A system boiler works with a separate hot water cylinder to store and deliver large volumes of hot water. They're the preferred choice for larger homes and families with high hot water demand.
How a system boiler works
The boiler heats water and circulates it through the central heating system in the same way as a combi. But instead of heating domestic hot water on demand, a system boiler heats a large volume of water and stores it in an insulated cylinder — typically 150–300 litres.
When you open a hot tap, hot water flows from the cylinder at full mains pressure (if an unvented cylinder is fitted). The boiler monitors the cylinder temperature and periodically reheats it to maintain the set temperature.
Key advantages
- Multiple simultaneous outlets: With 250 litres of stored hot water, you can run two showers, a bath, and the kitchen tap at the same time.
- Mains pressure hot water: An unvented cylinder delivers powerful showers without a pump.
- Solar thermal compatible: A system boiler can work alongside solar thermal panels to pre-heat the cylinder.
The one downside
You can run out of hot water if demand exceeds the cylinder capacity. With a well-sized cylinder, this is very rare — but it can happen during busy mornings in large households.