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Making the move to 70:40

Significant efficiency improvements in heating systems can be achieved by reducing flow:return temperatures to 70°C:40°C. Lars Fabricius explains how this influences almost every aspect of system design.
There is growing recognition that the majority of heating systems operate more efficiently with lower return water temperatures.

This is evidenced by the guidance in the latest version of CIBSE AM12/2013 Combined Heat and Power for Buildings. This states: 'It is recommended that, for new systems, radiator circuit temperatures of 70ºC (flow) and 40°C (return) are used, with a maximum return temperature of 25°C from instantaneous domestic hot water heat exchangers.'

However, the benefits of lower return temperatures are not confined to CHP and district heating (DH). Heating systems using condensing boilers and/or heat pumps will operate more efficiently when systems are designed to operate with flow:return temperatures of 70°C:40°C.

Achieving this has important implications for the design of the whole system.
For example, weather compensation plays a key role as it will automatically reduce flow temperatures when ambient temperatures rise.

And, of course, a reduced flow temperature will result in a reduced return temperature as long as sufficient heat is removed from the water at the terminal unit.

The most obvious way to improve the efficiency of heat exchange at the terminal unit is to reduce the flow rate, so that the flow water spends more time in contact with the exchanger and is able to lose more heat to the space.

The same principle applies when plate heat exchangers are used for the domestic hot water (DHW) rather than hot water cylinders.

As CIBSE AM12 states: 'It is preferable to adopt a variable volume control system for the DH system and all of the building heating systems connected.

This will ensure that pumping energy is minimised through reducing the volume of water to be pumped and the pressure drops to be met, and also reduces heat losses through ensuring that return temperatures remain low under part load conditions.'

Using lower, variable flow rates has a knock-on effect on other aspects of the system.

For instance, variable flow rates require a variable speed pump and, as noted in CIBSE AM12, this enables reduced pump sizes and reduced pump energy consumption.

However, variable speed pumping will result in varying pressures at different points of the system and two-port valves such as thermostatic radiator valves (TRVs) are not designed to function with variable pressures.

Consequently, this may impact on their ability to control temperature effectively. Indeed, this is a common problem with many multi-residential and other distributed heating systems.

The answer is to include differential pressure control valves (DPCVs) within the heat interface units in heating and hot water circuits, to enable the two-port control valves to operate as designed.

The DPCVs respond dynamically to peaks and troughs in pressure throughout the day, maintaining constant pressure differential and optimising system performance.

Lower flow rates will also influence pipe sizes as the internal diameter of traditional copper pipework is too large for efficient performance of ultra-low flow systems.

The most sensible solution is to use flexible piping systems with a smaller internal diameter.

Commissioning with ultra-low flows When flow rates are reduced, commissioning engineers often find themselves dealing with ultra-low flows (<0.015 l/sec), which are too low to measure accurately and consistently with standard flow measurement devices.

CIBSE Code W therefore states: 'Specifiers are required to identify an appropriate commissioning method for ultra-low flow system designs.'

To address this issue, commissioning modules specially designed for use with ultra-low flow systems can be employed.

These modules overcome the problems of ultra-low flows by connecting groups of terminal units to a single, compact manifold that includes a differential pressure control valve (DPCV).

The module then acts as a 'control centre' for commissioning, enabling flow measurement by subtraction - one of the methods recommended in the latest revisions to CIBSE Code W.

With the subtraction method for commissioning ultra-low flow systems, there are three simple steps:

1. The flow through all of the terminal units connected to the commissioning module is measured to obtain a combined flow rate

2. With one terminal unit isolated, flow rate is then measured through the remaining terminal units

3. The difference in flows between 1 and 2 is equivalent to the flow rate of the isolated unit

A whole system approach
CIBSE AM12:2013 represents a sea-change in the approach to the design of heating systems and sets new benchmarks for what constitutes best practice in the design of CHP and District Heating schemes.

It is also clear that achieving 70:40 flow:return temperatures will bring efficiency benefits to many other types of heating systems.

Delivering these efficiencies in practice requires a whole-system approach that incorporates DPCVs, variable speed pumps, ultra-low flow commissioning modules and use of flexible pipe.

Key benefits of using flexible pipe

  • Connection is only required at the ends

  • Faster installation, fewer connections, no elbows or bends

  • Consistent with BSRIA guidelines for reducing installation times

  • Fewer connections reduce the risk of leaks

  • Easy to fit, with no requirement for specialist skills

  • Robust design, resistant to rough handling on site

  • Rated for temperatures up to 95°C and a working pressure of 12bar

  • Life expectancy of 50 years, full guarantee for 10 years

  • Gas-tight against oxygen, reduced risk of corrosion

  • // The author is managing director of SAV Systems //
    14 August 2013


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