Types of condensing boilers: Regular, Combi and System

Condensing boilers are between the most efficiency types of modern boilers. They operate with high efficiencies (above 90%...) and though they require a higher initial investment, they reward their buyers with much smaller energy bills.

Replacing old boilers

If your boiler is more than 10 or 12 years old, replacing it with a modern condensing boiler can reduce your heating bills by a third or more. In other words: fuel savings will quickly repay your extra initial cost.

Just take into account the types of condensing boilers: regular (also named open vent boilers and heat only boilers), combination (or combi) and system (also called sealed system boilers).

Combination boilers

Combination boilers do not use cylinders or water tanks. They give instant hot water and aren't based on stored hot water unlike other types of boilers.

Advantages of combination boilers

Since other types of boilers store the hot water in a cylinder for ulterior uses, and since the hot water is not used immediately, some heat is wasted and inefficiency is inevitable. That’s why combination (combi) boilers are in many cases the most efficient boilers.

But all depends ultimately on the size of your house and on features as the design of your heating system. Not all existing heating systems are compatible with combination boilers, and there are situations where other types of condensing boilers are preferable.

A qualified installer may help you on this issue, but you may take into account the tips and indications we list below.

Regular boilers (or open vent boilers or heat only boilers)

A typical regular boiler system – often named open vent boiler and heat only boiler - provides central heating and hot water incorporating a feed and expansion cistern, and also a hot water cylinder (often installed around or in the airing cupboard) and water tanks (usually located in the loft).

This is the design of old conventional boilers, but it can also be used by new high-efficient condensing boilers.

(Sealed) System boilers

System boilers also referred to as sealed system boilers, use hot water cylinders as regular boilers. But, unlike regular boilers, they do not need any water tank in the loft.

System boilers vs. regular boilers

System boilers are usually more efficient than regular boilers, since most major components of the heating are built in (which also allows an easier, quicker and eventually more efficient installation). They also remove the need for an expansion cistern and a feed, and also the need for water tanks, which allow faster responses and lower running costs.

Guidelines for choosing the type of condensing boiler

There are no absolute rules or guidelines to identify the ideal type of condensing boiler to a home’s particular needs. Manufacturers differ on their solutions, and there are different technological responses. Larger manufacturers as Bosh or Vaillant – with their different versions of high-efficient boilers for each situation - are good examples of those different solutions.

There are, however, some broad principles, that you may want to consider:

1- if you live in an apartment or other type of home with little space and no roof or roof space, then the combination (combi) boilers are the most suitable type to you.

2- if you have a loft space and do not want to use it with water tanks, then you should implement a combi boiler.

3- if you have low water pressure, then regular boilers and especially system boilers are often the most suitable.

4- if you have more than two bathrooms and large needs of heat and hot water, then system boilers might be the best solution.

5- if you have a very large house (e.g. with 3-4 bedrooms detached or semi-detached from bathrooms), than system boilers (and eventually high efficient regular ones) can be the best solution.