What Is a Basis of Design?

A mechanical basis of design states outdoor conditions, load assumptions, ventilation rates, system type, and redundancy. An electrical one states service size, demand assumptions, and spare capacity. Architectural BODs are less standardized and often show up as narratives of assemblies. The point of writing it down is that a later reader can see why the schedule says 40 tons, not only that it does.

When design changes, the BOD has to change. An equipment schedule that was updated in CDs and a BOD still describing the DD system will fail a commissioning review for a reason that looks like a paperwork nit and is actually a design-record split. One of them is wrong. Find out which before the submittal is approved.

A section that says "basis of design: manufacturer A, model B, or equal" was edited around that product’s dimensions, connections, and performance. An "or equal" product has to fit the same space and meet the same spec paragraphs, not merely the same marketing category. The drawings may have been coordinated to the basis-of-design dimensions. A smaller nameplate and a larger casing is how substitutions blow up a mechanical room.

Some owners prohibit naming products. The section is then purely performance, and there is no basis-of-design product to be equal to. Reviewers should not invent one. Other owners require three named manufacturers. That is a proprietary-spec choice with a political and procurement history. The substitution procedure in Division 01 still applies.

The owner’s project requirements say what success is. The basis of design says how the team intends to get there. Commissioning reviews the design for gaps between them. If the OPR requires N+1 cooling and the BOD shows a single chiller, that is a design comment, not a construction punch item. Waiting to discover it at functional testing is how the gap becomes a change order.

Owners who never write an OPR still have requirements, scattered through the program and meeting notes. The BOD then becomes the only consolidated statement. That is fragile. A short OPR and a BOD that cites it are easier to commission than a heroic narrative nobody approved.

Product data should be reviewed against the spec section and, where the dimensions matter, against the drawings that used the basis-of-design size. A cover letter that says "meets the basis of design" is not the comparison. The comparison is capacity, electrical characteristics, weight, and clearance.

If the basis-of-design product is discontinued, the contractor’s substitution request should say so and propose a product against the criteria, not against the discontinued model number alone. The engineer updates the BOD or the schedule when the substitute is accepted, so the record matches what will be installed.