The Architect Doesn't Design the Plumbing System But Must Understand Its Space and Performance Requirements
Plumbing system design is the plumbing engineer's domain - the architect does not size pipes, calculate water supply demand, or size domestic hot water heaters. But the architect must provide the plumbing engineer with the program information they need to design the system: fixture count by type and location, building height, distance from main to building, expected occupancy and use patterns. The architect also coordinates the plumbing engineer's work with the architectural plan - locating restroom cores, mechanical rooms, and plumbing chases; coordinating horizontal drain line routing with structural bays; and ensuring that plumbing fixture locations shown on the architectural plan match the plumbing plan. The ARE PPD tests plumbing system fundamentals as engineering knowledge that architects need to participate in the building systems design process and to check the plumbing engineer's space requirements against the architectural design.
Fixture Units
Plumbing fixture units (pfu) are a standardized measure of fixture water demand used to size water supply and drainage piping. Each fixture type has an assigned fixture unit value (water supply demand units, or drain fixture unit value for drainage). For water supply: a private lavatory = 1 wsfu; a flush valve water closet = 10 wsfu; a shower = 2 wsfu. The total wsfu count for a building determines the water service pipe size. Fixture units account for the intermittent and probabilistic nature of fixture use - not all fixtures operate simultaneously, so a building with 1000 total fixture units does not need a pipe sized for 1000 simultaneous uses. The probability of simultaneous use is embedded in the wsfu system.
Water Supply Pressure
Water supply pressure must be adequate at the highest and most remote fixture. Minimum pressure at flush valves is typically 35-45 psi; at lavatory faucets, 15-20 psi minimum. If street pressure is too high (above 80 psi), a Pressure Reducing Valve (PRV) is required - excess pressure damages fixtures and fittings and increases water consumption. If pressure is inadequate for upper floors (pressure drops approximately 0.43 psi per foot of elevation), a booster pump system is required. For tall buildings, pressure zones are established - the building is divided vertically into pressure zones each served by its own booster pump, preventing excessively high pressure at lower floors while maintaining adequate pressure at upper floors.
Drainage Fundamentals
Drainage systems operate by gravity - all horizontal drains slope to vertical stacks at a minimum slope of 1/4 inch per foot for pipes up to 3 inches in diameter. Vent stacks provide air to the drainage system (preventing siphonage of trap seals) and must be routed from drain branches to the exterior above the roof. The architect must provide adequate space for these systems: plumbing chases for vertical stacks (typically 4-8 inches), horizontal drain routes between structure (must not penetrate shear walls or be routed through areas without adequate slope), and vent penetrations through the roof.
Key Exam Points
- Fixture units: measure of water demand; wsfu for supply sizing; drain fixture units for drainage pipe sizing.
- Water pressure: 15-80 psi at fixtures; PRV if above 80 psi; booster pump if pressure inadequate for upper floors.
- Pressure zones: tall buildings require zones to manage pressure variation by floor height.
- Horizontal drain slope: 1/4 inch per foot minimum for pipes ≤ 3 inches diameter.
- Vent stacks: provide air to drainage system; prevent trap siphonage; routed to exterior above roof.
AREprep's PPD building systems content covers plumbing fundamentals, fixture unit calculations, water pressure requirements, drainage system principles, and the coordination between plumbing and architectural design - giving ARE PPD candidates the plumbing system knowledge the exam tests in building performance questions about water supply, pressure, and drainage system design.
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