Birling Consulting Ltd
Birling Consulting Ltd

Common Controllability Issues

The following are some typical examples of common controllability issues:

  • Interaction between primary and secondary circuits due to poor hydraulic design. See BRECSU GIR40 and CIBSE Guide F for typical examples of good design. It is surprising how much such basic poor design still continues.
  • Poor header design, flows need to be at one end and returns at the other, If one circuit returns prior to the take-off for another circuit, the second circuit's flow temperature will be affected. Basic engineering, but I have seen this on major projects involving dozens of engineers within the last few years, without anyone realising until commissioning.
  • Inappropriate Control Strategies - Controls systems integrators normally do their best to try to make systems work but can resort to little more than a time delay before supplementary heat sources are enabled. The supplementary heat sources then frequently provide most of the heating. A recent review of an existing biomass heating system with the gas fired boilers only inhibited on a time delay on start-up had a biomass utilisation of only 13%!
  • Poor multiple chiller hydraulic design, the most common issue is dilution of flow temperature by passing the return CHW through an off-line chiller and mixing with the on-line chiller to achieve the required flow temperature. This results in significantly higher energy use to achieve the desired flow temperature. Again basic engineering, but frequently wrong.
  • Modern boilers and chillers are normally more efficient at part load than at full load. Therefore, traditional sequence control systems are not the most efficient form of control. The most effective control strategies for low energy/low carbon operation will require appropriate system hydraulics - we have some simple solutions.
  • Integration of low carbon technologies, such as biomass boilers are poorly understood and can result in inappropriate flows through the fossil fuel boilers when trying to maintain the correct operation of the biomass boiler and instable control through complicated temperature based control of heat stores and auxiliary boilers - again we have developed some robust energy efficient solutions.
  • The pressure loss through boilers varies considerably from one supplier to another, even for the same type of boiler. This can be by a factor greater than 10 to 1 for the same rated output. This should be taken into account in boiler selection and hydraulic design.
  • CHP systems are often poorly integrated, resulting in inefficient and unreliable operation, with CHP as a lead supplementary boilers need to be carefully controlled and use of condensing boilers reviewed as often they will not work in condensing mode dependent upon system design. A recent system redesign resulted in CHP units working continuously instead of unlikely to ever operate due to temperature limitations found on page 95 of the O&Ms.
  • Boilers that vary the flow rate in parallel with the burner firing rate can cause significant hydraulic problems, dependent upon the system design. However, variable flow boilers can be sucessfully integrated into systems saving primary pump costs and offering greter flexibility of operation.
  • Inadequate consideration of flow rates of modern high differential temperature boilers and systems.
  • Many condensing boiler installations rarely work in condensing mode as, apart from startup, the return water temperature is normally above 54C due to the system design. The necessity for low return water temperatures to enable condensing mode is still often overlooked.
  • Two stage burners should normally be avoided, as the second stage must be set to a lower temperature than the first stage for two stage operation, and significant temperature variations will occur in many applications. They can be made to work effectively with some forms of sequence control but modulating burners provide far more effective control for very little extra cost. 
  • The use of PICVs (Pressure Independent Control Valves) is becoming more common, however selection should be carefully considered as some have high pressure losses, particularly on larger sizes. I have used a combination of two and three port valves to provide good variable flow control with low pumping energy in the past, but now normally use PICVs, however minimum pump flows must be considered.
  • Separate systems providing heating and cooling that are not interlinked will invariably result in heating and cooling fighting each other - these still get installed, despite Part L requirements to avoid.
  • Heating/Cooling systems with different response times will always be difficult to control, even if interlocked.
  • Poor plant selection resulting in unnecessarily high differential pressures in variable flow systems, one item can significantly affect the energy consumption of the complete system. We have control strategies to minimise the impact, if this cannot be avoided.
  • Pumps being used for multiple functions, such as minimum flows through boilers/chillers plus maintaining minimum differential pressures on other parts of the system require very careful control strategies. Where possible we design systems to avoid potential conflicts in plant demands. These can normally be controlled, but can result in unnecessary energy use and/or potentially instable operation if not carefully engineered.
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