Glossary for gas engineers.
Short, plain definitions with links to the official source, and to a free tool where there is one.
ACS (Accredited Certification Scheme)
The nationally recognised assessment scheme that gas engineers pass to show competence for the categories of work they do.
Annual boiler service
A yearly inspection and maintenance visit to keep a boiler working safely and efficiently, usually following the manufacturer’s service instructions.
Benchmark checklist
An industry commissioning checklist and service record supplied with many boilers, filled in at installation and at each service.
Calorific value (CV)
The energy released by burning a set volume of gas, usually in MJ/m³. It varies a little by area and day.
CCN1
The core domestic natural gas safety assessment under ACS. Most domestic gas engineers hold it and reassess every five years.
CO/CO2 ratio
Carbon monoxide divided by carbon dioxide in the flue gases. It shows combustion quality independent of how much excess air is present.
CP12 (landlord gas safety record)
The record of the annual gas safety check that landlords must arrange. “CP12” is the old form number that most people still use for it.
Flue gas analyser
A portable electronic instrument that samples flue gases to measure oxygen and carbon monoxide and calculate carbon dioxide, the CO/CO2 ratio and efficiency.
Gas rate
The volume of gas an appliance burns per hour, measured at the meter, usually in m³/h. It is used to work out the appliance’s heat input.
Gas Safe Register
The official list of gas businesses and engineers legally allowed to work on gas appliances and installations.
GIUSP (Gas Industry Unsafe Situations Procedure)
The industry procedure engineers follow when they find an unsafe gas situation, including how to classify it and what action to take.
Heat input (gross and net)
The rate at which an appliance takes in energy from its fuel, in kW. Gross includes the heat in the water vapour in the flue gases; net leaves it out.
Making Tax Digital for Income Tax
HMRC’s requirement for many sole traders and landlords to keep digital records and send quarterly updates using compatible software.
Room-sealed and open-flued appliances
Room-sealed appliances take combustion air from outside and send flue gases outside through a sealed case. Open-flued appliances take their air from the room.
Tightness test
A pressure test that checks a gas installation for leaks, using a pressure gauge on the installation pipework.