What Is Solar Thermal?

 

Solar thermal is a renewable-energy technology that captures heat from sunlight.

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Roof-mounted collectors absorb solar energy and use it to heat a fluid circulating through the system.

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That heated fluid passes through a heat exchanger within a hot-water cylinder.

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Heat is transferred into the stored domestic hot water without the solar fluid itself mixing with the water coming from the taps.

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The basic process is:

sunlight → solar thermal collector → heated fluid → hot-water cylinder → hot water for the property.

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Solar Thermal Panels

 

The equipment installed on the roof is commonly referred to as solar thermal panels or solar collectors.

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Their purpose is to absorb as much useful solar heat as possible.

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Unlike photovoltaic panels, they do not produce electricity.

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Instead, they heat a circulating fluid.

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This fluid carries the captured heat from the roof to a hot-water cylinder inside the building.

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Solar Thermal vs Solar PV

 

This is one of the most important distinctions.

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Solar PV produces electricity.

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Solar thermal produces heat.

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A photovoltaic system can supply electricity to:

lights, appliances, batteries, EV chargers, air conditioning and heat pumps.

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A solar thermal system is much more specialised.

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Its main role in a domestic property is normally heating water.

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Neither technology is automatically better than the other.

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They simply perform different jobs.

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How Does a Solar Thermal System Work?

 

A typical solar thermal system contains several connected components.

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These can include:

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• solar thermal collectors

• circulation pipework

• heat-transfer fluid

• pump

• controller

• temperature sensors

• hot-water cylinder

• heat exchanger

• expansion vessel

• safety equipment.

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When the collectors become warmer than the water in the cylinder, the controller activates the circulation pump.

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The heated solar fluid then flows from the collectors to the cylinder.

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Heat passes through the cylinder's heat exchanger into the stored water.

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The cooler solar fluid then returns to the roof to be heated again.

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This cycle continues while useful solar energy is available.

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What Is a Solar Thermal Collector?

 

A solar thermal collector is the roof-mounted component that captures solar heat.

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Two common types are:

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Flat-Plate Collectors

These resemble large dark panels.

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A transparent covering allows sunlight to enter while helping retain heat within the collector.

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Beneath the glazing is an absorber plate connected to pipework containing the heat-transfer fluid.

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Flat-plate collectors have a relatively simple appearance and can integrate neatly with many roofs.

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Evacuated-Tube Collectors

Evacuated-tube systems use multiple glass tubes.

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Each tube is designed to reduce heat loss and capture solar energy efficiently.

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They have a more distinctive appearance than flat-plate systems and may perform differently under particular conditions.

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The appropriate collector type depends upon the building, available space, system design and budget.

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Solar Heating Panels

 

The term solar heating panels generally refers to solar thermal collectors.

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They are designed specifically to collect heat.

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That makes them particularly suitable where the property has a regular demand for hot water.

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Solar thermal does not normally replace the entire heating system.

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Instead, it supplements another source of heat.

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Solar Thermal Hot Water System

 

A solar thermal hot water system is normally designed to provide part of the property's annual domestic hot-water requirement.

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During bright summer weather, the solar system may provide a substantial proportion of the required heat.

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During winter, solar contribution is lower and the conventional heating system will generally provide more of the energy required.

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The boiler, immersion heater, heat pump or other heating source therefore remains important.

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Solar thermal should be thought of as reducing the amount of conventional energy needed rather than necessarily replacing it.

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Solar Thermal Energy

 

Solar thermal energy is simply heat captured from solar radiation.

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Even when the outdoor air temperature is relatively cool, solar collectors can absorb useful energy when sufficient sunlight reaches them.

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This means the system responds to available solar radiation rather than simply to outside temperature.

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However, solar thermal output varies considerably throughout the year.

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Summer normally provides far greater useful output than the darker winter months.

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Does Solar Thermal Work in the UK?

 

Yes.

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Solar thermal systems can operate effectively in the UK climate.

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They do not require Mediterranean temperatures.

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What they require is solar radiation.

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However, the seasonal nature of UK sunlight means that summer production will normally be much stronger than winter production.

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This needs to be reflected in the design.

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The system should not be sized on the assumption that it will independently provide all hot water throughout the year.

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Solar Thermal in Summer

 

Summer is where solar thermal can perform particularly strongly.

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Longer daylight hours and stronger solar radiation allow the collectors to deliver substantial amounts of heat.

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During good conditions, the conventional boiler may need to contribute relatively little towards domestic hot water.

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This can be particularly attractive during periods when the central-heating system would otherwise be operating solely to heat water.

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Solar Thermal in Winter

 

Solar thermal continues to collect available solar energy during winter, but production is significantly lower.

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Days are shorter and the sun is lower in the sky.

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The conventional heating system therefore plays a much greater role.

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This seasonal imbalance is fundamental to solar thermal design.

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The system should complement the property's main heat source rather than being expected to replace it completely.

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Can Solar Thermal Heat a House?

 

Technically, solar thermal energy can contribute towards space heating.

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However, domestic systems in the UK are most commonly designed principally for hot-water production.

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There is an important reason.

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The greatest demand for central heating occurs during winter, when solar energy availability is lowest.

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The greatest solar thermal production occurs during summer, when little or no space heating is usually required.

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For this reason, domestic hot water is generally the more natural application.

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Solar Heating Systems

 

The phrase solar heating systems can cover several arrangements.

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At the simplest level, a property may have solar thermal collectors feeding a dedicated hot-water cylinder alongside a gas boiler.

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More complex systems can potentially integrate with:

heat pumps, thermal stores, swimming pools or other heat demands.

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The right system depends upon the building and how hot water and heating are currently provided.

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The Solar Hot Water Cylinder

 

The hot-water cylinder is a critical part of the installation.

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A conventional cylinder may not always be suitable for adding solar thermal.

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Solar cylinders normally have an appropriate heat-exchanger arrangement allowing the solar circuit and conventional heating system to operate together.

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A typical arrangement may use:

one heat exchanger connected to the solar collectors and another connected to the boiler or other heating source.

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This allows solar energy to heat the stored water whenever available, with the conventional system providing additional heat when required.

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Do I Need a New Hot Water Cylinder?

 

Possibly.

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Properties with suitable existing cylinders may sometimes be adaptable, but many installations require a cylinder designed for solar thermal operation.

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Properties using a combi boiler can require more significant alterations because they may not currently have a conventional stored hot-water cylinder.

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The existing heating and hot-water system should therefore be assessed before deciding whether solar thermal is practical.

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Solar Thermal and Combi Boilers

 

A combi boiler normally heats water directly when a hot tap is opened.

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There is therefore no large conventional hot-water cylinder storing water.

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Solar thermal generally works most naturally with stored hot water.

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Adding solar thermal to a property with a combi boiler may consequently require a different arrangement, potentially including a new solar-compatible cylinder.

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That increases the scale and cost of the project.

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It is one reason why solar thermal is not equally suitable for every house.

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Solar Thermal and Conventional Boilers

 

Properties already using a conventional or system boiler with a hot-water cylinder can potentially provide a more straightforward starting point.

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The solar system can preheat or heat the stored water.

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The boiler then provides additional heat when solar output is insufficient.

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This allows the two technologies to operate together automatically.

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Solar Thermal and Heat Pumps

 

Solar thermal can also be considered alongside a heat pump.

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The design needs to account for the way domestic hot water is stored and heated.

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In some properties, photovoltaic panels may instead be used to generate electricity that contributes towards operating the heat pump.

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This illustrates why the entire energy system should be considered rather than looking at individual technologies in isolation.

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Solar Thermal or Solar PV?

 

For many homeowners, this is the big question.

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Solar thermal is very efficient at performing one particular job:

capturing solar heat for hot water.

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Solar PV is much more flexible because the electricity produced can be used for many purposes.

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Solar PV electricity can potentially supply:

appliances, lighting, battery storage, EV charging, air conditioning and heat pumps.

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This flexibility is one reason PV has become such a major part of modern home-energy planning.

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However, solar thermal can still be attractive where a property has substantial and consistent hot-water demand.

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Solar Thermal and Battery Storage

 

Solar thermal itself does not charge an electrical battery because it produces heat rather than electricity.

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Its equivalent form of storage is effectively the hot-water cylinder.

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Heat collected during the day is stored within the water until required.

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This is known as thermal storage.

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The principle is similar to battery storage in one respect:

energy is captured when available and retained for later use.

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The difference is that the stored energy is heat rather than electricity.

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Solar Thermal and Solar PV Together

 

It is possible to install both solar thermal and solar PV on the same property.

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One system produces hot water.

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The other produces electricity.

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However, both technologies compete for roof space.

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Where roof area is limited, the designer needs to determine which use of that space provides the greatest benefit.

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For some properties, maximising PV and using electricity flexibly across the house may be preferable.

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For others with unusually high hot-water demand, solar thermal may justify part of the available roof area.

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How Much Roof Space Does Solar Thermal Need?

 

Solar thermal generally requires less roof area than a large PV installation because its purpose is principally to satisfy hot-water demand.

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The required collector area depends upon factors including:

number of occupants, hot-water consumption, collector type, roof orientation and cylinder capacity.

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An oversized system is not necessarily desirable.

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If the system produces more summer heat than the household can use or store, that energy provides little additional benefit.

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Correct sizing is therefore important.

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Which Direction Should Solar Thermal Panels Face?

 

Collectors need good solar exposure.

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South-facing roofs can provide excellent performance, although south-east and south-west orientations can also be suitable.

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Other orientations may still work depending upon the property.

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Shading should be considered carefully.

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Nearby trees, chimneys and buildings can all reduce the amount of useful solar energy reaching the collectors.

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Solar Thermal on Flat Roofs

 

Solar thermal collectors can be installed on flat roofs using mounting frames.

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This allows the collector angle and orientation to be selected independently of the roof pitch.

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The system needs to consider:

wind loading, structural capacity, waterproofing, access and shading.

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As with photovoltaic panels, roof condition should be checked before installing equipment expected to remain in place for many years.

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Solar Thermal on Pitched Roofs

 

On pitched roofs, collectors can be mounted above the roof covering or integrated more closely into the roof construction.

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Fixings need to transfer loads safely back into the supporting structure.

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The roof must remain weatherproof.

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For new roofs or major refurbishment projects, integrating the solar system during the roofing work can sometimes produce a neater installation.

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Solar Thermal for New Builds

 

A new house provides an opportunity to design solar thermal into the building from the beginning.

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The roof orientation, plant space, pipe routes and hot-water cylinder can all be considered before construction.

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However, solar thermal should still be compared with the alternatives available for the complete energy strategy.

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A new home may also be considering:

solar PV, battery storage, heat pumps, EV charging and mechanical ventilation.

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The correct decision depends upon how all of these systems work together.

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Solar Thermal During an Extension or Refurbishment

 

Major building works can provide an excellent opportunity to assess solar thermal.

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The roof may already be changing.

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Pipe routes may be accessible.

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A new utility or plant room may be being created.

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The hot-water system may already require alteration.

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This can reduce the disruption involved compared with installing solar thermal after the building has been fully refurbished.

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The same principle applies to solar PV, batteries, air conditioning and EV charging.

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Major refurbishment is often the right moment to consider the property's whole energy strategy.

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Solar Thermal for Large Families

 

Solar thermal can be particularly attractive where there is substantial daily hot-water consumption.

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A large household may require significant quantities of hot water for:

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showers, baths, washing and general domestic use.

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Because that demand exists throughout the year, the property may be able to make useful use of the heat generated by the collectors.

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System sizing should nevertheless be based upon realistic consumption rather than simply installing the largest possible collector area.

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Solar Thermal for Hotels and Commercial Buildings

 

Solar thermal can also be useful beyond domestic properties.

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Commercial buildings with high hot-water demand can provide interesting opportunities.

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Examples might include:

hotels, leisure centres, care homes, changing facilities and certain hospitality businesses.

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These properties may use considerably more hot water than an ordinary house.

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Where hot-water demand remains high during the summer, solar thermal can potentially be particularly useful because this coincides with the strongest solar availability.

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Solar Thermal for Swimming Pools

 

Swimming pools can provide another application for solar thermal energy.

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Pools require substantial quantities of heat.

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Demand can also coincide well with periods of strong sunshine, particularly for outdoor pools.

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The type of collector and system design may differ from normal domestic hot-water installations.

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A specialist assessment should establish the appropriate arrangement.

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Solar Thermal System Controls

 

Modern solar thermal systems include controls that monitor temperatures within the collectors and hot-water cylinder.

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The circulation pump normally operates only when useful heat can be transferred.

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There is little benefit in circulating fluid from a cold roof through a warmer cylinder.

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The controller therefore compares temperatures and operates the pump according to the available solar energy.

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Solar Thermal Pumps

 

The solar circuit requires a circulation pump to move heat-transfer fluid between the collectors and cylinder.

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This pump uses a relatively small amount of electricity.

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It must be appropriately selected for the system and capable of operating with the solar fluid and expected temperatures.

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The pump station may also incorporate gauges, valves and other system components.

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Heat-Transfer Fluid

 

The liquid circulating through the roof collectors is not normally the domestic water coming from your taps.

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Solar systems generally use a specialised heat-transfer fluid.

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This allows the circuit to operate safely under varying temperatures and helps protect the system during cold weather.

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The fluid transfers heat through the cylinder's heat exchanger without mixing with the domestic hot water.

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Solar Thermal Expansion Vessel

 

Fluid expands as it becomes hotter.

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The solar thermal system therefore requires provision to accommodate changes in fluid volume and pressure.

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An expansion vessel forms part of this safety arrangement.

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The system also requires appropriate valves and controls.

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Solar thermal installations operate at potentially high temperatures, so correct design and commissioning are important.

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Can Solar Thermal Overheat?

 

Solar thermal collectors can become very hot during periods of strong sunshine.

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This is particularly relevant when there is little hot-water demand.

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For example, a household might go away on holiday during the summer while the collectors continue receiving strong sunlight.

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The system therefore needs to be designed to manage high temperatures and periods when the stored heat cannot be used.

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Correct sizing is important partly for this reason.

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Solar Thermal Maintenance

 

Solar thermal systems require more mechanical components than a simple roof-mounted PV array.

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Maintenance can include checks to:

solar fluid, pumps, pressure, controls, expansion vessel, insulation and collectors.

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The condition of the heat-transfer fluid may need to be monitored and serviced according to the system requirements.

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A properly maintained installation can continue operating for many years.

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Solar Thermal Panel Cleaning

 

Collectors exposed to normal rainfall may require relatively little cleaning.

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However, significant dirt, bird deposits or other contamination can reduce solar absorption.

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Cleaning should be undertaken safely.

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Roof access should not be attempted casually simply to wash the collectors.

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Specialist maintenance may be appropriate where access is difficult.

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Solar Thermal System Lifespan

 

A well-installed system can provide many years of service.

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However, different components have different working lives.

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Roof collectors themselves may remain serviceable for a long period.

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Pumps, controllers, valves and other mechanical components may require replacement earlier.

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The maintenance history and availability of replacement components are therefore relevant when assessing an older installation.

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Solar Thermal System Cost

 

The cost of a solar thermal system varies according to the building and installation.

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Factors can include:

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• collector type

• collector area

• roof access

• scaffolding

• hot-water cylinder

• pipework

• controls

• existing heating system

• plant-room alterations

• complexity of installation.

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A property already undergoing major heating or refurbishment work may sometimes be easier to adapt than a fully finished house requiring extensive alterations.

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Solar Thermal Installers Near Me

 

When searching for solar thermal installers near me, it is worth recognising that solar thermal requires a different skill set from ordinary solar PV installation.

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The contractor needs to understand:

roof installation, plumbing, hot-water storage, controls and renewable heating.

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An installer specialising entirely in photovoltaic electricity may not necessarily undertake solar thermal work.

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Ask specifically about experience with the type of thermal system being proposed.

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Choosing a Solar Thermal Installer

 

A competent installer should assess the whole property before recommending a system.

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That assessment should consider:

hot-water demand, roof orientation, collector area, cylinder capacity and the existing boiler or heating system.

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The quotation should explain what equipment is being proposed and what alterations will be required within the house.

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A system based purely upon the maximum collector area that fits on the roof may not represent good design.

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Solar Thermal Planning Permission

 

Many straightforward roof-mounted renewable-energy installations may benefit from planning allowances, but the position depends upon the building, location and installation.

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Additional consideration may be required for:

listed buildings, conservation areas, prominent roof slopes and unusual installations.

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The planning position should therefore be checked where necessary before work starts.

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Solar Thermal on Listed Buildings

 

Solar thermal may be possible on some listed properties, but it requires careful assessment.

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Collectors can alter the appearance of a historic roof and their installation can affect building fabric.

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Less prominent roof slopes or outbuildings may sometimes provide alternatives.

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The heritage impact should be considered before the system design is finalised.

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Solar Thermal in Conservation Areas

 

Conservation-area properties may also require greater attention to visual impact.

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A rear roof slope may present a very different planning situation from a prominent front roof facing the street.

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The collectors should therefore be positioned with both performance and appearance in mind.

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Solar Thermal and Roof Condition

 

Before fitting thermal collectors, the roof should be in suitable condition.

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The installation is intended to remain in place for many years.

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Placing collectors onto a roof that will soon need substantial repair can create unnecessary future cost because the equipment may have to be removed and reinstated.

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Where roof replacement is already planned, coordinating the two projects can be sensible.

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Is Solar Thermal Efficient?

 

Solar thermal can be highly effective at converting available solar energy into useful heat.

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But efficiency alone does not determine whether it is the best investment.

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The useful question is:

Can the property actually use the heat being produced?

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A highly efficient system producing excess hot water during summer provides limited additional benefit once the cylinder is already hot.

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This is why matching collector area to hot-water demand is so important.

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Is Solar Thermal Still Worth Considering?

 

Yes, particularly for properties with substantial and consistent hot-water demand.

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However, the decision should now be considered alongside the growing range of electrical technologies available.

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Solar PV has the advantage that its electricity can be used for many different purposes.

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A homeowner considering a broader energy strategy may want electricity for:

battery storage, EV charging, air conditioning, heat pumps and household appliances.

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Solar thermal remains excellent at producing heat, but its output is less flexible.

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The right choice therefore depends upon the property and what the owner is trying to achieve.

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Solar Thermal or a Bigger PV System?

 

This is increasingly a useful question.

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Imagine a roof with limited space.

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Part could be allocated to solar thermal collectors and part to photovoltaic panels.

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Alternatively, the entire available roof could potentially be used for PV.

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The electricity could then contribute towards domestic hot water through an immersion heater or heat pump while also being available for other loads.

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There is no universal answer.

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But the comparison should be made before committing valuable roof area to either technology.

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The Whole-Home Energy Strategy

 

Modern homes are becoming increasingly interconnected electrically.

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A future-focused energy system might combine:

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solar PV

battery storage

EV charging

air conditioning or air-to-air heat pumps

efficient domestic hot-water production

smart controls.

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Solar thermal can still form part of that picture, particularly where hot-water demand justifies it.

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But it should be considered within the wider strategy rather than installed simply because the roof receives sunshine.

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Planning a Solar Thermal System?

 

A good solar thermal system starts with understanding the property's hot-water demand.

 

Roof orientation, available collector area, shading, hot-water storage and the existing heating system should all be assessed before equipment is selected.

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It is also worth comparing solar thermal with solar PV and considering how the property's energy requirements may change over the coming years.

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If you are planning an extension, refurbishment, heating upgrade or wider energy improvements, that can be an ideal time to consider the options together.

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APN Solar can help homeowners and property owners connect with experienced renewable-energy specialists who can assess the property and provide proposals for solar thermal panels, solar PV and wider home-energy improvements.

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