Can I have two showers running simultaneously on a combi?

Limited — 30kW combi DHW typically 12 l/min (one good shower at 8 l/min + sink at 3 l/min just OK); two showers (16 l/min combined) exceed DHW capacity = flow drop on both. 35–40kW combi handles two showers marginally. Two simultaneous showers reliably need system boiler + unvented cylinder.

Should I convert from heat-only to system?

Often yes — at boiler replacement, convert from heat-only (cold tank loft + vented cylinder) to system (mains-pressure unvented cylinder, no cold tank). Benefits: better DHW pressure + flow, no loft tank (no Legionella risk, no frost risk, no leak above ceiling), more loft space. Premium £450–£950 for conversion above like-for-like boiler swap; usually worth it.

Will gas boiler be banned?

New dwellings: gas connection prohibited from June 2025 (Future Homes Standard 2025). Existing dwellings: gas boiler replacement banned proposed for 2035 (under consultation — date may slip). Repairs + servicing existing gas boilers permitted indefinitely. Plan major renovation for ASHP if 2026+ — premium upfront but future-proof + BUS grant + lower running cost on time-of-use tariff.

Combi System Heat-Only Boiler London | Builderr

Combi, System, or Heat-Only Boiler — Which for a London Renovation?

London boiler types: combi (instantaneous DHW from mains — small home, 1 bathroom, no cold tank, no cylinder — most common new install £1,800–£3,800); system (stored DHW in unvented cylinder, multi-bathroom multi-shower, mains-pressure hot water — £3,500–£6,500); heat-only / regular (open-vented system with cold storage tank loft + vented cylinder — heritage / large old systems being phased out). ASHP increasingly default for new install — gas boiler ban for new dwellings from 2025; existing dwellings 2035+.

Combi boiler

Combi (combination) boiler: single appliance providing heating + instantaneous domestic hot water (DHW) from mains. No hot water cylinder, no cold water storage tank. DHW heated on demand as mains water flows through plate heat exchanger inside boiler. Models: Worcester Bosch Greenstar 30i/35i, Vaillant ecoTEC 832/835/838, Baxi Platinum+ 33/40, Ideal Vogue 32/40. Output: 24kW (1 bathroom small flat), 30kW (small house, 1 bathroom + 1 en-suite), 35kW (3-bed terrace, 2 bathrooms not simultaneous), 40kW+ (4-bed with 2 simultaneous showers — borderline; system better). Pros: compact, no tank + cylinder space, lowest install cost (£1,800–£3,800 supplied + installed), instant unlimited hot water (until simultaneous demand exceeds DHW output), mains-pressure hot water. Cons: simultaneous demand limit (2 hot taps running drops flow rate significantly), depends on mains pressure (low pressure = poor DHW performance), inefficient for very low DHW demand (boiler fires for tiny demand), service life 8–12 years (heat exchanger fouling). Best for: 1-bathroom flat, small 2-bedroom house, where space premium + simultaneous DHW demand modest.

System boiler

System boiler: provides heating to radiators/UFH + heats stored DHW in unvented hot water cylinder. No cold storage tank loft (mains-pressure throughout). Models: Worcester Bosch Greenstar System 25Si/30Si, Vaillant ecoTEC plus 412/415/418/424. Unvented cylinder (Megaflo, OSO, Heatrae Sadia Megaflo): 150L (2 bath family), 210L (3-bath family), 300L (4+ bath large home), at mains pressure (3–6 bar typical) = high-pressure DHW + simultaneous multi-shower without flow drop. Cost £3,500–£6,500 supplied + installed (boiler + cylinder + airing cupboard cylinder plinth + filling loop + expansion vessel). Pros: high simultaneous DHW demand handled (limited only by cylinder volume + heat-up time); mains-pressure throughout = excellent shower performance; well-suited to family home with 2+ bathrooms. Cons: cylinder takes airing-cupboard or plant-room space; heat-up time 30–60 min from cold; more components = more service items. Best for: 2+ bathroom homes with simultaneous demand expectation; large family; powerful shower preference.

Heat-only + ASHP transition

Heat-only (regular) boiler: traditional open-vented system — boiler heats radiator + vented hot water cylinder; cold water storage tank in loft feeds cylinder (gravity-fed); separate small expansion tank for heating circuit. Heritage systems — being phased out in new install (combi or system + unvented cylinder preferred). Retained where: lead pipework limits mains pressure conversion; heritage Listed Building where cylinder cupboard + loft tank in original location; part of phased refurbishment retaining existing emitters. ASHP transition: gas boiler ban for new dwellings April 2025; existing dwellings boiler replacement 2035+ (under consultation). ASHP install for retrofit: see [[heat-pump-vs-gas-boiler-london]] + [[ashp-vs-gshp-london-renovation]]. Typical ASHP install £8,500–£14,500 with BUS grant £7,500 (eligibility: replacing gas boiler, MCS installer). Builderr default new install today: ASHP preferred for whole-house renovation + extension; gas combi/system where ASHP not feasible (no outdoor unit space, owner preference, retrofit constraint).