Can I undersize the cylinder?

Marginal undersize OK with quick reheat (gas boiler 90 min); ASHP undersize problematic (3–4 hour reheat from cold). Common error: 150L cylinder for 3-bed family (2 simultaneous showers exhaust within 8 min, no reheat ready for next shower). Cost difference 150L → 210L is £150–£250 — pay the small premium for headroom.

Vented vs unvented — which?

Unvented for all modern install — mains pressure throughout, no loft tank (no Legionella, no frost, no leak risk), space efficiency, modern simultaneous demand handling. Vented (open-vented) only for heritage retention where pipework or LBC prevents conversion.

How long does cylinder last?

Megaflo + OSO + Joule: 20–25 year stainless steel warranty + 5 years immersion + parts. Real life: 25–35 years if water quality OK (London hard water — fit limescale reducer to prolong life by 5–10 years); 15–20 years in high-mineral water without softener. TPRV + expansion vessel 10–15 year life. Service annually for maximum life + warranty validity.

Hot Water Cylinder Sizing London | Builderr

How Big Should a Hot Water Cylinder Be in a London Renovation?

London hot water cylinder sizing: 150L for 1–2 person flat (1 bathroom); 210L typical 3–4 person family (2 bathrooms); 300L for 5+ person or 3+ bathroom; 400L+ luxury whole-house with simultaneous multi-shower demand. Add 30–50L for ASHP system (longer heat-up time from lower flow temperature). Unvented (Megaflo, OSO, Heatrae Sadia, Joule) preferred — mains pressure throughout. Vented only where pipework prevents mains pressure conversion.

Sizing calculation

Hot water demand: 45L per shower (8 minutes × 8 l/min × 70% hot proportion), 80L per bath, 5L per hand wash, 12L per dishwasher cycle, 30L per kitchen sink wash. Daily total per person: 70–120L typical. Family of 4: 280–480L daily DHW = 3–5× cylinder recovery cycles per day at typical 2-hour reheat time. Sizing rule: cylinder volume should comfortably cover peak hour (morning shower rush) without exhausting hot water + recover within 2 hours for next peak. 1–2 person flat: 150L cylinder (Megaflo CL150, OSO Saga 150). 3–4 person family with 2 bathrooms: 210L (Megaflo CL210, OSO Super 200). 4–5 person family with 3 bathrooms simultaneous demand: 300L (Megaflo CL300, OSO Saga 300, Joule TwinCoil 300). Luxury whole-house multi-bath simultaneous: 400–600L (Megaflo CL400/CL500). ASHP system: add 30–50L over equivalent gas boiler size; ASHP cylinder Heatrae Sadia Megaflo Eco SolarReady or Joule Cyclone Renewable specifically designed for heat pump.

Cylinder selection

Unvented cylinder: stainless steel pressure vessel, mains-pressure DHW (3–6 bar typical), no loft tank, relief via temperature + pressure relief valve to outside, expansion vessel, immersion + boiler/ASHP coil(s). Brands: Megaflo (Heatrae Sadia, 25-year warranty), OSO Hotwater (Norwegian premium), Joule (Irish value), Telford Tempest, Range Tribune. G3-certified installer mandatory (HSE Approved Building Regs Part G3 unvented installation — competent person scheme; £450–£1,200 install). Coil arrangement: single coil (gas boiler or ASHP only), twin coil (gas + solar thermal, or ASHP + immersion backup, or two heat sources for redundancy), triple coil (ASHP + solar + immersion). Anti-Legionella: storage temperature 60°C+ minimum; cycle to 65°C weekly. Tundish: visible discharge of TPRV — continuous drip indicates fault. Annual service mandatory under warranty.

Plant room + location

Cylinder location: airing cupboard upstairs preferred (gravity feed + minimal pipe run to bathrooms); ground-floor plant room/utility acceptable but longer hot water dead-leg (mitigate with secondary return loop). Cylinder dimensions: 150L Megaflo 1290mm × 580mm diameter; 210L 1370mm × 615mm; 300L 1670mm × 685mm; 400L 1780mm × 750mm. Plant room minimum: 200mm clear all around for service access. Floor loading: 150L cylinder full = 175kg; 300L = 325kg; 500L = 525kg — concrete slab acceptable; suspended timber floor needs joist check. Secondary return loop: 22mm flow + 15mm return + bronze circulating pump (Grundfos UPS) + temperature-controlled run — £450–£1,200 install. Future-proof for ASHP retrofit: specify Megaflo Eco SolarReady size for heat pump even if installing gas boiler today — easier 2030+ swap to ASHP.