A Glossary of Boiler Jargon

Oil Boilers A-Z


A No-Nonsense Boiler Jargon Buster

To most of us the boiler and heating system that keeps us cosy during the long, cold winter months and delivers hot water through the tap on demand is a mysterious set of clever components and operations that is best left to the experts.

When your boiler engineer chats to you about your oil-fired boiler, perhaps you nod and smile without really understanding how this vital appliance does its job. To help you fathom out how your boiler operates we’ve put together this “plain English” A-Z Glossary of Oil Boiler and oil-fired heating system Terminology:

Oil Boilers A-Z

Bio-Liquid Blend

Bio-liquids are renewable fuels made from recycled cooking oil or plants. When blended with mineral heating fuels like kerosene they are used to power oil boilers and furnaces, burning more cleanly than pure heating oil or gas.


This is the outer shell which contains a single skin oil storage tank. The bund is there as a safeguard against pollution if there is an oil spill or leak from the tank.


The burner is the oil boiler component that provides the heat, by combusting oil with air or oxygen.

Combi (Combination) Boiler

This is a type of boiler which delivers hot water instantly on demand from taps, or for heating purposes. Suitable for small to medium homes.

Condensing Boiler

A condensing boiler (gas or oil) converts water vapour condensation into heat, recovering some of the lost heat from waste gases, thereby improving energy efficiency by producing more usable heat.


The boiler flue is the equivalent of the boiler’s chimney. It is a piece of pipework which channels away the waste gases and water vapour (and some other unpleasant gases like carbon monoxide and nitrous oxide) that is emitted by the boiler’s operation. The flue carries the emissions safely outside where they can be released into the atmosphere at a safe distance.


This is a light, low-sulphur, clean-burning oil which is used in most types of domestic oil heating and cooking appliances. It is also known as 28 second, Class C2 oil to BS2869. By contrast Gas Oil, which is used for large, non-domestic boilers, is thicker and gives off more soot and emissions.


This is a time-activated switching device that regulates when the boiler turns on to deliver heating and/or hot water. It can vary from a simple time switch to a device with multiple functions to allow full control of the heating system.

Regular Oil Boiler

A boiler designed to heat water only, using the water to circulate around the heating system with an external pump, valves and controls.

Room Thermostat

A room thermostat works by sensing the air temperature in the room and switching on the heating whenever the temperature drops below the thermostat setting – then off again when the set temperature is reached. The thermostat enables you to maintain a comfortable temperature.

SEDBUK Efficiency

Remember when boilers were rated with letters – A, B, C, D, E etc – to designate their energy efficiency? Well, nowadays they are rated according to a percentage score, and this is known as a SEDBUK (Seasonal Efficiency of a Domestic Boiler in the UK) efficiency rating. The top rating is 90% plus. Legislation requires all newly installed boilers to be at least 88% efficient.

System Oil Boiler

Ideal for homes with several bathrooms and a high demand for hot water the system boiler uses water direct from the mains water supply and heats water inside a water cylinder, which can then be delivered to taps, bath, shower and kitchen sink.

TRVs (Thermostatic Radiator Valves)

These nifty valves enable you to control the temperature in various rooms. You’ll find them fitted on the side of your radiators. They are self-regulating valves that work by changing the flow of hot water into a radiator. When the room temperature changes a capsule in the head of the valve contracts or expands, moving a pin in the valve body which causes it to open or close accordingly. They work particularly well as part of a smart heating system, and can produce significant energy savings.


By making use of various thermostatic controls you can institute zoned heating in your home, so that you can set different temperatures for different rooms, ensuring energy efficiency.

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