Boiler-fed coil · Immersion backup · G3 certified
Indirect Unvented Cylinder London
Boiler-fed indirect unvented cylinders for gas system and heat-only boilers. Standard specification for 2+ bathroom homes. Megaflo Eco Indirect, Telford Tempest, Gledhill Stainless Lite. Building Notice and G3 certificate included.
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Indirect unvented cylinders — the mains-pressure standard for boiler-fed homes
An indirect unvented cylinder heats the stored water via an internal coil connected to the primary side of a gas or oil boiler. The boiler circulates hot primary water through the coil, which transfers heat to the stored domestic hot water through the coil wall. Because the primary circuit is a closed sealed system (not the drinking water) and the DHW is at mains pressure downstream of the pressure-reducing valve, the two circuits are properly separated for hygiene while sharing the boiler as the single heat source. This is the standard cylinder specification for any UK home with a gas system boiler or heat-only boiler.
The reason to specify an indirect rather than a combi boiler (which has no cylinder at all) is straightforward — simultaneous hot water delivery. A combi boiler modulates its output to match the flow rate at the tap or shower, which means a 24kW combi delivers about 10 L/min at 40°C rise. That is comfortable for one shower. When a second shower opens simultaneously, the flow rate at each falls sharply because the boiler cannot heat 20 L/min at once. An indirect cylinder holds 150–300 litres of pre-heated water at 60°C, so simultaneous demand is served from the storage volume at the full mains flow rate at each outlet — 12–16 L/min at each of two showers running at the same time.
The four common London scenarios where indirect unvented is the correct specification: gas system boiler feeding 2+ bathrooms, heat-only (regular) boiler retrofit from a loft-fed vented system to mains pressure, large family home with 4+ bedrooms and multiple ensuites, and HMO or serviced-apartment property with boiler-fed heating and DHW plus statutory Legionella controls. In each of these scenarios the indirect topology matches the heat source (gas) to the storage volume (cylinder) via a proper coil arrangement, delivering mains-pressure DHW at every outlet.
Every indirect unvented cylinder we install is by a G3 competent engineer (UK Certification Ltd certificate 136359 issued 8 September 2025, expiry 18 August 2030) with Water Regulations 1999 competency (WaterSafe registration, certificate 136356 same period). Every install includes a Building Notice submitted to the local authority Building Control team, a G3 competency certificate, a secondary 3kW Incoloy immersion factory-fitted as backup DHW, and a 25-year cylinder warranty from the manufacturer. Public liability £5,000,000 via SiriusPoint through Eaton Gate MGU, policy BE26ACTT000000018221, period 07/05/2026 to 06/05/2027.
When an indirect unvented cylinder is the right specification
System boiler with 2+ bathrooms
Combi boilers struggle to deliver simultaneous hot water to two bathrooms at flow rates the occupants notice. A system boiler feeding an indirect unvented cylinder decouples the storage volume from the boiler flow rate — you get mains-pressure hot water at every outlet simultaneously, at the shower head performance of the mains supply.
Heat-only (regular) boiler retrofit to mains pressure
Traditional gravity-fed hot water systems with a loft tank and heat-only boiler suffer chronically weak shower pressure (0.1–0.3 bar from the header tank). Replacing the loft-fed vented cylinder with an indirect unvented cylinder delivers 3–4 bar at the shower head from the mains, without changing the boiler.
Large family home with high simultaneous demand
4+ bedroom Victorian and Edwardian London terraces with 2 or 3 bathrooms need cylinder capacity plus boiler heat input capacity to deliver comfortable simultaneous showering. An indirect unvented with a 24kW+ system boiler and a 210L+ cylinder is the sizing that matches this demand.
HMO with multiple bedsits or ensuites
HMOs above 6–8 occupants specifying a boiler-fed cylinder benefit from the indirect topology — the primary boiler heats the cylinder efficiently, and a secondary immersion is available as backup DHW. Full L8 Legionella controls achievable with rapid stored-volume reheat.
Coil types — single, twin, high-recovery, solar-ready
The coil configuration matters more than the cylinder brand — it determines reheat speed, dual-fuel compatibility, and future-proofing for solar or heat-pump retrofit.
Standard single-coil
One coil connected to the boiler primary circuit. Simplest, cheapest, and the default specification for most single-fuel gas system boilers. Coil surface area typically 1.4–2.6 m² depending on cylinder size.
Twin-coil (dual heat source)
Two coils — typically one connected to the boiler primary and one connected to solar thermal panels or a heat pump. Enables the property to use whichever heat source is cheapest or most available. Sized so the solar/heat-pump coil is lower in the cylinder and the boiler coil is higher.
High-recovery coil
Larger coil surface area (up to 4.0 m²) for faster reheat — a full 210L cylinder reheats from cold in 25–30 minutes rather than the standard 40–45 minutes. Specified for HMOs, high-occupant homes, or properties running back-to-back showers.
Solar-ready twin coil
Twin coil sized specifically for a solar thermal retrofit — solar coil bottom of cylinder (larger surface, low-temperature transfer), boiler coil top. Standard practice on 2010-onwards eco-refits under the RHI scheme.
Sizing guide — cylinder capacity plus boiler heat input
Rule of thumb: 50L per occupant for baths, 30L for showers only. Boiler kW rating must match the coil surface area — undersized boiler leaves the cylinder cold-recovering slowly; oversized boiler cycles inefficiently.
| Occupants / property | Cylinder size | Boiler capacity |
|---|---|---|
| 1–2 people 1-bed flat | 120–150L | 18–24 kW system |
| 2–3 people 2-bed flat | 180–210L | 24–28 kW system |
| 3–4 people 3-bed home | 210–250L | 28–32 kW system |
| 4–5 people 4-bed home | 250–300L | 32–35 kW system |
| 5+ people or HMO | 300–400L | 35+ kW system, consider twin-coil |
Six specification highlights
Duplex stainless coil
Modern indirect cylinders use duplex or 316L stainless steel coils resistant to London's hard-water scale deposits. Standard 25-year cylinder warranty covers coil failure.
Coil primary connection sizing
Standard indirect cylinders use 22mm compression connections on the coil primary side. On high-flow boilers or long primary runs we increase to 28mm to minimise pressure loss and maximise heat transfer.
Motorised valve control
Two-port zone valve on the coil primary controlled by the cylinder thermostat. Standard specification. On S-Plan systems the DHW valve operates independently of the heating zone valves.
Secondary Incoloy immersion backup
Every indirect cylinder we install has a secondary 3kW Incoloy immersion factory-fitted as backup DHW. If the boiler fails on a cold weekend, the immersion keeps hot water available while the boiler is repaired.
Factory-fitted G3 safety kit
Pressure-reducing valve, expansion vessel, single-check valve, tundish, T&P relief valve, expansion relief valve — all factory-installed. AK re-torques and pressure-tests every connection on install.
G3 competency certificate + Building Notice
Every install completed by a G3 competent engineer (UK Certification Ltd certificate 136359 issued 8 September 2025, expiry 18 August 2030) with the Building Notice submitted to local authority Building Control.
Cost — indirect unvented cylinder installation
| Scope | Price (inc. VAT) | Includes |
|---|---|---|
| Indirect unvented cylinder — supply and installation (150L) | £1,750–£2,050 | Cylinder (Megaflo Eco Indirect 150L or equivalent), single coil, 3kW immersion backup, G3 safety kit, install, Building Notice, G3 certificate |
| Indirect unvented cylinder — supply and installation (210L) | £1,950–£2,350 | Cylinder 210L, single coil, 3kW immersion, install, Building Notice, G3 certificate |
| Indirect unvented cylinder — supply and installation (250L) | £2,150–£2,550 | Cylinder 250L, single coil, 3kW immersion, install, Building Notice, G3 certificate |
| Twin-coil indirect (solar or heat-pump ready) | +£350–£550 | Twin-coil cylinder upgrade — bottom coil for solar/heat-pump, top coil for boiler |
| High-recovery coil upgrade (fast reheat) | +£200–£350 | Larger surface-area coil variant for HMOs / high-occupant homes |
| Replacement indirect (like-for-like same location) | £1,950–£2,650 | Cylinder replacement, coil connections re-made, safety kit refresh, waste haulage of old unit |
| Vented-to-unvented conversion (removal of loft tank) | +£350–£550 | Loft-tank decommission, cold-feed re-plumb from mains, discharge pipework to external tundish outlet |
| Annual G3 service | £150–£195 | Coil performance test, immersion check, T&P relief test, expansion vessel pre-charge, tundish clear, service report |
Real London indirect cylinder installs
Hampstead terraced house — combi boiler upgrade to indirect
Existing 24kW combi boiler struggling to deliver simultaneous hot water to two bathrooms. Replaced with a 30kW system boiler and 210L Megaflo Eco Indirect cylinder in the airing cupboard. Both bathrooms now deliver mains-pressure hot water simultaneously. Total £4,850 including full G3 kit, immersion backup, and Building Notice.
Ealing 1930s semi — heat-only boiler retrofit to unvented
Existing vented cylinder with loft tank feeding 3 bedrooms and one bathroom at 0.15 bar shower pressure. Kept existing 28kW heat-only boiler, replaced vented cylinder with 250L Telford Tempest Indirect on the ground floor airing cupboard. Loft tank decommissioned. Shower pressure now 3.2 bar from the mains. Total £2,850 including conversion and Building Notice.
Notting Hill house — twin-coil upgrade for solar thermal retrofit
Existing 210L indirect cylinder scheduled for solar thermal panel install on south-facing rear roof. Replaced with twin-coil 250L Gledhill Torrent — bottom coil connected to solar loop, top coil to system boiler. Solar panels install separately by roofing specialist. Total cylinder swap plus twin-coil upgrade £2,650.
Indirect unvented cylinder installation across every London borough
Frequently asked questions
What is the difference between indirect and direct unvented cylinders?
Do I need a Building Notice for the install?
What is the immersion backup for if the cylinder is boiler-fed?
How is the boiler connected to the cylinder — S-Plan or Y-Plan?
Can I upgrade my existing vented cylinder to indirect unvented?
What certification do you hold for unvented cylinder work?
What cylinder brand do you recommend for indirect installs?
How long does the install take?
Do you carry public liability insurance?
How quickly can you attend for an emergency indirect cylinder failure?
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Book an indirect unvented cylinder install
G3 certificate, Building Notice and immersion backup included. Written quote within 24 hours.
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