Heat networks are becoming increasingly popular for multi-dwelling developments. Ian Robinson, technical manager at Heatrae Sadia, explains more about sizing the system.
Simple installation and maintenance requirements, energy efficiency and reduced costs are some of the factors making heat networks an increasingly desirable option for multi-dwelling developments, but what goes into a successful system?
The success of a heat network largely lies in it being sized and designed appropriately for the demand of the building, and it is therefore important for specifiers to ensure that time has been taken to understand both the peak and base loads.
When looking at peak loads it is critical to understand and apply appropriate coincidence factors, both inside and outside the apartment, to take account of the diversified hot water demand and space heating loads. When looking at how the system will cope with the base load, it is important that as much system turn down is built into the design as is feasibly possible.
There has been some confusion in the industry as to how to calculate the diversity of the system. While the latest UK guidance (BS EN 806 and BS8558) for sizing hot water loads can lead to oversizing, the Danish standard (DS439) offers a different solution and is being adopted throughout the industry. It is understood that using DS439 leads to significant reductions in the size of the central plant and heat network, especially in larger developments.
However, even DS439 is not a blanket solution. The designer must establish who will be using the building and what the likely tapping patterns are, and then establish what the likely impact will be on the system as a whole.
Oversized systems can have damaging consequences in the long run, as it won’t just be the boilers that are too large – inevitably elements such as the pipework and distribution pumps will increase, to the detriment of the system.
At the same time, the performance of an oversized system can be compromised because of its limited ability to modulate down in line with the daily and seasonal load profiles. It is important that a strategy to cope with the base loads is built into the system and sharing the peak load across more boiler modules is one way of doing this, but consideration must be given to how quickly the boilers can ramp up and down to meet fluctuating system demands.
For a more responsive system, designers should consider making the system more flexible by incorporating thermal buffer vessel. Here the buffer vessel is fed from the boilers and the buffer vessel subsequently feeds the system, thus any delay in the boilers responding to the changing demands of the system can be accommodated, leaving the more agile pumps to ramp up and down quickly and efficiently.
It is also worth noting that when using instantaneous hot water HIUs there must always be a flow of primary water around the system capable of delivering heat and inhibitor treated water to every extremity of the building. To achieve this, it is good practice to install a bypass (constant flow valve) at the end of each branch, which should be collectively sized to protect the pumps or jockey pump (if fitted) at the minimum duty point.
Heatrae Sadia’s Special Applications Team is experienced in providing expert support at the design stage of schemes both large and small, and its Hi-Max Instant ID HIU carries the NEMKO N mark and is approved by KIWA. For more information call 01603 420 220 or visit www.heatraesadia.com.
