The core challenge for large families
Hot water demand peaks — two showers, a bath running, and someone washing up simultaneously — is where a combi boiler's limitations show. A combi heats water on demand at a fixed flow rate (typically 10–13 L/min). A system boiler draws from a stored cylinder of typically 150–210+ litres at mains pressure.
Simultaneous usage
Combi boiler: two showers running simultaneously draws approximately 18–20 L/min — more than a combi can deliver comfortably. System boiler with 210L unvented cylinder: can supply multiple outlets at full pressure simultaneously. System wins decisively for simultaneous usage.
Morning rush hour
Large family, 3 bathrooms in quick succession: the system boiler's stored cylinder handles this without any pressure drop. A combi can handle sequential use but struggles if two bathrooms are running at once.
Running costs
System boiler: hot water in the cylinder slowly loses heat (typically 1–3°C/hour). Cylinder heat loss can add £50–£100/year. Combi: only heats when tap is open — no standby loss. Combi wins marginally on running costs.
Space
Combi: no cylinder needed. System: requires an airing cupboard or utility room space for the cylinder.
Verdict
3+ bedrooms, 2+ bathrooms, family of 4+: system boiler with a 210L unvented cylinder is almost always the right choice. The simultaneous hot water delivery difference is significant and daily.