Why is One Skimmer Hole Plugged? Common Pool Problems Explained

Owning a swimming pool is a true pleasure, but it also comes with regular maintenance and a few mysteries. One question that often confuses pool owners is: Why is one skimmer hole plugged? You might notice a plug or cap blocking one of the holes in your pool’s skimmer, and wonder if this is a mistake, a sign of a problem, or simply normal practice. The answer is not always simple. In fact, understanding skimmer holes and their purpose is essential for anyone who wants a clean, healthy, and efficient swimming pool.

In this article, you’ll get a complete explanation of skimmer holes, why one might be plugged, and how this affects your pool’s performance. Whether you’re a new pool owner or someone who’s managed a pool for years, these insights will help you avoid common mistakes, make smart choices, and keep your pool in top shape.

What Is A Skimmer And What Does It Do?

A pool skimmer is a built-in part of your pool’s filtration system. It’s usually located on the side of the pool, just below the water’s surface. The skimmer’s job is to pull in water and catch debris like leaves, bugs, or dirt before these things reach your pool pump.

Inside the skimmer, there’s a basket that traps larger debris. Water flows from the pool, through the skimmer, and onward to the pump, filter, and back into the pool. By catching debris early, the skimmer helps keep your pool clean, protects the pump from clogs, and reduces the load on the main filter.

Most skimmers have two holes at the bottom. These are sometimes called “ports” or “suction lines. ” Understanding what each hole does is the first step to knowing why one might be plugged.

The Two Holes In A Pool Skimmer: An Overview

If you look inside your pool skimmer (with the basket removed), you’ll see two holes at the bottom. These are called the skimmer hole and the main drain hole (or equalizer line). Here’s what they do:

• Skimmer Line (Suction Line): This is the main hole that pulls water from the pool’s surface into your pump and filter. It’s always open during normal operation.
• Main Drain or Equalizer Line: This second hole connects either to the main drain on the pool floor, or to an equalizer line. Its purpose depends on your pool’s design.

You might see both holes open, or one blocked with a plug or plate. The reason for this depends on how your pool’s plumbing is set up.

Why Is One Skimmer Hole Plugged?

The main reason one skimmer hole is plugged is that it’s not always needed for your pool’s circulation system. Pool builders often block the second hole intentionally, depending on your pool’s specific plumbing and safety needs.

Let’s break down the main reasons why one skimmer hole might be plugged:

1. Not All Pools Use Both Lines

Many pools only use one skimmer line to pull water into the pump. The second hole, which might connect to a main drain or equalizer, is not always needed. If your pool doesn’t use the main drain through the skimmer, the builder will plug the hole to prevent air from entering the system.

2. Preventing Air Leaks

If both holes are open but only one is connected to suction, the other can suck in air. Air in the pool pump system is a problem—it can cause the pump to lose prime, make noise, or even stop working. Plugging the unused hole prevents these air leaks.

3. Safety And Building Codes

Some local building codes require that unused ports are plugged for safety. For example, the Virginia Graeme Baker Pool & Spa Safety Act in the U.S. requires specific suction and anti-entrapment measures. Plugging the extra hole can help meet these standards.

4. Equalizer Line Not In Use

The second hole is sometimes plumbed as an equalizer line, which is a safety feature. If your pool’s system doesn’t use this line, it should be plugged to avoid leaks and air entry.

5. Simplifying Maintenance

Having only one open line makes it easier to troubleshoot and maintain your pool. If there’s a problem with suction, you have fewer lines to check.

Skimmer Line Vs. Main Drain/equalizer Line

It’s important to know the difference between the two lines at the bottom of your skimmer. Here’s a quick comparison to make it clear:

In most modern pool builds, the main drain is plumbed separately from the skimmer. But in older pools, the main drain line often connects through the skimmer, using the second hole.

The Equalizer Line: Purpose And Use

The equalizer line is a second pipe that runs from the pool wall (just below the skimmer) to the skimmer’s second hole. Its main purpose is safety. If the water drops below the skimmer’s opening—say, due to a leak or heavy splash—the equalizer line allows water to keep flowing to the pump. This prevents the pump from sucking air and running dry, which can cause expensive damage.

However, not all pools have or need an equalizer line. If your pool doesn’t have one, or if it’s not connected, the second hole should be plugged.

When Is The Equalizer Line Used?

• In commercial pools, equalizer lines are required by code.
• In residential pools, they’re less common, especially in newer builds.
• If your pool’s skimmer basket is always under water and you don’t see a small grate or pipe below the skimmer, you probably don’t have an equalizer line.

Common Skimmer Hole Configurations

Every pool is a bit different, but here are the most common setups and why one hole might be plugged:

• Single Skimmer Line Only: The second hole is plugged because it’s not used.
• Skimmer and Main Drain Combined: Both holes are open, a diverter plate (like a float valve) is used to balance flow.
• Skimmer with Equalizer Line: Both holes are open, equalizer line is connected for safety.
• Skimmer with Plugged Equalizer: Only the skimmer line is in use, equalizer line is plugged.

Below is a summary in a clear format:

How Does A Plugged Skimmer Hole Affect Pool Operation?

Plugging one skimmer hole is usually the correct and safest option, but it does affect how your pool runs.

Positive Effects

• Prevents Air Entry: Keeps the pump primed and running smoothly.
• Reduces Leak Risk: Unused lines can develop leaks over time. Plugging them prevents water loss.
• Simplifies Troubleshooting: With only one suction line, it’s easier to find and fix problems.

Negative Effects (if Done Incorrectly)

• Losing Equalizer Safety: If your pool was designed to use an equalizer line, plugging it removes an important safety feature.
• Reduced Main Drain Flow: In pools where the main drain is plumbed through the skimmer, plugging the wrong hole stops water flow from the pool floor.

Non-obvious insight: Some pool owners accidentally plug the wrong hole, which can cause poor circulation or even pump damage. Always confirm which line is in use before plugging any hole.

How To Identify Which Skimmer Hole Should Be Plugged

If you’re not sure which hole to plug, follow these steps:

• Turn off the pump.
• Remove the skimmer basket.
• Look for water movement in each hole when the pump is running.
• If only one hole pulls water, that’s your active skimmer line.
• The hole with no suction is likely unused and should be plugged.

Warning

Never plug a hole that’s actively pulling water. This can damage your pump or cause dangerous suction in other parts of your pool.

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What Happens If Both Skimmer Holes Are Open?

Leaving both holes open when only one is connected to suction is a common mistake. Here’s what can happen:

• Air Gets Into the System: Air can be drawn in, leading to loss of prime.
• Pump Noise and Inefficiency: The pump may make loud noises or run weakly.
• Possible Damage: Pumps are designed to move water, not air. Running them dry can cause overheating and failure.

Practical tip: Always check your skimmer holes after opening your pool for the season, or after repairs. Sometimes plugs fall out or get lost during cleaning.

When Should Both Skimmer Holes Be Open?

Both skimmer holes should only be open if:

• Your pool uses a diverter valve (also called a float valve) to control flow from both the skimmer and main drain.
• Your pool has an active equalizer line.
• You have confirmed both lines are connected and in use.

If you’re not sure, consult your pool’s design plan or a qualified pool technician.

The Role Of Skimmer Hole Plugs

Skimmer hole plugs are usually made of rubber or plastic. They’re designed to create a tight seal in the unused skimmer port. Here’s why they matter:

• Seal Out Air: Prevent air from entering the suction line.
• Prevent Debris Entry: Stop leaves or dirt from getting into unused lines.
• Block Water Loss: If an unused line develops a leak, the plug keeps water in the pool.

How To Install A Skimmer Plug

• Clean the hole to remove dirt or debris.
• Choose the correct size plug.
• Press or screw the plug firmly into the unused hole.
• Check for leaks or air bubbles after installation.

Expert tip: Over time, plugs can wear out or become loose. Check them every few months, especially before pool season.

Diverter Valves And Float Valves Explained

Some pools use a diverter valve (also called a float valve or skimmer weir) in the skimmer. This device balances how much water the pump pulls from the skimmer and the main drain.

• Diverter valves sit at the bottom of the skimmer, covering both holes.
• They usually have an adjustable plate or float that opens or closes each port.
• This helps control water flow for better circulation.

If you have a diverter valve, both skimmer holes might be open. However, the diverter ensures air doesn’t enter the system.

Skimmer Hole Plugging And Pool Winterization

When you close your pool for the winter, plugging skimmer holes is part of the process. Here’s why:

• Prevents Freeze Damage: Water left in pipes can freeze, expand, and crack your plumbing.
• Keeps Out Debris: Plugs stop leaves, insects, and animals from entering during the off-season.

To winterize:

• Lower the pool water below the skimmer.
• Remove the skimmer basket.
• Insert a winterizing plug (sometimes called a “gizzmo”) into the skimmer holes.
• Add pool antifreeze if needed.

Non-obvious insight: Some plugs are designed to absorb ice expansion, protecting your skimmer from cracks.

Common Mistakes With Skimmer Hole Plugs

Even experienced pool owners make errors with skimmer plugs. Avoid these common mistakes:

• Plugging the Active Line: Always confirm which line is in use before installing a plug.
• Using the Wrong Size Plug: A loose plug will not seal air or water effectively.
• Ignoring Old or Cracked Plugs: Replace worn plugs each season.
• Not Checking After Installation: Always check for air leaks after installing or replacing a plug.

Signs Of A Problem With Skimmer Holes

Sometimes, pool issues are caused by problems with your skimmer holes or plugs. Watch for these signs:

• Bubbles in the pump basket: Air is entering the system.
• Water loss: A leak in an unused line can drain your pool.
• Pump losing prime: The pump sucks air and loses suction.
• Unusual noise: Air in the system causes rattling or whining sounds.

If you see these symptoms, check your skimmer plugs and lines first.

How To Troubleshoot Skimmer Hole Issues

If you suspect a problem, follow these steps:

• Turn off the pump.
• Remove the skimmer basket.
• Inspect both holes for debris, wear, or missing plugs.
• Run the pump and check for air bubbles in the pump basket.
• Try plugging one hole at a time and see if the issue improves.

If you can’t solve the problem, consult a pool professional.

Pool Types And Skimmer Hole Setups

Different pool types use different skimmer setups. Here’s a quick overview:

In-ground Pools

• Usually have two holes per skimmer.
• Main drain often plumbed separately.
• Equalizer lines less common in new builds.

Above-ground Pools

• Often have only one skimmer line.
• Second hole may be present but usually plugged.
• Main drain is rare.

Commercial Pools

• Must use equalizer lines by code.
• Both holes are generally open.

Each setup has its own needs. Always follow the manufacturer’s guidance.

Skimmer Hole Plugs Vs. Skimmer Plates

Some pool owners confuse skimmer plugs with skimmer plates.

• Skimmer plugs fit directly into the hole at the bottom of the skimmer.
• Skimmer plates are flat covers used to block the skimmer opening during winter.

Plugs seal the line; plates block the skimmer mouth.

How Skimmer Hole Plugging Affects Pool Cleaning

Proper skimmer hole setup is important for good water circulation. If you plug the wrong hole, debris may not be pulled in effectively, or your vacuum may not work.

Vacuuming tip: If you use a manual pool vacuum that connects to the skimmer, always check which hole is in use. Vacuuming with the wrong configuration can cause poor suction or air leaks.

Skimmer Hole Plug Materials And Lifespan

Skimmer plugs are usually made from:

• Rubber: Flexible, provides a tight seal, lasts 2-5 years.
• Plastic: Durable but can crack with age or freeze damage.

Cheap plugs can fail quickly. It’s worth investing in a quality plug, especially if you leave your pool open year-round.

Example: A $5 rubber plug may need replacement every two years, while a $15 heavy-duty plug can last five years or more.

Pool Codes And Regulations Around Skimmer Holes

Some areas have strict rules about pool plumbing and skimmer setups. For example, the Virginia Graeme Baker Pool & Spa Safety Act requires dual drains or other anti-entrapment measures in public pools.

Home pools: Local codes may require unused lines to be sealed or capped.

If you’re unsure about your area’s requirements, check with your local building department or visit CDC Pool Owner Resources for more information.

Upgrading Or Changing Skimmer Hole Setups

If you’re planning a pool renovation, you might want to change how your skimmer holes are used. For example, you can:

• Add a new main drain line
• Convert an unused equalizer line to an active suction line
• Install a diverter valve for better flow control

Tip: Always hire a professional for plumbing changes. Mistakes can lead to leaks or pump damage.

Skimmer Hole Plugging: Myths And Misunderstandings

There’s a lot of confusion about skimmer holes. Here are some common myths:

• Myth: “Both holes must always be open.”

Truth: Most pools use only one open hole.

• Myth: “Plugging a hole will damage the pump.”

Truth: Plugging the unused hole prevents air from entering and protects the pump.

• Myth: “All pools need an equalizer line.”

Truth: Equalizer lines are required in commercial pools, but not in most home pools.

Understanding these facts helps avoid costly mistakes.

Professional Advice For Pool Owners

If you’re unsure about your pool’s skimmer setup, here’s what you should do:

• Review your pool’s blueprint or owner’s manual.
• Take photos of your skimmer and plumbing layout.
• Ask a professional if you plan to change plugs or lines.
• Regularly inspect your skimmer holes and plugs for wear or leaks.

Pro insight: Sometimes, leaks in old pools are caused by cracks in unused equalizer lines. Plugging these lines can stop water loss without expensive repairs.

Frequently Asked Questions

Why Do Some Skimmers Have Two Holes?

Many skimmers are built with two holes to provide options for connecting both a skimmer line and a main drain or equalizer line. Not all pools use both, but having two holes makes the skimmer more versatile for different plumbing setups.

Should I Plug Both Holes In My Skimmer During Winter?

Yes, you should plug both holes when winterizing your pool. This helps prevent water from entering the pipes, which can freeze and cause damage. Use special winter plugs designed for this purpose.

How Do I Know If My Pool Uses The Equalizer Line?

Check for a small grate or pipe below the skimmer basket. You can also consult your pool’s plumbing diagram. If you don’t see a second line connected, it’s likely not in use and can be plugged.

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What Happens If I Plug The Wrong Skimmer Hole?

Plugging the active skimmer line will prevent water flow to the pump, which can cause the pump to run dry and possibly burn out. Always identify which line is active before installing a plug.

Is It Safe To Leave One Skimmer Hole Open All Year?

Yes, as long as it’s the active suction line and the other hole is properly plugged. Regularly inspect the plug for wear and replace it if necessary to ensure a tight seal.

Taking care of your skimmer holes may seem like a small detail, but it makes a big difference in your pool’s health and your peace of mind. By understanding why one skimmer hole is plugged, you’ll avoid common problems, extend the life of your equipment, and enjoy a cleaner, safer pool all season long.

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