Basis of Design and Or-Equal: What You Can Actually Substitute
- 1 day ago
- 5 min read
The equipment schedule says RTU-1 is a Trane Precedent. You sell Daikin. Can you quote it?
The schedule can't answer that. It names a selection; the specification governs whether you're allowed to deviate from it, and the two documents are frequently read as if they say the same thing. They don't, and the gap between them is where quotes get disqualified at submittal.
What “basis of design” actually means
When an engineer designs a mechanical system, they have to select real equipment to get real numbers. Airflow, capacity, dimensions, weight, electrical draw, sound data — all of it comes from a specific manufacturer's published performance data. That selection becomes the basis of design: the product the drawings were engineered around.
It is a design reference, not automatically a purchasing requirement.
Its practical function is to define the performance envelope. When the schedule lists a rooftop unit at 3,000 CFM and 7.5 tons, it's telling you the system was laid out assuming that unit's footprint, curb dimensions, weight, and electrical characteristics. Anything you propose instead has to work inside that envelope — not just match the headline capacity.
That distinction matters more than most people treat it. A competing unit that hits 3,000 CFM but is four inches taller, two hundred pounds heavier, and needs a different curb adapter isn't an equal in any meaningful sense, even though the performance columns match.
Where the actual rule lives
Not on the drawings. The specification decides.
Look at the front of the relevant section — Division 23 for HVAC, Division 22 for plumbing — and at Division 01 general requirements, which usually carries the substitution procedures for the whole project. You're looking for one of a few patterns:
Proprietary specification. Names one manufacturer with no substitution language. You quote that product or you don't quote. Rare on public work, common on projects with an owner standard — a hospital that runs one brand across its campus, a school board with an existing maintenance contract.
Basis of design with named acceptable manufacturers. Names the BOD product plus two or three others deemed acceptable. If you're on that list, you're in without a substitution request. If you're not, you're treated as a substitution regardless of how good your product is.
Basis of design with “or equal.” Names the BOD and permits equivalent products, subject to approval. The word doing the work is approval — it means the engineer decides, after the fact, whether your product qualifies.
Performance specification. Describes required performance without naming products. The most open, and the rarest on mechanical work.
Read this before you price, not after. Which pattern applies changes whether your quote is competitive or dead on arrival.
“Or equal” and “approved equal” are not the same
These get used interchangeably in conversation and they carry different weight.
Or equal generally means an equivalent product is permitted, with equivalence assessed at submittal — after award. You bid your product, you win the job, and then you find out whether the engineer accepts it.
Approved equal typically means the equivalence has to be established before bid, through a formal pre-approval process with a deadline. Miss the deadline and your product isn't eligible, no matter how well it performs.
The pre-approval deadline is the trap. It's usually stated in Division 01 or the instructions to bidders, it's commonly around ten days before bid date, and it passes quietly while everyone is focused on the drawings. Reps who assume they can sort it out at submittal discover the window closed a week and a half before the bid.
What “equal” is actually measured against
This is where substitutions get rejected, and it's rarely about capacity.
Engineers evaluate against the full envelope, which includes things no one checks when they're matching CFM:
Physical fit — dimensions, service clearances, whether it still fits the mechanical room or the roof opening
Weight and curb — structural loading, whether it lands on the existing curb or needs an adapter
Electrical — MCA and MOCP, whether the scheduled feeder and breaker still work
Sound — often specified as an NC level, and often the quiet reason a substitution fails
Efficiency — minimum EER, IEER, or SEER, sometimes driven by code or an energy model rather than the engineer's preference
Controls compatibility — whether it integrates with the specified BAS protocol
Warranty terms — the spec may require a longer term than your standard
Serviceability — local parts and service availability, sometimes explicit
A substitution that matches the performance columns and fails on sound or controls integration is still a rejected substitution. When you assemble a request, address the whole list rather than the two or three columns that look most relevant.
Who carries the cost when it's accepted
Worth understanding before you propose anything, because it's usually stated in Division 01 and rarely read.
Standard substitution language puts the burden of accommodation on the party proposing it. If your unit needs a different curb, a modified structural opening, a resized feeder, or a controls integration the design didn't allow for, those costs are typically yours — not the owner's and not a change order.
Some specs go further and make the substituting party responsible for the engineer's redesign fees and any resulting coordination changes across other trades.
That reframes the maths. A product that's four thousand dollars cheaper but triggers a curb adapter, an electrical revision, and a re-review isn't cheaper. Reps who price the equipment delta without pricing the accommodation win the quote and lose the margin.
Reading it in order
A workable sequence when a schedule names a product you don't sell:
Find the substitution clause in Division 01 before reading anything else. It governs the whole project.
Read the relevant section's manufacturer list. Are you named? If yes, most of this is moot.
Check for a pre-approval deadline and diarize it immediately. This is the item most often missed.
Pull the full performance envelope from the schedule — not just capacity, but dimensions, weight, electrical, sound, efficiency.
Compare your product against all of it, honestly, including the things likely to fail.
Price the accommodation, not just the equipment.
Assemble the request with side-by-side data against every specified parameter.
The unglamorous part
Every step above depends on having the scheduled data in front of you, accurately, for every tag on the job. On a set with forty scheduled units across six schedule sheets, that's a lot of transcription before you've made a single commercial decision.
That extraction is what HyperQuote does — it reads the schedules and the specs and hands you the full envelope per tag, so the time goes into evaluating substitutions rather than typing them into a spreadsheet.
If you want the schedule side in more depth, start with How to Read an HVAC Equipment Schedule, or Equipment Schedules: All You Need to Know for the fundamentals.
This describes how substitution language generally works in North American commercial construction. Specs vary, and the governing document on your project is always the project's own. Nothing here is legal advice on a specific dispute.
