What is the correct order to design a building plumbing system?

Start with fixture unit counting and code selection (IPC or UPC), then size the water supply system and analyze pressure, then lay out the drainage/waste/vent (DWV) system, then size the water heating system using the hot-water demand established during supply sizing, then design any special systems the building type requires (backflow prevention, grease interceptors, medical gas, storm drainage), and finish with testing, disinfection, and commissioning before occupancy.

Why can't I size the water heater before finishing water supply sizing?

Water heater sizing depends on the hot-water fixture count and the hot-water Water Supply Fixture Unit (WSFU) demand, both of which are established during the water supply sizing step. Sizing a water heater before that step is complete means sizing it against a guess rather than a calculated peak demand, which typically results in an oversized or undersized system.

Do drainage/vent design and water supply design happen at the same time?

They can proceed in parallel once fixture counts and locations are reasonably fixed, since both draw on the same fixture schedule. In practice many designers complete them together rather than strictly sequentially, but both still depend on Step 1 (fixture counting and code selection) being finished first.

Which special systems does every plumbing project need?

Backflow prevention is nearly universal — almost every building has at least one hazardous cross-connection requiring an assembly. Grease interceptors, medical gas, and coordinated storm drainage design are building-type-specific: they apply to food service, healthcare, and site-coordination contexts respectively, not to every project.

Can testing and commissioning be skipped or shortened to save schedule time?

No. Testing, disinfection, and commissioning verify that the installed system actually performs as designed — pressure integrity, absence of cross-connections, and potable water safety. Problems found after occupancy (leaks, contamination, failed backflow assemblies) are far more expensive and disruptive to fix than the time spent testing before occupancy.

What is the single most common sequencing mistake in plumbing design?

Sizing the water heater before fixture counting and water supply sizing are complete. This skips the step that establishes actual hot-water demand, producing a heater sized to an assumption rather than a calculated peak load.

Engineering·9 min read·July 22, 2026

🗺️ Plumbing System Design: A Complete Workflow Guide

The roadmap connecting fixture counting, water supply sizing, DWV design, water heating, special systems, and commissioning into one coherent plumbing design process — with the correct order and why it matters.

📖Plumbing Systems — Illustrated Guide (Full Access)

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Why a Roadmap, Not Another Technical Guide

This studio already covers the individual technical pieces of plumbing design in depth: code selection, water supply sizing, drainage and venting, water heating, backflow prevention, grease interceptors, medical gas, storm drainage, and testing/commissioning. Each of those articles answers "how do I design this subsystem correctly?" This guide answers a different question: what order do I tackle these in, and how does the output of one step become a required input to the next?

Topics covered

plumbing system designplumbing design processplumbing workflowfixture unit countingplumbing design sequenceplumbing engineering roadmap