Combis work as sealed systems, providing hot water for both the taps and central heating system, heating the water directly from the mains as and when it is needed — meaning there is no need for a hot water storage cylinder, or a cistern in the roof space.
Combis are quicker, easier and cheaper to fit than system boilers, as well as space saving due to the lack of a cylinder or cistern. Water is delivered at mains pressure, so you can enjoy a more powerful (although not ‘power’) shower.
It’s a priority system, so it only satisfactorily deals with one heating need at a time. While fine for small families with one bathroom, larger families will experience poor flow rates when multiple outlets are used at once. Performance is also dependant on the diameter of the pipe entering the property: if it’s less than 22mm, then a combi is a bad choice.
System boilers are fitted to sealed heating systems, but unlike combis work on the principle of storing hot water in a cylinder, so they can feed several outlets at once at mains pressure. There’s no need for a cistern in the loft and the expansion vessel is built in.
Ideal for larger homes with higher demands, and as they have most of their major components built in (i.e. expansion vessel and pump), installation is quicker, cheaper and neater. Flow rates are usually high as water is delivered at mains pressure, and hot water is instantaneous.
Will run out of hot water if overused. Some installers claim they are more complex and prone to problems than regular boilers, such as pressure loss.
Regular boilers are now largely bought as replacements for homes with an open-vented heating system (i.e. supplied by means of a feed and expansion cistern in the roof space, meaning the system is open to air). Like system boilers, they work on the principle of stored water and require a separate hot water cylinder.
The water out of the taps will be at a good flow rate (not to be confused with pressure) and hot water can be supplied instantaneously. This is the ideal setup for a ‘power’ shower, which requires a cold water feed from the cistern and a separate electric pump
They’re more expensive to install, needing more components and pipework, and also take up more space. They can suffer from low pressure if the cistern is not located high enough, meaning additional shower boosters may be required. Hot water can run out.