A combi boiler is a shortened version of the words combination boiler which is one of the most popular and commonly installed boilers in the UK. They are popular because of their high efficiency qualities, money saving features and also space-saving characteristics.
With ordinary boilers, a copper cylinder heats the water in a storage tank. With combi boilers, they are designed to heat water for your radiators as well as heating the water you use in your home. This is where they get the name combi or combination because they combine both features in one small boiler unit. They heat water on demand rather than heating a whole tank of water. So when you turn a tap on the boiler will heat the water instantly.
With the radiators in your home the water inside them is recycled through the system back to the boiler and is re-heated again in the boiler. As long as the boiler is working, you will have unlimited hot water and heat in your radiators.
The beauty of these boilers is that they are much more efficient that traditional boilers with saving energy and gas costs. The feature of a combination boiler that makes them more efficient than the old standard systems is that with old systems heat is lost in the flue gases. With the combination boiler, a portion of the flue gas heat is extracted and used. In most homes in the UK, a water heater is one of the largest users of power and in many cases it is wasted energy. If you're out at work in the daytime, a conventional system still burns power to keep the water hot in the tank.
There are no pipe works and the need of space for a water tank isn't required. A small cupboard in the kitchen will be large enough. Combi boilers don't need an extra pump to increase water pressure as they work perfectly well with the normal pressure you get from your ordinary household tap.
Ok so that's the pros, what are the cons? Well they are great for a small family home or homes with one bathroom. If you have more than one bathroom say with two showers, only one shower could be used at a time. So for larger homes it is better to store water.