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What is a Combi Boiler? – Easy Guide

Combi Boiler

We recently wrote a useful guide about Combi Boiler Leaks, which looked into detail about central heating boiler leaks, how you spot them and how leaks in them are commonly found.

In this guide, and as a supplement to our previous article, this is a simple and easy guide as to how combi boilers (or combination boilers to give them their full name) work and what they can do. This article is just a basic guide to help highlight basic functionality and some of the key components vs a conventional hot water and central heating system, in a clear and concise manner.

Combi Boilers
Combi Boilers – Example House

As we always say, gas combi boilers need to be installed and maintained by Gas Safe certified engineer (do not do this yourself), but this diagram above shows a elements of a very basic layout of a combi boiler system in a UK house over two storeys.

How Does a Combi Boiler Work?

Using a gas combi boiler as an example, they are one of the most common systems installed in homes and businesses in the UK and, unlike a conventional boiler, do not generally have a hot water tank on the system. Instead, combi boilers produce hot water (more or less instantly on-demand).

The word ‘combi’, short for combination, comes from the fact that they both provide hot water for the central heating system and the hot water to the taps in your property.

Gas Combi Boiler
Gas Combi Boiler – Design (simplified)

As you can see from the diagram above, there are a several combi boiler components:

  1. The main body of the combi boiler
  2. This includes the combi boiler control panel and;
  3. The combi boiler pressure gauge
  4. Also the combi boiler display panel – showing things like F1 boiler errors
  5. The mains water supply – which feeds the water to the boiler
  6. The gas mains supply – to provide gas to the internal burner
  7. The (outward) ‘flow’ water pipes to radiators
  8. The (inward) ‘return’ water pipes back from radiators
  9. The hot water pipes from the boiler feeding taps etc.
  10. The condensate overflow pipe (shown above going outside)

Related to this, see our guide on how to find a water pipe.

There are also some other elements, not specifically shown above:

  • Boiler Gas Valve – to control the pilot and burner
  • Pressure Relief Valve (PRV) – a safety valve mechanism
  • Tundish Drains – to see if water is flowing out of the PRV
  • Expansion Vessel – to allow for expansion in the system pressure
  • Filling Loop – to re-pressurise the combi boiler
  • Heat Exchanger – transferring heat from the flame to the water
  • Boiler Fans – to vent gases from the flue (and separately for combustion)
  • Flue – including venting the gases from your boiler to the outside
  • PCB – printed circuit boards with the controls or ‘brain’ of the boiler
  • Circulating Pump – to circulate water in the system
  • Diverter Valves – to control if water is for heating or hot water
  • Temperature Sensors – safety controls and to monitor water temperature
  • Thermocouples – sensing device which helps to control the gas flow linked to;
  • The Pilot Light or Ignition Spark – to light the flame burners to heat water
  • Thermostat – external to the boiler to help measure property temperature
  • Other Boiler Components – various depending on the design and model

Combi Boiler Component Parts
Combi Boiler Component Parts

So hopefully you can see how, even though combi boilers are relatively simple to operate and use, they contain a lot of component parts and, ‘under the bonnet’, are complicated devices – another reason why you must use an accredited Gas Safe engineer who is qualified. Important with safety in mind in particular.

Clearly, what components each different make and model of combi boiler contain will vary, so a Worcester Bosch Combi Boiler will differ to an Ideal Boiler, Glow Worm Boiler, Vaillant Boiler, Baxi Boiler, Ferrolli Boiler and so on!

We discuss combi boilers in our new tool to help understand when Boiler Pressure Lowers and our excellent guide to Central Heating Leak Detection.

Electric Combi Boilers

Although gas combination boilers (or even LPG – Liquid Petroleum Gas) tend to be the most common in the UK, you can get electric combi boilers too. They function in a similar way but with electrical heating rather than gas. They have certain advantages vs gas:

  • They can be installed where you do not have a gas supply
  • They are often quieter than gas systems
  • They tend to be more simply designed, with less things to break
  • There’s no carbon monoxide risks (or flue for that matter)
  • The upfront cost can be less in some instances
  • They can be linked to solar panel systems in some instances

That said there are some disadvantages of electric systems:

  • They can be much more expensive to run (depending on gas vs electric prices)
  • There are less options and models on the market
  • May not be suitable for larger properties and higher demands
  • May put a lot of strain on the electrical system
  • Not necessarily greener (depends on the source of electric!)

Boiler Losing Pressure
Why my boiler is losing pressure.

Combi Boiler Leak Detection

Because this type of boiler is common in the UK, we are very familiar with leaks from combination boiler networks. People are often alerted to the fact that they may have a leak because of something that has alerted them on the boiler display – like a Vaillant F75 Error indicating a loos in pressure, which can be from a water leak.

Clearly, not only can a leak cause the pressurised system to stop working but resolving it, by finding the leak if there is one, can be tricky as pipes are often hidden in walls, floors and ceilings (causing ceiling water damage). This is where leak detection specialists like ourselves come into action.

We can help find water leaks by using a number of professional leak detectors, including:

  • Thermal imaging Cameras
  • Acoustic Listening Devices
  • Pipe Tracing and Detectors
  • Moisture Meters for Moisture Mapping
  • Pressure Testing Systems and Devices
  • Water Meter Monitoring to check usage and loss
  • Tracer Gas Leak Detection, useful for small leaks
  • Various Other Leak Detectors

You can read about this more in our page about Trace and Access Services.

Other Articles Relevant to Combination Boilers

We also have a useful guide regarding Vaillant F22 Boiler Fault.

Combi Boiler – FAQs

What is the downside of a combi boiler?

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One of the main downsides of a combi boiler is that they can struggle to supply volumes of hot water if multiple demands are being placed on the system. Related to that, they may be slower at, for example, filling a bath. They may not be able to supply a power shower too. Another disadvantage is that they can be complex and therefore costly to install and maintain (with lots of potential components to go wrong) and need regular servicing.

Are combi boilers being phased out?

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Currently, new homes will likely not be able to have natural gas combination boilers and will therefor require an alternative heating an hot water system. However, this ban applies to new homes so if you have a combination boiler in your property, you will not be required to scrap or replace your current system. This means that combination boilers will be around for some time to come.

What will replace gas combi boilers?

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There are various alternatives to replace natural gas combination boilers in your home or business. These include air source heat pump systems, ground source heat pump systems, hybrid heat pumps, solar panel systems, hydrogen boiler systems. Each system has their own set of costs as well as pros and cons to them. Plus, the technology is progressing constantly. Expect to see improvements in these systems over time and other new ones!

If you have need help finding a water leak at your home with our leak detection service, including Thermal Imaging Leak Detection – get in touch with our friendly team today. We offer these services to both domestic and commercial customers, including commercial leak detection. We are renowned for delivering a first-class exceptional service.

As experts in both water damage restoration and leak detection, we have you covered. logo logo