Solar Thermal Systems: A Plumber's Guide to Hot Water from the Sun

Solar thermal is one of the most cost-effective renewable technologies for domestic hot water in the UK. Unlike solar PV, which generates electricity, solar thermal directly heats water using energy from the sun — with efficiencies of 70–80% compared to around 20% for PV panels. For plumbers looking to diversify into renewables, it's a natural extension of hot water and heating skills, and with the right MCS accreditation, it opens access to grant funding for clients.

This guide covers how solar thermal systems work, the two main collector types, system design principles, installation requirements, and what engineers need to know about controls, commissioning, and maintenance.

How Solar Thermal Works

A solar thermal system uses roof-mounted collectors to capture heat from solar radiation. A heat transfer fluid (typically a glycol/water mixture) circulates through the collectors, absorbs heat, and transfers it to a purpose-designed hot water cylinder via a heat exchanger coil.

The key components are:

• Solar collectors — mounted on the roof (or occasionally on a ground frame or integrated into the roof pitch)
• Heat transfer fluid — glycol/water mix, typically 30–40% propylene glycol for UK climate freeze protection down to around -15°C
• Pump station — a circulating pump, flow meter, pressure gauge, safety valve, and non-return valve in a single manifold unit
• Solar cylinder — a twin-coil cylinder with a lower coil for the solar circuit and an upper coil for the boiler or immersion backup
• Expansion vessel — solar-rated vessel sized for the system volume and stagnation temperature (higher specification than standard heating vessels)
• Solar controller — a differential temperature controller that starts and stops the pump based on the temperature difference between collector and cylinder

The controller runs the pump when the collector is a set number of degrees warmer than the bottom of the cylinder (typically 5–8°C differential to start, 2–3°C to stop). When the sun isn't shining sufficiently, the pump stops and the boiler or immersion provides top-up heat.

Flat Plate vs Evacuated Tube Collectors

There are two main collector technologies used in the UK:

Flat Plate Collectors

Flat plate collectors consist of an absorber plate (typically copper with a selective coating) inside an insulated, glazed frame. The glazing is low-iron tempered glass that admits solar radiation but minimises thermal losses.

Advantages:

• Lower cost per m² than evacuated tubes
• More robust — less vulnerable to hail and physical damage
• Better performance in warm, sunny conditions
• Integrated design — can be roof-integrated (in-roof) for a neater appearance
• More predictable stagnation behaviour — when overheating occurs, flat plates are more tolerant

Disadvantages:

• Less effective in cold, diffuse light conditions (overcast UK winters)
• Heat losses are higher because the entire glazed area loses heat to the environment

Evacuated Tube Collectors

Evacuated tube collectors use rows of glass tubes, each containing an absorber strip within a vacuum. The vacuum dramatically reduces convective and conductive heat losses, making evacuated tubes significantly more efficient at lower temperatures and in diffuse light.

Advantages:

• Higher efficiency in cold and overcast conditions — better UK winter performance
• Lower heat losses due to the vacuum insulation
• Can achieve higher temperatures — useful for space heating contributions
• Modular design — individual tubes can be replaced if damaged

Disadvantages:

• Higher cost per m²
• Stagnation temperatures can be very high (>200°C) when the cylinder is fully heated — requires careful system design with adequate expansion capacity and high-temperature rated components
• Hail damage can break individual tubes
• Some designs are more complex to install

For most UK domestic installations: Both technologies are widely used. Evacuated tubes offer better annual yields in the UK climate (particularly in winter months), while flat plates are often preferred for roof-integrated aesthetics and simpler stagnation management.

System Sizing

The MCS (Microgeneration Certification Scheme) Solar Thermal Standards provide the definitive framework for sizing UK installations. Key sizing principles:

• Collector area: Typically 1–2 m² of flat plate per person, or 0.8–1.5 m² of evacuated tube per person. For a family of four, a 4 m² flat plate or 3–4 m² evacuated tube installation is typical.
• Cylinder volume: The solar cylinder should provide approximately 50–80 litres per person, with the solar coil occupying the lower portion and the backup coil or immersion in the upper section. Minimum 200 litres for a family of four — 250–300 litres is more typical.
• Don't oversize: An oversized system will frequently reach stagnation (where the cylinder is fully heated and the collectors have nowhere to dump heat). This stresses the system and is harder to manage than a correctly sized installation.

The annual solar fraction — the proportion of domestic hot water energy provided by solar — is typically 50–70% in the UK, depending on location, occupancy pattern, system size, and collector orientation.

Orientation and Pitch

Optimal collector performance requires:

• Orientation: Due South (±45°) — significant deviation from South reduces annual yield. East or West-facing installations are viable but with a meaningful performance penalty.
• Pitch: 30–50° from horizontal is optimal for annual yield in the UK. Steeper pitches (50–70°) reduce summer overheating but improve winter performance — sometimes preferable for evacuated tube systems where summer stagnation is a concern.
• Shading: Any shading on the collector (chimneys, trees, adjacent buildings) has a disproportionate impact. Carry out a shading analysis — the MCS standards require this.

Pipework and Circuit Design

Solar Circuit Pipework

The solar primary circuit (between collector and cylinder) requires:

• Copper tube — 15mm or 22mm depending on flow rates. Plastic tube is not suitable for the solar primary circuit due to high operating temperatures (stainless steel or copper only).
• Insulation — solar-rated insulation capable of withstanding temperatures up to 200°C. Standard foam insulation will melt at stagnation temperatures — use armaflex or equivalent rated insulation throughout, including outdoor sections.
• Outdoor sections — insulation must be UV-stabilised or protected with an outer casing where exposed to sunlight.
• Maximum pipe length — keep the solar circuit pipe run as short as possible to minimise heat losses and system volume. Longer runs require larger expansion vessels.

Glycol Concentration

The heat transfer fluid must be mixed to the correct concentration for the installation location. In most of the UK, 30–35% propylene glycol provides frost protection to around -15°C. Higher concentrations reduce heat transfer efficiency and should be avoided unless genuinely required by climate.

Use propylene glycol (food-safe), not ethylene glycol (toxic). Solar-specific glycol formulations include inhibitor packages designed for the collector materials and operating temperatures.

Expansion Vessel

The expansion vessel for the solar primary circuit must be:

• Rated for a maximum temperature of at least 130°C (many solar vessels are rated to 160°C)
• Sized for full stagnation — not just normal operating expansion. For evacuated tubes especially, the entire circuit may flash to vapour during prolonged summer stagnation, and the vessel must accommodate this without the safety valve lifting repeatedly.
• Positioned on the cold side of the pump (on the return from the cylinder, before the pump inlet) — this prevents cavitation and ensures the vessel sees the lower operating temperature.

Controls and Differential Temperature Controllers

The solar controller compares the temperature at the top of the collector with the temperature at the bottom of the cylinder (where the solar coil delivers heat). Standard settings:

• Switch-on differential: 5–8°C (pump starts when collector is this many degrees warmer than cylinder base)
• Switch-off differential: 2–3°C (pump stops to prevent heat loss back to collector)
• Maximum cylinder temperature: Typically set to 60–65°C. The controller stops the pump when the cylinder reaches this temperature, allowing the boiler or immersion to provide any remaining top-up. This is the anti-Legionella setpoint — the upper cylinder should reach 60°C regularly.
• Maximum collector temperature: High-temperature protection — the pump may run briefly on summer nights to dump excess heat to the cylinder if the collector temperature becomes dangerously high.
• Anti-freeze function: The pump runs briefly when the collector approaches freezing temperatures to circulate warm fluid from the cylinder.

Modern solar controllers also provide yield monitoring (kWh display), fault diagnostics, and holiday/boost modes.

Legionella Considerations

Solar thermal cylinders must reach 60°C regularly to control Legionella risk. Design considerations:

• The upper cylinder section (above the solar coil) should be heated to 60°C by the boiler or immersion at least once a week — most controllers provide an automatic pasteurisation cycle.
• The solar coil occupies the lower cylinder volume — this zone will fluctuate in temperature with solar gain. Do not allow the lower zone to sit at 30–50°C permanently (the Legionella growth range) without the full cylinder being brought to temperature.
• Document the pasteurisation strategy as part of the commissioning record.

MCS Certification and the Renewable Heat Incentive

To qualify for government funding schemes and to carry out solar thermal work commercially in the UK, installers must hold MCS accreditation for solar thermal (MCS 012 standard). This requires:

• Level 2 or 3 qualified plumber/heating engineer
• Solar thermal-specific training (City & Guilds 6089, BPEC Solar Thermal, or equivalent)
• Registration with an MCS-approved certification body

The Renewable Heat Incentive (RHI) scheme closed to new applicants in 2022 and was replaced by the Boiler Upgrade Scheme — which currently does not include solar thermal. Installers should check current government guidance for any new funding available at the time of installation. Even without direct subsidy, solar thermal provides genuine long-term fuel bill savings and strong client interest as energy costs remain high.

Commissioning

Solar thermal commissioning includes:

1. Pressure test the solar primary circuit (typically at 1.5x working pressure)
2. Flush the circuit to remove flux and debris
3. Fill with correctly mixed glycol solution — record the concentration with a refractometer
4. Pressurise to working pressure (typically 2.5–3 bar cold)
5. Commission the controller — set differentials, maximum temperatures, and test pump operation
6. Verify flow rate (typically 40–60 litres/hour per m² of collector) using the flow meter on the pump station
7. Complete MCS commissioning documentation
8. Provide full handover documentation to the client including system schematic, maintenance requirements, and controller operation guide

Ongoing Maintenance

Annual solar thermal maintenance should include:

• Visual inspection of collectors — check for broken glass, tube damage, soiling
• Check system pressure (cold) — should be at commissioning pressure; significant drops indicate a leak or expansion vessel fault
• Test glycol concentration and pH with a refractometer and test strips — glycol degrades over time and should be replaced typically every 5–10 years
• Check expansion vessel pre-charge pressure
• Verify controller is functioning correctly and yield data looks reasonable
• Check pump station components — non-return valve, safety valve, flow meter
• Inspect all pipe insulation for degradation, particularly at outdoor sections

Plumbing Products from APM

APM Electrics Plumbing stocks a wide range of plumbing and heating products relevant to solar thermal installations, including copper tube and fittings, expansion vessels, pressure relief valves, and system accessories. Browse our heating collection for expansion vessels and system components, or visit our trade counter for same-day collection.

Summary

Solar thermal is a well-established, cost-effective renewable technology that plays to the strengths of qualified plumbers and heating engineers. Key points:

• Evacuated tubes outperform flat plates in UK diffuse conditions; flat plates are more robust and better for in-roof aesthetics
• Size correctly — 1–2 m² per person, 50–80 litres cylinder volume per person
• Use copper tube and high-temperature-rated insulation throughout the solar primary circuit
• Size the expansion vessel for stagnation, not just normal operation
• Set controller for regular 60°C pasteurisation to manage Legionella risk
• MCS accreditation is required for commercial solar thermal installation and customer-facing certification
• Annual maintenance should include glycol testing and expansion vessel checks