Combi boiler or conventional?

An in-depth look at combi boilers vs traditional or conventional boilers

Combi boilers are utilised to supply the household with both heat and hot water. These special types of boilers have been gaining popularity over the years in many homes because they are efficient and take up little space. Usually small and compact, these boilers can be placed in discreet areas on the kitchen or bathroom wall. Combi boilers are energy-saving because they do not store water, and they heat only the amount that is needed for tasks such as cooking and bathing.

In some cases, these boilers can be very easy to install. This because there is no need to install a cylinder & complicated controls, such as Y plans & S plans.

Unlike the conventional boiler systems which require complex installation, combi boiler systems are more convenient to install. There is no need for a tank dedicated to hot water since water is heated directly and produced almost instantly. One only has to link the combi boiler directly to the main water source and there will be an endless supply of hot water.

best combi boiler

Advantages of Combi Boilers

  1. Compact & space saving One of the many features of combination boilers is that they are small and compact. Because of this, they take up little space, and you will not have to worry about a cupboard or flood space area being taken up by bulkier boilers, additional pipework, and water tanks. Water storage, though, may be a better option when your house has more than one bathroom.
  2. Can be easier to install depending on current setup.
  3. Pressurised – All of your radiators will be under pressure, meaning better flow & higher efficiency.
  4. Everything in one place, generally simple to repair & maintain if you choose a credible brand.
  5. Can be more efficient than their cylinder counter-part syle system.
  6. Never run out of water! As there’s no store water, you never need to worry about running out.


Bear in mind, house size, flow rate & demand impact.

The make of the combi boilers can vary from one another. More powerful units, like the Vaillant 938 or Worcester Highflow can deliver around 20 liters per minute at a 35-degree rise. Other combi boilers, for example, a (24KW) Vaillant boiler 824. Will deliver around 11 liters per minutes at a 35-degree rise. When I say rise, I mean water temp is coming in at X (say 10 degrees)  means a 24KW boiler can heat this at 11 liters per minute to (add 35 degrees) to 45 degrees. However, if you run the tap faster this will drop, eg running a shower at 15 liters per minute might only give you 30 degrees. Therefore a small boiler is not recommended for busy large households and for very busy households, a combi boiler may not work well at all.

Combi boilers can definitely save money and energy for the household and even the commercial establishments. It may even surprise you that you can eventually save money enough to buy yourself a combi boiler unit. You can check out a good number of retail outlets and online stores when you want to buy a combi boiler unit.

Disadvantages of Combi Boilers

  1. Busy households, with appliances on & multiple outlets running, will cause this type of boiler to struggle. You can counter this with a store water style combi eg the Vaillant 938 or Highflow mentioned above. You must have the correct pressure coming in!
  2. No back up, stored systems generally have an immersion back up, to keep you going if the boiler goes off. with a combi, if it goes down, everything goes down!
  3. The flow of hot water is generally affected by the weather outside the house. This means that there is less flow when the weather is cold. This happens because the time it takes for water to be heated increases, owing to the cold temperature outside the house.

Combi boiler negatives

Installation Cost of Combi Boilers

Combi boilers are popular such that over 50% of the total number of new boilers installed in the households all over the UK is the combi type. There must be a very good explanation for this. Combi boilers are efficient, energy-saving, easy to install, compact, and take up little space. These attractive features have made combi boilers more popular than the traditional types of boilers.

Often install costs are cheaper due to the lack of complicated controls, airing cupboards and water storage cylinders. That cost in materials and labour to install is generally 30-40% more.

Boiler Maintenance 

A Combi boiler should be properly and regularly maintained so that it will last you a long time and you’re kept safe from poisonous gasses. For this, you should see to it that the central heating services that you hire are Gas safe registered. Servicing nowadays is more of an MOT checklist, looking for faults more than the old days, cleaning a boiler out, it’s also generally mandatory to keep a boiler covered under warranty.

We’ll be writing a servicing guide soon.


So, you probably came to this post with an older system looking to see the pro’s and cons. The answer is, (I have 17 years experience in heating as of writing this). If you live in a house smaller than 4 bedroom and people aren’t desperate to shower all at the same time AND you have good mains pressure coming in. 




  • […] Vaillant and Worcester Bosch are both domestic heating and hot water product manufacturers. Both are highly regarded brands with consistently high customer satisfaction scores. Both make all types of boiler, conventional boiler, combi boilers & system boilers. When trying to decide between a Vaillant and a Worcester boiler, there are a few things to consider, e.g. price, size, control, and energy efficiency. First, we should get an overview of the two companies and the range on offer. Ps if you’re interesting the in the: Pros & Cons of Combi boilers VS Conventional boilers . […]

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