Conventional Boiler vs Combination Boiler: Pros And Cons - DSB Ltd
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Combination boiler installation from DSB

Conventional Boiler vs Combination Boiler: Pros And Cons

Conventional Boiler vs Combination Boiler: Pros And Cons

When looking at the differences between a Combination Boiler vs Conventional Boiler, it’s important to bear in mind your requirements, and also the suitability for your home.

Not all homes are the same, and family sizes differ, so the demands specific to each home will determine your needs for a Boiler system.

Below, we outline the main differences between the two, and the pros and cons associated with each.


Conventional Boilers (open vent)

Conventional Boilers are great for larger houses which have multiple bathrooms.

These boilers feature both a cold water tank, a feed and expansion tank, AND a hot water cylinder.

Both tanks are stored in the loft, with the cylinder being stored in the house below (the cold water needs gravity to reach the cylinder).

As the water is cold when it’s fed into the cylinder, it does mean you have to wait for the water to heat.


Larger houses

A home that has two or more bathrooms has a higher demand for hot water. Conventional boilers store the hot water until it runs out. If two hot taps are on at the same time, you’ll find no change to the water pressure.


Replacing the boiler

Unless you’re replacing your boiler with another Conventional Boiler, it’s usually necessary to replace the whole heating system and its components. This includes pipework, radiators, and the boiler itself.


Due to their size (two tanks plus one cylinder), a lot of space is needed.

Hot water

With the hot water being stored, once it runs out it takes time to reheat.

Cylinder insulation

The cylinder needs to be insulated and if this isn’t done well, the stored hot water will lose its heat.


Combination Boilers (combi)

These boilers are both a water heater AND a central heating boiler contained in one system.

They don’t have a tank or cylinder and can be either gas or electric.

As you have access to unlimited hot water, Combi Boilers are the most popular type of boiler.



Combi Boilers must have an efficiency rating of 92%, therefore, changing from a Conventional Boiler to a Combination Boiler will generally reduce your utility bill.


As everything is stored in one unit, they are cheaper to run due to the hot water not constantly being heated. They’re also quicker and easier to install.


Hot water is unlimited and instant due to them always being on standby.


Combi Boilers are the ideal fit for any kind of home as they’re small enough to fit inside a cupboard.

Cleaner water

As there’s no tank, the water runs directly from the mains eliminating the risk of “sludge”, consequently resulting in safer water.


Running the water straight from the mains means the water pressure rarely drops.


No tank

No water tank means no hot water storage. If the boiler breaks, there’ll be no hot water until it is fixed.

Multiple Taps

As there is only one unit, running hot water from multiple taps will affect the water pressure. This also means if your house has more than one bathroom, you won’t be able to simultaneously run the bath or shower.

Water pressure

If the water pressure in your area is naturally weak, a pump will help increase this.


To replace a Conventional Boiler with a Combi Boiler will cost you time and money as the whole heating system is likely to need replacing.


System Boiler

Aside from the two more popular options, there is also a third option; a System Boiler.

Typically, these are a cross between a Combi Boiler and a Conventional Boiler with the water feeding from the mains into a hot water cylinder.


Water pressure

As the water is fed straight from the mains, the water pressure is high.

High demands

The presence of a hot water cylinder means the hot water meets the demands of a larger home.



System Boilers require more space as they are larger than Combi Boilers.

Cylinder size

The space available depends on the size of the cylinder you’ve installed. This consequently affects how much hot water is available.


If you need further help and advice regarding your Boiler, or simply want to enquire about updating yours, contact us to chat to one of our highly skilled engineers.