Covering all 32 London boroughs[email protected]24/7 · Available Day & Night
GuidesBlogAboutContactCall 020 7101 0629

Solar & heat pump ready · Dual heat source · G3 certified

Twin Coil Cylinder London

Twin coil unvented cylinders for solar thermal, heat pump and dual-fuel homes. Two coils, two heat sources, one storage volume. Gledhill Torrent, Telford Tempest, Megaflo Eco Solar, Vaillant AroSTOR. Building Notice and G3 certificate included.

Get a Twin Coil Quote

Twin Coil Install Quote

Your browser may ask to share your location — this helps us find the nearest available plumber faster. You can decline and still send the form.

Twin coil cylinders — dual heat source, one storage volume

A twin coil unvented cylinder has two internal coils. Each coil is connected to a different heat source, and both coils share the same stored DHW volume. The topology exists because there is no single heat source that is the best answer for the whole year in a UK home. Solar thermal delivers essentially free hot water for six months of the year but nothing on a January morning. An air-source heat pump has an excellent coefficient of performance (COP) at 45°C flow but a poor one at 65°C. A gas boiler is unmatched for delivering high-temperature top-ups on demand but expensive per kWh compared to a heat pump when operated well. Twin coil cylinders let you combine these — solar or heat pump on one coil for most of the year, gas boiler on the other coil for peak demand and Legionella pasteurisation cycles.

The layering matters. The lower coil, connected to the low-temperature heat source (solar or heat pump), sits at the bottom of the cylinder where the coldest water is. This maximises the temperature differential and therefore the heat transfer rate for a low-grade heat source. The upper coil, connected to the higher-temperature heat source (boiler), sits at the top where the hot draw-off comes from. The boiler heats only the top portion of the cylinder to 60°C+ when tap demand requires it, rather than heating the whole cylinder — which is what unlocks the efficiency saving. Water leaves the tap from the top of the cylinder at boiler temperature; the middle and bottom of the cylinder stays at the lower solar/heat-pump temperature until called for.

The four common London twin coil scenarios: solar thermal retrofit on a rear-facing south-oriented roof; ASHP install for space heating with gas boiler backup for winter peak; ground-source heat pump plus immersion for Legionella pasteurisation; biomass boiler with oil or gas backup. In every case the twin coil delivers meaningful running-cost saving over a single-coil cylinder heated by the fossil-fuel source alone, while retaining the reliability and Legionella compliance the fossil-fuel backup provides.

Every twin coil 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, twin cylinder thermostats with independent setpoints, and where applicable an emergency top-mounted 3kW Incoloy immersion for backup Legionella pasteurisation. Public liability £5,000,000 via SiriusPoint through Eaton Gate MGU, policy BE26ACTT000000018221, period 07/05/2026 to 06/05/2027.

When a twin coil cylinder is the right specification

Solar thermal retrofit

Property with south-facing rear roof space suitable for solar thermal panels (typically 2–4 m² collector area for a family home). Twin coil cylinder allows the solar loop to heat the bottom half of the cylinder during daylight and the boiler to top up the top half during morning and evening peak demand.

Air-source heat pump plus gas boiler backup

ASHP installed for space heating plus backup gas boiler for winter peak. Twin coil allows the ASHP to heat DHW at moderate temperature (48–50°C) via the bottom coil for most of the year, with the boiler taking over via the top coil during December-February when the heat pump COP falls.

Ground-source heat pump primary + immersion backup

GSHP delivering low-flow, high-efficiency space heat plus DHW. Twin coil configuration means the GSHP heats the cylinder at optimal 45–50°C without stressing the compressor to high output, and a 3kW immersion element at the top handles Legionella pasteurisation cycles at 60°C.

Biomass boiler homes

Rural London-fringe properties with a wood-pellet or log biomass boiler backup often specify twin coil so the biomass primary can be switched off during summer while a secondary electric coil handles low DHW demand. Dual-fuel resilience.

Coil placement — why bottom is solar/HP and top is boiler

Bottom coil (larger surface area)

Connected to the lower-temperature heat source — solar thermal, air-source heat pump, ground-source heat pump. Placed at the bottom of the cylinder because the coldest water in the storage column is at the bottom, giving the widest temperature differential and highest heat transfer efficiency to a low-grade heat source.

Top coil (smaller surface area)

Connected to the higher-temperature heat source — gas boiler, oil boiler, biomass. Placed at the top of the cylinder because the boiler can rapidly heat the top portion of the stored water to 60°C+ for the tap draw-off, without needing to heat the entire cylinder volume.

Cylinder thermostats — two, not one

Two thermostats — one at the bottom coil level (drives the solar/HP loop demand), one at the top coil level (drives the boiler demand). Independent setpoints let you optimise: solar/HP runs whenever the panel or heat pump can deliver useful heat; boiler only runs when the top-of-cylinder temperature falls below the tap-draw setpoint.

Twin coil brands we install

Gledhill Torrent Twin Coil

Standard workhorse twin coil for solar thermal and heat pump retrofits. 250L, 300L, 350L variants. 25-year cylinder warranty. Widely stocked across London merchants.

Telford Tempest Twin Coil

Good-value duplex stainless. 210L to 400L. Strong solar coil surface area for retrofit. Twin thermostat kit factory-fitted.

Megaflo Eco Solar

Heatrae Sadia twin-coil variant of the market-standard Megaflo. Best spare-parts backing in London for future service work. 210L, 250L, 300L.

Vaillant AroSTOR (heat-pump specific)

Heat-pump-optimised cylinder with integrated ASHP control. Larger bottom coil sized for low-flow-temperature operation. Compatible with Vaillant aroTHERM ASHP range.

Joule Cyclone Twin

Premium high-recovery twin coil for large homes and HMOs. 4.0 m²+ coil surface area for fast solar recovery. Common on eco-refurbishment specifications.

Cost — twin coil cylinder installation

ScopePrice (inc. VAT)Includes
Twin coil cylinder — supply and installation (210L)£2,250–£2,650Cylinder (Megaflo Eco Solar 210L or equivalent), factory G3 kit, twin thermostat, immersion backup, install, Building Notice, G3 certificate
Twin coil cylinder — supply and installation (250L)£2,450–£2,850Cylinder 250L, factory G3 kit, install, Building Notice, G3 certificate
Twin coil cylinder — supply and installation (300L)£2,650–£3,150Cylinder 300L, factory G3 kit, install, Building Notice, G3 certificate
Solar thermal ready twin coil (larger bottom coil surface)+£250–£450Upgraded bottom coil (3.0–4.0 m² surface area) for effective solar coupling
Heat-pump-specific twin coil (Vaillant AroSTOR or similar)+£350–£650Heat-pump-optimised cylinder with integrated ASHP interface
Replacement single-coil to twin-coil conversion£2,450–£3,250Cylinder replacement, primary re-plumb for two coils, second thermostat, control wiring
Annual G3 service (twin coil)£175–£220Both coil performance tests, immersion check, T&P relief test, expansion vessel pre-charge, tundish clear, service report

Real London twin coil installs

Wandsworth eco-refurb — solar thermal retrofit twin coil

Victorian terrace on a rear extension with south-facing roof. 3.2m² Solimpeks solar collector paired with 250L Gledhill Torrent Twin Coil. Bottom coil solar loop (glycol), top coil connected to existing 28kW system boiler. Twin thermostats — solar demand runs from March to October, boiler top-up only in winter mornings. Estimated 55% DHW gas saving vs single-coil equivalent. Total cylinder install £2,750, solar collector separate.

Richmond detached — Vaillant ASHP with gas boiler backup

Existing 32kW gas system boiler kept as backup. Vaillant aroTHERM 7kW ASHP installed for space heat. 300L Vaillant AroSTOR twin coil cylinder — bottom coil for ASHP (48°C flow), top coil for gas boiler (65°C flow when required). Legionella cycle runs weekly via top-coil boiler pasteurisation. Cylinder install £3,150 including twin-thermostat control panel.

Muswell Hill 4-bed — biomass boiler primary, oil backup

Rural-fringe property with existing 25kW biomass log boiler as primary heat, 30kW oil boiler as winter backup. Fitted 350L Telford Tempest twin coil — bottom coil for biomass, top coil for oil. Twin cylinder thermostats prioritise biomass when the accumulator tank has heat available; oil only kicks in on the top coil when biomass is offline for reloading. Cylinder install £2,950.

Twin coil cylinder installation across every London borough

Barking and DagenhamBarnetBexleyBrentBromleyCamdenCroydonEalingEnfieldGreenwichHackneyHammersmith and FulhamHaringeyHarrowHaveringHillingdonHounslowIslingtonKensington and ChelseaKingston upon ThamesLambethLewishamMertonNewhamRedbridgeRichmond upon ThamesSouthwarkSuttonTower HamletsWaltham ForestWandsworthWestminster

Frequently asked questions

What is a twin coil cylinder and when do I need one?
A twin coil unvented cylinder has two internal coils, each connected to a different heat source. The most common combinations: (1) solar thermal panels on the bottom coil with a gas boiler on the top coil, (2) an air-source or ground-source heat pump on the bottom coil with a gas boiler on the top coil as winter backup, (3) a biomass boiler with a fossil-fuel backup. Twin coil is the correct specification whenever the property has two independent heat sources that need to feed the same DHW cylinder — you get the efficiency of the low-temperature source (solar or heat pump) for most of the year, plus the reliability of the fossil-fuel backup for peak demand or cold weeks.
Why is the solar or heat pump coil at the bottom of the cylinder?
Two reasons. First, the coldest water in the cylinder is at the bottom (hot water rises, cold sinks). Placing the low-temperature heat source at the bottom gives it the widest temperature differential to work with — a solar collector delivering 45°C flow to a bottom coil surrounded by 15°C mains-fill water transfers heat efficiently. Second, layering matters. The bottom coil heats the lower half of the cylinder to a moderate temperature (typically 40–50°C), while the top coil connected to the boiler heats only the top half of the cylinder to 60°C+ when needed. The stratification means the boiler is not heating water the solar has already warmed, which maximises the solar contribution.
Does a twin coil cylinder need two thermostats?
Yes — two independent cylinder thermostats are standard. The lower thermostat controls the solar/heat-pump loop demand (typically calling for heat whenever the collector or heat pump can deliver above the cylinder bottom temperature). The upper thermostat controls the boiler demand (typically calling for heat only when the top-of-cylinder water falls below the tap-draw setpoint of 60°C). Twin thermostats are what allow the two heat sources to operate independently on their own priorities — running one thermostat off both coils defeats most of the efficiency benefit of the twin arrangement.
Can I retrofit a twin coil cylinder without installing solar or a heat pump yet?
Yes — many customers install a "solar-ready" or "heat-pump-ready" twin coil cylinder now as part of a general cylinder replacement, with the second coil capped off for future connection. This future-proofs the property for a later solar thermal or heat pump retrofit without needing to change the cylinder again. The cost premium of the twin coil over a single coil equivalent is typically £350–£550, which is small compared to the £2,000+ cost of replacing a single-coil cylinder later when the solar or heat pump is added.
How does a twin coil handle Legionella pasteurisation?
Legionella controls under HSE ACOP L8 require the cylinder to reach 60°C at least weekly to pasteurise the stored water. On a twin coil the top-coil boiler handles the pasteurisation cycle by heating the top portion of the cylinder to 60°C+ for the required minimum 5-minute soak period. Even properties running the solar or heat pump loop at 45–50°C during normal operation can achieve the Legionella cycle via the boiler on the top coil — this is the practical reason the boiler backup is retained even in eco-refurbishments. Some specifications add a 3kW Incoloy immersion at the top of the cylinder as an emergency Legionella pasteurisation heater.
What size cylinder do I need for a twin coil solar install?
Solar thermal delivers more useful energy to a larger cylinder because the additional storage volume can absorb more of a peak solar day's heat gain. Rule of thumb: 60–80 litres per m² of solar collector area. A typical London family home with a 3.0 m² solar collector benefits from a 250L cylinder rather than a 210L. On very ambitious solar installs (5+ m² collector) we specify 300L or 350L cylinders to avoid the collector "stagnating" (overheating) during peak summer days.
What certification do you hold for twin coil installs?
HWSS G3 competency certificate (UK Certification Ltd certificate 136359 issued 8 September 2025, expiry 18 August 2030) — required for any unvented cylinder work. Water Regulations 1999 competency (WaterSafe registration, certificate 136356 same period). For solar thermal integration we work with an MCS-accredited solar thermal partner where the solar loop needs to remain within the MCS installation certification for RHI or other grant purposes.
Do you carry public liability insurance?
Yes — £5,000,000 public liability via SiriusPoint International Insurance Corporation (UK Branch) acting through Eaton Gate MGU Ltd, policy number BE26ACTT000000018221, current period 07/05/2026 to 06/05/2027.
How long does the install take?
A like-for-like twin coil replacement (existing twin coil cylinder swap) typically 5–7 hours. A single-coil to twin-coil upgrade with the second coil connected to an existing solar loop 6–8 hours. A full solar-ready twin coil install with the second coil capped for future 5–7 hours. All installs include the Building Notice submission on the same day as completion.
What is the annual service for a twin coil cylinder?
Standard G3 service items — T&P relief valve test, expansion relief valve test, expansion vessel pre-charge check, tundish clear, immersion element check, cylinder anode (where fitted) — plus twin-coil specific items: both coil performance tests (temperature differential across each coil at rated flow), both thermostat setpoint verification, primary strainer clean on each coil, glycol antifreeze concentration check on the solar loop (where applicable). Service annually, £175–£220.

Related services

Hot water cylinder replacement — main hub

Full cylinder replacement process.

Indirect unvented cylinder

Single-coil boiler-fed equivalent.

Direct unvented cylinder

Electric-only equivalent for all-electric properties.

G3 unvented cylinder engineer

The G3 certification behind every install.

Book a twin coil cylinder install

G3 certificate, Building Notice, twin thermostat control panel included. Written quote within 24 hours.

Request a Quote
Call 020 7101 0629