The challenge of scale
A combi boiler heats water on demand at a fixed flow rate — typically 10–13 litres per minute. For a family of 2 in a 2-bed flat, this is plenty. For a family of 5 in a 4-bed detached with 2 bathrooms, it's a daily frustration.
Morning rush hour
Two bathrooms in use simultaneously: shower 1 demands 8 L/min, shower 2 demands another 8 L/min — total 16 L/min. A combi boiler supplying 12 L/min will noticeably drop in pressure and temperature across both showers. A system boiler drawing from a 250-litre unvented cylinder can supply both at full mains pressure simultaneously.
Bath + shower simultaneously
Filling a bath (typically 15–20 L/min) while someone showers: a combi simply cannot cope. Pressure and temperature will drop sharply on the shower. A system boiler with a large cylinder handles this without any issue.
Morning recovery
After the morning rush, a 250-litre cylinder needs to reheat. With a modern system boiler this takes approximately 30–45 minutes. However, for most families the cylinder is large enough that it doesn't run out at all during a typical morning.
Running cost difference
System boilers have standby cylinder heat loss (typically £50–£100/year extra). But for large families, the hot water demand is so frequent that cylinder losses are minimised — the water is used before it cools.
Space
System boiler: requires a hot water cylinder (typically airing cupboard size). Combi: no cylinder. For large families, the performance gain far outweighs the inconvenience of housing a cylinder.
Verdict
4+ person household with 2+ bathrooms: system boiler with 210–250-litre unvented cylinder. The simultaneous hot water delivery difference is experienced daily. This is not a close call for large families.