Gas and heating engineers A–Z dictionary

A

Air source heat pump

An air source heat pump uses electricity to move heat from outside air into a building. In an air-to-water system it supplies water-based heating and usually a hot water cylinder. An air-to-air system distributes warmth through indoor air units. These arrangements meet different needs and should not be treated as identical products.

B

Balanced

In heating, adjusted so flow is distributed as designed. It does not simply mean every valve is equally open.

Bleeding

Releasing trapped air from a heating component by the appropriate method. It is not a cure for every cold radiator.

Boiler replacement

Boiler replacement exchanges a heat source as part of a compatible heating and hot-water system. The work may also require controls, flue, pipework, condensate, treatment and commissioning changes. A new appliance alone does not prove retained radiators, cylinder or distribution are suitable or that every existing comfort problem will disappear.

Boiler service

A boiler service is planned maintenance and examination carried out to the boiler manufacturer’s instructions. For a gas boiler, use a Gas Safe registered engineer competent for the appliance. A service is not simply a visual look at the casing and is not automatically the same booking as a landlord’s gas safety check.

C

Carbon monoxide

Carbon monoxide, or CO, is a poisonous gas that cannot be seen or smelled. If exposure is suspected, go outside and seek medical advice promptly; do not re-enter until advised. Call 999 for serious symptoms described by the NHS, such as breathing difficulty, sudden confusion, loss of consciousness, weakness or chest pain.

Combi boiler

A combi, or combination boiler, provides central heating and heats domestic hot water on demand within one appliance. A conventional combi normally avoids a separate hot water cylinder. Combi describes its functions; condensing describes heat recovery from combustion gases. A boiler can therefore be both a combi and a condensing appliance.

Condensate

Liquid produced as combustion water vapour condenses in suitable boilers. Its disposal route differs from a pressure-relief discharge.

E

Expansion vessel

An expansion vessel provides a compressible gas space to accommodate changes in water volume as a system heats. A flexible diaphragm or similar separation commonly keeps the gas and water apart. Vessels used for sealed heating and those suitable for drinking-water services are not interchangeable merely because their size or colour looks similar.

F

Flow temperature

Temperature of heating water leaving the heat source. Its useful setting depends on emitters, heat loss, controls and system design.

Flue

The route carrying combustion products away from an appliance. Clearances, integrity and termination need competent assessment for that appliance.

G

Gas emergency

A suspected gas escape needs prompt emergency action, not routine fault diagnosis. Leave danger and call from a safe place: in Great Britain, 0800 111 999 is the gas emergency number; Northern Ireland uses 0800 002 001. Call 999 where there is immediate danger or serious illness. Do not investigate a leak yourself.

Gas Safe Register

Gas Safe Register is the official register of businesses and engineers authorised for relevant gas work. Registration must cover the particular gas work being carried out, not just an unrelated appliance type.

Gas safety record

A gas safety record documents the checks and findings for the gas appliances and flues within its stated scope. For landlords, the required record is often informally called a CP12 or gas safety certificate. It is not the same as an invoice, boiler service sheet or a promise that no future fault can occur.

Getting into gas engineering

Gas engineering is safety-critical work within a defined appliance and system scope. Entry requires relevant learning, supervised experience, assessment and the appropriate registration arrangements. A manufacturer's attendance certificate or a general plumbing course does not make a novice competent to undertake independent paid gas work.

H

Heat emitter

A component delivering heat to rooms, such as a radiator or underfloor circuit. Its output depends on operating conditions.

Heat loss calculation

A heat loss calculation estimates how much heat a building or room needs to maintain a chosen indoor temperature under defined outdoor conditions. It informs heating design rather than simply matching a new appliance to the old one.

Heat only boiler

A heat-only boiler, often called a regular or conventional boiler, supplies heat to a suitable external heating and hot-water arrangement. The name distinguishes it from a combi, which produces domestic hot water on demand. It does not, by itself, specify every pump, tank, cylinder or pressure arrangement in the home.

Hot water cylinder

A hot water cylinder stores heated water for domestic use. It may be vented or unvented and may be heated directly by an immersion heater, indirectly through a heat exchanger, or by a combination of sources. The cylinder's usable service depends on its system design, not simply the volume printed on its label.

L

Lockshield

A radiator valve used in adjustment of the water flow distribution, normally distinct from the user's thermostatic room control.

P

Primary circuit

The heat-source side of an indirect heating arrangement. It should not be confused with the domestic water delivered to taps.

R

Radiator balancing

Radiator balancing adjusts water distribution through a heating system so that radiators receive an appropriate share of flow. It is different from bleeding air out of a radiator.

Room thermostat

A control responding to room temperature. Its location and interaction with other controls affect the system's behaviour.

S

Sealed system

A heating-water arrangement without an open expansion connection, using appropriate pressure and expansion provisions. It needs the correct safety devices.

System boiler

A system boiler heats a central heating circuit and normally works with a separate hot water cylinder. It generally incorporates key heating system components within the boiler casing.

T

Thermostatic radiator valve (TRV)

A TRV controls flow through a radiator in response to the temperature around its sensing head. It helps regulate a room; it does not directly select the boiler's water temperature.

Tundish

A tundish is an open-ended fitting which provides a visible break in a discharge route. In domestic plumbing it is often seen in the safety discharge pipework of an unvented hot-water cylinder. Its purpose is not simply to join two pipes: the visibility and separation are part of the intended safety arrangement.

Z

Zone valve

A valve controlling flow to a defined heating or hot-water zone. A motorised valve can form part of the control sequence; diagnosing it involves both hydraulic and electrical checks by competent people.

Sources and further reading

Linked article sources support the technical background; informal usage is editorial and varies by region. Full specification and legal requirements must be checked in the original.

• www.hse.gov.uk/gas/domestic/faqownerocc.htm
• Gas Safe Register: official registration check
• Gas Safe Register: public safety FAQs
• National Careers Service: occupational routes and duties
• HSE: gas registration and safety
• HSE: record contents, timing, issue and retention
• NHS: suspected CO exposure and medical response
• HSE: CO sources and combustion risks