Effective pressurisation design and accurate expansion vessel sizing are essential for ensuring the long-term health, efficiency and cost-effectiveness of water-based heating systems.
Two key standards provide guidance in this area: BS7074:1989 Application, selection and installation of expansion vessels and ancillary equipment for sealed water systems – Code of practice for domestic heating and hot water supply, and BS EN 12828:2012+A1:2014 Heating systems in buildings – Design for water-based heating systems. While both aim to achieve stable system operation, they adopt different methodologies, leading to varying outcomes when applied to the same system.
Both begin by requiring the calculation of the system’s water volume and the amount of thermal expansion resulting from temperature changes between filling and operation. But then the guidelines diverge in their treatment of reserve volume, vessel sizing methodology, and fill pressure.
Under BS7074, 10% is added to the calculated expansion volume to account for system variability. However, this standard provides no guidance on fill pressures, leaving its interpretation open. BS EN 12828 requires the addition of 0.5% of the total system volume to the expansion calculation as a reserve. This ensures the expansion vessel always contains water, which is critical for maintaining stable pressure and preventing air ingress during cooling or other operational changes.
Both standards aim to achieve stable system operation, but adopt different methodologies
The approach to sizing expansion vessels differs significantly. BS7074 suggests multiplying the calculated expansion volume by three and rounding up to the nearest standard vessel size. This approach can become problematic for larger systems or those with significant height differences. Iterative recalculations may be needed to achieve the desired working pressure.
BS EN 12828 determines vessel size by first calculating the minimum and maximum working pressures. From this, the standard calculates the maximum permissible percentage of vessel usage and derives the minimum required vessel size in a single step. This method is more precise and avoids inefficiencies associated with undersized vessels.
System setup is critical for stable performance. Both standards agree on the importance of maintaining positive pressure under cold, static conditions to prevent air ingress. They specify that the minimum system pressure must be at least 0.2 bar above the static height of the system. However, approaches to fill pressure differ substantially.
BS7074 does not provide specific guidance on fill pressure, and the common practice of equating fill pressure with vessel gas pressure often results in empty vessels at startup. This can cause pressure fluctuations and air ingress, particularly in taller systems or chilled water applications. Furthermore, this setup leads to challenges with pump operation. As pumps react to system pressure, they require a stable differential to prevent excessive cycling. Without sufficient water in the vessel, pumps may experience rapid start-stop cycles, increasing wear and energy use.
In contrast, BS EN 12828 specifies that the fill pressure must exceed the vessel’s gas pressure to ensure water enters the vessel. This prevents sudden drops in pressure and reduces the likelihood of air ingress. By maintaining a stable reserve volume within the vessel, this standard supports consistent pump operation. The differential pressure required to prevent pump cycling —typically 0.2 bar — is more reliably maintained, improving energy efficiency and reducing wear on components.
The differences between these standards has practical implications. Systems designed under BS7074 may experience frequent air ingress, which accelerates corrosion, increases chemical consumption and shortens system lifespan. In addition, no guidance on fill pressure and reserve volume can lead to operational inefficiencies, such as excessive pump cycling.
BS EN 12828 offers a more comprehensive framework. Its emphasis on maintaining adequate reserve volume and stable pressure ensures better reliability, reduces maintenance frequency, and extends component life. The standard may require a larger initial investment in vessel capacity, but long-term benefits outweigh costs.
Choosing the appropriate standard depends on the system’s specific requirements and operational goals. While BS7074 offers simplicity, its limitations can lead to long-term inefficiencies and reliability issues. BS EN 12828, with its more precise calculations and detailed guidance, provides a robust solution, particularly for those with complex operational demands.
By understanding the differences between the standards, designers and operators can optimise performance, and reduce life-cycle costs.