Combi boiler advantages and limits
Advantages: cheapest install (£2,400–£3,400), smallest footprint (no cylinder), instant hot water (no waiting for cylinder heat), no Legionella risk. Limits: DHW flow rate capped by boiler output (e.g. 30kW combi delivers 12 L/min at 35°C rise). One shower at 8–10 L/min uses most of capacity; second shower will drop flow significantly. Mains cold pressure must be 1.5+ bar — many old London terraces deliver below this. Combi can't easily integrate solar thermal or low-temperature heat pumps.
System boiler advantages
System boiler heats water in a cylinder; cylinder stores 150–300 litres for instant access. Two showers simultaneously? No problem (cylinder discharges at mains pressure 16–24 L/min). Larger families with peak morning hot water demand? System wins. Integration with solar thermal, heat pump or future solid fuel? System is the upgrade path. Downsides: cylinder takes ~600×600×1500mm space (typically in utility, airing cupboard or large bathroom). Cost £2,800–£4,200 boiler + cylinder.
Renovation decision framework
1 bath, 1–2 occupants → combi (35kW for power shower preference). 1 bath, 3+ occupants → combi (35kW) or small system + 180L cylinder. 2 bath, any occupants → system + 210L cylinder, or combi 35kW if mains pressure excellent. 3+ bath → system + 250L+ cylinder, full stop. Future-proofing: planning a heat pump in 5–10 years? Install system + cylinder now (heat pumps need cylinder). Planning solar thermal? System. Off-gas grid? Different conversation — typically heat pump or oil boiler.
