Twin coil solar · MCS partner · G3 certified
Solar Thermal Cylinder London
G3-certified twin-coil solar thermal cylinder installation and retrofit. Bottom coil solar, top coil boiler backup — 55–65% DHW gas saving in summer on a properly-sized 3 m² + 250L system. MCS-partner integration for RHI-eligible installs.
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Solar thermal cylinders — the cylinder half of a solar hot water retrofit
A solar thermal cylinder is a twin-coil unvented cylinder specifically sized and configured for solar hot water. The bottom coil (larger surface area, sized for the low-temperature 45–65°C flow that UK solar collectors typically produce) is connected to the solar loop. The top coil (smaller surface area, sized for the high-temperature 65°C+ flow from a gas or oil boiler) is connected to the backup boiler. Twin independent cylinder thermostats let the solar run whenever the collector is delivering useful heat, with the boiler only topping up the top portion of the cylinder when the solar has not delivered enough. In a properly-installed system this arrangement delivers 55–65% of summer DHW demand from solar, with the boiler taking over as needed in shoulder seasons and winter.
The cylinder is only half the installation. The other half — the collector, roof brackets, flow-and-return lines, solar station, glycol antifreeze, and MCS certification — is the province of a specialist solar thermal contractor. Our practice at AK is to handle the cylinder-side works to G3 standard, and to partner with an MCS-accredited solar thermal specialist for the roof-side works. This gives the customer a single-point contract for the combined install, with the specialist certifications each properly held. Where the customer wants the Renewable Heat Incentive (RHI) or an energy-company grant, the MCS certification is mandatory — the partner delivers that.
The commercial case for solar thermal versus solar PV has changed over the last decade. In the early 2010s, dedicated solar thermal was clearly the more efficient answer for DHW-only applications — 60% conversion of solar energy to hot water versus PV\'s 20–25% conversion to electricity. Since 2020 the price of solar PV has fallen dramatically while solar thermal costs have stayed similar, and modern smart cylinders like Mixergy can use PV excess with near-100% efficiency at the immersion element. For a completely new install today, PV plus a Mixergy or similar smart cylinder is often the more cost-effective answer. For retrofits where the customer is replacing a failing cylinder anyway and has south-facing roof space that would suit a solar thermal collector, dedicated solar thermal can still be the right answer — particularly where the customer values the direct hot-water focus of the system.
Every cylinder-side install is delivered 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). Public liability £5,000,000 via SiriusPoint through Eaton Gate MGU, policy BE26ACTT000000018221, period 07/05/2026 to 06/05/2027. MCS certification for the roof-side works from our accredited partner.
Sizing guide — cylinder capacity plus solar collector area
Solar thermal delivers more useful energy to a larger cylinder because the additional storage volume absorbs a peak solar day\'s heat gain. Rule of thumb: 60–80 litres per m² of collector.
| Collector area | Cylinder size | Typical occupants | Summer saving |
|---|---|---|---|
| 2.0 m² collector (starter install) | 210L twin coil | 2–3 occupants, 1 bathroom | 40–50% DHW gas summer |
| 3.0 m² collector (standard family) | 250L twin coil | 3–4 occupants, 2 bathrooms | 55–65% DHW gas summer |
| 4.0 m² collector (large family / high hot water use) | 300L twin coil | 4–5 occupants, 2–3 bathrooms | 60–70% DHW gas summer |
| 5.0 m² collector (very ambitious) | 350L twin coil (high-recovery) | 5+ occupants or B&B use | 70%+ DHW gas summer, stagnation risk |
The six components of a solar thermal cylinder system
Twin-coil unvented cylinder
250L or 300L twin-coil variant sized for solar. Bottom coil (larger surface area, low-temperature) for solar loop, top coil for boiler backup. Gledhill Torrent, Telford Tempest Twin, or Megaflo Eco Solar. See /twin-coil-cylinder-london for the twin-coil detail.
Solar thermal collector (flat plate or evacuated tube)
Flat-plate collectors — cheaper, more resilient in London wind conditions, better for large-area installs. Evacuated tube — more efficient in low-light conditions, better performance in shoulder-season UK weather. Both work for our installs.
Solar station (pump, expansion vessel, air separator)
Compact wall-mounted unit near the cylinder containing solar loop pump, dedicated expansion vessel (sized for the glycol volume), air separator, and safety valves. Simplifies solar loop maintenance and troubleshooting.
Glycol antifreeze fill
Propylene glycol antifreeze mixture (typically 40% glycol / 60% water) in the solar loop — prevents freezing in London's occasional -8°C winter mornings. Non-toxic (food-grade glycol) so no risk if the coil ever leaks into the stored DHW. Refresh every 5–7 years.
Solar controller
Programmable controller reading collector temperature and cylinder bottom temperature, running the pump only when the collector is hotter than the cylinder by a set differential (typically 6°C on, 2°C off). Modern controllers also log energy delivered per day for MCS RHI reporting.
Twin cylinder thermostats
One thermostat at the solar coil position (drives the pump demand indirectly via the controller), one at the boiler coil position (drives the boiler DHW zone valve). Independent setpoints let the solar do the majority of the work with the boiler only topping up as needed.
The six-stage install workflow
Property survey and roof assessment
Site survey to determine south-facing roof space availability, shading from surrounding buildings and trees, roof pitch (30–40° optimal for London), and structural fixing feasibility. Cylinder location assessed for twin-coil replacement or new install.
MCS partner engagement (if RHI or grant required)
Where the customer needs MCS certification for the Renewable Heat Incentive, energy company grants, or property-value uplift documentation, we engage our MCS-accredited solar thermal partner. AK handles the cylinder-side works; the MCS partner handles the collector and MCS certification. Combined quote provided.
Cylinder replacement to twin-coil
Existing single-coil unvented or vented cylinder replaced with twin-coil variant. Bottom coil connections plumbed for the solar loop, top coil connections to existing boiler primary. Both coil thermostats fitted with independent setpoints.
Solar collector install (MCS partner or AK-integrated)
Roof-mounted collector fixed via building-regs-approved brackets. Flow and return lines run from roof to cylinder location via loft or external wall route. Glycol fill added.
Solar station install and pipework balancing
Solar station wall-mounted near the cylinder. Pump balanced for flow rate (typically 0.4–0.8 L/min per m² of collector). Solar controller configured with property-specific setpoints and MCS reporting where required.
Commissioning and MCS certification
System filled, purged of air, glycol concentration verified, controller test cycle run. G3 certificate for cylinder-side works, MCS certificate from partner for collector-side works, Building Notice submitted to Building Control. Full commissioning report to customer.
Cost — solar thermal cylinder installation
| Scope | Price (inc. VAT) | Includes |
|---|---|---|
| Twin-coil cylinder replacement (solar-ready, not yet solar-connected) | £2,450–£2,950 | Twin-coil cylinder (250L or 300L), bottom coil capped for future solar connection, G3 kit, Building Notice, G3 certificate. See twin-coil page. |
| Cylinder side of solar retrofit (existing collector, cylinder swap only) | £2,650–£3,250 | Twin-coil cylinder replacement, solar loop reconnection, solar station commissioning |
| Full solar thermal install with MCS partner (cylinder + collector) | £6,850–£10,850 | AK cylinder side plus MCS partner collector side. Combined quote and single-point delivery. |
| Solar station replacement (pump, expansion vessel) | £850–£1,350 | Pump and station swap on existing solar loop, glycol drain and refill, pressure test |
| Annual solar cylinder service | £175–£220 | Standard G3 items plus solar loop glycol check, pump flow test, controller readings review |
| Glycol drain and refresh (every 5–7 years) | £350–£550 | Loop drain, flush, fresh 40/60 glycol / water fill, pressure test |
Real London solar thermal cylinder installs
Fulham eco-refurb — 3.2 m² flat plate + 250L Gledhill Torrent twin coil
Full eco-refurb combined with solar thermal retrofit. AK swapped existing 210L Megaflo Indirect for 250L Gledhill Torrent Twin Coil. MCS partner installed 3.2 m² Solimpeks flat-plate collector on south-facing rear extension roof. Solar station commissioned, glycol filled. First-summer measured performance: 62% of DHW demand delivered by solar, gas top-up cut correspondingly. Total combined cost £8,650 including MCS certification for RHI. Annual RHI payment applied.
Wandsworth Victorian terrace — solar-ready twin-coil replacement (future-proof)
Customer replacing a failed 15-year-old Megaflo cylinder as urgent replacement, wanted to future-proof for a possible solar thermal install within 2 years. AK fitted a 250L Telford Tempest Twin Coil with bottom coil capped for later solar connection. Boiler side connected as normal indirect. Total £2,750 including Building Notice. Deferred solar install planned for spring 2027.
Notting Hill Georgian townhouse — 4.0 m² evacuated tube + 300L Megaflo Solar
High-usage 5-person family with 3 bathrooms. Existing gas boiler kept. 4.0 m² evacuated-tube collector on hidden mansard roof (planning-friendly), 300L Megaflo Eco Solar twin coil. First summer measured 68% DHW demand from solar. Family reporting winter gas usage reduced but boiler still primary role Nov-Feb. Combined cost £10,250 including MCS certification and RHI application. Property value uplift documented for future sale.
Solar thermal cylinder installation across every London borough
Frequently asked questions
What is a solar thermal cylinder and how is it different from a standard cylinder?
How much gas will solar thermal save me?
Is solar thermal or solar PV a better investment?
Do I need MCS certification for a solar thermal install?
What happens if the solar loop leaks into the cylinder?
How often does the solar loop need servicing?
What certification do you hold for the cylinder-side works?
Do you carry public liability insurance?
How long does the install take?
What if my roof does not face south?
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