In the planning and design phases, their duties typically include creating drawings and diagrams of building HVAC (heating, ventilation, and air conditioning), and refrigeration, and plumbing systems. The HVAC and refrigeration systems include chillers, boilers, air-handling, ventilation, pumping, exhaust systems, cooling towers, and more. The plumbing systems typically include plumbing fixture selections and water, gas, and sanitary piping design.

In the construction and verification phases, commissioning agents confirm the architects’ and engineers’ designs meet the owner’s project requirements, and that the contractors have installed and programmed systems correctly according to the design. These tasks are performed through rigorous testing of the building equipment and systems before occupancy to ensure that everything performs as expected.  In the post-construction occupancy phase, mechanical engineers provide services for buildings that are in-use through operational maintenance as well as retrofit services to increase performance and reduce energy consumption. The construction and post-construction roles have more on-site responsibilities while the pre-construction roles have responsibilities that are typically performed in an office.

There are buildings and systems which may require specialized plumbing system design professionals, for example, manufacturing piping systems that include hazardous chemicals.

Additionally, mechanical engineers can perform Commercial Building Energy Modeling, which constructs engineering and economic models to represent the performance of buildings in order to evaluate and quantify the impact of policy, design, retrofit, and operational decisions. This role is often referred to as energy engineer. All of these positions have an impact on the process of producing efficient and sustainable buildings.