Pros and Cons of Combination Boilers

Over half of the new boiler sales in the U.K are currently of the combination type.

Commonly known as the combi, the combination boiler basically heats water as it is required, either by the hot water taps, or the central heating system. It is known as a sealed system, as the hot water expands into a sealed expansion tank. In a traditional central heating system, the boiler heats the water at regular intervals and this water is then stored in a copper cylinder for use on demand.

There a number of combi boiler types.

The most popular is the combination (instantaneous) boiler. As the name suggests, it generates instant hot water from the cold water mains and supplies the hot taps on sinks, baths and showers.

Next comes the combination (instantaneous condensing) boiler. As we have seen, instantaneous refers to instant heat, whereas the condensing part refers to the greater efficiency of the boiler which extracts further energy from the burning and exhaust gases.

A further variation is the combination storage boiler, which still provides instantaneous heat, but has a capacity for storing the heated water for later use. A large version of this is the combined primary storage unit, which is a combi complete with very large hot water storage.

And, of course, to follow the logic of combi boiler development there’s the combined storage condensing, which is instantaneous, is very efficient and stores the heated water.

Now, there are a number of pros can cons when using a combi boiler.

Firstly, lets look at the pros.

A basic combi system has fewer bits and pieces (no cylinder, or cold water tank feed system for example), so the installation often costs less than a more traditional system. The installation is therefore potentially easier and quicker.

It also uses energy more efficiently, as it only heats the water when needed, and does not progressively go cold in the water cylinder.

And one of the key benefits, mains pressure on all hot and cold outlets.

Now the cons.

Combi boilers don’t usually have quite the same consistently high hot water temperature and pressure as the traditional system. This is usually more noticeable in the winter months.

And if you have a large property, with a number of taps providing water for numerous sinks, showers and baths, the combi boiler might not be able to cope with the demand for large volumes of hot water across the whole system.

Technically, the combi boiler has two heat outputs, one for the domestic hot water (“DHW”) and one for the central heating (“CH”). The latter is usually lower than the former, which means that for large houses, the combi system might struggle to heat it.

Water pressure is also important and will have to be checked, as the Combi is directly fed with cold water from the mains.

One of the main cons is the combi’s vulnerability to hard water, which effects most properties in the U.K. The heat exchanger within the boiler is particular prone and many consumers consider fitting water softening devices.

A further problem is that should the system fail, there is no electric immersion heater, which is often a very useful back-up.

And whereas installation should be quicker, sometimes the installer will have to fit an upgraded diameter of gas supply pipe.

The top of the range combi boilers can of course compete with the best open systems, but that depends on the budget available.

Choosing between a combi and more traditional boiler really comes down to the size of the property and the demand for hot water from both the occupants of the property and the central heating system.