A combination, or combi, boiler works as both a water heater and central heating boiler, all in one compact unit. Instead of having one boiler to heat water for the radiators and separate cylinder for hot water for bathing, the combi boiler uses one heat source for both, and also saves space. It is said to be the U.K.’s best-selling type of boiler due to their reliability and durability.
Combi boilers are more efficient than other boiler types since there is no water being stored, and thus there is no heat loss from the storage tank and the need to reheat water is eliminated. This saves on energy consumption. It delivers hot water to the taps and shower at main pressure without the need for a pump; however, the flow of hot water may be restricted when there are several taps running.
There are some disadvantages to the combi boiler. First, flow rate is not the same with a system that has a storage cylinder. Also, there may be a lower amount of water at a given time due to heat on demand. This means that the combi boiler may not be ideal for big homes where there may be high hot water demand at any one time.