How many times have you looked forward to relaxing in a hot bath only to find when you run your bath there is no hot water left in the system? This is where the combination boiler, or combi, scores over the regular boiler. The combination boiler provides domestic hot water on demand at all times of the day and night. Hot water for space heating e.g. radiators is provided on a separate circuit. The boiler can be set to provide space heating continuously, or not at all, or it can be programmed to switch on and off at times of the customer's choosing.

Since the combination boiler is fed directly from the main water supply it does not need an external cold-water tank, nor does it need a separate ''header'' tank for the heating circuit, nor does it need an external hot water cylinder to store hot water for domestic use. Thus, space is saved and the risk of flooding that can occur with a traditional system if any of these storage vessels fracture or develop a leak is eliminated. Another advantage of the combination boiler is that it has no external motorised valves, another item that can go wrong or develop a leak.

With regard to efficiency because the combination boiler does not store hot water externally there is no external hot water cylinder (nor the pipes that go with it) to loose heat to the surrounding air; consequently, systems employing combination boilers tend to be more efficient The efficiency is higher still if the combination boiler is one of the condensing models.