If you own a swimming pool or maintain one for clients, you know that proper function is key to a clean, safe, and enjoyable experience. One of the more curious features found in modern pool systems is the presence of two small holes located under the skimmer basket. At first glance, they may seem insignificant or even like a design flaw. However, these holes—commonly referred to as the main drain line and the auxiliary drain or vacuum line—play critical roles in the overall operation of the pool’s circulation and filtration system.
In this article, we’ll explore in detail what those two holes are, how they function, why they’re important, and how to troubleshoot them if issues arise. Whether you’re a pool owner, a technician, or someone learning about residential plumbing, this guide aims to provide you with a comprehensive understanding of this key feature of in-ground pool design.
Introduction to Pool Skimmer Baskets and Plumbing
Before diving into the holes themselves, it’s essential to understand how skimmer baskets fit into the overall pool system.
The skimmer basket is typically located at the side of an in-ground pool and sits partially submerged in the water. It serves as the first line of defense in the pool’s filtration system by drawing in surface water and trapping large debris—such as leaves, bugs, and twigs—before it reaches the pump and filter.
Within the skimmer, you’ll find a floating weir that controls the flow of water and helps prevent debris from escaping back into the pool. Most importantly, within or directly beneath the basket, pool experts often notice two small openings. These are more than just extra holes—they are part of the complex plumbing that keeps a pool circulating evenly and efficiently.
The Two Holes: What They Are and Why They Exist
The two holes under the pool skimmer basket are:
- The main drain line inlet
- The auxiliary (or vacuum) line inlet
While their function can seem similar, each serves a unique purpose in the circulation system.
1. The Main Drain Line (Primary Inlet)
The main drain line is typically the larger or primary of the two holes and is usually connected to a drain located at the bottom of the pool. This drain helps draw water from the deep end, contributing to overall water circulation and helping prevent stagnant or colder pools of water from forming.
Functions of the Main Drain Line
- Facilitates water movement from the bottom of the pool to the pump
- Helps ensure even temperature and chemical distribution
- Provides a secondary intake point alongside the skimmer basket
Despite its location in the skimmer, this inlet technically draws water via connection to the main pool drain. It works in tandem with the skimmer to pull water through the system effectively. However, modern safety regulations increasingly promote the use of dual main drains, especially in public pools, to prevent suction entrapment hazards.
2. The Auxiliary Drain (Vacuum Line Inlet)
The auxiliary drain, sometimes labeled as the vacuum port, is a secondary inlet typically located just below or beside the main drain line hole. This port connects to a dedicated vacuum line or serves as an emergency drain in some pool models.
When using a manual pool vacuum, some systems allow the operator to either connect directly to the skimmer vacuum port or bypass the skimmer entirely by hooking into a dedicated vacuum line. However, in some cases, this auxiliary line simply allows water to flow in from the bottom of the skimmer wall to enhance circulation.
Purpose of the Auxiliary Drain
- Enables connection to a vacuum system without using the skimmer basket
- Assists in deeper water turnover in pool systems with minimal main drains
- May provide a backup intake if the skimmer basket or main drain becomes restricted
This secondary port becomes especially important when vacuuming large quantities of debris that might clog the skimmer basket. By using the auxiliary line for suction, the pool cleaner can operate more efficiently without constant basket cleaning.
How These Two Inlets Work Together
In a typical pool circulation system, water is drawn in through both the skimmer basket and the main drain at the bottom of the pool. The skimmer collects surface debris and pulls in the top layer of water, while the main drain supports deeper water movement.
The auxiliary line, depending on how the system is configured, may assist in either circulation or vacuuming. If the auxiliary line is plumbed as a direct vacuum inlet, it can serve as an independent suction line to a pool cleaner. If plumbed as an extra drainage point, it helps maintain consistent flow and prevent vacuum loss.
Together, these two under-skimmer inlets help ensure balanced circulation and more efficient cleaning.
The Role of Equal Water Flow and Suction Balance
Proper water balance is essential for:
- Effective filtration
- Clear and sparkling water
- Energy-efficient pump operation
If the skimmer is the primary suction point and the main drain has limited flow, the pump may pull air instead of water, causing the pump to lose prime or overheat—both of which lead to damage and costly repairs. Therefore, the presence of two inlets helps prevent such imbalances.
Understanding Skimmer Weir Interaction
The floating weir found within the skimmer basket regulates the flow rate of surface water entering the system. In the event of high suction from either the main or vacuum line, the weir adjusts to reduce backflow of debris. This dynamic system helps maintain the efficiency of the pump’s suction.
If only one inlet were present, it would be more likely for turbulence or suction issues to interfere with the weir’s operation and allow debris to flow back into the pool after being skimmed.
Proper Maintenance of the Two Skimmer Inlet Holes
Because these two inlets are integral to the pool’s circulation and vacuum operation, regular maintenance is crucial to preventing performance issues and costly repairs.
Inspection and Cleaning
These holes can become clogged through algae buildup, mineral scaling, or debris infiltration. Especially in regions with hard water or high mineral content, scaling inside the pipes can restrict flow or even block one or both lines completely.
Steps to Clean Skimmer Lines:
- Turn off the pump and filter system.
- Remove the skimmer basket and inspect the two openings.
- Use a plumbing brush or pool-safe pipe cleaner to clear moderate blockages.
- Apply a descaling solution if buildup is present.
- Use a garden hose or jet nozzle to flush remaining debris out of the lines.
For deeper blockages or suspected breakage, a professional may use a drain snake or a hydro-jet to clean the line thoroughly.
Preventing Algae and Biofilm Buildup
Algae can quickly form inside these lines, especially when the pool sits idle or during winter months. Because these inlets are located just below the skimmer basket, sunlight doesn’t always reach them, giving algae an opportunity to thrive.
Regular brushing and circulation can help. Additionally, using weekly enzyme cleaners or clarifiers can break down organic matter that might otherwise accumulate in these lesser-seen components.
Checking for Suction Leaks
If the pool pump runs with significant air intake, it could signal a leak in the skimmer area. Since both the main drain and vacuum lines are in close proximity, it’s wise to check for loose fittings, cracked gaskets, or cracked skimmer housing around these areas.
A common sign of leakage near the skimmer is bubbles in the return jets and the pump strainer basket being half-filled with air. If detected, close the valve and inspect or replace the skimmer lid, gasket, or O-rings accordingly.
Common Problems and Troubleshooting
Even with proper maintenance, issues can arise. Below are some common issues related to the two skimmer inlet holes and their possible causes or remedies:
Problem: Weak or No Suction from the Skimmer
Possible Causes:
- Blockage in either the main drain or auxiliary line
- Air leak near the skimmer lid or fittings
- Incorrect valve positioning
Troubleshooting:
Check for debris buildup, clean skimmer thoroughly, inspect and replace the skimmer lid gasket if needed, and check the multiport valve or diverter valve settings.
Problem: Excessive Debris Re-entering the Pool from the Skimmer
The weir inside the skimmer should prevent this from happening, but if the auxiliary or main inlets are pulling too much suction, the weir may not function properly.
Cause:
Too much suction pull from auxiliary lines or imbalance in the system’s water intake.
Solution:
Adjust the water flow by changing the valve settings to balance skimmer and main drain suction. In some systems, a smaller flow adjustment screw (usually embedded in the skimmer housing) can be turned to reduce vacuum strength and restore proper function.
Problem: Pool Cleaner Won’t Suction Properly from the Vacuum Line
If the cleaner is connected via the auxiliary line but doesn’t perform well, it may be due to an internal blockage or cross-flow from the skimmer line.
Fix:
Ensure the vacuum line is entirely dedicated, not shared with the main skimmer. If sharing lines is necessary (e.g., through a multiport diverter valve), confirm that the lines aren’t cross-draining or causing suction loss.
Professional Pool Plumbing and Skimmer Design
While DIY maintenance helps, understanding or designing a professional-grade skimmer system with dual inlets requires some technical insight.
Professional pool builders typically calculate the number and placement of skimmer lines based on:
- Pool volume
- Surface area
- Water turnover requirements
- Plumbing layout and pipe sizing
Most often:
-
Pools under 20,000 gallons will typically have:
- One skimmer with two suction holes
- One or two dedicated main drains
-
Larger pools may require separate vacuum ports or dedicated cleaning circuits.
When a professional installer integrates both the main drain line and vacuum line into the skimmer assembly, it’s usually to streamline access and reduce component clutter. It gives the owner or user flexibility in managing suction source options.
Are the Two Holes Always Present?
It’s important to note that not all skimmers are built with two inlets. Here’s a breakdown based on types:
| Skimmer Type | Has Two Inlets? | Use Case |
|---|---|---|
| Standard residential in-ground skimmer | Yes | Common in pools with separate main drains or vacuum lines |
| Single-port skimmer | No | Older, basic pool systems with minimal plumbing |
| Commercial or heavy-duty skimmers | Yes (and sometimes more) | High-efficiency facilities or public pools requiring extra suction points |
If you’re uncertain about your skimmer’s plumbing configuration, consult a pool technician or check the original pool schematics.
Summary and Benefits of Understanding the Dual Inlets
Knowledge of the two holes under your pool skimmer basket not only empowers pool owners to better maintain their systems but also helps technicians identify issues quicker and more accurately.
Key benefits include:
- Better water circulation and filtration
- Improved vacuuming efficiency
- Avoidance of costly pump maintenance or breakdowns
- Stronger overall pool hygiene and sanitation
Modern in-ground pools rely on strategic design to maximize circulation and cleanliness. By including a main drain line and an auxiliary drain or vacuum line within the skimmer area, engineers and builders help homeowners and professionals access a flexible and durable system that supports both daily operation and deep cleaning.
Conclusion: The Hidden Power of Dual Skimmer Inlets
What many may dismiss as quirky plumbing turns out to be a feature by design. The two holes under the skimmer basket, namely the main drain line inlet and the auxiliary vacuum line inlet, play crucial roles in making your pool system run as efficiently as possible. Whether you’re dealing with everyday operation, troubleshooting performance hiccups, or just want to understand your equipment better, knowing what these inlets are for—and how they work—is an investment in better pool management.
By paying attention to the small details, including those two holes under the skimmer basket, you ensure your pool operates smoothly, all season long.
Whether you’re a weekend pool owner or a full-time pool service provider, this understanding will give you a professional edge and peace of mind.
If you found this article informative, consider sharing it with others who may benefit from understanding this often-overlooked pool feature.
What are the two holes under the skimmer basket in a swimming pool?
The two holes located beneath the skimmer basket in a swimming pool are commonly referred to as the “main drain” and the “equalizer” or “backup drain.” The main drain is typically connected to the pool’s circulation system and plays a key role in drawing water from the bottom of the pool for filtration. The equalizer line, on the other hand, is positioned slightly below the skimmer and acts as a backup to prevent the pump from drawing air in case the water level drops below the skimmer intake.
These two lines work together to maintain proper water flow and prevent damage to the pump. In some installations, especially older pools, the equalizer line may connect to the same pipe as the skimmer, allowing water to be drawn from both the surface and lower points of the pool. Understanding the function of each line helps pool owners maintain balanced water circulation and avoid potential equipment issues.
Why does my pool skimmer have two openings underneath the basket?
The dual openings under a pool skimmer basket are designed to enhance the efficiency and safety of the pool’s circulation system. One opening, the main drain, pulls water from the bottom of the pool to ensure even water turnover and filtration. The second opening, known as the equalizer line, serves as a safeguard, maintaining water intake even if the pool’s water level drops slightly. This helps prevent the pump from running dry and protects the system from damage.
These two lines are essential for creating a balanced plumbing system. Without an equalizer line, the skimmer could draw in air if the water level is too low, leading to loss of prime or damage to the pump. The presence of two lines under the skimmer basket ensures consistent water flow, supports the pool filter’s performance, and helps maintain optimal chemical balance throughout the pool.
How do the two holes under the skimmer basket affect pool circulation?
The two holes under the skimmer basket—main drain and equalizer—play crucial roles in ensuring balanced and continuous circulation throughout the pool. The main drain pulls water from the deeper areas of the pool, which is typically cooler and may carry debris that settled at the bottom. This water is then filtered and returned to the pool through the return jets, promoting thorough mixing and better temperature distribution.
Meanwhile, the equalizer line helps maintain consistent suction even when the water level fluctuates, which is especially important in pools with variable usage or evaporation. Together with the skimmer, which captures surface debris, these two openings work synergistically to ensure that all parts of the pool are serviced by the circulation system. By combining surface and subsurface water intake, the entire body of water is filtered more efficiently, reducing the risk of stagnant zones or chemical imbalances.
What happens if one of the holes under the skimmer basket is blocked?
If one of the two holes under the skimmer basket becomes blocked, it can disrupt the proper flow of water through the pool’s circulation system. A blockage in the main drain line might reduce the pump’s efficiency, leading to poor turnover, uneven chemical distribution, or difficulty in vacuuming the pool. Similarly, if the equalizer line is clogged, the skimmer may lose prime when the water level is low or during heavy surface debris events, leading to air being drawn into the pump.
It’s important to regularly inspect and clean the lines under the skimmer basket to ensure optimal performance. Signs of a blockage include reduced water flow, loss of suction at the skimmer, or unusual pump noises. In some cases, a blockage can cause the pump to overheat if not drawing enough water. To resolve the issue, you may need to clear the blockage manually or hire a professional to inspect and clean the lines more thoroughly.
Can I close off one of the holes under the skimmer basket?
Yes, it is possible to close off one of the holes under the skimmer basket, but it is generally not recommended unless under specific circumstances. The two openings work together to ensure balanced water flow and prevent the pump from running dry. Closing off the main drain line removes one of the main pathways for water to circulate from the bottom of the pool, which can result in stagnant zones or longer filter cycles.
However, in some pool maintenance or repair scenarios, such as during vacuuming or winterization, a pool owner may temporarily block the equalizer line to increase suction from the skimmer. If you do choose to close off one of the lines, it’s crucial to monitor the system closely to ensure the pump continues to prime and function properly. Consult a professional if you’re unsure how closing a line may impact your specific pool system.
How do I know if the two lines under the skimmer basket are working correctly?
To determine if the two lines under the skimmer basket are functioning properly, begin by observing the skimmer’s suction power when the pump is running. If both the main drain and equalizer lines are open, you should feel consistent suction at both the skimmer opening and the main drain located at the bottom of the pool. If suction at the skimmer is weak or inconsistent, it may indicate a blockage or line issue.
Additionally, check for signs of reduced water flow through the pump and filter. Air bubbles in the pump basket, unusual pump noise, or a sudden increase in cleaning time could indicate a problem with one of the intake lines. If needed, perform a dye test near the main drain or equalizer line to see if water is being drawn in properly. For more accurate diagnostics, consider hiring a pool professional to inspect and test the system using specialized tools.
What is the purpose of having an equalizer line along with the main drain under the skimmer?
The equalizer line, found in many pool skimmers, serves as a backup water intake that prevents the skimmer from drawing in air if the water level drops below the skimmer’s normal operating level. Alongside the main drain, it ensures that water continues to flow into the pump even under less-than-ideal conditions. This is especially important in pools exposed to evaporation, frequent use, or heavy backwashing that can lower the water level.
By working in tandem with the main drain, the equalizer line contributes to a more stable and reliable circulation system. The main drain draws water from the bottom of the pool, balancing temperature and improving filtration, while the equalizer maintains operational continuity during fluctuating water levels. Together, they enhance the efficiency and safety of your pool’s plumbing system, ensuring that the pump always has water to process and reducing the risk of equipment damage.