Swimming pools are intricate systems designed to maintain clean, balanced, and safe water for both recreational and therapeutic use. A key component of this system is the pool’s main drain, which plays a crucial role in water circulation and filtration. However, many pool owners—and even experienced swimmers—are curious about where water goes after it travels from the main drain.
This article will take a comprehensive and illuminating journey through the complete lifecycle of water from the main drain of both residential and commercial pools. We’ll explore the mechanics of water flow, the components involved in the filtration system, and the ultimate destinations of pool water after it passes through the main drain. By the end, you’ll have a thorough understanding of how your pool system works and its impact on maintaining optimal pool conditions.
Understanding the Role of the Main Drain in Pool Systems
Before exploring where the water goes, it’s essential to understand the function of the main drain within the pool’s plumbing configuration.
What is a Main Drain? The main drain is a crucial component typically located at the lowest point in the pool. It acts as a primary intake point for water, drawing it downward toward the pool’s circulation system. Unlike the skimmer, which collects floating debris, the main drain is responsible for drawing in water from the lower depths of the pool.
Why It’s a Safety Feature: Beyond circulation, the main drain serves an important safety function. It must be installed with a vacuum breaker or anti-entrapment device—as required by the Virginia Graeme Baker Pool and Spa Safety Act (VGBA)—to prevent children or swimmers from being sucked in due to improper pressure balance.
How the Main Drain Facilitates Circulation
When the pool pump system is activated, it creates suction that pulls water from several paths:
- From the pool skimmers (which collect surface water)
- From the main drain (drawing bottom water)
- And in some systems, from additional sources like water features or spa return jets
In most systems, the main drain is set up to circulate up to 80% of the water from the floor level. This helps in preventing stagnation in the deeper areas of the pool, ensuring even chemical distribution, and avoiding cold or warm layers beneath the surface.
Where Does the Water Go After Leaving the Main Drain?
Once water leaves the main drain, it follows a structured path through the pool’s plumbing and filtration system. Let’s break down the typical journey step by step.
1. Water Flows Through Pipes to the Pump
From the main drain, water enters the underground plumbing system. This system is designed with slope gradients and PVC pipes that direct water toward the filtration equipment—most importantly the pool pump.
The pump is the heart of the filtration system: It generates the necessary pressure and flow to move water through the entire system.
The water from the main drain is combined with water coming from the skimmer in what is often a junction box before entering the pump.
2. Pump Forces Water into the Filter
After being pulled into the pump, the water undergoes pressurization and is pushed into the pool’s filtering system, which comes in three primary types:
- Sand filters: These use layers of sand and gravel to trap debris and contaminants as water passes through.
- Cartridge filters: These use a disposable cartridge that filters out dirt, algae, and fine particles from circulating water.
- DE (Diatomaceous Earth) filters: These are made with a fine powder-like substance that coats grids, effectively catching the finest particles down to 2-5 microns.
The choice of filter can impact the water quality and maintenance frequency but in all cases, their main role is consistent: remove debris before returning clean water to the pool.
3. Water May Go Through the Heater or Chlorinator
Depending on the system setup, pool water may pass through additional components after filtration:
Pool Heater
If the pool is equipped with a heater, the water flows through it next. Whether gas-powered, heat pump-based, or solar-heated, these devices elevate the water temperature to a comfortable level.
Chemical Feeder or Chlorinator
After heating, many systems pass water through a chlorinator or automatic chemical feeder. In this stage:
- The system introduces chlorine, salt, or other sanitizers to kill bacteria and algae
- Some systems include automated sensors that determine chemical balance in real-time and dispense treatment chemicals accordingly
This ensures that pool water is clean, balanced, and safe for swimmer contact immediately before it returns to the pool.
4. Water Returns to the Pool Through Return Jets
After being filtered, heated, and chemically treated, the now-clean water returns to the pool through strategically placed return jets located on the pool walls. These jets not only reintroduce the water but also agitate the pool to keep things well-circulated and oxygenated.
The Full Cycle: Visualizing the Flow
To better understand how water flows from the main drain to completion, here’s a simplified table showing the entire sequence:
| Step | Location in System | Action Taken |
|---|---|---|
| 1. | Main Drain | Draws in bottom water |
| 2. | Plumbing Pipes | Transfers water to pump and skimmer junction |
| 3. | Pool Pump | Filters and pressurizes water |
| 4. | Filtration System | Cleans water of debris |
| 5. | Heater / Chlorinator | Adds heat and/or sanitizers |
| 6. | Return Jets | Reintroduces clean and warm water into pool |
This cycle repeats every 8 to 12 hours to maintain proper turnover rates and effective sanitation.
Factors That Affect the Water Flow From Main Drain
Several factors can affect how efficiently water flows from the main drain and through the filtration system. These influence both water quality and equipment performance:
1. Water Level and Skimmer Interaction
The pool water level must be maintained at a level that allows the skimmer to function. If the water level is too low, the main drain might suck in air, which can damage the pump or cause loss of prime. Conversely, if the water level is too high, the main drain pulls minimal water.
2. Valves and Water Distribution
Most multiport valve systems allow directing water or controlling the proportion of water from the skimmer vs. the main drain. Adjusting these can determine whether main drain flow is full capacity or partially restricted, depending on whether the pool is being backwashed or under standard filtration.
3. Pump and Filter Efficiency
A clogged filter or malfunctioning pump can restrict the main drain’s water flow. Over time, wear and tear on pool equipment—including the impeller, seals, or electric motor—can significantly alter performance and flow consistency.
4. Valve Type and Use
Especially in larger pools or commercial systems, automatic valve controllers can manage fluid dynamics based on the presence of swimmers, time of day, or seasonal demand. These ensure optimal main drain and skimmer utilization without overloading the pump with unnecessary suction.
What Happens During Pool Draining, Repairs, or Winterization?
Under certain conditions, the water from the main drain is not just part of normal circulation. Instead, it is used for draining or preparing the pool for long-term inactivity.
1. Pool Drainage
Some scenarios where pool water exits through the main drain permanently or temporarily:
Regular maintenance: Draining the pool to clean or repair its structure, especially in concrete and vinyl pools, requires connecting a submersible pump directly to the drain to siphon water out.
Winterizing your pool: In cold climates, main drains are often used in conjunction with other drainage fittings to lower the water level before adding winter covers and plugs.
2. Backwashing and Filtration Cleanouts
Backwashing—a process where pool water is forced through the filter in reverse—is an essential process for maintaining sand or DE filters. During this cycle, the main drain may:
- Temporarily lose suction
- Reverse some flow to expel dirt and debris into the waste line
This helps ensure consistent system performance and filtration capacity.
3. Emergency or Overflow Conditions
If the pool overflows due to rain or mechanical issues, the main drain can assist in alleviating pressure by allowing rapid passage of water out of the pool.
However, this is generally secondary to dedicated overflow drains or troughs designed specifically for such purposes.
Pool Water Disposal and Environment Considerations
If you’re draining your pool or performing a major maintenance event, understanding where this water goes on a larger scale is key.
Environmental Impact: Pool water (especially chlorinated water) should not be dumped directly into storm drains or local waterways without proper treatment. The chlorine, algaecides, and other chemicals can endanger aquatic life and contaminate local water sources.
Most areas require homeowners to follow local municipal guidelines for water disposal, which can include:
- Letting water sit to dechlorinate for several days
- Using chemical neutralizers such as sodium thiosulfate
- Redirecting water flow into a sewer system (if permitted)
Safe Disposal Methods:
- Backyard soakage: Treated water can be directed into planted areas or grass safely for small amounts, as soil and plants act as natural filters.
- Municipal sewer line: Some jurisdictions allow discharge into a sanitary sewer (but not storm drains), assuming the water is non-toxic and free of debris.
- Hiring a professional disposal company: Larger commercial pools may require licensed hazardous waste disposal.
Modern Innovations in Pool Drain Usage
Advancements in pool technology have affected how main drains are used and managed in newer pool systems.
1. Main Drain Covers and Safety Plates
Since the VGB Act of 2007, main drain covers have become a regulatory requirement in public and many private installations. These covers prevent entrapment and must meet specific standards for surface area, non-perforation risk, and secure installation.
2. Secondary or Auxiliary Drains
In commercial pools, multiple main drains are often installed to prevent vacuum entrapment. These drains are connected through separate plumbing systems and valves, so if a drain is blocked, the water can redirect safely.
3. Automation and Smart Control Systems
Many pools now use smart pool systems that can:
- Detect low flow or abnormal suction levels
- Alert owners when main drain efficiency decreases
- Automatically redistribute suction force between skimmer and drain
These systems are particularly useful in preventing equipment damage and providing data on pool performance over time.
4. Enhanced Pool Bots and Robotic Cleaning
While not replacing the function of the main drain, advanced pool cleaning bots work synergistically by reaching areas (like steps and corners) that suction from the drain might not serve well. This comprehensive cleaning ensures that all parts of the pool benefit from the circulation and treatment cycle.
Conclusion: Maximizing the Function of Your Pool’s Main Drain
The main drain is not just a simple hole at the bottom of your pool—it is a crucial part of a dynamic and scientifically designed system that ensures continuous water movement, filtration, heating, and sanitization. From the depths of the pool to the return jets, the journey water takes is a carefully calculated process that helps maintain a clear, balanced, and enjoyable environment for every swimmer.
By understanding where the water goes from the main drain and how that fits into the broader pool system, homeowners, commercial operators, and even safety advocates are better equipped to maintain efficient and hazard-free pool environments. Whether it’s regular operation, seasonal preparation, or environmental responsibility, every drop of pool water has a purpose and a path—and it all starts at the main drain.
For optimal performance and safety, routine inspections, maintenance, and adherence to local regulations are key. In modern pool care, knowledge of your system is more than just technical—it’s a way to keep the water clean, safe, and continuously flowing right back to you.
Where does water go from the main drain in a pool?
The water that flows from the main drain in a swimming pool is typically directed toward the pool’s filtration system. As part of the pool’s circulation system, the main drain channels water into the pump, which then pushes it through the filter to remove debris and contaminants. This process ensures that the water is cleaned before being circulated back into the pool through the return jets.
In some cases, the water from the main drain may also be connected to a separate sump pit or vacuum release system. This setup helps prevent entrapment hazards by allowing water to be drawn from multiple locations and managing flow rates. By balancing the suction and improving safety, these systems help provide a more efficient and safer circulation of pool water.
Why is the main drain important to a pool’s circulation system?
The main drain plays a crucial role in a pool’s overall water circulation. Because it is located at the lowest point of the pool basin, the main drain efficiently captures debris and cooler water that settles there. This contributes to an even turnover of water, which helps maintain uniform temperature and chemical distribution throughout the pool.
Additionally, by working in tandem with the skimmer, the main drain ensures comprehensive water movement. While the skimmer collects floating debris and surface water, the main drain focuses on deeper water layers. Together, they support continuous filtration and effective water quality management, making the main drain an essential component in a balanced and functional pool system.
Can water from the main drain be redirected or disconnected?
Water from the main drain should not be arbitrarily redirected or disconnected, as doing so could disrupt the pool’s balance and create safety hazards. Many modern pools have interconnected drainage systems that include safety features like a secondary drain or vacuum release systems. Disconnecting the main drain can interfere with those mechanisms, increasing the risk of entrapment or damaging the pump.
However, in certain situations, such as repairs or maintenance work, an experienced technician might temporarily redirect or isolate the main drain for safety purposes. It’s vital that any such action is done according to industry standards and local regulations. Always consult a certified pool professional before making modifications to ensure compliance and continued safe operation.
What happens if the main drain stops functioning properly?
If the main drain becomes clogged or stops functioning correctly, it can significantly impact the pool’s circulation. Poor water flow leads to uneven filtration, which may result in cloudy water, algae growth, and an imbalance in chemical levels. Additionally, water might not be properly pulled from the lower end of the pool, disrupting overall turnover and hygiene.
There may also be safety concerns if the main drain’s suction is not working as intended. Entrapment risks can increase if a single drain system is compromised, making it crucial to address malfunctions promptly. Checking for obstructions, ensuring proper connections, and maintaining equipment can help prevent such issues. In case of persistent problems, a certified pool technician should be consulted.
How does the main drain contribute to pool safety?
The main drain contributes to pool safety in multiple ways, primarily by reducing the risk of suction entrapment. Modern pool systems often include a dual-drain design where two separate main drains share the suction load. This setup significantly reduces the chance of a body part or object becoming trapped against the drain, preventing potential injury or drowning incidents.
Additionally, many systems feature vacuum release mechanisms or anti-entrapment covers that comply with federal safety standards. These design improvements ensure that if one drain becomes blocked or obstructed, the system automatically equalizes the pressure, making suction safer. Properly maintained main drains, therefore, serve both operational and life-saving functions in residential and public pools.
Is the main drain used during regular pool cleaning?
Yes, the main drain is actively involved in regular pool cleaning as part of the circulation and filtration system. When the pool pump is running, it pulls water—and suspended particles—from the main drain, allowing debris and contaminants to be filtered out. This continuous process helps keep the pool water clear and clean over time.
In addition, when using pool cleaning devices such as automatic cleaners, the main drain works with skimmers to maintain balanced water flow. However, manual vacuuming often bypasses the filter and directs debris through the pool cleaner hose to the filtration system or directly into the waste line, depending on the system. The main drain supports this activity by ensuring efficient water movement during the cleaning process.
Do all pools have a main drain?
Not all swimming pools are equipped with a main drain. While many in-ground pools include at least one main drain for effective circulation and safety, above-ground pools typically do not have them. Additionally, some older or smaller in-ground pools may lack a main drain or have outdated systems that do not meet current safety standards.
Installation of a main drain depends on various factors including local pool regulations, construction standards, and design preferences. Many newer residential and public pools are built with at least two drains or anti-entrapment systems to meet modern safety guidelines. If a pool does not have a main drain, alternative measures must be taken to ensure proper water circulation and maintenance of water quality.