How to Make Sure Water Is Out of Pool Lines Before Winter

Let's build a foundation for a safer winter pool closing by focusing on something you cannot easily see: the water hiding inside your plumbing. A pool can look completely ready for winter while several underground pipes still contain water that could freeze and expand. Knowing how to make sure water is out of pool lines before winter can help prevent cracked fittings, damaged equipment, and frustrating surprises when you reopen in spring. The key is understanding how to clear each line, recognize the signs that water has been displaced, and keep it from flowing back in.

Why Getting Water Out of Pool Lines Matters

Water expands when it freezes. Inside a swimming pool's plumbing, that expansion can put considerable pressure on PVC pipes, elbows, valves, and fittings. If trapped water has nowhere to expand, something may crack.

Underground plumbing deserves particular attention because the damage may remain invisible until the pool is running again. A broken return line beneath a concrete deck can be considerably more difficult to repair than a cracked fitting beside the pump.

Not every drop of moisture must disappear from every pipe. The practical goal is to remove bulk water from sections exposed to freezing temperatures and protect the remaining plumbing according to the pool's design and equipment instructions.

How Can You Tell When a Pool Line Is Clear?

When air is pushed through a water-filled pipe, the discharge usually changes as the water is displaced. You might initially see a strong surge of water, followed by sputtering, mist, and vigorous bubbling. Eventually, the discharge becomes a steadier stream of mostly air.

Step 1: Identify Every Pool Plumbing Line

Before connecting any blower, determine which pipes serve the different parts of your pool. This is especially important on pools with several valves or an attached spa.

Common plumbing circuits include:

• Skimmer suction lines
• Main drain suction lines
• Wall return lines
• Dedicated vacuum or cleaner lines
• Attached spa suction and return lines
• Waterfall, fountain, or other water feature plumbing

Some pools have separate pipes for each opening. Others have multiple returns connected through a shared underground branch. That difference affects how you direct airflow and verify that every section has been cleared.

Label the valves before changing their positions. If you cannot confidently identify a pipe, do not assume that bubbles appearing somewhere in the pool mean every connected line has been winterized.

Step 2: Shut Down Equipment and Prepare the Plumbing

Turn off the circulation pump, heater, and related equipment at their appropriate disconnects before beginning. Follow the manufacturer's instructions for the correct closing sequence.

Adjust the pool water level only as required by your particular pool, cover, and winterizing method. Some installations can be winterized without lowering water below every return. Draining too much water can create separate problems, particularly with vinyl liners, fiberglass pools, and high groundwater conditions.

Remove the return eyeball fittings or other components specified by your closing procedure. Prepare correctly sized winterizing plugs and the appropriate skimmer protection devices.

Use a pool-specific, high-volume, low-pressure blower suitable for your system. Do not assume a conventional high-pressure air compressor is safe. Excessive pressure can damage plumbing or equipment and create an injury hazard. Follow equipment-specific pressure restrictions, isolate sensitive components, and never pressurize a sealed filter tank or other vessel contrary to manufacturer instructions.

Step 3: Blow Out the Return Lines

Return lines carry filtered water back into the pool. Because they often run underground, trapped water in a low section may not be visible from either end.

Connect the approved blower at the designated winterizing connection. Configure the valves so air reaches the intended return circuit without passing through equipment that should be isolated.

As air moves through the pipe, watch the return openings. Water may initially gush or sputter out. Continue according to the blower and plumbing instructions until the outlet shows sustained airflow with little or no bulk water being discharged.

Install the correct winterizing plug while airflow is maintained, following the approved procedure for that fitting. This helps prevent pool water from immediately flowing back into the cleared line.

If several returns share one pipe, air may escape through the easiest opening first. Plugging that outlet can redirect airflow toward another branch. Work through every return rather than treating the first bubbling outlet as proof that the entire network is clear.

Step 4: Clear and Protect the Skimmer Lines

Skimmer plumbing brings surface water toward the pump. A skimmer line may be buried below the deck, while the skimmer housing itself sits closer to freezing surface conditions.

Direct air through the appropriate suction circuit and observe the skimmer opening. Once the heavy water discharge has stopped and airflow becomes steady, seal the connection using the specified winterizing plug or skimmer protection device.

Remove standing water from the skimmer body as directed by your closing procedure. A properly fitted skimmer ice-expansion device may help protect the housing if water freezes inside it.

What pool owners often miss: A pool with two skimmers may have separate suction lines. Clearing one skimmer does not necessarily clear the other. Check every skimmer connection individually.

Step 5: Understand Why the Main Drain Is Different

The main drain is usually located on the pool floor, far below the water surface. Unlike a wall return, it generally cannot be plugged from the pool side during ordinary winterization.

Many inground pools use an air-lock method instead. Air is introduced through the main drain circuit until a strong stream of bubbles rises from the drain. The appropriate isolation valve is then closed while the blower is still supplying air, trapping an air column that helps keep pool water from refilling the plumbing.

This procedure depends on the actual plumbing layout and a valve that seals properly. A leaking valve can allow the line to refill. Some pools also have interconnected drains or specialized suction arrangements that require a different method.

If you cannot identify the main drain valve or determine whether an air lock can be maintained, have a pool professional handle this part of the closing.

Step 6: Drain the Equipment, Not Just the Underground Pipes

Clearing the pool lines does not automatically empty the pump, filter, heater, or other equipment.

Pump housings, filter tanks, heater headers, chlorinators, and exposed plumbing sections may hold water even after air has moved through the underground lines.

Follow each manufacturer's winterization instructions and remove the designated drain plugs when required. Pay attention to equipment installed below surrounding pipe elevations, where water may collect after the main plumbing has been cleared.

Store removable plugs and small fittings together in a labeled container so they are available when the pool reopens.

Why Bubbles Alone Do Not Prove Every Pipe Is Empty

A steady stream of bubbles is encouraging, but it does not reveal everything happening underground.

Consider a return pipe that dips beneath the deck before rising toward the pool wall. Air may establish a pathway through the pipe while some water remains in the lowest section.

Another complication involves shared return branches. Air may escape through one outlet while a different branch still contains substantial water.

This is why effective winterization combines adequate airflow, correct valve isolation, careful observation, and immediate sealing. Where the plumbing design allows, clearing each branch separately gives more useful information than blowing air indiscriminately through the entire system.

Common Mistakes That Leave Water Behind

Should You Use Pool Antifreeze?

Pool antifreeze may provide additional freeze protection in plumbing circuits where its use is appropriate. It is not a replacement for correctly clearing and sealing lines when the system requires blowout winterization.

Use only antifreeze specifically labeled for swimming pool plumbing, in the quantity and manner directed by its manufacturer. Never use automotive antifreeze.

Antifreeze is not automatically suitable for every pipe. Main drain lines, for example, typically require a different winterization approach because they connect directly to the pool and cannot be sealed at the drain opening in the usual way.

What About Above-Ground Pools?

Above-ground pools may be simpler to winterize because portions of their plumbing are exposed and removable.

Depending on the installation, flexible hoses can be disconnected and drained, while the pump and filter can be emptied and stored or protected according to manufacturer directions.

However, an above-ground pool with rigid PVC plumbing, buried sections, or a complex equipment pad may still require a dedicated line-clearing procedure. Do not assume that disconnecting a hose drains every low point in the system.

How to Check Your Work Before Covering the Pool

Once the lines are cleared, perform a final inspection before completing the winter closing.

• Confirm that every identified suction and return circuit was addressed.
• Check that winterizing plugs are seated and secured correctly.
• Verify that the main drain isolation valve is in its required winter position.
• Inspect skimmer protection devices and remove remaining standing water as directed.
• Confirm that equipment drain plugs have been handled according to their instructions.
• Look for exposed pipes or low fittings that might still contain water.
• Record unusual bubbling, poor airflow, or valves that do not seem to seal properly.

If one line continues to discharge significant water, produces unexpectedly weak airflow, or cannot be reliably isolated, investigate before calling the winterization complete. A professional can help determine whether the issue involves valve configuration, trapped water, or an obstruction.

What If You Notice Unexplained Water Loss Before Closing?

Winterizing protects plumbing from freezing, but it does not repair an existing leak. If your pool has been losing water unexpectedly, consider investigating before it is covered and the plumbing becomes harder to access.

A simple first-step tool such as the Mini Bucket Test can help compare normal evaporation with possible leak-related water loss. It cannot identify the leak location or confirm that a plumbing line is damaged, but it may help you decide whether further investigation is worthwhile.

If water loss appears connected to pump operation or a particular plumbing circuit, document that pattern and discuss it with a qualified pool leak detection professional before winter closing.

When Should You Hire a Pool Professional?

Professional winterization is worth considering when the plumbing layout is unfamiliar, the pool has an attached spa or several water features, the main drain cannot be isolated confidently, or you are unsure how to protect a particular piece of equipment.

A qualified pool technician can identify the correct airflow paths, choose equipment compatible with the plumbing, and check that each circuit is protected without relying on dangerous pressure levels.

The Bottom Line: Clear, Seal, and Verify

Making sure water is out of pool lines before winter requires more than watching a few bubbles appear. Each plumbing circuit needs the appropriate clearing method, and each cleared line must be protected against water returning.

Pay particular attention to shared return branches, separate skimmers, main drain air locks, and equipment that retains water in its housing. A careful closing procedure now can reduce the risk of discovering freeze-damaged plumbing when swimming season returns.