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Last Updated on November 27, 2023

How Does a Combi Boiler Work?

In summary, whichever fuel a combi boiler is running on, whether it’s mains gas, oil or LPG, it enters a sealed combustion chamber inside the boiler and an electric ignition sets it alight.

The primary heat exchanger within the boiler is then heated up to heat incoming water up to 60 to 85 degrees Celcius, flowing through all the radiators and heating up your home.

A secondary heat exchanger is used to divert central heating water to heat up fresh water from your mains water feed for your hot water taps and showers.

Combi is short for combination, a combi boiler combines home heating and on-demand hot water within the same compact unit.

That’s the basic explanation of how a combi boiler works.

In direct contrast, open vent (also called regular, standard or conventional) boilers and system boilers heat water that’s stored in a tank. A combi boiler works by using heat exchangers as already discussed.

Learn more about heat exchangers in this guide >

Heating the Radiators – Primary Heat Exchanger

The primary heat exchanger heats the water that goes into the radiators. That’s how a combi boiler works to heat your house. This water travels through the radiators and is recycled back into the boiler. The boiler keeps firing to reheat that water and keep it at around 60 to 85 degrees Celsius. This water can get quite dirty, as it cycles through the system constantly.

Clean Hot Water – Secondary Plate Heat Exchanger

The water that travels through the radiators and recycles back to the combi boiler is undrinkable. In order to give you clean hot water, the combi boiler pauses the central heating and diverts this water through to the secondary heat exchanger. This then transfers the heat through the exchanger to the incoming fresh cold water. When this switching happens, you will hear some boilers switching on and off as you open the taps.

DO NOT try to Install a Combi Boiler Yourself

How combi boilers are installed can be very complicated and requires a Gas-safe engineer. Sometimes it’s assumed that because combi boilers are compact, and don’t require extra tanks, it should be easy enough to install it themselves.

It’s not easy, and you shouldn’t try it unless you are Gas-safe.

Not only is it extremely dangerous because of the fuel, but it’s also illegal to install a combi boiler if you’re not certified. Illegal gas workers are prosecuted on a daily basis, and for good reason.

Ignoring the law and our advice to try and save some money by breaking the law and installing the boiler yourself is dangerous. You may be in breach of the law and can face hefty fines and possibly prison.

There are “armchair experts” online who may try to convince you to save money and install a boiler yourself. Please be warned, you will be putting yourself and your family, tenants or property buyers in danger.

No matter how good your understanding of how a combi boiler works is, below are some reasons why you should NEVER install a boiler yourself:

  • A Gas Safe Registered Engineer spends 5 years studying and hit their true level of experience in 5-10 years.
  • Flue Gas Analysers are required by law, which cost between £600 & £1000.
  • Flues Gas Analysers need to be calibrated annually and hold a certificate.
  • The suitability of the boiler for the property needs to be checked.
  • Boiler siting needs to be safe and comply with Gas Safe & Building Regulations.
  • Manufacturer-specified appropriate materials and fittings have to be used.
  • If you’re not Gas Safe Registered and make a new gas connection to a boiler you will face heavy fines or prison.
  • Equipment like Manometers test the gas supply and make sure the pressures are safe.
  • Gas integrity needs to be tested prior to connecting the boiler, and again once connected.
  • Let-by test & tightness tests are required.
  • Flow rates need to be checked and adjusted.
  • Gas Safe Engineers need to gas rate the boiler and check gas pressures and flue integrity.

After reading all of the above, if you still decide to chance-it and install the boiler and make gas connections yourself, you will invalidate the boiler warranty.

If you would like a Gas Safe Registered Installer to install your combi boiler or a certified heating expert to survey your property or answer further questions on how a combi boiler works, start by getting an online quote – start below.

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Which is the Best Combi Boiler?

It’s important to choose the right type and capacity (kW / kilowatt) boiler for your home. If you would like to get into the details of which combi boiler is best please read our guide >

On top of the capacity of the boiler, it’s also crucial to choose the right manufacturer. So, which is the best combi boiler manufacturer in the UK?

According to our installers, and our in-house heating experts and engineers, the answer to this question would have been different 10-15 years ago. But in 2021, the answer is pretty simple and straightforward. We think Viessmann are the best combi boiler manufacturer not just in the UK market, but in the world right now.

Every Viessmann boiler has a patented Inox-Radial Stainless Steel Heat Exchanger inside every boiler throughout their boiler range. These are built in-house in the Viessmann factory in Germany – something no other manufacturer can boast about.

Along with other unique innovations like PRV (pressure release valve) and condensate combined into one unit, and in-built shock arresters + 10 or 12 year warranties (when installed by Boilerhut), you really can’t go wrong with Viessmann.

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