Combi boiler
In plumbing · also called Combination boiler
A boiler that provides central heating and also heats water straight from the mains as it is used, with no cylinder or loft cistern. Hot water takes priority over heating, and because its output is fixed, the flow at the taps drops when more than one outlet is open or the incoming water is colder.
Using Combi boiler in a plumbing system
Updated
A distinction worth remembering
A combination boiler delivers a set amount of heat, so the water it can supply depends on how much is flowing and how cold it arrives. Compare that with what a household needs at once. The parts behind the casing are gas work, so a learner's checks stay on the water side.
Questions to take further
- Will a more powerful combi fix a weak incoming water supply?
- No. A combi can only heat the water that reaches it. When the main delivers too little, the supply needs assessing first, and a stored hot-water system may suit better.
Sources
- HSE: domestic gas safety FAQs — Gas Safe registration and checking the categories of work on an engineer's ID card.
- Skills England: plumbing and domestic heating technician — The systems, materials, planning and customer responsibilities in the England occupational standard.
Where Combi boiler fits in the learning path
The Hot Water Systems section covers these topics:
- Hot Water Cylinders
- Combis and Instant Hot Water
- Hot Water Temperature Control
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- Combis and Instant Hot WaterHot Water Systems
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The valve inside a combi boiler that sends heat either to the central heating or to the hot-water heat exchanger, giving hot water priority while a tap is open. One that sticks can leave radiators warming while a tap runs, or the hot water lukewarm. It sits inside the casing, which in Great Britain is work for a Gas Safe registered engineer.
Diverter valve
Read the Diverter valve entryCompare related terms
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Diverter valve
The valve inside a combi boiler that sends heat either to the central heating or to the hot-water heat exchanger, giving hot water priority while a tap is open. One that sticks can leave radiators warming while a tap runs, or the hot water lukewarm. It sits inside the casing, which in Great Britain is work for a Gas Safe registered engineer.
Read the Diverter valve entryCondensate pipe
The pipe that carries the slightly acidic water a condensing boiler produces away to a drain. Where it runs outside, it can freeze in very cold weather and stop the boiler working. Pouring warm, not boiling, water along the frozen length is a common first step that makers' instructions often describe.
Read the Condensate pipe entryFlow rate
How much water comes out of an outlet in a given time, usually measured in litres a minute by timing how long a container of known size takes to fill. It is different from pressure: a supply can have good pressure and poor flow if a pipe is narrow, partly blocked or shared with other outlets.
Read the Flow rate entryBefore you start: some plumbing work is regulated or notifiable
Some water fittings work needs notice to, and consent from, the water supplier before it starts. Check the rules where you live.
Gas work is for competent workers covered by a Gas Safe registered business in Great Britain. Northern Ireland has separate gas regulations. Do not work on gas pipes or appliances yourself.
In England, installing an unvented hot-water system over 15 litres is notifiable building work: use a competent installer registered with a competent-person scheme, or tell building control before work starts. Leave this work to a suitably skilled person.
Wales, Scotland and Northern Ireland have their own water and building rules; check which apply before work starts. Electrical connections belong with an electrician.
This guide does not qualify you or make you competent, approved or registered for plumbing, gas or unvented work.
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