The short answer
A combi boiler burns gas to heat water in an internal heat exchanger and uses a diverter valve to switch between heating your radiators and heating domestic hot water on demand. There is no storage cylinder: when you open a hot tap, the boiler fires up and heats cold mains water as it flows through. This makes combis compact and space-efficient. For help choosing one, see our guide on combi vs system vs regular boilers.
Combination boilers now account for the majority of new boiler installations in the UK, yet many homeowners are unsure exactly what happens inside the unit when they turn the heating on or open a tap. Understanding the basics — the burner, the heat exchanger, the diverter valve and the plate heat exchanger — helps you recognise when something is wrong, follow an engineer’s diagnosis and make sense of a repair or replacement quote.
Combi boiler at a glance
- Heat source Gas burner (mains gas or LPG)
- Central heating Primary circuit → radiators
- Hot water Plate heat exchanger — mains cold in, hot out
- Storage None — heats on demand
- Key component Diverter valve switches between CH and DHW
- Efficiency 90–94% on modern condensing models
The primary circuit: central heating
When the room thermostat calls for heat, the boiler’s gas burner fires and heats water in the primary heat exchanger. This heated water circulates through the primary circuit — around the radiators and back to the boiler — driven by the built-in pump. The flow temperature (the temperature leaving the boiler) is typically set to 60–80°C for traditional radiator systems, though modern condensing boilers run more efficiently at lower flow temperatures of 50–60°C on well-designed systems with larger radiators. A modern combi also has a modulating burner, meaning it can vary its firing rate to match demand rather than running full-blast and then shutting off, which improves efficiency and comfort.
The diverter valve: switching priorities
When you open a hot tap, the boiler detects the water flow and the diverter valve switches the primary circuit away from the radiators and through a plate heat exchanger instead. This is the combi’s key piece of domestic hot-water (DHW) engineering: the plate heat exchanger is a compact stack of thin metal plates through which the hot primary circuit water and the cold mains water flow in opposite directions, separated by the plates, exchanging heat without mixing. The mains water exits as hot water to your tap; the primary water returns to be reheated. Because heating is paused while this happens, you may notice radiators cooling slightly during a long shower — this is normal behaviour, not a fault.
| Mode | Diverter valve position | Hot water goes to |
|---|---|---|
| Central heating only | CH position | Radiators |
| Domestic hot water | DHW position | Plate heat exchanger → taps |
| Both called at once | DHW takes priority (most models) | DHW first, then CH resumes |
Condensing technology: how efficiency reaches 90%+
Since 2005, virtually all new gas boilers in the UK must be condensing type. A condensing boiler has a secondary (condensing) heat exchanger in the flue path that recovers latent heat from the water vapour in the exhaust gases, which would otherwise escape to the atmosphere. This is why condensing boilers produce a white plume of steam from their plastic flue terminal — the exhaust gases are cool enough (50–55°C or less) for the water vapour to condense, releasing extra heat. A non-condensing boiler exhausted at 180–200°C, wasting a large portion of the gas burned. For the condensing effect to work efficiently, the return-water temperature from the radiators should be below about 55°C — another reason low-temperature system design matters.
Common combi faults and what they signal
Understanding how a combi works makes fault symptoms easier to interpret. If you have no hot water but radiators still heat, the diverter valve or plate heat exchanger is the likely cause. If you have no heating or hot water at all and the boiler shows a lockout code, check the pressure gauge (it should sit between 1 and 1.5 bar when cold) — low pressure often causes lockouts and is usually resolved by represssurising through the filling loop. Noisy operation — banging, kettling or gurgling — can indicate limescale on the heat exchanger, sludge in the system, or trapped air. A Gas Safe registered engineer should diagnose and carry out any work on the internal components.
How long does a combi last?
A well-maintained combi boiler installed by a Gas Safe registered engineer and serviced annually typically lasts 10–15 years. Some models from reputable manufacturers last 20 years with proper care. The main life-limiting components are the heat exchanger, the pump and the diverter valve. Annual servicing checks these components, cleans the burner, checks seals and tests the safety devices. If your boiler is over 10 years old and requiring repeated repairs, a replacement is likely more cost-effective — see our guide on signs you need a new boiler and the guide on new boiler costs. This is general information, not advice for your specific installation; all gas work must be carried out by a Gas Safe registered engineer.
Time to replace your combi?
If your boiler is over 10 years old or needs frequent repairs, it may be more economical to replace it. Compare quotes from Gas Safe registered engineers.
Frequently asked questions
Why does my combi boiler make my radiators go cold when I run a tap?
This is normal. When you open a hot tap, the diverter valve switches the boiler’s primary circuit to heat domestic water instead of circulating through the radiators, so they cool slightly. The heating resumes once you turn the tap off.
Why is my combi boiler pressure low?
Combi boilers lose pressure over time through small leaks, bleeding radiators or the pressure-relief valve activating. You can repressurise via the filling loop (check your boiler manual), but if pressure drops repeatedly a Gas Safe engineer should investigate the cause.
Can a combi boiler supply two showers at once?
Most standard combis struggle with simultaneous high-flow outlets because output is limited. A high-output combi (35 kW+) helps, but for consistently high simultaneous demand a system boiler with a stored cylinder is usually more practical.
How often should a combi boiler be serviced?
An annual service by a Gas Safe registered engineer is recommended. This checks the burner, heat exchanger, seals, gas pressure and safety devices and keeps the manufacturer’s warranty valid on most models.
Sources & further reading
- Gas Safe Register — guidance on boiler operation, safety and annual servicing
- Energy Saving Trust — condensing boilers and heating efficiency
- HHIC — Heating and Hotwater Industry Council, combination boiler consumer guide
- GOV.UK / Building Regulations Approved Document L — boiler efficiency requirements for dwellings
This is general information, not advice for your specific property or installation. Costs, timescales and outcomes vary with your home, system condition and chosen engineer. All gas boiler work must be carried out by a Gas Safe registered engineer.