Glossary · Boilers and boiler types
What is a gas boiler?
A gas boiler burns natural gas, or LPG in homes without a mains supply, to heat the water that warms your radiators and, depending on the type, your taps. Modern gas boilers are condensing models. By law, only a Gas Safe registered engineer may work on one in a home.
Key points
- Gas boilers come in three main types: combi, system and heat only (regular), which differ in how they supply hot water.
- Since 1 April 2005 all new gas boilers installed in the UK have had to be condensing models.
- Approved Document L sets a minimum efficiency of 92% (ErP) for gas boilers heating radiators or underfloor heating in existing homes.
- Anyone employed to work on gas appliances in a home must be Gas Safe registered, and HSE advises an annual service.
- A flue carries exhaust gases outside, so a blocked or damaged flue is a carbon monoxide risk.
How a gas boiler works
A gas boiler burns gas to heat water. The hot water then runs through a sealed heating circuit to your radiators. Worcester Bosch notes that gas is the most common fuel used in the UK for hot water and heating, and that if you are on the gas grid you will probably have a gas boiler.[6]Worcester Bosch, Regular boilers
Modern gas boilers are condensing boilers. A condensing boiler recaptures heat from the combustion gases that would otherwise be lost through the flue.[5]Baxi, What is a condensing boiler and how does it work? As the gases cool, water vapour turns into liquid, called condensate, and drains away through a separate pipe. See condensate pipe and heat exchanger for the detail. Vaillant says that since 1 April 2005 all new gas boilers installed in the UK have been required by law to be condensing boilers.[2]Vaillant, What is a condensing boiler?
Types of gas boiler
| Type | How it supplies hot water | Needs a cylinder? |
|---|---|---|
| Combi (combination) | Heats water as you use it | No |
| System | Heats a stored cylinder, pump and expansion vessel inside the boiler | Yes |
| Heat only (regular) | Heats a stored cylinder, with tanks in the loft | Yes |
Baxi describes combi boilers as the most popular type in the UK, with everything needed for heating and hot water in one compact casing.[1]Baxi, What are the different types of boilers? System boilers include the expansion vessel and pump and need a cylinder but no loft tanks. Heat only boilers need a feed and expansion tank and a cold water tank in the loft, plus a cylinder and a separate pump.[1]Baxi, What are the different types of boilers? Read more in how a combi boiler works, system boiler and regular boiler.
Efficiency rules and safety
Approved Document L, the building regulations guidance for England, sets a minimum efficiency of 92% (as defined in ErP) for a gas-fired wet heating system, such as radiators or underfloor heating, in an existing home. It allows lower figures, 78% for natural gas and 80% for LPG, only in exceptional circumstances.[3]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments If a gas combi boiler is installed in an existing home, at least one extra efficiency measure should also be fitted. The options include weather compensation, load compensation and a smart thermostat with automation and optimisation.[3]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments
The legal position on who does the work is clear. HSE says anyone employed to work on gas appliances in domestic premises must be a Gas Safe registered engineer and competent in that area of gas work.[4]Health and Safety Executive, Domestic gas: frequently asked questions Do not open the boiler casing or touch gas pipework yourself. You can check an engineer’s registration on the Gas Safe Register. See Gas Safe Register for how.
HSE also strongly advises an annual service by a Gas Safe registered engineer.[4]Health and Safety Executive, Domestic gas: frequently asked questions Our boiler service page covers that.
If you suspect a gas leak, HSE says to call National Grid’s Gas Emergency Freephone number, 0800 111 999, open all the doors and windows and shut off the gas supply at the meter control valve if you know where it is.[4]Health and Safety Executive, Domestic gas: frequently asked questions
Example
Dev lives in Bradford in a three-bedroom house with a 20-year-old gas boiler. It is a non-condensing model, and his engineer proposes a modern replacement. Approved Document L expects a replacement to reach 92% ErP efficiency unless exceptional circumstances apply.[3]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments If the new boiler is a combi, the engineer should also fit one of the extra measures, such as weather compensation or a smart thermostat.[3]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments Dev checks the engineer’s Gas Safe ID card before work starts, as HSE advises.[4]Health and Safety Executive, Domestic gas: frequently asked questions Our boiler installation and combi boiler pages explain what happens on the day.
What people get wrong
People often think a gas boiler is any boiler that heats a home. In fact the fuel is only part of it. Combi, system and heat only boilers are all gas boilers, so the type you need depends on your hot water demand and your space, not on the fuel.
Baxi lists combi, system and heat only as the types of boiler a home can have.[1]Baxi, What are the different types of boilers? A second mistake is to treat the service as optional. HSE says it strongly advises that all gas appliances, flues and pipework are serviced at least annually. It also says every year around 7 people die from gas-related carbon monoxide poisoning.[4]Health and Safety Executive, Domestic gas: frequently asked questions See carbon monoxide for the warning signs. Questions about future rules for gas boilers are covered under gas boiler ban.
The bottom line
A gas boiler burns gas to heat your radiators and hot water, and in the UK it is almost always a condensing model. It comes as a combi, system or heat only type. England sets a 92% ErP minimum for replacements. Only a Gas Safe registered engineer may work on it, and an annual service is advised.
Common questions
What are the main types of gas boiler?
Combi, system and heat only (also called regular). A combi heats water as you use it. System and heat only boilers heat a stored cylinder. Baxi sets out the pros and cons of each on its types page.
Are all new gas boilers condensing?
Yes. Vaillant says that since 1 April 2005 all new gas boilers installed in the UK have been required by law to be condensing boilers. A condensing boiler recovers heat that would otherwise leave through the flue.
Who is allowed to work on a gas boiler?
Only a Gas Safe registered engineer. HSE says anyone employed to work on gas appliances in domestic premises must be Gas Safe registered and competent in that area of gas work.
How often should a gas boiler be serviced?
HSE strongly advises that gas appliances, flues and pipework are installed, regularly maintained and serviced at least annually by a Gas Safe registered engineer.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Baxi. What are the different types of boilers?. Checked 11 November 2026. ↩
- Vaillant. What is a condensing boiler?. Checked 11 November 2026. ↩
- Ministry of Housing, Communities and Local Government. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩
- Baxi. What is a condensing boiler and how does it work?. Checked 11 November 2026. ↩
- Worcester Bosch. Regular boilers. Checked 11 November 2026. ↩