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H ow do Combi Boilers work is a question we are asked over and over again by those looking to change their boilers and improve the quality of their home heating and hot water. If you have been researching boilers for a while now, you probably have asked this question yourself and haven’t found a satisfactory answer yet.

The truth is, most people have a basic idea of how a boiler works (it heats water up by a combustion process and then drives it into radiators to bring your home to a comfortable temperature), but Combi Boilers are a bit different from Standard (Regular) or System Boilers

Today’s post will look into the specifics of Combi Boilers, what makes them different from other types of boilers, how the Combi Boilers work and how to identify them, so that you can make the best decision about your home heating system. We have a lot ahead of us, so let’s start!

Combi vs Regular vs System boilers: What's the main difference?

The main difference between Combi Boiler and any other boiler that is found on the market is that Combi Boilers don’t have to store hot water due to their design.

Combi boilers combine two heat exchangers within the same compact unit, becoming highly efficient heaters that can provide hot water for the owners to use and at the same time are able to heat up the radiators. This is the concept behind the name (as you might have already guessed, combi is short for combination) which is main difference between combi and any other boilers found on the market.

However, there are a few advantages combi boilers have when compared to system and standard boilers, which might be enough for you to go with a combi boiler when the time of picking a new boiler comes.

Advantages of Combi Boilers

The mechanics and the design of Combi Boilers are brilliant: they do not require any water storage when compared to Standard or System Boilers: Combi Boilers receive cold water supply from the mains and heats it up on demand, whenever you need it.

Reduced Size

Combi Boilers are very well sized, and they can fit in kitchens, utility rooms or any other tight spaces at home which is an issue with Standard Boilers (also called Open Vent or Regular boilers) and System Boilers, both of which store hot water in tanks before distributing it through the house.

Combi Boilers are a very popular choice for people living in flats, or smaller sized houses as they do not need to worry about deciding where they will have to fit large water cylinders that Regular and System Boilers require.

Savings on Monthly bills

Another benefit of having a Combi Boiler installed in your household, has to do with your monthly bills. As all the water in the household will now be heated by the boiler and supplied straight away, there is no need of any water pumps that Standard old boilers require, which translates into savings on energy bills.

Installation costs

Combi Boilers are also cheaper to install, as they don’t require the extra equipment and pipe work that is needed when standard and system boilers are installed

hot water

Disadvantages of Combi Boilers

In no way are saying that combi boilers are a panicle of engineering and that they can be fitted into any household. Combi boilers do have some limitations which is worth exploring.

Large households

Combi boilers are not suitable for large households with more than two bathrooms. And most of the readers will ask why. The reason behind this statement is simple: the lack of pressure from the mains. As we mentioned earlier on, Combi boiler takes the water from the mains and distributes through the household, and if the home is a well sized one, there will not be enough pressure to supply a number of taps of showers at the same time. This can particularly be frustrating when people are taking showers and they receive droplets.

A water pump could be installed to aid this issue, but again, it will be an added cost.

Old pipework

What about if you have a household with old pipework? Well, there might be an issue installing a combi boiler due to the fact that old pipework might not be able to cope with the mains pressure. This applies to both radiators and taps. If you are keen, you can get new pipework installed, but with the downside of an added cost.

Combi Boilers vs Combi Condensing Boilers

Before we jump into this section, it is worth mentioning that most of new boilers, including combi boilers, will fall under the class of ‘condensing boilers’, as this is just a term to specify additional boiler efficiency features.

Thanks to their much more effective ‘heat exchangers’, Combi Condensing boilers harness the hot gasses produced by the combustion of fuel towards the heating of water. In older boilers, these gasses would go straight out the flue and be wasted, but Combi condensing boilers condense the water vapour from the heated exhaust gasses, and reintroduce it in the heating system. Great idea!

New boilers installed after 2005, mostly will be Combi Condensing boilers, due to the regulations that came out that year stating that all new Combi boilers must be condensing type. This is good news for all of us, as Combi Condensing boilers are even more efficient and represent the benefits of lower energy bills for your economy, and a reduced carbon footprint for the environment.

If you recently got a new boiler installed and you are wondering if your boiler engineer installed a condensing boiler, then do not worry! It must have been a condensing combi boiler due to the regulations that came out in 2005 stating that all new combi boilers must be condensing type!

Combi Condensing boiler efficiency

Having a highly efficient Combi boiler in your household is the best way of saving money on energy bills, while you help the planet by reducing pollution. Heating accounts more than the 50% of your annual electricity bill, so even if you are not worried about global warming you may want to think about changing your old boiler and saving a few pounds (up to £ 400) this year.

Boiler efficiency ratings and markings

SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) measures boiler efficiency mainly focusing on how well they convert fuel into heat. SEDBUK use two different ratings

  • SEDBUK 2005 – uses letters to rate efficiency, being A-rated boilers above 90% efficient) and
  • SEDBUK 2009 – rates the efficiency as a percentage

Some say that condensing boilers reach up to 99% efficiency. We think that, realistically, modern Combi Condensing boilers work at around 90% efficiency for the majority of installations that happen within our business, including top of the range boilers. Older, non-condensing boilers offer on average 75% efficiency and older types offer around 55%-60% efficiency.

new combi boiler savings

Combi Condensing boilers for larger homes

Combi Boilers are perfect for small properties, but there are also Combi Boilers suitable for larger properties with more than one bath. They are called Storage Combi Boilers, and they house a water tank within the same unit. This makes them a bit bigger than standard Combi Boilers, but they are great option, as they can supply hot water anywhere in your household at the same time without losing pressure.

Are Combi Condensing boilers hard to install?

Due to their smaller size, Combi boilers are easier to install than Standard boilers could ever be. Nevertheless, you will have to hire a professional worker to install it for you due to the safety reasons, and it’s also a legal requirement to get the boiler installed by a certified gas engineer.

To know more about boilers installation and our boiler services, get in touch with us today and get a free quote from Boiler Repairs in Glasgow.


So hopefully we have answered any of your questions about Combi boilers and Combi condensing boilers. Should you have any further questions, or if you think we have missed something important, please do not hesitate contacting us and we will do our best answering your questions and suggestions!

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