Commercial Plumbing Design Guide: Key Considerations for Engineers and Architects

Gravity Drainage vs. Vacuum Drainage

Gravity Drainage Systems – The traditional gravity drainage system uses gravity to move wastewater and waste down pipes to a municipal main or a lower sewage site.

Vacuum Drainage Systems – A vacuum drainage system utilizes negative pressure to move waste and wastewater through a closed network of pipes. Unlike a gravity-based system, it can move waste against gravity, around obstacles, including up slopes or even vertically.

Vacuum drainage should not be confused with two related systems. Vacuum sewer systems collect wastewater at a community or municipal scale, outside individual buildings, while medical vacuum systems provide clinical suction in healthcare settings. This guide covers vacuum drainage within buildings.

Is the project a new-build or a conversion/adaptive use?

For new builds with straightforward layouts, gravity drainage is usually the simplest and lowest-cost option. In conversions and adaptive reuse, existing slabs, structural elements, and pipe elevations often make it difficult to achieve the continuous slope gravity drainage needs. This is where vacuum drainage is most often considered, because its smaller pipes can run horizontally or overhead. For more information, read our article on what a vacuum drainage system is.

Are there terrain or geographic complexities?

Gravity drainage depends on pipes maintaining a continuous downward slope at a specific grade, so site conditions below grade can rule it out or make it costly. Bedrock, too little or too much slope, shifting ground and high water tables are the most common constraints. Because vacuum drainage does not rely on gravity, it can reduce or avoid many of them.

Each of these site conditions is covered in detail, with project examples, in Vacuum Drainage vs. Gravity Drainage for Underground and Below-Grade Construction.

Does the remodel involve structural or heritage constraints?

For remodelling projects, vacuum plumbing systems may solve problems related to:

• Incorrect invert elevations – Pipe inverts at the wrong elevation disrupt gravity flow. Correcting them requires a major redesign, significant demolition and construction, or a workaround such as a vacuum drainage system.
• Difficult structural elements – Examples include post-tensioned slab foundations (slab-on-grade foundations reinforced with high-strength steel cables encased in plastic sheathing), central steel columns, primary concrete piers, and lateral-resistance shear walls, all of which require high cost, effort, and time to modify.
• Historic building restrictions – Historic structures often have restrictions on what can be changed. Vacuum plumbing system benefits include:
  • No gravity slope required
  • No floor trenching needed
  • Protection of historic flooring, tiles, and structural foundations
  • Ability to bypass hazardous materials, such as asbestos

What are the options when existing pipe inverts don’t work for gravity drainage?

When existing pipe elevations won’t support gravity flow, design teams typically consider multiple approaches and often in combination: re-routing or re-grading gravity pipework, raising floors or fixtures locally to gain fall, pumped systems such as sewage ejectors, or vacuum drainage.

When is gravity-based plumbing the better choice?

• New-build projects (standard low-rise) – These projects typically have short pipe runs, straightforward layouts, and reliable water and sewer access. Gravity-based drainage offers relatively low upfront costs and conventional installation and maintenance.
• Residential projects with traditional layouts (suburban) – These projects typically have ample land, conventional sewer access, and standardized designs. Gravity-based plumbing and wastewater systems integrate well with existing infrastructure and provide predictable, straightforward operation.

When is vacuum-based plumbing the better choice?

• Adaptive reuse and conversions – Existing buildings may have structural constraints, historical-preservation requirements, or limited opportunities for adding plumbing stacks and other infrastructure. Vacuum systems can provide a flexible, lower-impact retrofit option.
• High-rise and dense urban settings – Projects with long vertical runs, limited service space, or challenging routing can benefit from vacuum technology, which can transport wastewater efficiently while reducing the amount of conventional infrastructure required.
• Healthcare and institutional facilities – These facilities often require high standards of hygiene, controlled flushing, and reliable drainage performance. Vacuum systems can support water-efficient operation while helping reduce blockages and the potential for improper use.
• Stadiums, arenas, and event venues – High-occupancy facilities experience significant fluctuations and peaks in demand. Vacuum systems can help manage these demands while reducing water consumption and supporting rapid, reliable drainage.
• Remote and off-grid sites – Where municipal water and sewer networks are unavailable or difficult to access, vacuum systems can provide greater flexibility in system design, improved water efficiency, and compatibility with decentralized or self-contained treatment. They can also be advantageous on hilly, rocky, or otherwise challenging terrain.
• Sustainability-focused projects – Projects pursuing aggressive water- and energy-efficiency goals or certifications such as LEED, BREEAM, or WELL* may benefit from vacuum plumbing. By using substantially less water for flushing and enabling more flexible system layouts, vacuum technology can contribute to overall resource efficiency and potentially reduce a building’s environmental footprint.

Additional Considerations for Engineers and Architects

Broadly, when planning projects, considerations about vacuum-based plumbing systems versus gravity-based plumbing systems include:

• Early multi-disciplinary coordination. It is important to coordinate and align mechanical, electrical, and plumbing design early in a project to avoid spatial complications and expensive retrofits.
• Occupancy-based sizing. Standards such as REHVA, ASHRAE** or local building codes should be considered to size fixtures accurately based on maximum occupant loads instead of over-designing infrastructure.
• Specialized facility demands. Consider unique space and equipment needs for projects, such as specialized medical gas lines for healthcare spaces and grease interceptors for commercial kitchens.
• Sustainability and efficiency. To meet LEED certification credits or other sustainability certifications, consider integrating low-flow fixtures, wastewater recycling loops, or rainwater harvesting implementation.
• Accessibility and maintenance. Locate shut-off valves, cleanouts, and mechanical chases with accessibility in mind to simplify maintenance and repairs, and future tenant buildouts.

There is a lot for engineers and architects to consider when planning a new project. Vacuum drainage systems aren’t the right solution for every construction or remodeling project. But for many projects, they can be the perfect solution for common challenges.

To learn more about Evac’s line of total vacuum solutions, contact us today!