Indirect Unvented Cylinder London
Boiler-fed unvented hot water cylinders with immersion backup, installed across all 32 London boroughs. Megaflo Eco, Telford Tempest, Gledhill StainlessLite Plus, Kingspan Albion and OSO Super S specialists. Fully fitted from £1,495 inc. VAT — Part G3 notified, benchmark commissioned.
Same-day quotes. Like-for-like swaps typically 4–6 hours on site. Conversions from vented systems carried out the same day where access permits.
An indirect unvented cylinder is heated by your boiler through an internal coil, with an immersion heater fitted as backup. It is the most common cylinder type in London homes with a system or regular boiler — strong mains-pressure flow at every outlet, lower running cost than direct electric, and a 25-year tank warranty when serviced annually. Fully fitted in London from £1,495 (120L) to £2,595 (300L), inc. VAT and Part G3 notification.
What is an indirect unvented cylinder?
An indirect unvented cylinder is a sealed, pressurised hot water store that holds heated water under mains pressure with no loft tank and no open vent. The word indirect refers to how the water is heated — through an internal copper coil fed by your boiler — rather than directly by an immersion element. The boiler primary flow enters the top of the coil at 75–80°C, transfers heat to the surrounding stored water, and returns to the boiler 15–20°C cooler. The coil and the stored water never mix.
An immersion heater is always fitted to an indirect cylinder as a backup — typically a 3kW electric element positioned near the top of the tank, on its own fused isolator. It is designed for boiler-down emergencies: enough to heat the upper third of the tank to 60°C in about 90 minutes, sufficient for one shower or a half-bath. It is not a substitute for the boiler — running on immersion alone is 3× the cost of gas, and most cylinders will not hold full set-point on immersion against simultaneous demand.
Indirect unvented is the default specification for the vast majority of London 2-bed-plus homes with a system or regular (heat-only) boiler. It pairs the mains-pressure performance of unvented with the running-cost efficiency of gas. The dominant brands across the city are Heatrae Sadia Megaflo Eco, Telford Tempest, Gledhill StainlessLite Plus, Kingspan Albion Ultrasteel and OSO Super S. All five are duplex stainless steel construction with 25-year tank warranties when serviced annually. For the wider unvented family see our unvented cylinders London hub.
Indirect vs direct — which do you need?
The single most common question on the phone. The answer almost always comes down to whether you have a working boiler and a gas supply.
Indirect (this page)
- Heated primarily by your gas / oil / heat-pump boiler via an internal coil
- Immersion heater fitted as backup (typical 3kW element near the top of the tank)
- Lower running cost — uses boiler fuel rather than electricity
- Most common type in 90%+ of London 2-bed-plus homes
- Best paired with a system or regular (heat-only) boiler
Direct (electric only)
- Heated only by one or two immersion elements — no boiler coil
- Best for properties without gas (top-floor flats, off-grid annexes)
- Higher running cost on standard electricity tariff
- Often paired with Economy 7 or solar PV diverter for off-peak heating
- Simpler install — no primary pipework or motorised valve
Sizing guide — indirect cylinder by property
The Hot Water Association rule of thumb is 35–45 litres per occupant plus 60 litres per bathroom. Below is the practical London sizing matrix, with coil surface area and recovery time on a typical 24kW system boiler. Recovery is the time taken to bring the tank from cold to 60°C — the smaller the coil and the bigger the tank, the longer it takes.
| Size | Best for | Coil area | Recovery | From |
|---|---|---|---|---|
| 120L | 1 person, 1 bathroom | 0.45 m² | ~22 min (24kW boiler) | £1,495 |
| 150L | 2 occupants, 1 bath | 0.55 m² | ~28 min | £1,595 |
| 180L | 3 occupants, 1–2 bath | 0.65 m² | ~33 min | £1,745 |
| 210L | 3–4 occupants, 2 bath | 0.75 m² | ~38 min | £1,895 |
| 250L | 4–5 family, 2 bath | 0.85 m² | ~45 min | £2,195 |
| 300L | 5+ bed / 3 bath / freestanding bath | 1.05 m² | ~54 min | £2,595 |
* Inc. VAT, fully fitted, like-for-like swap. Add £350–£500 for vented-to-unvented conversion. Recovery times assume a 24kW boiler at full output with a clean coil and BS 7593-dosed primary circuit.
Brand comparison — which indirect cylinder?
Across roughly 90% of London installs we specify one of five brands. All are 25-year tank warranty, duplex stainless steel, factory-foamed and WRAS approved. The right pick depends on cupboard footprint, recovery requirement and parts-stocking pragmatism.
Heatrae Sadia Megaflo Eco
25-year tank · 2-year partsBest for: Most London 2–4 bed flats and houses. Strongest parts network across the M25.
Coil: Single high-efficiency coil, factory-foamed
Default specification on most ERL like-for-like swaps. Vessels, PRVs and immersion elements on every engineer van.
Telford Tempest
25-year tank · 2-year partsBest for: Mid-spec installs and conversions. Excellent value-to-warranty ratio.
Coil: Single coil, 22mm primary tappings
Strong choice for first-time vented-to-unvented conversions. Generous boss layout for re-routed pipework.
Gledhill StainlessLite Plus
25-year tank · 2-year partsBest for: High-recovery requirements — homes with system boilers above 28kW.
Coil: High-output coil, fastest recovery in class
Picked when a family of five wants back-to-back showers without recovery wait.
Kingspan Albion Ultrasteel
25-year tank · 2-year partsBest for: Tight cupboards — slim 475–550mm diameter footprint options.
Coil: Single high-performance coil
Specified often for Victorian London airing cupboards too narrow for a Megaflo 250L.
OSO Super S Indirect
25-year tank · 5-year partsBest for: Premium installs, heavy simultaneous-use loads, larger period properties.
Coil: Spiral coil, very high recovery
Premium price (~+15%) buys longer parts warranty and superior temperature stratification.
Indirect unvented cylinder cost London — 2026
Every quote is a fixed figure given on site after a 30-minute survey. No call-out fee on quoted work, no parking add-ons, no out-of-hours premium on scheduled installs.
| Job | What's included | Typical cost |
|---|---|---|
| 120L indirect unvented — supply & fit | Cylinder + G3 inlet group + expansion vessel + T&P valve + tundish + disposal + Part G3 notification + benchmark. | From £1,495 |
| 150L indirect unvented — supply & fit | Like-for-like swap, fully commissioned, 12-month workmanship guarantee. | From £1,595 |
| 180L indirect unvented — supply & fit | Standard London 3-bed family install. Primary coil connections, motorised valve check, dose to BS 7593. | From £1,745 |
| 210L indirect unvented — supply & fit | Typical for 3–4 bed with two bathrooms. Recommended Megaflo Eco or Gledhill StainlessLite Plus. | From £1,895 |
| 250L indirect unvented — supply & fit | 4–5 bed family / multi-bath property. Megaflo +£250 premium upgrade available. | From £2,195 |
| 300L indirect unvented — supply & fit | Larger period homes, freestanding baths, 3+ bath properties. Includes mains pressure verification. | From £2,595 |
| Vented-to-unvented indirect conversion | Strip loft tank + open vent, fit unvented cylinder, new G3 group, discharge route, Building Control notification. | From £2,495 |
| Annual G3 service — indirect cylinder | 12-point check: vessel pre-charge, T&P lift, PRV pressure, coil flow/return, immersion backup function, anode, benchmark refresh. | £155 |
| Immersion backup heater replacement | Drain to immersion boss, fit new element + thermostat, refill, vent, electrical safety test. | From £225 |
| Motorised valve replacement (indirect coil feed) | Replace failed 2-port or mid-position valve on the primary coil circuit, recommission. | From £245 |
* Inc. VAT. Building Regulations Part G3 notification included on all installs. Megaflo Eco premium upgrade +£250. Full price list on the pricing page.
Need a fixed-price quote for your property?
Call 0207 046 1363Our 5-step install process
A like-for-like indirect swap is one engineer on site for a half-day to full day. A conversion from vented is closer to a full day. Every install ends with a benchmark logbook completion and Part G3 notification — no exceptions.
Survey & isolate
30–45 minCold mains pressure measured (must be ≥1.5 bar dynamic), discharge route confirmed to outside, boiler output and motorised valve checked. Water and electrics isolated, old cylinder drained via gravity or pump.
Strip out
45–90 minOld cylinder, immersion wiring, primary coil pipework and any redundant gravity components removed and taken off-site for recycling. Airing cupboard cleaned and prepped.
Cylinder & G3 group install
2–3 hoursNew indirect cylinder positioned and levelled. Cold inlet group (stop, strainer, PRV, check valve, expansion vessel) fitted, primary flow and return repipied to coil tappings, immersion backup wired through fused isolator.
Discharge pipework
45–90 minTundish fitted with 300mm visible air gap, D2 discharge pipe routed in 22mm copper to BS EN 1057, continuous fall to a visible external termination above the gully. Single biggest compliance fail on inherited installs.
Commission & certify
45–60 minSystem filled, vented, primary side dosed per BS 7593:2019, run-up to 60°C, T&P witnessed lift, vessel pre-charge confirmed at 3.0 bar, benchmark logbook signed, Part G3 notified via WaterSafe / BESCA — certificate posted within 30 days.
G3 compliance & benchmark commissioning
Every indirect unvented cylinder install we deliver sits on top of a small stack of statutory rules. Each one is documented on the benchmark logbook that ships with the cylinder — the same logbook the manufacturer asks for if they're ever called on the 25-year tank warranty.
Approved Document G3 — Hot water supply and systems
Indirect unvented cylinders over 15L are a controlled service under Building Regulations Part G. Installation, commissioning and modification must be carried out by a person holding a current G3 ticket (BPEC HWSS, LCL Awards or City & Guilds 6189) and the work notified to Building Control either directly or through a competent person scheme.
HHIC Benchmark Commissioning Checklist
Every indirect unvented cylinder install is signed off on the manufacturer's Benchmark logbook — flow/return temperatures, pre-charge, T&P lift, set-point and discharge route documented. The signed logbook validates the 25-year tank warranty; missing entries void it.
Treatment of water in central heating systems
The indirect primary circuit feeding the cylinder coil must be dosed to BS 7593:2019 — inhibitor at correct concentration, and where appropriate, a magnetic filter fitted. Skipping this voids the boiler warranty as well as the cylinder warranty.
Water Supply (Water Fittings) Regulations 1999
Every new unvented install is notifiable to the water undertaker. Backflow protection (single check valve minimum, double check on category 3 risks) and WRAS-approved wetted components are mandatory.
18th Edition Wiring Regulations
The immersion backup circuit must be RCD-protected, on a dedicated fused spur, sized to the element rating (typically 13A for 3kW), bonded to the local earthing terminal in line with BS 7671 + A2:2022.
Legionella control
Stored water held at or above 60°C, distribution at 50°C within one minute of opening any outlet. On HMOs and care settings the legionella risk assessment under HSG274 / L8 is filed alongside the G3 certificate.
Common faults & worked examples
Five anonymised jobs from the last twelve months that show the typical failure modes on indirect unvented systems — and what each one costs to put right.
Indirect coil scaled up — slow recovery, no hot water in the morning
Hard-water London property, 12-year-old Megaflo indirect. Recovery time crept from 35 minutes to over 2 hours. Diagnosed primary coil scaling — boiler flow temp at 75°C but cylinder stat opening repeatedly. Drained primary side, descaled coil via inhibitor + filtration, refilled to BS 7593. Recovery restored to 38 minutes. Cost: £285 inc. VAT.
Motorised valve stuck — no hot water but heating works
Wandsworth 3-bed terrace, S-plan system. Hot water demand calling but the cylinder cold. Diagnostic showed the 2-port motorised valve on the coil feed seized closed. Honeywell V4043 replaced, wiring tested, hot water restored within 90 minutes on site. Cost: £245 inc. VAT.
Immersion backup failure during boiler breakdown
Hackney mansion-block flat, 180L Telford indirect. Tenant lost hot water Friday evening when the boiler PCB failed. Immersion backup tripped the consumer unit on first use — element earth-leaking. Replaced element + Sunvic thermostat same-day, full hot water restored that evening while boiler repair was scheduled for Monday. Cost: £235 inc. VAT.
Tundish dripping after heat-up — waterlogged vessel
Islington Victorian conversion, 210L indirect. Constant T&P discharge through the tundish on each boiler heating cycle. External expansion vessel pre-charge collapsed to 0.4 bar (target 3.0). 18L vessel fitted external to the cylinder, recharged, system retested. Cost: £210 inc. VAT — typical and one of the three most common faults.
Discharge pipe non-compliant — failed insurance survey
Kensington flat, 150L Kingspan Albion. Building insurance survey flagged the D2 discharge pipe terminating into a soil pipe with no air gap. Re-routed 22mm copper through external wall, continuous fall, visible termination 100mm above gully with mesh. G3 paperwork refiled. Cost: £380 inc. VAT.
Booking an indirect cylinder install?
Free 30-minute on-site survey. Fixed written quote on the day. Like-for-like installs typically booked within the week, conversions within 7–10 working days. G3 notification and benchmark logbook completion included on every job.
Areas covered across London
Indirect unvented cylinder installs and servicing across every London borough. Heaviest volumes from Camden, Westminster, Islington, Wandsworth, Kensington & Chelsea and Hammersmith & Fulham. Same-day call out across Zone 1–2, next-day across Greater London.
Frequently asked questions
What is an indirect unvented cylinder?
What's the difference between an indirect and direct unvented cylinder?
How much does an indirect unvented cylinder cost to install in London?
Can I use my existing boiler to feed a new indirect cylinder?
What size indirect cylinder do I need?
Does an indirect unvented cylinder need a G3 engineer?
How long does the immersion backup last on an indirect cylinder?
How often does an indirect unvented cylinder need servicing?
What is a coil and why does it matter on an indirect cylinder?
What happens if the motorised valve to the coil fails?
Can I fit an indirect unvented cylinder in a top-floor flat?
Do you notify Building Control?
What warranty do I get on an indirect cylinder install?
Why is the tundish dripping on my indirect cylinder?
How long does a new indirect unvented cylinder last?
Question not answered? Call 0207 046 1363 and talk to a G3 engineer directly — no call centre, no triage.
Service area
Service Area — All London Boroughs
G3 qualified indirect unvented cylinder engineers based in central London. Same-day cover across all 32 boroughs and the City. Average response time inside Zone 2 is 2 hours for emergencies; like-for-like installs typically booked the same week.
Replacing an indirect cylinder this month?
G3 qualified engineers across every London borough. Free 30-minute survey, fixed written quote on the day, like-for-like installs from £1,495 fully fitted. Building Regulations Part G3 notification and benchmark commissioning included. £5M public liability, fully insured.
London 247 Home Services Ltd · Company No. 17120057 · £5M public liability · G3 qualified · 24/7 emergency line