Do I need a water tank or cylinder for my boiler, and what size does my household need?
It depends on your boiler type - combi boilers don't need one, while system and regular boilers rely on a cylinder sized to household demand and the number of bathrooms in use.
A combi boiler heats water on demand and has no need for separate storage, which is why many smaller properties are set up this way. A system or regular boiler works alongside a hot water cylinder instead, and older regular-boiler setups sometimes also have a cold water storage tank in the loft. There's no single correct cylinder size - it depends on the number of bathrooms, how many people are drawing hot water at once, and how the household actually uses it day to day. Undersizing leaves you running short at busy times; oversizing costs more to install and run than necessary. This is one of the first things worth discussing at survey stage, alongside boiler type if you're deciding between the two as part of a wider installation.