Blowing Out Main Drain Inground Pool: Expert Guide for Homeowners

Getting your pool ready for winter means doing things right—especially when it comes to your main drain. If you live in a place where it gets cold, water left in your pool pipes can freeze, expand, and cause cracks or leaks. One of the most important steps is blowing out the main drain in your inground pool. This process might sound complicated, but with the right guidance, you can do it safely and protect your pool for years to come.

In this guide, you’ll learn exactly how to blow out your main drain, why it matters, what tools you need, and the common mistakes to avoid. You’ll also get answers to questions that often confuse new pool owners. Whether you’re doing this yourself or checking the work of a professional, understanding the process gives you peace of mind.

Let’s dive in.

Why Blowing Out The Main Drain Matters

Your pool’s main drain sits at the deepest point. Its job is to pull water from the bottom, helping with circulation and cleaning. When winter comes, water in this drain line can freeze if not removed. Frozen water can easily crack pipes or fittings, leading to expensive repairs in spring.

Blowing out the main drain is not just about removing water. It’s about making sure the line stays dry and blocked off, so water can’t sneak back in during the winter. Missing this step means you risk hidden damage that’s tough to spot until you start your pool next season.

Many new pool owners think just lowering the water level is enough. But the real danger is in the pipes you can’t see. Air is your best friend for winterizing, as it pushes water out and fills the space, leaving no room for ice.

How Pool Plumbing Works: Understanding The Main Drain

Before you start, it helps to know a bit about your pool’s plumbing. Most inground pools have three main types of pipes:

• Skimmer lines – pull water from the surface
• Return lines – send clean water back into the pool
• Main drain line – pulls water from the bottom

The main drain connects to your pool’s pump and filter system. On most pools, all pipes meet at a multiport valve or a set of valves near your equipment pad. Some pools have a dedicated main drain line; others combine it with the skimmer (called a “shared line”).

If you’re not sure which valve goes to what, check your pool’s plumbing diagram or ask your builder. Knowing which pipe is which is key to blowing out the right line.

Tools And Materials You Need

You don’t need a lot of fancy gear to blow out your main drain, but having the right tools makes the job faster and safer. Here’s what you’ll need:

• Air compressor or a specialized pool blower (like a Shop-Vac or Cyclone)
• Winterizing plugs (also called gizmos or expansion plugs) for returns and skimmers
• Thread tape (Teflon tape) for sealing plugs
• Safety goggles and gloves
• Pool antifreeze (optional but recommended for main drain lines in very cold regions)
• Socket wrench or screwdriver (for removing drain plugs on equipment)
• Pool plumbing diagram (if available)

A pool blower is better than a regular air compressor because it moves a lot of air at low pressure, which helps push out water without damaging pipes. However, many homeowners use a standard air compressor with good results—just keep the pressure below 20 PSI.

Step-by-step Guide: Blowing Out The Main Drain

Let’s break down the process into clear steps. Take your time—rushing leads to mistakes.

1. Lower The Pool Water Level

Start by lowering the water level in your pool. For winterizing, you want the water at least a few inches below the skimmer mouth. Use your pool pump’s “waste” setting or a submersible pump. Don’t drain the pool completely—just enough to keep water out of the skimmer and return lines.

• In most climates, 4–6 inches below the skimmer is enough.
• Never drain below the main drain. The main drain stays submerged.

2. Turn Off All Power

Safety first! Shut off the breaker to your pool equipment. This prevents accidents with pumps, heaters, or lights while you work.

3. Remove Equipment Drain Plugs

Next, open all drain plugs on your pump, filter, and heater (if you have one). Water trapped here can freeze and break expensive parts. Keep the plugs in a safe place.

4. Set Valves For Main Drain Only

Go to your pool’s valve system. Close the valves for the skimmer and return lines, leaving only the main drain valve open. This way, all the air you blow in will go straight to the main drain.

Tip: If your main drain shares a line with the skimmer, set the valve to “main drain only.” If you have a “Y” diverter, turn it fully toward the main drain.

5. Connect The Air Source

Attach your air compressor or blower to the pump’s drain plug port (where you removed the plug earlier). Some people use the skimmer opening, but connecting to the pump is better for main drain lines.

• Use duct tape or a rubber fitting to seal the connection if needed.
• Set your compressor to deliver low-pressure, high-volume air.

6. Blow Out Main Drain Line

Turn on the air. You’ll see bubbles start to rise from the main drain at the bottom of the pool. This bubbling means air is pushing water out of the line.

• Let the air run until the bubbles turn steady and strong—usually 2–3 minutes.
• The goal is a column of air, not just a few bubbles.

Insight: Many beginners stop too soon. Keep blowing until you’re sure all water is out. In cold climates, it’s smart to blow for a full minute after bubbles appear.

7. Plug Other Lines

While the air is still running, quickly plug the skimmer and return lines at the pool wall with winterizing plugs. This keeps air in the pipes and water out.

• Use thread tape on plug threads for a tight seal.
• If you have multiple returns, plug them all.

8. Seal Main Drain Line

Most main drains can’t be plugged at the pool. Instead, you seal the line using the valve at the equipment pad.

• With air still blowing, slowly close the main drain valve. This “air locks” the line so water can’t get in.
• For extra protection, add pool antifreeze to the main drain line before blowing it out, especially if your winters are severe.

Pro tip: Not all pool valves seal perfectly. If yours leaks, consider replacing the valve or using a plug at the equipment pad end.

9. Add Pool Antifreeze (optional)

In very cold regions, pool antifreeze adds extra safety. Pour it into the main drain line before you start blowing, or after blowing if your valve allows. Use only pool-safe antifreeze—never car antifreeze, which is toxic.

10. Final Checks

Double-check all plugs, valves, and equipment before covering your pool. Make sure nothing is left open or loose.

• Store all removed plugs and parts in a labeled bag for spring.
• Note any leaks or odd sounds for repair.

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How Long Does It Take?

For most inground pools, blowing out the main drain takes about 30–60 minutes, including setup and cleanup. The actual blowing part is only a few minutes, but careful prep saves you headaches later.

Larger pools or complicated plumbing systems may take longer, but the steps are the same.

Comparing Methods: Air Compressor Vs. Pool Blower

Some homeowners wonder if they need a special pool blower or if their garage air compressor will do the job. Here’s a quick comparison:

Insight: Pool blowers are best for safety and speed. Air compressors work but must be carefully regulated—never use more than 20 PSI to avoid bursting pipes.

Main Drain Plumbing Types

Not every pool is built the same. Understanding your main drain’s plumbing helps you blow it out correctly.

Dedicated Main Drain Line

This setup has a separate pipe from the main drain to the equipment pad. It’s easy to identify and blow out.

Shared Line (main Drain + Skimmer)

Some pools combine the main drain and skimmer into one pipe (often labeled “MD/SK”). These require setting the valve to main drain only and sometimes plugging the skimmer to force air to the drain.

Multiple Main Drains

Larger or newer pools may have two main drains for safety. Blowing out is the same, but watch for air coming from both drains.

Here’s a look at common configurations:

Tip: Always check your pool’s owner manual or ask a pro if you’re not sure about your plumbing.

Mistakes To Avoid When Blowing Out Main Drain

Many pool owners make the same errors, leading to leaks or damage. Here’s what to watch for:

• Stopping too soon – If you stop blowing when you first see bubbles, water may remain in low spots. Keep blowing until bubbles are strong and steady.
• Using too much pressure – High air pressure can crack pipes or fittings. Stay below 20 PSI.
• Forgetting to air lock – Closing the main drain valve while blowing is key to “air locking” and keeping water out all winter.
• Skipping antifreeze – In very cold regions, a bit of pool antifreeze can protect against tiny water pockets you can’t remove.
• Not sealing plugs tightly – Loose winterizing plugs let water seep in and freeze.
• Ignoring plumbing type – Not all pools have the same main drain setup. Make sure you know yours before starting.
• Not checking for leaks – Listen and look for leaks at valves, plugs, and fittings. Fix these before winter hits.

When To Call A Professional

Blowing out your main drain is not hard, but if you’re unsure, don’t risk your pool. Call a certified pool technician if:

• You can’t identify your pool’s plumbing layout
• Your valves don’t seal properly
• You have complicated or older equipment
• You don’t have the right blower or compressor

A pro can do the job quickly and safely, often for less than the cost of a major repair.

Winterizing: Complete Pool Closing Checklist

Blowing out the main drain is just one part of closing your pool for winter. Here’s a quick checklist for a safe shutdown:

• Balance water chemistry – Adjust pH, alkalinity, calcium, and chlorine.
• Clean the pool – Remove leaves, brush walls, and vacuum the bottom.
• Lower water level – As described earlier.
• Blow out lines – Main drain, skimmer, and return lines.
• Plug all inlets/outlets – Use winterizing plugs.
• Add winter chemicals – Algaecide, pool antifreeze (if needed).
• Remove equipment – Ladders, skimmer baskets, and cleaners.
• Cover the pool – Use a winter or safety cover.
• Store parts safely – Label bags for spring.

Missing any step can lead to big problems once things thaw.

Main Drain Blowing For Saltwater Pools

Saltwater pools use the same main drain and plumbing as standard chlorine pools. The blowing process is identical. However, saltwater systems are more sensitive to leaks, and equipment is more expensive to repair.

• Be extra careful with salt cell and control box—remove and store indoors if possible.
• Make sure all wiring is dry before covering.

Environmental Impact And Safe Chemical Use

Always use pool-safe antifreeze—never dump car antifreeze down your pipes, as it’s toxic to animals and plants. Winterizing chemicals should be measured carefully and kept out of drains and streams.

If you drain water to waste, check local laws. Some cities have special rules about pool water disposal.

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Case Study: Main Drain Freeze Damage

Consider the story of a homeowner in Michigan who skipped blowing out the main drain. The winter was harsh, and by spring, the pool lost 2 inches of water per day. A leak test showed a cracked main drain pipe, buried 6 feet deep.

The repair cost over $3,000—much more than a winterizing service. The lesson? Taking time to blow out the main drain saves money and headaches.

Non-obvious Insights For Beginners

• Air lock is your insurance – Many new pool owners don’t realize that closing the main drain valve while blowing air creates an “air lock.” This lock is what keeps water from sneaking back in all winter, even if the pool cover lets some rain or snow through.
• Pipe slopes matter – If your main drain pipe slopes upward from the pool to the equipment pad, water can get trapped in low spots. That’s why it’s important to keep blowing for a full minute after bubbles appear, not just until the first sign of air.

Comparing Diy Vs. Professional Service

Should you do this yourself or hire a pro? Here’s a quick look at the differences:

If you’re comfortable with basic tools and careful, DIY is a good option. If you’re new to pools, a professional gives you certainty.

Safety Tips For Main Drain Blowing

• Always wear safety goggles and gloves—pressurized air can kick up debris.
• Keep children and pets away from the equipment pad.
• Double-check that all power is off before starting.
• Never use a compressor above 20 PSI for pool pipes.

Where To Learn More

For more details on pool winterizing, visit the SwimmingPool.com Winterizing Guide, which covers a wide range of inground pool types and safety advice.

Closing your pool might seem like a chore, but blowing out the main drain is a simple step that prevents big problems. With the right tools and careful attention, you can keep your pool safe all winter—and enjoy a smooth opening in spring.

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