Housing Retrofit: Solar hot water heating

• After insulation, air-tightness, ventilation control and the installation of an efficient heating system, producing domestic hot water (DHW) using solar energy is probably the next highest contributor to cost-effective energy efficiency.
• A well-designed and properly installed solar domestic hot water system will typically produce between 50 – 60% of a home’s annual hot water needs. In summer a solar collector will produce between 80 – 90% of requirements, with the figure falling to around 20% during the winter months.

solar hot water collectors



• The most effective orientation for a solar panel / collector (in the UK) is facing due south. Efficiency falls off progressively from collectors facing towards east or west – though practically, even if a collector faces either due east or west, this might be only around 80% of maximum efficiency.

• Lower efficiency from less-than optimum orientation can be matched by increasing the size of the collector.

• If a pitched roof faces east and west, consider mounting panels on both pitches. Collector controls can be programmed to favour the better performing panel as the sun tracks around the house to the south.

optimising solar gain - roof facing

Location on the building

• The most common area to locate a solar panel/collector will be on the main roof of the building.

• Pitch: usually between 25-60ß. (The optimum angle for a south facing collector is 0.9 multiplied by the latitude + 29°. This maximises winter collection and reduces over-production in the summer.

• If for reasons of orientation or unsuitability of retrofit on the main roof, consider subsidiary roofs such as an extension. Even flat roofs can accommodate solar panel / collectors through the use of proprietary framework.

solar collector roof angle

• Orientation can be improved if a roof is to be replaced. Consider incorporating features (eg hips) that can optimise the orientation of collectors.

• Care should be taken to avoid shading by eg chimneys, dormers, adjacent buildings, trees etc.

• As part of the planned refurbishment, the immediate installation of a solar thermal system might be considered inappropriate. However, it might still be prudent to provide fixings and pipework to enable an installation to be easily installed at a later date.



Types of collector

• The choice of collector system will be between ‘flat plate’ and ‘evacuated tube’ technologies.

• Much debate surrounds the respective efficacies of the two systems. Specifiers will be recommended to review available evidence ahead of selecting a system.

• More information about how flat plate and evacuated tube systems work can be found at ‘Solar hot water collectors’.

solar collector tubes


Solar hot water systems



There are many permutations of components within solar thermal systems. The illustrations below are of typical installations and are for illustration purposes only. Some parts, controls and other components have been omitted for purposes of clarity.


The Twin-coil system

solar collector twin coil system

• Though ‘Direct’ systems (where water to be used for bathing and washing is heated directly through a solar collector) are often used, the prevalent systems in the UK are ‘Indirect’. The most basic indirect type of solar hot water system works through the heating of a liquid in the collector by the sun. The liquid then passes through a coil into a hot water cylinder. The water in the cylinder can either be used at this temperature, or raised to a higher temperature by a boiler or electric immersion heater.



The Pre-feed system

The Pre-feed system

• If the existing hot water cylinder is to be retained, the system most likely to be installed will be a ‘Pre-feed’ system.

• In a pre-feed system, a separate pre-heat cylinder is placed between the existing cold water feed and the existing hot water cylinder.

• This system has to be carefully designed to include a controller with shunt pumps between the cylinders.



The Combisystem (combining DHW and space heating)

The Combisystem (combining DHW and space heating)

• Although common in Scandanavia, combisystems have yet to make an impact on the UK market. This is likely to change as housing becomes more energy efficient and low-temperature heating systems become more viable.

• A combisystem is one that provides both domestic hot water and space heating.

• Typically, in a well-insulated house, a combisystem can contribute as much as 20-30% of the combined DHW and space heating requirement.

• In a combisystem there are two or more energy sources used to supply the heat. The solar collector will deliver heat as long as solar power is available, and the auxiliary source (gas or wood) supplements the missing solar power.

• The design of the storage tank and the water stratification therein is critical to the effectiveness of the system. A typical height / diameter ratio should be 2:5 or 3:1.

• The solar collector area supplying a combisystems will be typically 3x the area supplying a DHW systems alone




• A solar control system will be required seperately from other heating controls. The system is pre-set at the time of installation and will require no further user interaction.

• The control system monitors the collector and cylinder temperatures and starts the pump when there is useful solar energy to be gained. When there is no more useful solar energy the controller stops the circulating pump so that the water in the cylinder cannot be cooled.

• Safety features will ensure that the system temperatures can nvever get too high and that all components are protected against freezing.



Design considerations

• Panel sizing: Though a typical household collector will be around 4m2, it is important to calculate the size based upon likely demand. Sizing is important – if the area is too small there will not be enough heat in the hot water cylinder at a useable temperature, If the area is too large, energy will be wasted.

• Ensure that orientation of a panel anywhere other than due south is compensated for in the sizing of the panel.

• Check with the local planning authority that the collectors are acceptable.

• Calculate and provide for sufficient storage to accommodate all systems including solar. Ensure that sufficient space for water storage is allocated during space planning.

• Plan for a regular maintenance regime.




A useful introduction to the installation of solar collectors from the manufacturers Worcester Bosch.





Solar systems

• BS 5918:1989. Code of practice for solar heating systems for domestic hot water
• BS EN 12975-1:2006. Thermal solar systems and components. Solar collectors. General requirements
• BS EN 12975-2:2006. Thermal solar systems and components. Solar collectors. Test methods
• BS EN 12976-1:2001. Thermal solar systems and components. Factory made systems. General requirements
• BS EN 12976-2:2006. Thermal solar systems components. Factory made systems. Test methods
• DD ENV 12977-1:2001. Thermal solar systems and components. Custom built systems. General requirements
• DD ENV 12977-2:2001. Thermal solar systems and components. Custom built systems. Test methods
• DD ENV 12977-3:2001. Thermal solar systems and components. Custom built systems. Performance characterisation of stores for solar heating systems
• BS EN 15316-4-3:2007. Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies. Heat generation systems, thermal solar systems

Other useful standards

• BS 8211-1:1988. Energy efficiency in housing. Code of practice for energy efficient refurbishment of housing
• BS EN 832:2000. Thermal performance of buildings. Calculation of energy use for heating. Residential buildings
• BS EN ISO 13790:2008. Energy performance of buildings. Calculation of energy use for space heating and cooling
• BS EN 15316-1:2007. Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies. General
• BS EN 15316-2-1:2007 (3 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15316-3:2007 (3 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15316-4:2007 (6 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15459:2007. Energy performance of buildings. Economic evaluation procedure for energy systems in buildings
• BS EN 15377-3:2007. Heating systems in buildings. Design of embedded water based surface heating and cooling systems. Optimizing for use of renewable energy sources

Further information

• Solar Trade Association (www.solar-trade.org.uk)
• Boiler and Radiator Manufacturers' Association
• Hot Water Association (www.hotwater.org.uk)
• Heating & Hotwater Industry Council (www.centralheating.co.uk)
• Association of Plumbing and Heating Contractors (www.competentpersonsscheme.co.uk)


Solar thermal systems on GreenSpec





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