Review of BS 6644

by Michael Smith
NBS Information Specialist

BS EN 12828 Heating systems in buildings - Design of water-based heating systems has been in circulation in the UK since 2003; even then, it was a well established requirement in Europe. However, since its introduction to the UK, there has been confusion about its use. One of the main reasons for this appears to be that many crucial elements of BS EN 12828 have conflicted with some of the requirements of BS 6644. BS 6644 Specification for Installation of gas-fired hot water boilers of rated inputs between 70kW (net) and 1.8 MW (net) (2nd and 3rd family gases) is the main standard for installation of gas boilers above 70kW (net) in the UK.

Coverage

The wider ranging BS EN 12868 covers all aspects of designing for water-based heating systems; it also has specific implications for boilers over 300kW installed on sealed systems. Originally it was introduced in the UK to help safeguard against the over-heating and over-pressurisation of individual boilers.

As installers, consultants and designers of heating systems are most familiar with the requirements of BS 6644 and the crossover between the requirements of this and those of BS EN 12828 has caused some confusion, the industry has continued to observe what it knows best and not use the harmonised standard. As a consequence of this, some heating systems incorporating boilers over 300kW designed and installed in the UK, will not have been designed to the BS EN 12828 standard.

Fundamental review

This situation looks set to change within the next few years, BS 6644 is currently undergoing a fundamental review to incorporate the requirements of BS EN 12828 and other technical updates to harmonise the two standards and provide greater clarity for the heating and ventilation sector and construction industry generally.

At present the standards are in the hands of the responsible committees: RHE/24 Central heating installations and GSE/30 Gas installations.

These changes are being questioned, especially as BS EN 12828 has been so widely overlooked for the past eight years; however, there are important reasons why the proposed changes to the standards must be made, mostly relating to system and boiler safety.

Proposed changes

The main requirements of BS EN 12828 are for additional safety equipment to protect against overheating and over-pressurisation of individual boilers. Although cases of this happening are relatively rare in the UK, instances have been reported in Europe. In light of this the standard was introduced for prevention purposes here also.

The following items mention some of the areas that are expected to be covered in the review of BS 6644.

Flash traps

Under BS EN 12828, flash traps (liquid separators) are required to be fitted to the outlet of safety valves serving boilers in excess of 300kW. The point of a flash trap is to separate steam and water in the possible event of the safety valve discharging. The main reason for this requirement is that the boiling point of water increases as heating system pressure increases. Theoretically, the water in systems incorporating boilers over 300kW could reach temperatures that are well in excess of 100°C at the point that a safety valve discharges. As the liquid water exited into normal atmospheric pressure, it would immediately explosively evaporate to steam, with an expansion ratio of around 1700:1. This type of rapid expansion could cause serious damage or critical injury.

A flash trap allows the steam and water to separate by having an increased internal area with a pipe routed vertically up to atmosphere for steam and another discharge down to a safe level for the water. To install flash traps requires vents to outside atmosphere, and so it allows another possible path to compliance.

The alternative option is to double up on the safety chain equipment to ensure the likelihood of a safety valve ever having to discharge is remote. An additional limit thermostat and high-pressure limiter (in addition to those already in the boiler equipment) can also be fitted instead.

Safety temperature limiter

BS EN 12828 states that each heat generator shall be served by a safety temperature limiter, which includes its own specific sensor. It also states that if the heat generator is not equipped with a safety temperature limiter by the manufacturer, such a device should be fitted on the system flow pipe as near as possible to the heat generator, to restrict temperature rise to no more than 10°C after switching off the heating or fuel supply.

Pressure limiter

BS EN 12828 also states that each individual heat generator greater than 300 kW nominal heat output shall be served by a pressure limiter. If it is not equipped with a pressure limiter by the manufacturer such a device must be fitted to the system, as near as possible to the heat generator.

Water level limiter

Along with the requirements of BS 6644, a water level limiter must be fitted for boilers over 300kW or boilers installed in a position where the system is lower than the boiler (roof top installations, for example). Generally they incorporate a float activated cut off with a manual reset. It is important there is no automatic re-setting of the safety device, so the cause can be investigated properly should there ever be a problem.

Summary

As a consequence of the upcoming review of BS 6644, BSI hopes that all future installations incorporating boilers over 300kW will be designed and installed to the standard, but will be able to measure up to the safety factors given in BS EN 12828.

Further information

BS EN 12828:2003 Heating systems in buildings - Design for water-based heating systems

BS 6644:2005 Specification for installation of gas-fired hot water boilers of rated inputs between 70 kW (net) and 1.8 MW (net) (2nd and 3rd family gases)

Committee: GSE/30 Gas installations (1st, 2nd and 3rd family gases)
http://standardsdevelopment.bsigroup.com/Home/Committee/50000784

Committee: RHE/24 Central heating installations
http://standardsdevelopment.bsigroup.com/Home/Committee/50001173

Related NBS information:

Articles:

• Condensing boilers and domestic heating controls
• Domestic heating compliance: Solid fuel appliances

Selected links:

• Mechanical and electrical engineering
• HVAC

Added August 2011

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