ACS
AcronymAccredited Certification Scheme
The certification route by which gas operatives prove ongoing competence; reassessment is normally every five years.
Accredited Certification Scheme
The certification route by which gas operatives prove ongoing competence; reassessment is normally every five years.
The CO/CO₂ ratio at which you must act. Where no manufacturer's figure applies, BS 7967 sets it at 0.0040 for flued appliances (0.008 for flueless). At or below the level combustion is acceptable; above it the appliance is not safe to leave in normal use.
The CO/CO₂ ratio above which combustion is unacceptable. Where no manufacturer figure applies, BS 7967 sets it at 0.0040 for a flued appliance (0.0080 for a flueless one); above it, make safe and classify under GIUSP.
Natural air infiltration through the building fabric. Taken as about 35 cm², assumed to cover the first 7 kW net of an open-flued appliance.
Additional Emergency Control Valve
Fitted where the ECV is remote from the building, at the point the supply enters, so the occupier can isolate quickly.
How a burner gets its combustion air. A pre-aerated (aerated) burner mixes primary air with the gas before the flame, giving a crisp blue flame; a post-aerated burner takes all of its air (secondary air) at the flame itself, giving a softer, luminous flame as on many cookers and live-fuel-effect fires.
A visible gap where the condensate pipe joins an external drain or downpipe, preventing sewage or water backing up into the boiler if the drain freezes, blocks or floods — unless the boiler trap already has the specified seal.
A safety switch on a fanned-flue appliance that confirms the fan is producing flow. The safety circuit proves the switch is open (at rest) before start-up, so a switch stuck closed cannot let the appliance fire without proven draught.
A valve on many modern modulating boilers that keeps the air-to-gas ratio correct across the firing range. Adjust it only per the MIs and with a calibrated analyser — never by guesswork — and confirm the result by combustion analysis.
The landlord's legal duty under GSIUR to have every gas appliance and flue they provide checked for safety at least every 12 months by a Gas Safe registered engineer. It confirms operating pressure or heat input, combustion-air (ventilation) provision, flue effectiveness and safe operation, and is recorded on a Landlord Gas Safety Record (LGSR).
The gap between a flexible flue liner and the old chimney around it. At the base it must be sealed; an unsealed annulus lets cold air chill the liner and stall the draught, causing spillage.
A protective mechanism in electronic token, smart-prepayment or Quantum meters. Dropping the pressure during a test can trip it and lock off the supply until the energy supplier resets it, so plan tightness tests on these meters carefully.
HSE-approved guidance that supports a set of regulations. For GSIUR the ACOP and guidance is published as L56. Following an ACOP is not itself compulsory, but if you are prosecuted a court can treat a failure to follow it as evidence that you breached the regulation.
At Risk
Unsafe-situation classification under IGEM/G/11 where one or more faults could be a danger; turned off with permission and labelled.
A hazardous fibrous material found in some older building fabric and flue components. If you suspect it (for example in old flue pipework), keep it intact, double-bag it in heavy-duty plastic, use the right PPE and dispose of it through a licensed route — never break or sand it.
Atmosphere Sensing Device
Also called an oxygen-depletion device; shuts an appliance down before combustion products build up in the room.
Atmospheric Sensing Device / Oxygen Depletion System
A safety device that shuts an open-flued appliance down if the room's oxygen falls (the air vitiates); required for an open-flued appliance under 14 kW installed in a bedroom or bath/shower room.
A gas that harms by displacing oxygen rather than by poisoning. Carbon dioxide (CO₂) acts mainly as an asphyxiant in quantity; in CPA1 it serves as the reference against which CO (a toxic product of incomplete combustion) is judged.
Checking the air in a dwelling for CO/CO₂ — for example after a CO alarm activates or fumes are reported. Wear a personal audible CO alarm, check the air is safe before other work, sample at breathing height (about 1.5–2 m), and run a sweep test for at least two minutes.
A valve that keeps a minimum water flow when TRVs and zone valves close, protecting the pump and heat exchanger; fitted where the MIs require it.
A boiler fitted behind a gas fire within a builder's opening, historically providing heating and hot water (BS 5871-1, fire/back boilers). Now largely obsolete but still found during HTR1 work.
A sealed housing that lets an open-flued appliance behave like a room-sealed one. It takes its combustion air from a point adjacent to the chimney terminal through an air duct of at least 7.5 cm²/kW net, behind a draught-proofed, self-closing door interlocked with the appliance (with a 'keep shut' notice). Where a compartment side is within 75 mm of the appliance's hot surfaces, fire protection gives a half-hour delay.
A natural-draught room-sealed (Type C) flue whose air inlet and products outlet sit in the same pressure zone at one terminal, so wind affects both equally.
A self-sealing push-and-twist connector for a cooker flexible hose; its valve seals automatically when the cooker is unplugged. It must lock fully home.
The commissioning checklist / log book completed and signed at boiler commissioning and left with the customer; supports the warranty.
A control-wiring arrangement (room/cylinder thermostats and motorised valves) that stops the boiler firing with no heat demand. TRVs alone do not provide it.
An England Building Regulations standard (from 2018) setting minimum efficiency and controls for domestic boiler installs — e.g. a combi must have time and temperature control plus one efficiency measure (such as weather or load compensation, flue gas heat recovery, or smart controls). Confirm current requirements.
Flueing / Ventilation standard
Part 1 covers flueing and Part 2 ventilation for gas appliances of rated input not exceeding 70 kW net.
Gas fires & space heaters standard
Installation of fires, convector heaters, fire/back boilers and heating stoves (-1), ILFE (-2) and DFE (-3) appliances.
Cooker installation standard
Installation, servicing and maintenance of domestic gas cooking appliances.
The standard for larger/commercial gas installations. A single domestic boiler up to 70 kW net is covered by BS 6798, but a cascade of boilers whose aggregate input exceeds 70 kW falls under BS 6644, even if each individual boiler is domestic-sized.
Cooker hose & socket standard
The primary standard for domestic flexible hoses, end fittings and self-sealing (bayonet) sockets connecting a movable appliance such as a cooker; BS EN 14800 covers the corrugated stainless-steel safety hose variant.
Boiler installation standard
Installation of gas-fired hot water boilers of net input not exceeding 70 kW.
Installation pipework standard
Covers low-pressure gas installation pipework of up to 35 mm in domestic premises.
Water treatment standard
Cleaning, flushing and dosing of heating-system water with a corrosion inhibitor.
Combustion analysis standard
Use of combustion gas analysers and the CO/CO₂ ratio for assessing domestic appliances; basis of CPA1.
The standard for domestic corrugated stainless-steel safety hose assemblies connecting a movable appliance such as a cooker. The rubber flexible hose and self-sealing bayonet socket are covered by BS 669-1.
CO alarm standard
The performance standard for domestic CO alarms; at 100 ppm an alarm should sound between 10 and 40 minutes.
The European standard that an electronic combustion gas analyser (ECGA) used for CPA1 work should conform to. It specifies the performance of portable instruments measuring CO and CO₂ in flue gas.
The gas pressure at the burner test point, set to the appliance data-plate figure. On its own it does not prove the correct volume is burning — a blocked or wrong injector can change the rate without changing the pressure — so it is cross-checked by gas rating.
Comparing an analyser against certified reference gas of known concentration and correcting any drift. Required at least annually (and per the manufacturer); a reading from an out-of-calibration analyser is not valid.
soldered joint
A copper joint made by capillary action drawing solder into the fitting. It is permanent, so unlike a compression joint it may be concealed — but you never solder close to a meter without isolating it first.
A type of inset live fuel-effect (ILFE) gas fire with an outer firebox built into a purpose-made opening, normally fitted to a lined chimney. Some accept an optional flue kit specified by the manufacturer.
A flueless space heater that burns gas at low temperature on a catalyst pad; needs room ventilation and an atmosphere-sensing device (ASD).
A debris-collection space of specified volume below a fire fitted to a chimney, so falling debris cannot block the appliance.
The air pre-charge on the dry side of a diaphragm expansion vessel, set to match the system's cold fill pressure. If a sealed system's pressure relief valve lifts every time the boiler heats, the usual cause is an expansion vessel that has lost its charge or is undersized — re-pressurise or resize it rather than blaming the valve.
A permanent plate fixed in an accessible position recording a chimney's size, construction and lining, so its suitability for a gas fire can be checked later. Asbestos-cement must not be used for a new chimney, and raw poured-in-situ concrete is not a recognised lining.
A classification of masonry or lined chimneys by size. A Class 1 chimney is a full-size traditional chimney (around 175 mm / 7 in diameter); a Class 2 chimney is a smaller lined or precast flue (around 125 mm / 5 in). Open-flued fires such as DFEs must be matched to a suitable chimney class with adequate catchment.
The minimum gap that must be kept between a cooker's burners/hot surfaces and any combustible material, set by BS 6172 and the manufacturer's instructions. Typically there must be adequate clearance above a hob to a combustible surface or cooker hood (commonly at least 650 mm — verify the current standard and MIs), and a hob must not be fitted hard against a combustible side wall without the required side clearance.
A sealed plate closing a builder's opening behind a fire, providing the flue connection and any specified relief opening.
Carbon monoxide
Toxic, colourless, odourless product of incomplete combustion; binds to haemoglobin and gives no sensory warning.
A household electrical carbon monoxide alarm to BS EN 50291. It must give an audible warning — at 100 ppm it is designed to sound within 10–40 minutes (faster at higher levels). It is not the engineer's analyser, and a passive colour-change indicator card is not a substitute and is unsuitable for sleeping areas.
A carbon monoxide sensor in a concealed flue void wired to shut off the boiler if it detects CO. It is the recognised safeguard where a concealed room-sealed flue cannot be given inspection access.
Combustion performance ratio
The key indicator of combustion quality; action level 0.0040 in BS 7967 where no manufacturer's figure applies. Largely unaffected by air dilution.
Provides both central heating and instantaneous domestic hot water from one unit, usually mains-fed with no storage cylinder.
The fresh air an appliance needs to burn its fuel completely. It is supplied by purpose-provided ventilation (and, up to small inputs, by adventitious air). Too little combustion air causes incomplete combustion and carbon monoxide, and it must not be drawn from a bathroom.
A shared flue serving several dwellings. CFS(NV) is naturally ventilated; CFS(PP) is a fan-assisted positive-pressure manifold. A loose or uncapped connection is Immediately Dangerous.
Where an appliance is enclosed (e.g. an airing cupboard), the compartment needs a low-level vent for combustion air and a high-level vent to remove heat, sized to BS 5440-2 (larger when venting to a room than direct to outside).
A pipe joint sealed by a nut compressing an olive (ring) onto the tube. Because it can be re-tightened, a compression joint may be used only where it stays accessible; it is not permitted in a protected escape route.
A room-sealed flue that runs through a ceiling void, riser or other concealed space. It needs inspection hatches so the joints can be checked; where access is impossible, a CO void-monitoring shut-off is the recognised alternative.
Slightly acidic (pH ≈ 3–6) water from condensing flue gas. Drained in corrosion-resistant plastic, never copper or steel; external runs short, insulated, ≥32 mm.
A small automatic pump that collects condensate and lifts it to the drain where gravity discharge isn't possible (e.g. a basement boiler with the drain above it).
A water seal built into a condensing boiler's condensate outlet that stops flue gases escaping down the drain pipe. Must hold the condensate seal specified by the manufacturer.
A boiler that cools the flue products below their dew point (around 55 C) so the water vapour condenses, recovering its latent heat. Now required for most new and replacement domestic boilers; produces an acidic condensate that must be drained.
Air provided to keep an appliance or its compartment within safe temperature limits, separate from combustion air. A room-sealed appliance in a compartment needs cooling air only; an open-flued one needs both cooling and combustion air.
Council for Registered Gas Installers
The former gas registration body for Great Britain, replaced by the Gas Safe Register on 1 April 2009. Training material that still refers to CORGI is out of date and should be read with that in mind.
A chemical dosed into a heating system (to BS 7593) after cleaning and flushing, to prevent internal corrosion and sludge (magnetite).
Carbon dioxide
Product of complete combustion; an asphyxiant in quantity. Used as the reference in the CO/CO₂ combustion ratio.
Combustion Performance Analysis
The category covering use of a flue gas analyser to assess combustion, to BS 7967.
Combined Primary Storage Unit
A boiler type with a large internal thermal store providing both heating and mains-pressure hot water.
The tendency of a CO sensor (or indicator card) to respond to other substances — aerosol sprays and solvent vapours can cross-sensitise the sensor and give a false reading. Clear the air and re-check; these are not themselves CO sources.
Corrugated Stainless Steel Tube / Pliable corrugated tube
Flexible gas tube permitted internally when installed to its standard, bonded and mechanically protected through walls/floors.
Calorific Value
The heat released per unit of gas. Gross CV includes the latent heat of the water vapour; net CV excludes it.
Controls stored hot-water temperature, set to about 60 C — hot enough to limit Legionella but not so hot as to scald.
A movable plate that can restrict a flue. Any manually operated damper in the flue of a gas appliance must be removed or permanently fixed in the open position, so the flue cannot be accidentally closed off while the appliance runs.
The appliance rating plate, giving make/model, gas type, heat input (gross and/or net), burner pressure and other data needed to commission and check the appliance.
A drop in room pressure — typically caused by an extract fan — that can pull the products of combustion back down an open flue and make the appliance spill. The usual cure is extra air-vent free area (often around 100 cm²) to break the depressurisation; it is checked by the spillage test with all fans running.
The temperature at which water vapour in the flue products begins to condense (around 55 C for natural gas). A condensing boiler runs the return below this to recover latent heat.
Decorative Fuel-Effect appliance
An open-flued gas fire of input ≤20 kW covered by BS 5871-3; burns deliberately yellow and needs a suitable chimney with catchment space.
Domestic Hot Water
The hot water supply to taps and outlets, as opposed to the central-heating circuit.
The mixing of flue products with excess air or room air, which lowers both CO and CO₂ together. Because it affects them equally, the CO/CO₂ ratio is largely unaffected — which is why the ratio, not the raw CO, is used to judge combustion.
The safe discharge route from an unvented cylinder's safety valves. D1 runs from the valve to the tundish; D2 runs from the tundish to a safe, visible termination and is at least one pipe size larger than D1.
The gas pipe running along the street/network from which individual properties are supplied. Gas reaches a property from the distribution main through a service pipe to the meter.
The recognition that not all appliances run at full demand at once, so the simultaneous load can be less than the simple sum. Applied with care in pipe sizing, never as an excuse to undersize.
An open-flue feature that breaks downdraught; the usual combustion sampling point on an open-flued (Type B) appliance.
Sealing gaps around doors, windows and floors to reduce uncontrolled (adventitious) air leakage. Because it cuts natural infiltration, a well-draught-proofed room may need a larger purpose-provided air vent to keep enough combustion air.
Plasterboard fixed to a wall on battens or dabs, leaving a void behind. Gas pipe behind dry-lining must be encased in building material so that a leak cannot track unseen behind the board.
Electronic Combustion Gas Analyser / Flue Gas Analyser
The instrument used for CPA; should conform to BS EN 50379-3, zeroed in fresh air before use.
Emergency Control Valve
The consumer's means of shutting off the gas supply in an emergency; must be accessible and have a permanent notice nearby.
EAWR
The regulations governing electrical safety at work. Regulation 4 requires electrical systems to be constructed and maintained to prevent danger; Regulation 16 forbids working beyond your competence unless properly supervised.
The cell type used in most analysers for CO and O2. It reacts chemically with the gas to give a small electrical signal, and ages/drifts over a few years whether used or not — hence periodic calibration.
The measured pipe length plus an allowance for each fitting (elbows, tees, valves) expressed as extra metres of pipe. Used with the BS 6891 tables to check the pressure drop stays within 1 mbar.
ErP (Energy-related Products) energy labelling rates and labels boiler efficiency; it replaced the older SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) rating.
Extra air, or room air leaking into a sample, that lowers CO and CO₂ together. The CO/CO₂ ratio is used because it largely cancels this dilution.
A diaphragm vessel that absorbs water expansion in a sealed heating system (BS 7074).
Feed and Expansion cistern
On an open-vented system, the cistern at the highest point that tops up the system and takes up expansion.
A room-sealed flue in which a fan moves the combustion products, allowing longer flue runs and (per the MIs) smaller terminal clearances than a natural-draught flue.
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A flexible connector with a double-check valve used to add mains water to a sealed heating system and bring it up to pressure. It is closed (and ideally disconnected) after filling.
The connection used to top up a sealed heating system with mains water, commonly to around 1 bar cold (verify against the MIs). It must prevent backflow into the mains and is typically removed or disconnected after filling.
Sealing the gap where a pipe penetrates a fire-rated wall or floor with an approved material, so the penetration does not reduce the building's fire resistance or let fire and smoke spread between compartments.
Flame cooled below its ignition temperature (by a cold surface or excess air), causing incomplete combustion.
Flame striking a surface (e.g. a cold heat exchanger) before combustion completes, causing CO and soot.
A combustion fault where the flame lifts off the burner ports because gas velocity or aeration is too high, making the flame unstable and risking flame loss.
A fault where the flame lifts away from the burner ports and may go out — caused by too high a burner pressure (high gas velocity) or excess primary air. The opposite fault is light-back.
The appearance of the burner flame, judged against the MIs. DFE and ILFE fires are designed to burn yellow and luminous, unlike most appliances where yellow tipping warns of incomplete combustion.
An electronic flame-detection method used in many modern appliances: a flame conducts a small one-way (rectified) current via a probe, which the controls sense to keep the gas valve open. Faster to respond than a thermocouple.
A burner feature (retention ports/pilot tabs) that anchors the flame and stops it lifting off at higher rates or in draughts.
The flexible connection between a movable cooker and its bayonet point. It must be the correct type (BS 669-1, or a corrugated safety hose to BS EN 14800), in date, undamaged, and routed in a loop that keeps it clear of the oven flue and hot surfaces while letting the cooker be pulled out without strain. An old, kinked or perished hose restricts flow and is a leak and fire risk.
A sheet-metal box to BS 715 used to flue an open-flued gas fire where there is no existing chimney; the fire connects to it and a flue pipe carries the products to a terminal.
A smoke pellet proves the empty flue draws correctly and is unobstructed along its length, before the appliance is connected.
A flexible or rigid liner inserted into a masonry chimney to give a sound, correctly sized passage for an appliance's products.
The external fitting where a flue discharges products (and, on a room-sealed appliance, draws in air). Its position must meet BS 5440-1 clearances.
The European classification of appliance flue types. Type B (e.g. B11) is open-flued, drawing combustion air from the room — B11 is natural-draught with a draught diverter. Type C (e.g. C11 natural-draught, C32 fanned vertical, C7/VERTEX) is room-sealed, taking air from and discharging to outside through a sealed system.
A purpose-made point for inserting the analyser probe. On a room-sealed (Type C) appliance use the integral flue-gas sampling point or sample as the MIs direct; on an open-flued (Type B) appliance the usual point is the draught diverter.
A safety interlock used where a fan sits in the secondary flue of an open-flued system: it proves the fan is actually moving the products before the appliance is allowed to fire. Distinct from a boiler/ventilation interlock.
Discharges products into the room (e.g. a cooker); relies on room-volume ventilation and, on heaters, an ASD.
Cleaning a heating system to clear installation debris, flux and existing sludge before a new boiler is fired and the inhibitor is dosed. Power-flushing uses a pumped machine; it protects the new heat exchanger from blockage.
The actual unobstructed open area of a ventilator that air can pass through (less than its overall size). Ventilation requirements in BS 5440-2 are stated as a free area in cm².
Running an analyser in clean air — on power-up to zero the sensors, and before switching off to clear flue gas from them. Protects sensor life and sets the baseline.
A frost thermostat (often with a pipe thermostat) wired to bring on the boiler and pump in freezing conditions regardless of the programmer or room thermostat.
Flame Supervision Device
Holds the gas valve open only while it senses the flame; shuts the gas off on flame loss. Often thermocouple-based.
The artificial coals, pebbles, logs or ceramic radiants of a DFE or ILFE fire. They must be positioned exactly to the manufacturer's layout — incorrect placement disturbs combustion and can cause CO.
natural gas
The reference natural gas for UK domestic appliances — second-family gas G20, supplied at a nominal 21 mbar. Appliances are set up and tested against G20 figures in the manufacturer's instructions (LPG is third-family gas).
The competency required to install unvented hot-water storage systems under the Building Regulations — separate from gas registration.
The Building Regulations competency required to install an unvented (mains-pressure) hot-water storage cylinder. It is a separate competency from the gas work itself, reflecting the stored-energy hazard of mains-pressure hot water.
Fuel gases are grouped into families: 1st family (manufactured gases), 2nd family (natural gas) and 3rd family (LPG — propane and butane). UK domestic work is mostly 2nd and 3rd family.
Volume of gas used over time. Rate (m³/h) = (volume ÷ time in seconds) × 3600. Gross kW = m³/h × CV ÷ 3.6.
The official gas registration body for Great Britain, the Isle of Man and Guernsey. It replaced CORGI on 1 April 2009. Under GSIUR, anyone carrying out gas work for reward must be registered, and the public can check an engineer's registration and the appliance categories they hold.
The organisation that conveys gas through the network of pipes to a property (and runs the national gas emergency service). When a transporter first supplies gas to premises that contain an appliance which has not been commissioned, it must commission the appliance or seal it off.
Gas Industry Unsafe Situations Procedure
The procedure (IGEM/G/11) for classifying and acting on unsafe situations: ID and AR only.
Gross vs net calorific value
Gross counts the latent heat of water vapour in the products; net excludes it (vapour leaves as steam in a non-condensing appliance). For NG, net ≈ gross × 0.901.
The HSE guidance note for electrical test equipment. GS38-compliant test leads and probes are fused and finger-guarded with only a minimal exposed metal tip, to protect you when testing on or near live mains.
Gas Safety (Installation and Use) Regulations 1998
The principal law governing gas fittings and appliances in Great Britain, supported by the Approved Code of Practice and guidance L56.
A non-combustible base of defined size beneath a gas fire that protects the floor and combustibles from heat and debris (MIs / BS 5871).
The component that transfers heat from the burner products to the water; in a combi a plate heat exchanger heats the domestic hot water. Vulnerable to scale and sludge.
The energy entering an appliance per unit time (kW), found by gas rating: kW gross = gas rate (m³/h) × CV (MJ/m³) ÷ 3.6. Modern data plates usually quote net input; for NG, net ≈ gross × 0.901.
A device fitted to some cookers that shuts off the gas to the hob burners when the lid is closed, so gas cannot be left running under a closed lid.
Health and Safety Executive
The regulator that approves the gas registration body (currently Gas Safe Register) as the recognised 'class of persons'.
Health and Safety at Work etc. Act 1974
The enabling Act sitting above GSIUR; gas-specific duties flow from it.
Immediately Dangerous
Unsafe-situation classification under IGEM/G/11 for an installation that is, or could soon be, a danger to life or property.
Institution of Gas Engineers and Managers
Publishes key gas standards such as IGEM/UP/1B (tightness/purging) and IGEM/G/11 (unsafe situations).
Unsafe situations procedure
The current edition uses two classifications only — Immediately Dangerous (ID) and At Risk (AR).
The IGEM tightness-testing procedure for larger installations — used once the pipe exceeds 35 mm copper / DN32 steel or the installation volume exceeds 0.035 m³. Smaller domestic work uses IGEM/UP/1B.
Tightness testing & direct purging procedure
The procedure for tightness testing and purging small domestic-sized installations. Edition 4 sets permissible drop by Installation Volume (IV) and is mandatory from 1 October 2026 (Edition 3 withdrawn 30 September 2026).
Inset Live-Fuel-Effect fire
A gas fire covered by BS 5871-2; also designed to burn with a luminous decorative flame.
Combustion with too little air, or where the flame is disturbed, producing carbon monoxide (CO) along with soot and water. The three classic causes are lack of air (including vitiated, oxygen-depleted air), flame chilling and flame impingement. Complete combustion, by contrast, produces only carbon dioxide, water vapour, nitrogen and heat.
The precisely-sized orifice (jet) that meters the flow of gas into a burner. On a pre-aerated burner the gas jet leaving the injector also draws in primary air as it enters the mixing tube.
The drilled orifice that meters gas into the burner. Its size sets the gas rate at a given pressure; a partially blocked or incorrect injector changes the rate even when the burner pressure still reads correctly.
The volume of the pipework under test; used in IGEM/UP/1B Edition 4 to determine the permissible pressure drop.
A water heater that heats water on demand as it flows through, with no storage vessel (e.g. a combi's hot water, or a single-point/multipoint heater).
The heat released when water vapour condenses back to liquid. Recovering it is what makes a condensing boiler more efficient.
Leak Detection Fluid
An approved bubble fluid brushed onto joints to reveal escaping gas; used with the gas on across the ECV-to-regulator joints after a medium-pressure tightness test.
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Bacteria that can grow in stored water below about 60 C and cause Legionnaires disease; the reason hot-water cylinders are stored around 60 C.
Lower / Upper Explosive Limit
The flammable range of gas in air. For natural gas, roughly 5% (LEL) to 15% (UEL) gas in air. Also called LFL/UFL.
Checks the isolation valve/ECV is not passing gas into the test section; the only acceptable result is no rise in pressure.
Landlord Gas Safety Record
The annual safety-check record a landlord must complete and give to tenants (within 28 days / before occupation).
flashback
A fault where the flame burns back inside the burner towards the injector — caused by too low a burner pressure, blocked or damaged burner ports, or a flame speed higher than the gas velocity. It often gives a roaring or popping flame. The opposite fault is flame lift-off.
A combustion fault where the flame burns back inside the burner toward the injector — often from too low a gas rate or damaged ports — heard as a roar and able to damage the burner.
Liquefied Petroleum Gas
Propane or butane, supplied from a storage vessel; heavier than air, so it sinks into low points and drains.
An in-line filter on the heating return that captures magnetite (iron-oxide) sludge to protect the boiler and maintain efficiency. Recommended at installation under BS 7593.
Black iron-oxide sludge formed by internal corrosion; it blocks heat exchangers and radiators. Removed by flushing and prevented by an inhibitor.
Earthing the consumer's internal pipe within 600 mm of the meter outlet, before any branch (BS 6891 / BS 7671).
An instrument that measures gas pressure (e.g. operating or burner pressure), calibrated at least every 12 months. It measures pressure, not combustion — that is the analyser job.
Millibar
Pressure unit used in gas work. NG nominal at the meter ≈ 21 mbar; permissible installation drop to the appliance ≈ 1 mbar.
miniature circuit breaker
A resettable protective device that trips on overload or short circuit, replacing a rewireable fuse. It does not protect against electric shock the way an RCD does.
A valve or other device that lets the gas supply to an appliance be turned off. Every appliance should, where practicable, have a means of isolation at its inlet; a remote outdoor appliance needs an accessible control where the pipe leaves the building.
CH₄
The main constituent of natural gas (chemical formula CH₄). It is lighter than air (relative density about 0.6) so it rises and collects high up, and its flammability limits in air are roughly 5–15%.
Manufacturer's Instructions
The maker's installation/servicing instructions; always take precedence over the British Standard fall-back.
Medium-Pressure Isolation Valve
An isolation valve fitted upstream of the regulator on a medium-pressure meter installation.
The ability of a boiler to vary its output up and down to match demand, rather than simply cycling full-on/full-off. Improves comfort and efficiency and keeps the boiler condensing for longer.
Maximum Operating Pressure
The highest pressure at which a system is designed to run continuously; above about 75 mbar a domestic supply is treated as medium pressure.
A valve driven by a small motor that opens or closes a heating zone (or the hot-water circuit) on a signal from a thermostat or programmer. In S-plan and Y-plan systems these valves are part of the boiler-interlock wiring that stops the boiler firing with no demand.
An electrically operated 2-port or 3-port valve that opens or closes a heating/hot-water circuit on demand from a thermostat or programmer — a key part of achieving boiler interlock.
Medium Pressure / Low Pressure
Medium pressure is above 75 mbar; a regulator reduces it to the domestic operating pressure of about 21 mbar.
A single valve body that combines several controls — typically a governor (regulator), a flame supervision device and a solenoid — used on many modern appliances.
An instantaneous gas water heater that supplies hot water to several outlets from one unit, as opposed to a single-point heater serving one tap.
The free, 24-hour line to report a gas escape, a suspected carbon monoxide problem or a gas emergency in Great Britain: 0800 111 999. The operator can send an emergency engineer who has legal powers to make the situation safe, including disconnecting the supply.
Not to Current Standards
A former category withdrawn in 2015; non-compliances are still advised to the customer but NCS is no longer a formal class.
Non-Dispersive InfraRed
A sensor type required for measuring ambient room CO₂ — calculated-CO₂ flue analysers are not suitable for ambient air.
Natural Gas
Mains gas, mainly methane (CH₄), lighter than air. Nominal operating pressure ≈ 21 mbar at the meter.
voltage stick
A 'voltage stick' that indicates the presence of mains voltage without contact. Use it to prove an appliance's metalwork or pipework is not live before you touch it, because a fault elsewhere can make exposed metal live. It is a presence indicator, not a substitute for a GS38 test when working on live mains.
A valve that allows flow in one direction only. On a positive-pressure communal flue (CFS(PP)), every appliance exhaust needs a non-return valve so products cannot be pushed back into other dwellings when that appliance is off.
Oxides of nitrogen
Formed at high flame temperatures; a small part of the flue products alongside CO₂, water vapour and nitrogen.
tightness observation period
The timed part of a tightness test during which the gauge is watched for any drop (commonly about two minutes for a domestic natural-gas installation — confirm against the current IGEM/UP/1B). 'No perceptible movement' is taken as about 0.25 mbar on a water gauge or 0.2 mbar on an electronic gauge.
The pipe in an open-vented system that rises from the system and discharges over the feed-and-expansion cistern, giving heat and pressure a permanent safe escape path. It must never be valved or blocked.
Takes combustion air from the room and discharges via a flue; needs room ventilation and a spillage test.
A heating system that takes up expansion through a feed-and-expansion cistern at the high point plus an open vent (safety) pipe. It admits air, so is more corrosion-prone than a sealed system.
An opening window or door to outside required in any room with a flueless appliance such as a cooker, so combustion products can clear. The requirement scales with room volume — over 10 m³ generally needs only the openable window, while smaller kitchens also need a permanent air vent (verify against the current BS 5440-2).
The standing gas pressure supplied to the appliance — nominally 21 mbar at the meter for NG (acceptable about 19–23 mbar). BS 6891 limits the drop from meter to appliance to 1 mbar (2.5 mbar for LPG).
O₂
The gas (20.9% of fresh air) whose depletion in the flue stream most analysers measure in order to calculate CO₂. When positioning the probe you seek the highest steady CO₂, equivalently the lowest steady O₂, to confirm you are in the well-mixed products.
The filter that removes soot and dust from the flue-gas sample before it reaches the analyser's sensors. Replaced when dirty; the instrument must never be run without it.
A natural (fan-free) ventilation duct that uses stack effect to extract stale air. Keep a PSV outlet clear of an open-flue chimney outlet so the two don't set up a pressure gradient that pulls products back down the flue.
Polyethylene
The material used for underground (buried) gas service/installation pipe; not used above ground inside buildings.
Gauge readable movement (GRM)
Edition 4's numerical definition of gauge movement: up to 0.25 mbar on a water (fluid) gauge, or 0.2 mbar on a high-resolution electronic gauge, counts as 'no perceptible movement'. Above this on the pipework-only retest is a fail.
The largest pressure fall a tightness test may show and still pass. Under IGEM/UP/1B Edition 4 the figure (with appliances connected) is tied to the installation volume (IV) rather than a single fixed number; pipework alone must show no perceptible movement.
An IGEM/UP/1B Edition 4 step: after a test with appliances connected passes within the permissible drop, the appliances are isolated and the pipework alone is retested — it must show no perceptible movement, or it fails.
A compact stack of thin plates in a combi boiler that transfers heat from the central-heating water to the incoming mains water to make instantaneous hot water. Vulnerable to scaling in hard-water areas, so scale reduction may be needed.
A kit that redirects the visible condensate plume from a condensing flue terminal away from where it would cause a nuisance (BS 5440-1 / MIs).
Protective Multiple Earth
The common UK earthing arrangement where the supplier's combined neutral-and-earth (PEN) conductor splits into separate neutral and earth at the service head. Because earth and neutral share a conductor up to that point, correct main protective bonding of metal services is essential.
Parts per million
Concentration unit. 1% = 10,000 ppm, so to convert a CO reading in ppm to a percentage, divide by 10,000.
A flue formed from concrete or clay blocks built into a wall, common with back boilers and inset fires. Requires careful inspection for cross-leakage, correct route and a suitable terminal.
A flue built into a wall from purpose-made concrete or clay blocks, common with inset and decorative fuel-effect fires. It must be confirmed suitable for the specific appliance, with adequate catchment space, before use.
Press-fit (crimped) joint
A mechanical pipe joint made by crimping a gas-rated fitting; its yellow identification bands must remain visible after pressing.
Air mixed with the gas before the burner ports (pre-aerated burner), giving a crisp blue flame.
On an open-flued appliance, the flue between the appliance outlet and the draught diverter.
The gases produced when gas burns — mainly carbon dioxide, water vapour and nitrogen when combustion is complete, plus carbon monoxide when it is not. A flue's job is to carry the POC safely outside.
The time control that switches heating and hot water on and off to a schedule. Part of a compliant control system and a contributor to boiler interlock.
A control that sets the hours during which heating and hot water are available. A programmer can set heating and hot water independently; a time switch is a simpler single-channel version. Part of the boiler-interlock control arrangement.
A circulation route (such as a communal hallway or stairwell) that must stay usable in a fire. Gas pipe there may use only screwed or welded steel joints — no compression, capillary or push-fit — and any duct is vented high and low to outside.
Pressure Relief Valve (safety valve)
On a sealed system, typically set at 3 bar; discharges via a pipe to a safe, visible external point.
Purge Volume (PV)
The volume moved when purging an installation of air or gas; commonly taken as 1.5 × the Installation Volume (IV).
Displacing air with gas (or gas with air on decommissioning) so no flammable mixture is left in the pipework. Volume set by Installation Volume in UP/1B Ed 4.
A permanent, non-closable opening built through the structure specifically to supply combustion air — the only ventilation that counts toward an appliance's air requirement.
x
A gas fire that heats by radiation (glowing elements) and/or convection (warmed air). Covered by BS 5871-1.
Adjusting an appliance's heat input within the manufacturer's permitted range. For combustion testing, run a range-rated appliance at its maximum designed heat input (the worst case), then return the operating pressure to the user's as-found setting and re-test.
residual current device
A protective device that trips in milliseconds when it detects an earth-fault imbalance between live and neutral currents, guarding against electric shock. Distinct from an MCB (overload/short circuit) and a fuse (sacrificial).
Relative Density
Density relative to air. Natural gas ≈ 0.6 (rises); LPG ≈ 1.5+ (sinks). This changes how you search for and ventilate an escape.
The GSIUR duty to examine an appliance immediately after working on it. You must check the effectiveness of any flue, the supply of combustion air, the operating pressure or heat input (or both), and that the appliance operates safely. These checks do not apply when an appliance is being permanently removed.
The GSIUR duty on an employer or self-employed person to keep gas fittings, pipework and flues in a safe condition in any workplace under their control. (The landlord's equivalent duty for let homes is set out separately in the Regulations.)
The GSIUR requirement that gas fittings be of good construction and sound material and installed properly. Lead and lead alloys must not be used for gas pipes or fittings.
A boiler that only heats water and relies on an external pump and a separate cylinder; used on open-vented or sealed systems.
governor
A device that holds gas pressure roughly constant despite changes in flow. The primary meter regulator holds domestic supply at about 21 mbar and should droop slightly with flow (about 22 mbar at low flow to 20 mbar at full load). Gas escaping from its breather hole usually means a ruptured diaphragm. Its seal may be broken only by the gas transporter or an authorised person — being Gas Safe registered is not enough.
A device that controls and stabilises gas pressure. The meter regulator reduces the incoming service to the nominal working pressure (about 21 mbar for natural gas); appliance regulators set burner pressure. CCN1 covers checking and setting the meter regulator.
Under GSIUR, the person who occupies or is in control of premises (for example the occupier, or the owner where premises are unoccupied). You seek the responsible person's permission before disconnecting or turning off an unsafe appliance, and warning notices are issued to them.
Reporting of Injuries, Diseases and Dangerous Occurrences Regs
The separate reporting regime for certain gas incidents; distinct from the immediate make-safe duty.
Controls air temperature in a reference room and forms part of the boiler interlock; the room containing it should not have a TRV.
The size of a room in m³ (length × width × height), used to size ventilation for flueless appliances and cookers, which keep their products in the room. For example a cooker in under 5 m³ needs an openable window plus about 100 cm²; over 10 m³ needs only an openable window.
Takes combustion air from, and discharges products to, outside through a sealed circuit; needs no combustion ventilation to the room.
Common heating layouts. S-plan uses separate 2-port zone valves for heating and hot water; Y-plan uses a single 3-port mid-position valve. Both give full interlock when correctly wired.
The analyser's flue probe. Insert it about 200 mm into the well-mixed core of the products stream; too shallow (20–50 mm) sits in the dilution zone near the edge and reads low. Fine-tune its position for the highest steady CO₂ (lowest steady O₂).
Treating hard cold water feeding a combi's hot-water side (by scale-reducing or softening, per the MIs and BS 7593) to protect the plate heat exchanger. Excessive scale insulates a heat exchanger, making it overheat and ultimately fail.
Shared vertical flue/air systems for room-sealed appliances in multi-storey buildings; appliances draw air from and discharge into the common duct.
A pressurised heating system with no cistern; expansion is absorbed by a diaphragm expansion vessel and protected by a pressure relief valve (typically 3 bar), a gauge and a filling point.
Air supplied at or after the flame (post-aerated burner), as on a flueless cooker hob.
On an open-flued appliance, the flue between the draught diverter and the terminal.
The pipe that carries gas from the gas main in the street to the meter and ECV at a property (compare the distribution main, which runs along the street). A building or extension constructed over a live gas service pipe is normally classed At Risk and referred to the gas transporter.
An instantaneous gas water heater serving one outlet only (e.g. an over-sink heater), as opposed to a multipoint serving several.
A room used for sleeping, which carries a special GSIUR rule: an open-flued gas fire is allowed up to 14 kW net only if it is room-sealed or fitted with a suitable atmosphere-sensing device; over 14 kW net it must be room-sealed. A sleeping occupant cannot react to a build-up of products, so the appliance must protect itself.
A continuous tube protecting pipe through a wall or cavity, sealed at the inner face with a flexible fire-resistant compound.
Smoke-producing aids for flue testing: a smoke pellet for the flue flow test (to show the empty flue draws and is clear) and a smoke match held at the draught diverter for the spillage test.
A purpose-designed below-ground pit that disperses condensate into the ground, used as an external termination where no drain is available. Designed per BS 6798 / the boiler manufacturer's instructions.
A smoke match at the appliance proves combustion products clear into the flue and don't spill into the room. Carried out with extract fans running.
A short settling period before the tightness test is timed, letting gas temperature and pressure settle so a small temperature change is not mistaken for a leak.
A short settling time at the start of a tightness test (about one minute for domestic natural gas to IGEM/UP/1B Edition 4) that lets the gas temperature and pressure steady before the let-by check and the timed test begin.
A chain or bracket securing a freestanding cooker so it cannot tip when a load is applied to an open oven door or hotplate (BS 6172 / MIs).
The gas pressure measured with no gas flowing (all appliances off). Compared with the working pressure (appliances running) to check the regulator and the size of any pressure drop along the installation.
The exact air-to-gas ratio for complete combustion. Natural gas needs about 10 m³ of air (≈2 m³ oxygen) per 1 m³ of gas.
A water heater that heats and stores a volume of hot water ready for use, kept at temperature (around 60 °C) to limit Legionella.
Part of atmosphere analysis: moving the analyser slowly around, above and below an appliance for at least two minutes so small leaks around casings and draught diverters show up. Wear a personal audible CO alarm and confirm the air is safe before any other work.
A boiler with the circulating pump and expansion vessel built into its casing, used on a sealed system with a separate hot-water cylinder.
Cleaning a heating system (e.g. a dynamic flush or power flush) to remove installation debris and sludge before commissioning, after which inhibitor is dosed, per BS 7593.
The last-line safety valve on an unvented hot water cylinder, releasing water if temperature or pressure rises too far (typically around 90–95 °C or about 7 bar). Discharges via the tundish and discharge pipe to a safe place. A G3 requirement.
A safety valve on an unvented hot-water cylinder that releases water if temperature or pressure rises too high, discharging safely through a tundish and pipe. It is one of the layered controls (with a control thermostat and a non-self-resetting high-limit energy cut-out) that make an unvented cylinder safe.
A bonding lead clamped across a pipe before you break into it, so electrical continuity is maintained and you are not relying on the gas pipe to carry current while it is parted. It is removed after the joint is remade, and is different from permanent main protective bonding.
A cage fitted over a flue terminal that is low or vulnerable, protecting it from damage and keeping it clear.
A high-limit safety device on an unvented or storage system that cuts the heat source if the cylinder thermostat fails and the water overheats — a second layer of temperature protection.
A sensor used in many flame supervision devices. Heated by the flame it generates a small voltage (millivolts) that energises an electromagnet holding the gas valve open; when the flame goes out it cools, the voltage falls and the valve shuts (drops out).
A valve that blends stored hot water with cold to limit the temperature at the outlet (e.g. about 48 °C at baths) for scald protection, while the cylinder stays stored hot (≥60 °C) for Legionella control.
Proves the installation holds pressure with no perceptible drop. For domestic NG: stabilise, 1-minute let-by, then a 2-minute test.
series vent
An air vent in an internal wall that transfers combustion air from one room to another. It is sited low (not more than 450 mm above the floor), and where air passes through two or more internal walls the vents are increased by 50% (×1.5).
An adjustable background-ventilation opening, often in a window frame; because it can be closed it cannot be counted as combustion-air provision.
Thermostatic Radiator Valve
Gives local radiator control; fitted everywhere except the room with the controlling room thermostat. Does not by itself provide boiler interlock.
An open air-break in the discharge pipe of an unvented cylinder safety valve, giving a visible discharge to a safe point.
Mains-pressure stored hot water; needs layered temperature/pressure safety controls and a competent (G3) installer under Building Regs Part G.
Under-Pressure Shut-Off
A safety device that closes if gas pressure falls below a set value; it must be reset by hand once the cause is cleared.
A room-sealed flue arrangement that takes combustion air high up in outside air, with the draught break at least 300 mm above any insulation. Coded C7/C72.
Air whose oxygen content has been depleted, usually by recirculated combustion products; promotes incomplete combustion and CO.
A label attached to an appliance turned off under the GIUSP, stating it must not be used. A copy is left with the customer and the action recorded.
The chamber on an analyser sample line that removes condensate from the flue-gas sample so moisture can't reach and damage the sensors.
Boiler controls that adjust the flow temperature to outside (weather) or room (load) conditions, keeping the boiler modulating and condensing for efficiency. Recognised Boiler Plus efficiency measures.
A measure of the heat a gas delivers through a given injector — calorific value divided by the square root of relative density. Gases with the same Wobbe number behave similarly through the same appliance, which is why supply quality is controlled within limits.
operating pressure
The gas pressure measured at the appliance while it is running (about 21 mbar nominal for domestic natural gas). Low working pressure comes from undersized or blocked pipe or a faulty regulator — never from over-sized pipe. A step-change in pressure between two adjacent test points points to a local restriction or blockage there.
Yellow tips on an otherwise blue flame, indicating incomplete combustion (usually too little primary air) on most appliances. Note: DFE and ILFE fires are designed to burn yellow, so this does not apply to them.
Switching the analyser on and letting it set its baseline in fresh air, away from the appliance, before sampling. Zeroing in the flue or in the products would set a false baseline and corrupt every reading that follows.