Solar Thermal Systems: Flat Plate vs Evacuated Tube, Sizing & Building Regs

Quick Answer: Solar thermal systems use roof-mounted collectors to pre-heat domestic hot water, typically providing 50–70% of annual hot water needs in the UK. Flat plate collectors cost less and perform well in summer; evacuated tube collectors work better in low-light and winter conditions. Systems require a twin-coil or solar-dedicated cylinder, a solar pump station, and an expansion vessel. Unvented cylinders with solar require G3 qualification for installation.

Summary

Solar thermal is distinct from solar PV (photovoltaic). Solar thermal heats water directly using the sun's energy; solar PV generates electricity. In the UK, solar thermal typically uses glycol (antifreeze) in the collector circuit, which circulates through a heat exchanger coil in the hot water cylinder, transferring heat to the domestic water.

Solar thermal was popular in the 2000s under the Renewable Heat Premium Payment and later the Renewable Heat Incentive (RHI) schemes. With the RHI now closed to new applicants (March 2022), uptake has slowed somewhat, but solar thermal remains a cost-effective and low-carbon way to reduce energy bills, particularly for properties with high hot water demand or already-efficient heating systems (where solar thermal contributes a higher proportion of total heat demand).

For plumbers and heating engineers, the key competencies are: system sizing, collector selection, cylinder compatibility, safety device installation (for pressurised systems), and commissioning the controller.

Key Facts

• Flat plate collector — insulated frame with copper absorber plate and glass cover; lower cost; good summer output; output reduces significantly in winter/overcast conditions
• Evacuated tube collector — glass tubes with vacuum insulation; better in low-light conditions; 10–15% more efficient in winter; higher cost
• Glycol circuit — closed pressurised circuit using propylene glycol/water mix (typically 33–40% glycol for UK climate, giving freeze protection to -15°C to -20°C)
• Solar twin-coil cylinder — two heat exchanger coils: lower coil for solar; upper coil for boiler/heat pump backup; solar coil is usually larger surface area
• Cylinder sizing — approximately 40–80 litres per person for solar; for a family of 4: 160–200 litre solar-optimised cylinder
• Collector area — approximately 1–1.5m² per person; for a family of 4: 4–6m² flat plate or 3–4m² evacuated tube
• Solar fraction — proportion of annual DHW demand met by solar; typically 50–70% in UK
• Differential temperature controller — the solar pump station runs when collector temperature exceeds cylinder temperature by typically 5–10°C; shuts off when differential closes
• Safety thermostat — shuts off pump if cylinder exceeds 95°C (stagnation protection)
• Expansion vessel — sized for solar circuit; must accommodate fluid expansion AND stagnation pressure (when pump stops, glycol can boil if stagnation temperature high)
• Stagnation temperature — temperature a collector reaches when not circulating; can exceed 200°C for evacuated tubes in summer; system must be designed to handle this
• G3 qualification — required for unvented hot water cylinders including solar-dedicated unvented cylinders
• MCS certification — Microgeneration Certification Scheme; contractors must be MCS certified for domestic solar thermal installations if claiming any government support (no longer relevant to RHI but may be required for future schemes)
• Orientation — ideal is south-facing at 30–50° pitch; up to 45° from south and 10–70° pitch acceptable with reduced output

Quick Reference Table

Detailed Guidance

System Components and How They Work

A standard pressurised drainback or glycol indirect solar thermal system consists of:

1. Collectors — roof-mounted; absorb solar radiation and heat the glycol
2. Pump station — contains the circulation pump, flow meter, non-return valve, safety valve (set at 6–8 bar), pressure gauge, and often the differential temperature controller
3. Solar expansion vessel — sized for the glycol volume plus stagnation expansion; typically 18–35 litres for a domestic system
4. Pipework (solar primary) — insulated stainless steel or copper (copper in glycol is acceptable); must be insulated to prevent heat loss, particularly in cold roof voids
5. Twin-coil cylinder — primary heating coil (upper) and solar coil (lower); solar coil must be below the backup coil to take advantage of thermal stratification
6. Differential temperature controller — reads collector and cylinder temperatures; turns pump on and off

Glycol concentration: check annually with a refractometer. 33% propylene glycol gives freeze protection to approximately -15°C. Glycol degrades over time and should be replaced every 5–10 years.

Flat Plate vs Evacuated Tube — Choosing

Flat plate collectors:

• Lower upfront cost
• Simpler construction; fewer failure modes
• Lower stagnation temperatures (typically 120–180°C) — less stress on expansion vessel and glycol
• Good match for swimming pools, low-temperature applications
• Preferred by some installers because of lower maintenance complexity

Evacuated tube collectors:

• 10–15% better performance in diffuse light (overcast UK conditions)
• Individual tubes can be replaced if damaged (flat plate glass must be replaced entirely)
• Higher stagnation temperatures (up to 250°C for evacuated tubes) — requires robust expansion design
• Better performance in cold conditions (vacuum insulation means less heat loss)
• Higher cost; some designs have had issues with tube-to-manifold seal failure at stagnation temperatures

For most UK domestic installations, flat plate collectors are adequate and cost-effective. Evacuated tubes are worth considering for properties north of Manchester, for systems with significant winter demand, or where roof space is limited (evacuated tubes generate more output per m²).

Cylinder Compatibility and G3

A standard vented or unvented cylinder with a single coil is NOT suitable for solar thermal without modification. Requirements:

• Dedicated solar coil — positioned in the lower third of the cylinder (below the backup heat source coil), with at least 1.2m² of coil surface area
• Twin-coil cylinder — specifically designed for solar; most manufacturers produce these (Range Cylinders, Joule, Telford, Gledhill)
• Unvented option — for mains-pressure hot water; requires G3 qualification for installation; requires all mandatory safety devices (discharge valve, PRV, temperature-limiting thermostat, expansion vessel, tundish)
• Vented option — lower pressure, simpler; requires cold water storage tank

If the customer already has an unvented cylinder, check whether it has a solar coil (often marketed as "solar ready"). Many do — in which case, it is a matter of connecting the solar circuit to the lower coil.

Building Regulations and Planning

Planning permission: Usually not required for domestic solar thermal on a dwelling. Permitted development under the GPDO (General Permitted Development Order) applies if:

• Panels do not protrude more than 200mm from the roof plane
• Not on a Listed Building or in a Conservation Area
• Not on a World Heritage Site or SSSI

For flat roofs, panels may be mounted at an angle — check protrusion limits.

Building Regulations: Solar thermal connected to an unvented cylinder triggers G3 notification (unvented hot water system). The installer must hold the appropriate qualification (City & Guilds 6035 or equivalent) and notify the Local Building Authority.

Solar primary pipework (copper in the loft, through the building) should be fire-stopped at all penetrations and insulated to reduce heat losses per Approved Document L.

Controller Setup and Commissioning

After installation:

1. Fill and purge the solar circuit of air (critical — air prevents heat transfer)
2. Set differential temperature controller: typically ΔT on = 7°C, ΔT off = 3°C
3. Set maximum cylinder temperature: 60°C (Legionella prevention) to 90°C maximum
4. Set stagnation protection: if collector exceeds 120°C, pump can run briefly to cool system
5. Check glycol concentration with refractometer
6. Pressure test solar circuit: fill to 3 bar, check no leaks, then reduce to 1.5–2 bar working pressure
7. Check expansion vessel pre-charge: typically 1.5 bar (higher than heating system due to stagnation risk)
8. Run through one solar pump cycle and verify flow meter reading matches design flow rate

Handover documentation must include MCS compliance documents (if applicable), installer certificate, controller manual, glycol top-up instructions, and recommended maintenance interval (typically 3–5 years).

Frequently Asked Questions

Will solar thermal work in winter in the UK?

Yes, but with reduced output. On a clear winter day with good insolation, a flat plate collector will still generate useful heat, particularly in the morning when cold mains water enters the cylinder. Evacuated tubes perform better in winter. In overcast conditions (common in the UK from November to February), output may be negligible. This is why solar is described as "pre-heating" — the backup boiler or heat pump always provides the top-up needed.

Can solar thermal be combined with a heat pump?

Yes — solar thermal pre-heats the lower section of the cylinder; the heat pump serves the upper coil as backup. This combination can be particularly efficient because the heat pump operates with a warmer inlet temperature from the solar-preheated cylinder, improving COP. The cylinder must have three coils or a combined solar+ASHP coil arrangement; check with the heat pump manufacturer for compatibility.

What maintenance does a solar thermal system need?

• Annual check of glycol concentration with refractometer
• Annual check of expansion vessel pre-charge
• Annual check of controller settings and pump operation
• Visual inspection of collector, pipework insulation, and roof penetrations
• Glycol replacement every 5–10 years or when test shows degradation

Regulations & Standards

• Building Regulations Part G — hot water safety; unvented cylinders

• Building Regulations Part L — energy efficiency; solar contribution

• MCS MIS 3001 — Solar Thermal Installation Standard

• BS EN 12975 — Thermal solar systems and components; collector performance testing

• BS EN 12977 — Custom-built solar thermal systems; design and testing

• GPDO — General Permitted Development Order; planning exemptions for solar

• MCS Solar Thermal Standard MIS 3001 — MCS installation requirements

• Energy Saving Trust Solar Water Heating — Consumer guidance and savings data

• CIBSE AM3: Solar Heating Design — Design guidance for solar thermal systems

• BSI BS EN 12975 — Collector testing standard

• unvented cylinders — G3 requirements; mandatory safety devices

• hot water systems — Cylinder selection and system design

• heat pumps — Heat pump and solar thermal combination

• legionella management — Temperature requirements for solar systems