How does a combi boiler work?

how does a combi boiler work

Discover how does a combi boiler work and choose the right one for your home. Your home’s boiler is one of its more important aspects. There are heat only boilers, system boilers, and a third kind of boiler, commonly referred to as a combi boiler.

What is a combi boiler?

More formally known as a combination boiler, a combi boiler serves as both a central heating boiler as well as a water heater. They’ve become increasingly popular over the years thanks to the space they’re able to save by having both of these essential home services in one compact unit.

combiboilerThe benefits of a combi boiler

  • They provide instant on demand hot water
  • They’re compact and therefore don’t take up much space
  • They’re efficient and therefore economical
  • Can boost the value of a home

You might be wondering how they work, so let’s take a look at the intricacies of a combination boiler to see how they help your home.

How does a combi boiler work?

Boiled down to its most basic explanation, a combi boiler works how you might expect. Say you turn on a hot water tap in your home, this ignites the combi boiler’s gas burner and then a heat exchanger transfers this energy to heat up the water that is taken from the mains. This happens almost instantly, providing hot water very quickly.

A more in-depth look at a combi boiler shows that the burners within the boiler are activated when the valves are turned in central heating and hot water systems within a property. An open tap allows water to flow where it’s needed. When the flowing water passes over a paddle switch within the boiler, it will turn the burners on in the part of the boiler that heats water for the taps. The heat exchanger that was mentioned earlier is a small compartment that stores surplus heat.

An example of the heat exchanger in action would be extra heat from the central heating being used to heat the water for the taps. Because the central heating pipes and domestic hot water pipes both run through the heat exchanger, they can always heat each other and be hot at the same time, which is the core reason for owning a combi boiler.

Common Combi Boiler Systems

Gas Combi Boilers

Gas combis are the most popular option, using mains natural gas to fuel an internal burner and heat exchanger for heating and hot water. Clean burning, dependable and affordable to run, they are ideal for most homes.

Oil Combi Boilers

In areas not served by the gas grid, oil combis provide a viable alternative. An oil burner unit feeds the heat exchanger, while an oil storage tank stores fuel until needed. Oil must be delivered periodically to fill the external or internal tank.

LPG Combi Boilers

For off-grid homes, propane gas cylinders or large LPG tanks can supply fuel to an LPG combi boiler. As an alternative to oil or natural gas, LPG combis give reliable performance, but LPG delivery costs can be high.

Electric Combi Boilers

Electric heating elements replace the burner to warm water in an electric combi unit. Running on off-peak electricity can minimize energy costs. But electric combis are generally more expensive to buy and slower for heating water.

Combi boiler prices

The price of installing a combi boiler can vary, but on average you should expect to pay between £500 and £2000, depending on your property and other factors. For more information, view our full combi boiler price guide!

If you are looking for quotes from local combi boiler installers, just enter your details into our simple form and let us do the work. Trusted boiler companies will be in touch to talk you through options & quotes.

Choosing the Right Combi Boiler

Getting the size and features right for your home is key to ensuring your new combi performs efficiently and effectively:

Sizing Considerations

The heating and hot water output ratings should be suitable for the heat loss and hot water demands of your particular property. Bigger is not always better – an oversized boiler will cost more and operate less efficiently.

Consider: Floor area, levels of insulation, number of bathrooms/occupants, average hot water usage

Efficiency Ratings

The higher the efficiency, the lower your fuel consumption and carbon emissions. Choose an A-rated condensing combi for maximum efficiency – over 90% efficient compared to non-condensing models around 60-70% efficient.

Look for: A ErP rating for energy efficiency and low NOx emissions

Brands and Models

Popular quality brands include Ideal, Worcester Bosch, Viessmann, Vaillant and Baxi. Each offers a range of combi boiler models across different price bands and with varying features. Do your research to select the right one for your home.

Maintaining Your Combi Boiler

Annual Servicing

While reliable, an annual boiler service maximizes safety and efficiency. Checks include electrical wiring, ventilation, flue gas analysis, pipework, pressure readings and replacing parts if needed.

Bleeding Radiators

Bleeding trapped air out of radiators keeps your system running at peak efficiency. Bleed valves on top allow easy venting room-by-room.

Flushing the System

Over time, debris and corrosion inside pipes can block components and reduce heating performance. Periodic power flushing clears the entire system for a clean and efficient combi unit.

Fixing Common Combi Boiler Problems

Even quality combis can occasionally suffer faults or issues. Understanding the warning signs helps rapid diagnosis and repair:

Lockout or Fault Codes

A flashing digital display indicates a detected fault. Fault codes help engineers identify and resolve any issues – from an ignition or sensor failure to low water pressure or overheating.

Cold Spots in Radiators

Trapped air, sludge or a stuck valve prevents even water flow to all radiators. Bleed radiators and flush sludge to restore full circulation.

Strange Noises

From banging and hissing to loud knocking or whistling, odd boiler noises usually indicate component wear or a system pressure issue that requires checking.


Leaking joints, seals, valves or internal components allow water to drip externally. Have an engineer locate and repair any leaks before major damage can occur.


Quick Recap

As we’ve learned, a combi boiler provides central heating and hot water on demand in a compact, wall-mounted unit. Combining space heating and water heating with no need for tanks or cylinders, clever heat exchanger design allows combis to quickly deliver hot water or warmth when we need it. By choosing the right size and features for our homes, maintaining them properly, and getting faults checked, we can enjoy endless hot showers plus a cozy house year-round.

Key Takeaways

  • Combi boilers provide hot water and heating on demand without tanks, maximizing space
  • Condensing combis offer over 90% efficiency, cutting costs and emissions
  • Annual servicing keeps combis working safely and efficiently for longer
  • Understanding fault codes allows early diagnosis and repair of common issues
  • When properly maintained, a quality combi boiler will provide reliable heating and hot water for over 10 years


What is the lifespan of a combi boiler?

With regular yearly servicing, quality combis typically last 10-15 years. Premium models from reputable brands can even reliably last well over 15 years before needing replacement.

Are all combis condensing?

No. Non-condensing conventional combis have lower efficiency at around 60-70%. Condensing combis are 90%+ efficient, using a second heat exchanger to extract latent heat. Always opt for a condensing combi boiler.

What size tank does a combi boiler need?

Unlike traditional boilers, combis don’t need any tanks for hot water storage. Their compact all-in-one design eliminates the need for hot water cylinders and tanks.

Can a combi boiler supply multiple bathrooms?

Yes – a suitably powerful combi can supply hot water to multiple bathrooms and showers at once. Choose your combi’s flow rate based on the number of bathrooms/occupants and hot water usage levels.

Do I still need room thermostats with a combi?

No, combis have inbuilt controls to set heating timetables and temperatures room-to-room. However, extra smart thermostats can be useful for more advanced heating control.

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