What is the difference between a combi boiler and a conventional boiler?

As well as taking care of your heating needs, combi boilers provide instant hot water. They have the advantage of freeing up space in a home, because there's no need for a hot water cylinder (tank) like there is with a conventional boiler. 

In most cases, heating water instantly is more energy-efficient than 'stored' hot water systems. However, the flow of hot water is slower than if it was coming from a cylinder, so a bath will take longer to run. Some combi boilers can in any case also heat water in a cylinder.

Modern boilers generally no longer have tanks in the loft to 'pressurise' the system through gravity. Instead they are sealed systems, and typically only require manual topping up from the mains water supply when the internal pressure has dropped (usually because of tiny leaks). This is a simple operation that takes a few seconds, and involves opening a valve on a pipe below the boiler.

When chosing which boiler is best for your home, you first need to choose which type. There are three main types of boiler: combi, system and conventional.

Combi boilers

Combination boilers - more commonly known as combi boilers - are the most popular kind in the UK and provide heat and hot water with no need for water tanks or cylinders. You can pick either a gas or electric combi boiler.

Pros

• you get unlimited heat and hot water when you need it.
• there's no need for a tank in your loft.
• they don't take up much space.

Cons

The water pressure might be reduced if you need hot water from more than one tap at a time.

Combi boiler installation is quite tricky.

Suitable for

Almost any home, but best for homes where lots of people won't need lots of hot water at the same time.

System boilers

System boilers - also known as sealed system - come with a water cylinder (which usually sits in an airing cupboard) and no water tank.

Pros

• there's no need for a tank in your loft.
• you can get hot water from multiple taps at the same time.

Cons

• you don't get hot water instantly.
• the hot water can run out and you'll have to wait for it to reheat.
• you need to find room for the cylinder somewhere.

Suitable for

Homes which need to have hot water in more than one place at the same time.

Conventional boilers

Conventional boilers - also known as open vent or regular boilers - have both a cylinder and a tank.

Pros

You can get hot water from multiple taps at the same time.

Cons

• the hot water can run out and you'll have to wait for it to reheat.
• you need to find room for the cylinder and the tank.

Suitable for

Homes which need to have hot water in more than one place at the same time.

Condensing boilers

Condensing boilers aren't strictly a boiler type - instead it's an attribute your boiler can have, e.g. you can have a condensing combi boiler or a condensing conventional boiler.

Condensing boilers are very energy efficient because they capture some of the heat which would escape from the flue of a non-condensing boiler and re-use it.  This means they get more heat from the same amount of fuel, which will save you money on your heating bills.

All new gas boilers have had to be condensing since 2005 (although in exceptional circumstances non-condensing boilers are allowed).

Energy-efficient boilers

All new boilers are energy-efficient - since 2010 all new boilers must be A-rated for energy efficiency, or at least 88% efficient.

The energy efficiency rating system for boilers is called SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK).  All boilers are assessed and given a rating to help you pick a boiler that is energy-efficient, or see how efficient your existing boiler is.

Source: Uswitch