Heating · Glossary
System Boiler
A boiler that heats radiators and a separate unvented or vented hot water cylinder, with most components (pump, expansion vessel) built into the boiler casing.
A system boiler works with a separate hot water cylinder (vented or, more commonly today, unvented). Compared with the older heat-only design, the system boiler integrates the circulation pump, expansion vessel and motorised valves into the boiler housing, reducing pipe runs and cupboard clutter.
System boilers are the right choice for Cornwall properties with multiple bathrooms or high simultaneous hot water demand — a 200L unvented cylinder can deliver two showers at full mains pressure indefinitely. They also pair well with solar thermal, immersion heaters, and (with the right buffer arrangements) heat pump pre-heat.
Common 2026 specs we install include Worcester Bosch Greenstar 8000 Style, Vaillant ecoTEC pure and Viessmann Vitodens 200-W, paired with a Mixergy or Telford 200-300L unvented cylinder. See our boiler installation page.