Can I replace my old vented cylinder with an unvented?

Usually yes — most London houses have sufficient mains pressure (1.5+ bar static, 12+ L/min flow) for unvented. Survey mains supply first: pressure test at incoming stopcock. If pressure inadequate, install booster pump (£450–£950) or stay with vented + shower pump. Unvented installs require G3 certification, discharge pipe routing and removal of loft cold-water tank — typical retrofit £1,800–£3,200 all-in. Improved shower performance is the primary driver.

How often should I replace my hot water cylinder?

15–25 years for indirect cylinders (coil-heated); 10–18 years for direct cylinders (immersion). Replace when: visible corrosion or leaks; immersion heater repeatedly failing; insulation degraded (cylinder hot to touch); cylinder undersized for current family. Most failure modes are catastrophic (split tank, leak) so replace proactively at 15-year mark in family homes — cheaper than emergency replacement + flood damage.

Mixergy cylinder — is it worth £800 more?

For high-DHW-demand homes with variable consumption (family with teenagers), yes — Mixergy heats only the volume needed (30–40% energy saving vs traditional 100% heating). Smart app shows hot water available and usage patterns. For low-demand 1–2 person households, traditional cylinder + good insulation delivers similar energy efficiency at lower cost. Mixergy especially compelling with heat pumps where heating efficiency is critical.

Where should the cylinder go in a London terrace?

Airing cupboard in original location is standard — typically first-floor landing cupboard 700×700×1900mm, large enough for 180L cylinder + ancillaries. If converting airing cupboard to shower/storage, relocate to: ensuite cupboard, basement plant room, loft (with structural assessment for floor loading — 180L = 220kg full), or kitchen/utility room. Each metre of relocation adds £180–£420 of pipework. Avoid roof loft locations in heat pump installs (cold environment increases standing losses).

Hot Water Cylinder Replacement Cost London 2026 | Builderr

Cylinder types and what to choose

Unvented cylinder (Megaflo Eco, Telford Tempest, Heatrae Sadia Megaflo): pressurised system fed direct from mains; delivers full mains-pressure hot water to all outlets simultaneously (great for power showers and multi-bathroom houses). Stainless steel construction; £900–£2,200 supplied. Requires G3 Building Regulations compliance: discharge pipe to outside, expansion vessel, pressure relief valve, T&P valve; commissioning by G3-certified installer mandatory. Vented cylinder (Range Tribune HE, Albion Mainsflow): traditional copper or stainless cylinder with cold-water storage tank in loft; £550–£1,400. Lower pressure (gravity-fed, typically 1–2 bar at outlets) — limits shower performance unless pump fitted. Indirect cylinder: heated by coil from boiler or heat pump — single coil for simple system, twin coil for solar thermal + boiler combination, oversized coil specifically for heat pump (heat pump cylinders Telford Tempest HP, Mixergy HP). Direct cylinder: immersion heater only — appropriate for off-grid or low-DHW-demand homes. Choose: unvented indirect for most London family homes with high water pressure and multi-bathroom; heat-pump-ready indirect (180–250L with 3kW coil + immersion backup) if heat pump in plan.

Sizing — get this right

Sizing rule: 35–50L per person + 30L baseline. Family of 4: 170–230L. Family of 5: 205–280L. Higher end for power showers (12+ L/min) and multiple simultaneous bathrooms. Undersized cylinder = running out of hot water mid-shower; oversized = wasted heat (cycling losses 1–2 kWh/day) and physical space. Standard cylinder sizes 120L, 150L, 180L, 210L, 250L, 300L. Mixergy smart cylinders (mixergy.com) heat only top portion when small demand, full when needed — 30–40% energy saving vs traditional; £1,800–£2,800 supplied. Cylinder dimensions: 180L unvented ~1500mm tall × 600mm diameter; 250L ~1700mm tall × 650mm diameter — must fit airing cupboard (typical 700×700×1900mm). Space for cylinder + expansion vessel + secondary pump + smart controls: budget 800×800×2000mm cupboard. Recovery time: oversized boiler/coil heats 180L from 15°C to 60°C in 35–60 minutes (gas combi); heat pump 90–150 minutes (low coil temperature 45°C, slower).

Install, G3 and Building Regs

G3 of Building Regulations covers unvented hot-water systems and is the single most-failed inspection in domestic installs. Requirements: pressure relief valve to atmosphere via tundish to external drain (50mm air gap, copper-only pipe, dedicated route); T&P valve set 90°C 7 bar; expansion vessel sized at 8–17% cylinder volume; air gap above tundish; discharge pipe to outside terminating safely. Cost of full G3 install £400–£900 over and above cylinder supply; commissioning certificate £180–£280 issued by G3-certified installer. Discharge pipe routing is often the practical bottleneck in retrofits — pipe must run downhill to external air, no internal terminus permitted; typical Victorian terrace airing cupboard requires 4–8m of 22mm pipe routed externally. Cylinder location: airing cupboard ideal (insulated, central, accessible); loft acceptable but expansion losses higher; basement acceptable but pump pressures and discharge routing complex.

Heat pump cylinder specifics

Heat pump cylinders differ from gas-boiler cylinders. Larger surface area coil (3–5m² vs 1.5–2.5m²) — accepts heat at lower temperatures (40–50°C heat-pump output vs 65–80°C gas boiler). Cost premium £350–£800 vs standard cylinder. Sizing larger: heat pumps charge cylinder more slowly so larger buffer compensates — typical heat-pump cylinder 220–300L (vs 180–220L gas). Twin immersion options: 3kW lower (anti-Legionella weekly cycle to 60°C — heat pumps can't reach), 3kW upper (boost for surges). Hot water priority: heat pump diverts full output to cylinder when hot water demand triggered, suspending space heating. Programming: best to charge cylinder during off-peak electricity tariffs (00:30–05:30 Economy 7 or Octopus Cosy Tariff 04:00–07:00) to minimise running cost. Cylinder makers specifying heat-pump-ready: Mixergy HP, Telford Tempest HP, Newark Copper Cylinders Smart HP — typically £1,400–£2,800 supplied.

How Much Does a Hot Water Cylinder Cost to Replace in London?