One of the most important factors in the efficiency of any HVAC system is the selection and implementation of the proper control sequence of operations. Control sequences are the responsibility of the systems designer and are used to describe how systems shall operate. Key elements of any sequence include:

  • Equipment protection. Avoiding damage to ducts, coils, fans, pumps, chillers, etc., by verifying flows and positions prior to starting equipment.
  • Reliable operations. Detecting if a system element needs to be repaired or replaced and starting a backup unit if available. Examples of this include detecting fan and pump status, and looking at the pressure drop across a filter.
  • Comfort and safety. This is the main reason for any sequence! Examples of this include maintaining proper setpoints, managing outdoor air, tracking and controlling indoor environmental quality.
  • Efficiency. This may be the most difficult part of any sequence. Efficiency looks at many issues beginning with ventilation levels, use of economizers, preventing simultaneous heating and cooling, resetting temperature and static pressure settings, and coordination across systems such as chiller plants.

The process typically used to develop a control sequence starts with the systems design engineer, who needs to consider the owners’ requirements, codes and standards, and systems selected and then select and specify the sequence as part of an overall controls system design.

Sequences are typically described in a written format that becomes part of the project specifications. Controls contractors will typically review these sequences when bidding a project and again when developing the project submittals. A controls technician, working for the contractor, needs to interpret these written sequences and express them in the programming language used as part of the controls system. Ideally, sequences are then tested and reviewed during project implementation and commissioning.

Along the way, there are many things that can — and generally do — go wrong with control sequences. This can begin with challenges in developing the sequences and can continue with how the contractor interprets them and completes their implementation. While most systems generally are able to achieve the goals for equipment protection, reliability, and comfort, achieving optimal efficiency is a challenge due to difficulties that start in design and continue through the contracting process.


ASHRAE Guideline 36P

One potential solution for better design of control sequences is coming from the proposed ASHRAE Guideline 36 titled “High Performance Sequence of Operations for HVAC Systems.” This project has included committee work to define a series of optimized sequences as well as their associated points list and functional performance tests. The work being done on this project is very important for the industry and can provide the starting point for a greatly improved process for the development, testing, implementation, and validation of optimized control sequences. ES