Birling Consulting Ltd
Birling Consulting Ltd

Controllability Reviews

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.


There are many other factors that also affect performance such as commissioning, user interaction with controls, thermal characteristics of the building, etc. but controllability is often the Achilles Heel of building services. Poor controllability can lead to significant building performance gaps and protracted contractual disputes. In many instances the true cause of the problem is never established and sticking patch measures are often adopted to try to resolve issues.

A review of system controllability at an early stage in the system design process is invaluable to identify potential issues and revise the design as required. Often the effect on capital cost is negligible and commissioning problems are reduced, with lower overall costs to handover. The improvement in building performance can be significant, leading to satisfied occupants and/or operators and far less risk of delays and contractual issues.

A controllability review of existing systems can also be very valuable to identify fundamental issues that may not be apparent from site observations, or BMS data analysis.

To conduct an effective controllability review requires many years experience of both building services design and HVAC control design, plus hands on commissioning/problem solving experience. There are very few engineers with adequate experience for controllability reviews on more complex systems and recent experience has shown even highly experienced mechanical design engineers can make fundamental errors on relatively simple systems due to lack of understanding of controllability.

A controllability review should take into account the most efficient operation of plant, modern boilers and chillers are more efficient at low loads however, frequent stopping and starting at low loads is not efficient. Temperature limitations, rate of response, interaction with packaged plant controls, etc. should all be considered.


Controllability Reviews by Birling Consulting Ltd:

Graham Smith CEng FCIBSE MInstMC undertakes controllability reviews as part of a range of specialist professional services associated with building services and their control. Most services are provided via longstanding associations with clients, but new clients are always welcome.

Controllability reviews normally consist of:

  • Checks on buffer vessel/thermal store design and sizing relative to load profiles.
  • Checks on plant sizing relative to design loads.
  • Liaison with plant manufacturers where data is inadequate or options need to be explored.
  • Review of plant manufacturers data to determine performance characteristics, control interfaces/options, etc.
  • Review of proposed (or actual) control strategies
  • Review of schematic arrangements, particularly main plant hydraulic arrangements.
  • Sample checks on pipe/duct sizing.

Reports identify issues with plant selection, sizing, schematics/hydraulic design, controllability of plant to meet the range of load conditions, control strategies and any other observations. The report will include recommendations to overcome the problems identified and/or improve systems to provide stable efficient operation.

Costs for controllability reviews are normally relatively low and are dependent upon time taken. We have seen common issues many times and are familiar with a wide range of plant and systems, so can often identify issues quickly. It recently took five minutes to identify major flaws in a proposed chiller installation.


The better the standard of information provided, generally the less time taken to review the systems and establish any issues and their potential resolution. Normally a ‘desktop’ study and report is adequate, although on more complex schemes video conferences or meetings may be required to review options. Estimates can be provided on receipt of design documentation.


Following controllability reviews we can modify schematics, advise on plant reselection, specify control strategies, provide full controls/BMS specifications, attend site, etc.

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