Birling Consulting Ltd
Birling Consulting Ltd

Controllability - The Performance Gap

The building performance gap has been highlighted in recent years, this is often quantified in terms of energy efficiency, yet there are many other factors that affect stable, comfortable, energy efficient building operation.

Controls and their integration have been identified as principal cause of buildings performing poorly in numerous papers by Bill Bordass and others, BSRIA webinars on Soft Landings and the Australian NABERS programme. In 1994, BRECSU General Information Report 40 Heating systems and their control identified that 90% of systems were not properly controlled costing £500million in additional energy costs; this is still reported as a fact in current Carbon Trust Publications.

With increasing pressure for more efficient lower carbon solutions boxes are frequently being ticked for renewables, CHP, etc. These all have unique characteristics that must be considered to ensure controllable systems. For instance many CHP systems have maximum return water temperatures, biomass boilers respond slowly and need to dissipate residual heat on shutdown, etc. Systems are also becoming ever more complicated with thermal storage to increase utilisation of low carbon heat sources and provide additional capacity for peak loads on heat distribution networks.

Controls are often blamed when systems do not work; yet often systems, particularly bivalent systems, are uncontrollable or difficult to control due to plant characteristics, hydraulic design, etc. Plant manufacturers and specialist contractors guidance is often lacking (with some exceptions) as it normally aimed at the relevant item of plant without consideration of overall system operation. During the development of CIBSE AM15 Biomass Heating a high proportion of standard schemes that were proposed by manufacturers and specialist contractors were found to have fundamental flaws that affected overall system operation.
CIBSE AM15 Biomass Heating identifies the need for a controllability review, other CIBSE, BSRIA, etc. guidance identifies the need for checks that systems can be commissioned effectively throughout the design process.

Unfortunately, controllability reviews and commissioning design checks are rarely effectively (if at all) carried out and control specifications are often written stating the low carbon heat sources shall always be the lead heat source, but do not include principles of how this is achieved. 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%!

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