A set up that is rarely discussed is priority domestic hot water (PDHW) set up. PDHW means that when hot water is needed, the boiler temporarily stops providing heating (if it is on) and switches to producing hot water.
This is how a combi boiler works, but system and heat only boilers can also be set up to work like this with a hot water cylinder. The advantage is that it is possible to fit a smaller, more efficient, boiler with a smaller cylinder and often achieve better hot water performance than a very large combi boiler.
A common scenario is to fit a small cylinder, around 80 litres, which is set up to stay hot all the time, i.e. if some hot water is used or the temperature of the water drops due to little use over the day, the boiler will automatically switch to hot water mode and heat it up again. In this way, the boiler never spends a great deal of time heating up the hot water and is therefore never diverted away from heating long enough for the temperature of the home to drop.