Combis and Instant Hot Water
Hot Water Systems · Plumbing
A combination boiler heats mains water as it flows instead of storing it. This topic covers what that means for flow and temperature, how to compare it with a cylinder, and where a householder's checks stop.
A combi has one burner and a fixed output. Its diverter valve gives the taps priority, and its heat is shared across whatever water is flowing, so a second open tap or colder winter mains water means less hot water a minute.
A rough rule links the numbers: litres a minute is about the output in kilowatts times 14.3, divided by the temperature rise. Electric showers follow the same physics, which is why they run more gently.
Cleaning a scaled aerator or thawing a frozen condensate pipe with warm water sits on the water side. Everything inside the boiler casing is gas work; in Great Britain, businesses and self-employed people doing gas work have to be on the Gas Safe Register, so refer it to a Gas Safe registered engineer.
A fixed output is shared across the flow
Updated
How to reason about the system
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.
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Lessons on the plumbing path
- How a combi makes hot water
- Flow rate and temperature rise
- Combi or stored hot water?
- Water-side checks and the gas boundary
Terms to know
Before 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.