Why mechanical design should start at concept stage

There’s a pattern that repeats on building projects. A set of architectural drawings is nearly complete, design development is well advanced, and somewhere near the bottom of the consultant list is a note: engage mechanical consultant.

By that point the structural grid is locked, the ceiling zones are committed, and any plant room has been sized around an estimate rather than a design. Mechanical design often begins here — and it’s usually later than it should be.

The cost of starting late

When mechanical design starts after the architecture is substantially resolved, the job is no longer to design the best system for the building. It’s to design the best system that will fit the building as it stands. Those are very different briefs, and the consequences tend to show up in predictable ways.

Energy and compliance assumptions that don’t hold

Energy assessments prepared early in a project rely on assumptions about the building and its services. If the mechanical system that is eventually designed doesn’t match those assumptions, the assessment may need to be revisited — at the point where changes are slowest and most expensive.

Duct routes that fight the architecture

When ceiling zones, beams and service corridors haven’t been planned with ventilation in mind, ducts have to find their way around everything else. That usually means longer runs, more pressure drop, more fan energy, more noise — and sometimes bulkheads that were never on the architect’s drawings.

Plant space that is too small

Equipment is sized at detailed design, but plant space is drawn at concept. When the two finally meet and the equipment doesn’t fit, something gives: either the specification is compromised or the layout changes.

Allowances instead of a scope

If the mechanical scope hasn’t been defined during design development, tenderers fill the gap with allowances. Allowances are rarely accurate, and the difference tends to surface as variations once the job is underway.

What early engagement changes

• System type is agreed before anything is drawn around it. Whether a project suits ducted split systems, VRF, heat recovery ventilation with separate heating and cooling, or central plant has real consequences for ceiling heights, plant space, penetrations and energy use.
• Energy decisions are made together. Glazing, shading, insulation and the mechanical system interact. Considering them together gives the project team better information.
• Coordination happens on paper, not on site. Resolving where ducts, pipes, cable trays and structure fit is cheap in a model and expensive on site.
• The budget is based on a system. Even a concept-level design gives the team a considered estimate rather than an allowance.