing & purgingSee also Let-by, Purging, Installation volume, Perceptible movement
U
- U6 meteralso G4 meter, diaphragm meter
- The standard domestic diaphragm gas meter, able to pass up to 6 m³ of gas an hour. It is also called a G4 meter, and its serial number starts with G4. The electronic meter of the same size is called an E6.
- Taught in M04 Gas supply, meters & regulatorsSee also Meter regulator, Primary meter
V
- Vitiationalso vitiated air
- Air that has lost oxygen because products of combustion have mixed into it. Burning in vitiated air can lead to incomplete combustion.
- Taught in M11 Gas safety devices & controlsSee also Atmosphere-sensing device, Incomplete combustion
W
- Warning notice
- The written notice an engineer completes and leaves with the gas user or responsible person when they find an Immediately Dangerous or At Risk situation. It records what is unsafe, the category, what was done and what needs doing, and is signed by the gas user as a record that they received it. The engineer keeps a copy.
- Taught in M14 Unsafe situations & emergenciesSee also Immediately Dangerous, At Risk, Responsible person
- Wobbe indexalso Wobbe number
- The calorific value of a gas divided by the square root of its relative density. Gases with the same Wobbe index give the same heat input through the same burner at the same pressure.
- Taught in M02 Properties of gas & combustionSee also Calorific value, Relative density
- Working pressure
- Another name for operating pressure, the pressure while gas is flowing. IGEM/G/13 treats the two terms as the same, and IGEM/IG/1 Supplement 2 uses "working pressure" for the meter outlet. Others use it for the pressure at an appliance's inlet while it runs, which shows whether enough gas is getting through the pipework. Always say where it was measured.
- Taught in M03 Pressure, flow & measurementSee also Operating pressure, Pressure loss