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Published by Mo's Plumbing and Heating
How Does a Combi Boiler Work?
A combi, short for combination boiler, does two jobs from one appliance with no stored hot water anywhere. Understanding the one component that switches between those jobs explains most of what a combi does well, most of what frustrates people about them, and most of what goes wrong.
The Short Answer
A combi heats your radiators, and when you open a hot tap it stops heating the radiators and diverts all its output into heating the water flowing through to that tap instead. A component called a diverter valve does the switching. Because the water is heated as it passes through rather than stored, you never run out, and because the boiler can only heat water as fast as it arrives, the flow rate at your tap is limited by your incoming mains supply.
The Two Jobs and the Valve Between Them
In heating mode a combi behaves like any other boiler. It burns gas, heats water in a heat exchanger, and a pump circulates that water round the radiators and back.
Open a hot tap and everything changes. A flow sensor detects water moving, the boiler stops sending heat to the radiators, and the diverter valve redirects the flow so the boiler's output goes into a second heat exchanger instead. Cold mains water passes through that heat exchanger on its way to your tap and comes out hot.
That is the whole trick, and it explains the characteristic behaviour. The radiators go off while you run a hot tap, which is why they can feel cooler after a long shower on a cold morning. You never run out of hot water, because there is no store to exhaust. And the boiler is doing one thing at a time.
It also explains the commonest combi faults. The diverter valve is a moving part that switches thousands of times a year, and when it sticks you get one of two classic symptoms: hot water but no heating, or heating that comes on whenever you run a tap. Either points at the same component.
Why the Flow Rate Matters More Than the Boiler
This is the part that determines whether you will be happy with a combi, and it is about your house rather than the appliance.
A combi heats water as it flows through. To get water hotter, it either burns harder or lets the water pass more slowly. There is a limit to burning harder, so beyond that point the only way to reach temperature is to reduce the flow.
That means the hot water performance you experience is set by how much water arrives at the property and at what pressure. If your incoming mains delivers a modest flow, a bigger combi will not fix it, because the constraint is upstream of the boiler entirely. Paying for extra output you cannot feed is money spent on a number rather than on a shower.
It also means a combi serves one outlet well at a time. Running a second hot tap splits the same flow, so both get less. That is not a fault and it is not fixed by a larger appliance; it is inherent to heating water on demand.
Measuring the flow rate takes a minute with a jug and a watch, and it is genuinely the first thing to establish before choosing a combi. The cost guide for combis covers how.
How to Tell If You Have One
Surprisingly often unclear, particularly in a property that has changed hands or where nobody was left any paperwork. Four checks, in order of how conclusive they are.
- Look for a hot water cylinder. A large insulated cylinder in a cupboard means you do not have a combi. No cylinder anywhere in the property is close to conclusive that you do.
- Look in the loft. Tanks up there mean an older heat only system. A combi has no tanks at all, and neither does a system boiler.
- Look at the pipes under the boiler. A combi typically has five: gas, cold mains in, hot out to the taps, and flow and return for the heating. A system boiler has fewer, since it does not handle the taps directly.
- Check whether hot water is instant or needs warning. If you can run a hot tap at any time with no timer and no waiting, that is a combi. If hot water depends on a timer or a boost switch having run, there is a cylinder involved somewhere.
- Worth doing once and writing on something. It is the first question you will be asked on the phone about almost any heating fault, and knowing the answer shortens every conversation that follows.
What Combis Are Good and Bad At
Even-handed, because they are the default choice in this country and defaults get oversold.
Good at: never running out, freeing up the cupboard and the loft, taking up less space overall, and heating water only when it is actually wanted rather than keeping a cylinder warm around the clock. In a one or two person flat those advantages are real and the drawbacks rarely bite.
Bad at: simultaneous demand, which it cannot do well by design. Also poor where the incoming supply is weak, since performance is capped by the supply rather than the appliance. And there is no backup: a combi fault takes heating and hot water together, where a cylinder system usually still offers an immersion heater.
The lifespan question comes up constantly and the honest answer is that a well maintained combi is broadly comparable to other types, roughly ten to fifteen years, with servicing making a real difference at the top of that range. Combis have more moving parts doing more switching, so there is more to go wrong, but they are not inherently short lived.
The Faults Worth Recognising
Three that are specific to how a combi works, so they are handy to know as a combi owner rather than as general boiler knowledge.
Hot water fine, no heating, or heating that fires whenever a tap runs. Both are the diverter valve sticking in one position, and both are a repair rather than a replacement.
Hot water that runs hot then cold then hot again, often called cold water sandwich. Common on combis and usually a flow or temperature control issue rather than anything serious, though a pronounced version is worth having looked at.
Hot water that has gradually got weaker over years with no change to the heating. That points at the plate heat exchanger, the second one used for the taps, gradually scaling or blocking. It is a known combi wear point and it is repairable.
None of those apply to a system or heat only setup, because none of them have a diverter valve or a plate heat exchanger. That is the practical value of knowing which type you have.
Nobody Left the Paperwork
Working out what you have got is a bigger question in this city than the checklist above suggests, because of how much of the housing stock has moved between owners and tenants without documentation following it. A flat that has been let for fifteen years, sold twice and refurbished once will frequently have nothing at all in a drawer about the heating, and the boiler manual went out with a previous tenant.
That matters practically rather than administratively. The first thing anyone asks about a heating fault is what type of system it is, and the second is the make and model. Both are answerable in five minutes with a phone torch and a photograph of the sticker behind the front flap, and having them ready is the difference between a diagnostic conversation and twenty questions.
The pipe count is the most reliable check in a flat, because cupboards get repurposed. A cylinder cupboard converted into a wardrobe during a refit still has the pipework running through it, and a boxed-in section in a hall with pipes disappearing into it is worth a look before concluding there is no cylinder. We have found more than one cylinder sealed behind a stud wall by somebody who wanted the space and did not want the job of removing it properly.
It also matters for a sale or a let. A heating system nobody can describe is a question mark on a survey, and being able to say what type it is, what make, roughly what age and when it was last serviced is worth having written down somewhere that stays with the property rather than with you.
When to Call Someone Out
Worth calling for the specific combi faults above, since a sticking diverter valve or a scaling plate heat exchanger are both repairs rather than reasons to replace an appliance, and both are commonly misdiagnosed as the boiler being finished. Also worth a call if you genuinely cannot work out what type of system you have, because it takes an engineer a minute and it changes every subsequent conversation. Mo and Muhammad are Gas Safe registered for domestic work and cover combi repairs. Having the make, model and serial number ready from the sticker behind the front flap shortens the visit.
Boiler Installation in GlasgowCommon Questions
The full name for a combi. It combines two functions in one appliance: heating the radiators and heating your hot water on demand, with no separate cylinder. Combi and combination boiler mean exactly the same thing, and the shorter form is simply what everyone says.
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Read Next
Full guide
What Is a System Boiler?
A system boiler heats your radiators and heats a separate hot water cylinder, which stores hot water ready to use.
What Is a Condensing Boiler?
A condensing boiler extracts extra heat from its own exhaust gases by cooling them until the water vapour in them turns back into liquid.
What Is a Heat Only Boiler?
A heat only boiler heats water and nothing else.
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