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G3 qualified engineer commissioning a twin coil solar hot water cylinder in a London property
G3 Qualified — Solar Twin Coil & PV Diverter

Solar Cylinder Installation London

Twin coil cylinders for solar thermal and single coil cylinders with PV diverter. Telford, Joule, Gledhill, Kingspan and Mixergy specialists. Twin coil installs from £1,895 fitted, PV diverter terminal install from £495. Building Regulations Part G3 notification and Benchmark commissioning included on every job.

Same-day surveys across Greater London. Fixed-price written quote on site. 25-year tank warranty preserved on every install we sign off.

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G3 Qualified engineers
£5M Public liability
25yr Tank warranty
MIS 3001 standards
24/7 London cover
Quick Answer

A solar cylinder is a hot water store designed to be heated by solar input — either a twin coil cylinder for solar thermal panels (lower coil = solar, upper coil = boiler) or a single coil cylinder with a PV diverter that sends surplus solar PV electricity to the immersion heater. Installation in London is a controlled service under Building Regulations Part G3 and must be carried out by a G3 qualified engineer. Twin coil installs cost £1,895–£2,895 fitted; PV diverter terminal installs cost from £495.

What is a solar cylinder?

A solar cylinder is a hot water store engineered to accept heat from a solar source as well as (or instead of) a conventional boiler. There are two distinct flavours used across London: twin coil cylinders for homes with solar thermal panels on the roof, and single coil cylinders with a PV diverter for homes with solar PV panels. They look the same from the outside, but the internal engineering and the wiring at the back are very different.

A twin coil cylinder has two indirect heat exchangers stacked vertically. The lower coil, with a larger surface area (typically 1.4–1.8 m² for a domestic install), is connected to the glycol-filled primary loop that runs to the roof panels. Solar gain heats the bottom of the cylinder first. The upper coil, served by the existing boiler, only fires if the top thermostat falls below set point. In summer, the cylinder routinely hits 65°C on solar gain alone and the boiler stays off for weeks at a time.

A PV diverter cylinder is a standard single coil indirect cylinder with a second immersion boss. A diverter device (typically a myenergi Eddi or Solar iBoost+) clamps to the incoming mains tails, monitors export in real time and pushes surplus PV electricity to the immersion instead of the grid. This works because the avoided gas cost (25p+/kWh) is much higher than the Smart Export Guarantee tariff (typically 4–5p/kWh). On a 4kW PV array in London, a properly sized diverter heats the cylinder fully on most sunny days April to September.

Twin coil vs PV diverter — which solar cylinder do you need?

The right cylinder depends entirely on what solar generation you already have or plan to install. There is no single best answer — the table below summarises the four configurations we fit most often across London.

Cylinder typeBest forHow it heatsNotes
Single coil + PV diverterHomes with solar PV panels and a spare immersion bossMains pressure, single boiler/electric heat sourceCheapest solar-ready option. Diverter sends surplus PV export to the immersion instead of the grid. No solar thermal panels required.
Twin coil — solar thermalHomes with roof-mounted solar thermal collectors (flat plate or evacuated tube)Lower coil = solar primary loop, upper coil = boiler top-upSolar gain heats the bottom of the cylinder first. Boiler only fires if the top sensor falls below set point. Maximises free hot water in summer.
Twin coil — solar + heat pumpProperties pairing a heat pump with future solar thermalLarger solar coil for low-grade heat pump primary, second coil reserved for solarMCS-compliant sizing. Lower flow temperatures need oversized coils — typical heat pump cylinder is 0.4–0.6 m² coil per kW.
Smart / Mixergy solar variantFuture-proofed homes wanting app control and grid-flexible heatingSingle coil + smart controller blending PV, off-peak tariff and boilerHeats only the litres you need each day. Useful with Octopus Go / Cosy tariffs and existing PV. Premium spend with measurable energy savings.

If you're not sure which solar configuration fits your London property, call 0207 046 1363 and a G3 engineer will run through it on the phone in 5 minutes — no booking required.

Solar cylinder sizing — by property type

Sizing a solar cylinder follows the same volume-per-occupant rule as a standard unvented cylinder, plus a 20–30% uplift to bank the solar gain on sunny days for evening use. Get sizing wrong and you either run out of hot water mid-week, or you cycle the boiler unnecessarily because the cylinder is too small to hold the solar harvest.

PropertyStorage (litres)Recommended cylinder
1–2 bed flat, 1 bathroom150–180LTwin coil 180L or single + PV diverter
3-bed terrace, 1 bath + en-suite180–210LTwin coil 210L (lower solar coil ≥ 1.4 m²)
4-bed family, 2 bathrooms250LTwin coil 250L (solar coil 1.6–1.8 m²)
5+ bed home, 2–3 bathrooms300LTwin coil 300L, dual immersion, high-recovery solar coil
HMO / B&B (4–6 occupants)300L+ with secondary circulationTwin coil + secondary return loop, TMV2 outlets
Heat pump + solar PV combo210–300L (MCS sized)Heat pump cylinder with oversized coil (≥ 3.0 m²)

Solar cylinder installation cost London — 2026 prices

Every quote is a fixed figure given on site after a 30–45 minute survey. No call-out fee on quoted works. Prices include the cylinder, full G3 inlet group, expansion vessel, discharge route, Building Regulations notification, Benchmark logbook and a 12-month workmanship guarantee.

JobWhat's includedFrom price
180L twin-coil solar cylinder — supply & fitTelford Tempest Solar or Joule Cyclone Solar 180L, lower solar coil + upper boiler coil, full G3 inlet group, discharge route, Building Regs notification, Benchmark logbook.£1,895
210L twin-coil solar cylinder — supply & fit210L duplex stainless twin coil, dedicated solar return pocket, full inlet group, primary connections to existing boiler, commissioning to manufacturer flow rates.£2,095
250L twin-coil solar cylinder — 2-bath / family home250L cylinder sized for daily solar gain plus boiler top-up, twin thermostats, PV diverter terminal wiring where specified, full handover.£2,395
300L twin-coil solar cylinder — 5-bed / HMO300L Gledhill StainlessLite Solar or Telford Tempest 300L, high-recovery solar coil, dual immersion bosses, secondary circulation pump connection if required.£2,895
Single coil + PV diverter cylinder — 180LSingle coil indirect cylinder with second immersion boss wired to a PV diverter (myenergi Eddi or Solar iBoost+). Cylinder, second immersion element, terminal works to consumer unit feed.£1,795
Vented → solar twin coil conversionRemoval of vented cylinder and loft tank, repipe to mains-fed unvented twin coil, expansion vessel, G3 group, discharge to outside, Building Control notified.£2,795
PV diverter terminal install onlyWire a myenergi Eddi / Solar iBoost+ to an existing twin-immersion or spare boss. CT clamp to main tails, isolation, commissioning, MCS-style handover.£495
Solar pump station / cylinder swap (labour)Customer-supplied cylinder fitted by G3 engineer. Includes inlet group, primary connections, commissioning, G3 cert + Benchmark.£795

* Prices include VAT and Building Regulations G3 notification. Full price list on the pricing page. Vented-to-unvented conversion includes loft tank removal and full mains-pressure re-pipe.

Need a quote on a specific solar cylinder job?

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Installation process — survey to certificate

Twin coil solar cylinder showing lower solar coil and upper boiler coil connections
1

Survey & solar load check

30–45 min

Engineer attends and measures mains pressure (must hit 1.5 bar dynamic minimum), confirms cylinder location, checks discharge route to outside and inspects the existing solar primary loop or PV diverter feed. Call 0207 046 1363 to book the survey.

2

Fixed-price quote

On the day or within 24h

Written quote with cylinder model, coil size, expansion vessel rating, discharge route, controls and PV diverter (if applicable). No call-out fee on quoted works. Lead time on stocked cylinders is 3–7 working days.

3

Strip-out & repipe

2–4 hours

Existing cylinder isolated, drained and removed. Pipework re-routed in 22 mm copper to BS EN 1057. Solar primary flow/return brought to the lower coil tappings, boiler primary to the upper coil. New inlet group fitted: stop valve, strainer, PRV, single-check valve, expansion vessel, T&P route.

4

Cylinder fit & commissioning

2–3 hours

Cylinder lifted and levelled, primaries connected, discharge pipework run to a visible external termination, expansion vessel pre-charged to manufacturer spec (typically 3.0 bar). System filled, vented, pressurised, T&P witnessed-lifted, both thermostats set (60°C upper, solar sensor on lower).

5

Handover & certification

30 min

Building Regulations Part G3 notification filed via the Competent Persons Scheme. Benchmark logbook completed and signed. Customer demo on solar priority, holiday mode, immersion override and annual service requirement. Compliance certificate posted within 30 days.

G3 compliance & Benchmark commissioning

Every solar cylinder we install in London is signed off against the same statutory and product standards. The Benchmark logbook supplied with each cylinder is completed on site and registers the install with the manufacturer — it is the document that activates the 25-year tank warranty. The G3 Building Regulations notification is filed through our Competent Persons Scheme within 30 days, with the compliance certificate posted to the property owner.

G3

Approved Document G3 — Hot water supply and systems

All unvented solar cylinders over 15 litres are a controlled service. The engineer must hold a current G3 (BPEC, LCL Awards or City & Guilds 6035) ticket and notify Building Control directly or through a competent persons scheme.

MIS 3001

Microgeneration Certification Scheme — Solar thermal

Sets the install standard for solar thermal collectors and the cylinders they feed. We follow MIS 3001 sizing rules even on non-grant installs — minimum lower coil surface area, sensor pocket placement and primary loop pressure-test routine.

BS EN 12977

Thermal solar systems and components — custom-built systems

European standard governing the design, sizing and commissioning of solar primary loops. Determines the glycol concentration, expansion vessel sizing and stagnation safety margins on every twin-coil cylinder we fit.

BS EN 12897

Indirectly heated unvented storage water heaters

Product standard for the cylinder body itself. Defines the layered safety devices, vessel pressure rating and labelling required on every UK twin-coil unvented cylinder.

BS 7671

18th Edition Wiring Regulations (2018+A2:2022)

Governs the electrical work on every PV diverter install, immersion circuit and solar pump station — RCD protection, cable size, CT clamp installation and bonding to the main earthing terminal.

BS 5918

Code of practice for solar heating systems for domestic hot water

UK code of practice covering primary glycol selection, vessel sizing for stagnation, dual-stat wiring and the high-temperature cut-out logic on twin-coil solar cylinders.

G3 engineer completing Benchmark logbook on a solar cylinder install in London

Benchmark commissioning — what we record

Every solar cylinder install signs off against a 12-point Benchmark checklist. Nothing is judged by eye — pressures, temperatures, glycol concentration and electrical readings are measured, recorded and witnessed. The completed Benchmark logbook is your warranty trigger and the document your home insurer will ask for if a claim ever arises.

  • Cold inlet pressure

    Static and dynamic mains pressure logged. Must be ≥ 1.5 bar dynamic at the inlet group with the system flowing.

  • PRV outlet pressure

    Pressure-reducing valve set so working pressure sits between 3.0 and 3.5 bar, balanced with the cold side.

  • Expansion vessel pre-charge

    Pressurised to manufacturer-stated air pressure (typically 3.0 bar) with the system depressurised first.

  • T&P witnessed lift

    90°C / 7 bar safety device test-lifted on commissioning. Discharge confirmed running continuously to the external termination.

  • Solar primary loop pressure

    Filled to 2.0–2.5 bar cold with 40% propylene-glycol mix. Sight-glass clear, no air, pump speed set to manufacturer chart.

  • Solar high-limit cut-out

    Dedicated cut-out wired between the solar coil sensor and the differential controller. Trips at 95°C, manual reset.

  • Boiler stat / upper coil set-point

    Top thermostat set to 60°C with weekly anti-legionella raise to 65°C built into the controller schedule.

  • Anode protection

    Sacrificial magnesium or impressed-current titanium anode inspected and recorded — particularly on stainless cylinders fed from London hard water.

  • PV diverter wiring (where fitted)

    myenergi Eddi or Solar iBoost+ CT clamp installed on incoming live, dedicated circuit fused, immersion isolator labelled and tested to BS 7671.

  • Discharge pipe route

    Tundish 300 mm air gap confirmed, continuous fall, terminating at visible external location at low level. Internal voids and soil pipe routes are non-compliant.

  • Benchmark logbook

    All set-points, pressures, glycol concentration and cylinder serial recorded in the Benchmark logbook supplied with the cylinder.

  • G3 Building Regs notification

    Notification filed via our Competent Persons Scheme within 30 days. Compliance certificate posted to the property owner.

Real London jobs — worked examples

Every solar cylinder install is shaped by the property — solar input, cylinder location, mains pressure, discharge route. Five recent jobs from the last twelve months, with the brief, the work done and the final invoice.

Edwardian semi, Wandsworth — PV-only retrofit with diverter

4kW solar PV array on south-facing roof, owner asked us to capture the export rather than sell it cheap. Fitted a 180L single-coil unvented cylinder with twin-immersion boss, wired a myenergi Eddi to the spare element and CT-clamped the mains tails. Cylinder heats fully on sunny days, boiler stays off May to September. Total fitted price: £1,795 + £495 diverter install = £2,290 inc. VAT.

1930s detached, Barnet — solar thermal retrofit with twin coil

Existing 4-panel evacuated tube array on garage roof, original 210L single coil cylinder was 18 years old. Swapped for a Telford Tempest Solar 210L twin coil, lower solar coil 1.55 m², upper boiler coil 0.9 m². Primary loop reconnected, glycol topped to 40% concentration, pump speed reset to 3. Cylinder routinely hits 65°C on solar alone April–September. £2,095 fitted + £180 glycol service.

New-build conversion, Hackney — heat pump + solar PV cylinder

Air source heat pump going in alongside an existing 5kW PV array. Specified a 250L heat pump cylinder with 3.2 m² coil and a second boss for the PV diverter. Tank pre-heats from PV diverter when the sun is out, the heat pump tops up on the cheap off-peak window. £2,495 cylinder + £495 diverter = £2,990 inc. VAT.

Victorian terrace, Islington — vented to twin coil solar conversion

Loft tank removed, mid-1990s vented cylinder swapped for a 210L Joule Cyclone Solar twin coil. New G3 inlet group, discharge through the rear external wall to the gully, primary repipe to the existing solar thermal flat plate panels. Two-day job, full G3 paperwork, £2,795 inc. VAT.

Listed townhouse, Westminster — discharge route constraint

Cylinder cupboard sat 8 metres from the nearest external wall — outside the manufacturer's allowed discharge run. Fitted a Hotun dry tundish and routed the discharge via a sealed metallic duct in a fire-rated chase to the soil stack with air break. Sign-off from building control on the alternative route. £2,395 cylinder + £290 discharge works.

Solar cylinder survey this week?

Same-day surveys across Greater London. Twin coil cylinders stocked for 3–7 day installs. PV diverter terminal installs typically booked within a week. Fixed-price quote in writing before any work begins.

Call 0207 046 1363WhatsApp 07456 975436Book online

Frequently asked questions

What's the difference between a single coil and a twin coil solar cylinder?
A single coil cylinder has one indirect heat exchanger — it can be heated from one source (typically a boiler) plus an immersion element. A twin coil cylinder has two separate coils stacked vertically — the lower coil is dedicated to the solar primary loop (taking heat from roof-mounted thermal panels), the upper coil takes the boiler. Twin coil is the right choice if you have or plan solar thermal panels because it lets the cylinder heat from the bottom up on solar gain and only fires the boiler if the top sensor falls below set point. Single coil + PV diverter is the right choice if you have solar PV (electricity panels) and want to use surplus export to heat the immersion directly.
How much does a solar cylinder install cost in London?
A like-for-like solar cylinder swap in London starts at £1,895 fitted for a 180L twin coil, £2,095 for 210L, £2,395 for 250L and £2,895 for 300L. A vented-to-unvented twin coil conversion is from £2,795 because of the additional pipework, expansion vessel and discharge route. A single coil cylinder with a wired PV diverter is from £1,795 + £495 for the diverter terminal install. All prices include the cylinder, full G3 inlet group, discharge pipework, commissioning, Building Regulations notification and Benchmark logbook.
Do I need to be G3 qualified to install a solar cylinder?
Yes. Approved Document G3 of the Building Regulations applies to every unvented hot water vessel over 15 litres regardless of how it's heated. The engineer must hold a current G3 ticket (BPEC, LCL Awards or City & Guilds 6035), renewed every 5 years. Solar thermal-specific work additionally benefits from MIS 3001 / MCS competence, particularly the glycol management, stagnation protection and high-limit wiring. Anyone working on an unvented solar cylinder without a G3 invalidates the manufacturer warranty, the home insurance and the Building Control trail.
Will a solar cylinder work with my existing boiler?
Almost always, yes. A twin coil solar cylinder is heated by the boiler exactly the same way as a standard indirect cylinder — the boiler primary flow goes to the upper coil tappings via a motorised valve. The only complications are very old open-vented boilers that aren't suited to a sealed mains-pressure cylinder (these usually need a system boiler swap at the same time) and combi boilers, which don't use a cylinder at all. We confirm boiler compatibility on the on-site survey before quoting, so there are no surprises on install day.
What is a PV diverter and do I need one?
A PV diverter is a small device that monitors how much solar PV electricity your house is exporting back to the grid in real time and instead sends that surplus to your immersion heater. The two market leaders in the UK are the myenergi Eddi and the Marlec Solar iBoost+. On a sunny day a 4kW PV system can heat a 180L cylinder fully without the boiler firing once. You need a diverter if you have solar PV panels and want to capture the export value (which is typically 4–5p/kWh on SEG) as 25p+/kWh of avoided gas. Around 60% of solar PV homes in London now run a diverter.
Can I add a solar cylinder if I don't have panels yet?
Yes — and it's the smart move if you're planning solar PV or solar thermal within the next 5 years. Fitting a twin coil cylinder now means the second coil sits dormant until the panels go in, but the cylinder is already pre-piped and ready. The cost difference between a single coil and twin coil cylinder is typically £200–£400, and adding a coil to an existing cylinder is impossible — so doing it on day one is the cheap option. We see this regularly with homeowners running a boiler now but planning a heat pump or solar PV install in the next 2–3 years.
How much hot water will solar thermal panels actually give me?
In London, two flat-plate solar thermal panels (around 4 m² aperture) or a 20-tube evacuated tube array typically provides 50–60% of annual domestic hot water demand for a family of four. That breaks down to roughly 90–100% of demand May to August (cylinder reaches 60°C+ on solar alone), 40–60% April and September, and 10–30% the rest of the year. Solar PV with a diverter performs broadly similarly on annual gain but is more weather-resilient because PV panels still produce useful output on diffuse-light days when solar thermal panels stall.
What size solar cylinder do I need?
London sizing rule: 35–45 litres of stored hot water per person, plus 50 litres if there's a bath used daily, plus a 20–30% uplift if you have solar input (because you want to bank the solar gain on sunny days for evening use). For a 1–2 bed flat with shower-only that's a 180L twin coil. For a 3-bed terrace with one bath plus en-suite, 210L. A 4-bed family home with two bathrooms typically wants 250L. A 5+ bed property with three bathrooms moves to 300L. Heat pump cylinders are always oversized to compensate for the lower flow temperatures.
Does a solar cylinder need an annual service?
Yes, and the annual service is more involved than a standard unvented cylinder because the solar primary loop must also be checked. A solar cylinder service is £165 fixed in London and covers the standard 12-point unvented checklist (expansion vessel pre-charge, T&P witnessed lift, PRV outlet pressure, anode inspection, thermostat function, discharge route, all in line with BS EN 12897) plus the solar primary loop — glycol concentration and pH, pump speed, sensor function, high-limit cut-out, stagnation safety margin. Annual service is a condition of the cylinder's 25-year tank warranty.
Can a solar cylinder fit in a top-floor London flat?
Yes, very commonly. Solar PV + diverter is the most popular pairing in top-floor London flats because the cylinder lives inside the flat (typically the airing cupboard) and the PV array sits on the roof. The two checks before quoting are mains pressure (must hit 1.5 bar dynamic at the inlet group) and a viable T&P discharge route to outside. Where the discharge run exceeds the manufacturer's allowed length, a Hotun dry tundish or sealed metallic discharge route to the soil stack is an accepted alternative under Building Control sign-off.
What brands of solar cylinder do you fit?
We fit the main UK solar cylinder ranges: Telford Tempest Solar (twin coil, the workhorse for London retrofits), Joule Cyclone Solar (Irish-made, excellent insulation and recovery), Gledhill StainlessLite Solar (premium stainless with strong service network), Kingspan Albion Ultrasteel Twin Coil and OSO Super S Twin Coil for high-flow homes. For smart and grid-flexible installs we fit Mixergy Solar with the smart controller paired to PV diverter and off-peak tariff scheduling. Every cylinder comes with a 25-year tank warranty when annual-serviced.
How long does a solar cylinder install take?
A like-for-like twin coil swap (replacing one twin coil with another) is a single-day 4–6 hour job. A standard cylinder upgrade to twin coil (where the solar primary loop is already roughed in) is also a single-day job, typically 5–7 hours. A vented-to-unvented twin coil conversion is a 1–2 day job because of the new mains-pressure pipework, expansion vessel, discharge route and electrical works. A new install with no existing solar infrastructure (cylinder only, panels to follow) is a single day. We give an exact time estimate after the survey, not before.
Will I lose my warranty if a non-G3 plumber works on the cylinder?
Yes. Every UK cylinder manufacturer — Telford, Joule, Gledhill, Heatrae Sadia, Kingspan, OSO, Mixergy — states explicitly in their warranty terms that only a G3 qualified engineer may install, service or modify the cylinder. A non-G3 install is non-compliant with Approved Document G3 of the Building Regulations, voids the 25-year tank warranty and 2-year parts warranty, and is a red-flag finding on home insurance and conveyancing inspections. Always ask to see a current G3 certificate before any unvented work — we carry ours on every job.
What if my solar thermal panels overheat in summer?
Solar thermal stagnation is the technical risk of solar primary loops sitting in full sun with the cylinder already at set point — the glycol in the panels can reach 150°C+ and the pressure in the primary loop spikes. A correctly sized stagnation expansion vessel (sized to absorb the full panel volume at vapour state), a high-limit cut-out on the cylinder coil and good-quality glycol with a 5-year service interval handle this. We inspect every solar primary loop on annual service for glycol degradation and stagnation evidence (dark glycol, low pH, vessel pressure drop) and recharge or flush as needed.
Can a solar cylinder be combined with a heat pump?
Yes, and it's increasingly common. The MCS sizing rules for a heat pump cylinder require a much larger coil surface area than a boiler-fed cylinder (typically 3.0 m² or higher) because heat pumps run at much lower flow temperatures (45–55°C versus 70–80°C for a boiler). A purpose-built heat pump cylinder with a second coil reserved for solar thermal — or a single coil with a PV diverter wired to the immersion — is the standard combination. We MCS-size every heat pump cylinder against the property's heat loss and daily hot water demand before quoting.

Question not answered? Call 0207 046 1363 and speak to a G3 engineer directly — no call centre, no triage.

Service area

Service Area — All London Boroughs

G3 qualified solar cylinder engineers based in central London with same-day survey cover across all 32 London boroughs and the City. Twin coil cylinders stocked for 3–7 day installs. PV diverter terminal installs usually booked within the week.

Completed solar cylinder Benchmark commissioning certificate

Ready to fit a solar cylinder in your London home?

G3 qualified engineers across London. Twin coil cylinders for solar thermal, single coil with PV diverter for solar PV. Fixed-price written quote on site, Building Regulations G3 notification filed, Benchmark logbook signed on the day. £5M public liability, fully insured.

0207 046 1363WhatsApp 07456 975436Book online

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