How to Tell If Pool Lines Were Blown Out Correctly
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The solution is surprisingly simple: knowing whether your pool lines were blown out correctly comes down to checking a few important details from the winterization process. Properly cleared plumbing should have released water, produced a steady flow of air at the appropriate openings, and been sealed before pool water could flow back inside. Unfortunately, a pool can look perfectly closed for winter while water remains trapped in a vulnerable pipe. Understanding what successful line blowing looks like can help you recognize incomplete winterization before freezing temperatures create expensive problems.
If you watched someone close your pool, you may already have useful clues. Even if you missed the process, there are questions you can ask and visible details you can inspect without disconnecting plumbing or disturbing winter seals.
What Should Happen When Pool Lines Are Blown Out?
Blowing out pool lines means using an appropriate air source to push water out of plumbing that could otherwise freeze. This usually includes skimmer suction lines, return lines, and certain additional plumbing circuits. The main drain requires special handling because it remains connected to the water at the bottom of the pool.
During a successful blowout, water is initially forced from the line. As more water leaves, the discharge changes from a mixture of water and air to a relatively steady flow of air. At submerged pool openings, that airflow appears as continuous bubbling.
The important distinction is that seeing bubbles once is not enough to prove the entire line was properly winterized. The correct opening must produce sustained airflow, and the plumbing must then be sealed appropriately to prevent water from returning.
Quick Checklist: Signs of a Proper Pool Line Blowout
- Each applicable skimmer and return line was cleared individually or positively verified during the process.
- Water discharge changed to a steady stream of air or bubbles.
- Winterizing plugs were installed securely before water could refill the cleared lines.
- The main drain, when winterized using an air lock, was isolated with its valve while air was still flowing.
- Accessory plumbing, including spa jets or water features, received separate attention where needed.
- Pumps, filters, heaters, and other equipment were drained according to their manufacturers' instructions.
These signs provide useful evidence, but they are not a guarantee that every hidden section of plumbing is free of freeze risk.
Check Whether Every Return Line Was Cleared
Return jets send filtered water back into the swimming pool during normal operation. During winterization, air is commonly directed through the return plumbing until water is displaced and bubbles appear at the pool's return openings.
A frequent mistake is assuming that bubbling from one return means every return has been cleared. Pools with multiple returns may have a shared plumbing branch, and air naturally follows the path offering the least resistance.
For example, a pool with three wall returns might show vigorous bubbling at the nearest fitting while another outlet barely releases air. That second outlet should not automatically be considered clear. A technician may need to isolate or progressively plug the outlets to establish effective airflow through each branch.
After clearing the return lines, properly sized winterizing plugs should be installed before water can flow back into the plumbing. A return that was thoroughly blown out but left unsealed can refill, undermining the work.
Know What a Properly Winterized Skimmer Looks Like
Skimmers deserve special attention because their openings and plumbing are often near the water's surface, where freezing conditions can be particularly damaging.
When air is directed through a skimmer suction line, water should be expelled from the relevant opening until the flow becomes predominantly air. The line must then be sealed using a suitable winterizing plug or compatible skimmer protection device.
There is an important difference between protecting the skimmer body and protecting its underground suction pipe. A skimmer expansion device may help accommodate ice expansion inside the skimmer, but it does not compensate for an improperly cleared or unsealed suction line.
If your pool has two skimmers, ask whether both suction paths were addressed. Depending on the plumbing arrangement, clearing one skimmer does not necessarily establish that the other was winterized.
The Main Drain Is Different From the Other Lines
The main drain is one of the most misunderstood parts of pool winterization. Unlike a wall return, a submerged main drain generally cannot be plugged from the poolside during a routine closing.
For systems that require main drain winterization, a common procedure is to direct air through the drain line until a substantial, sustained column of bubbles rises from the drain. The technician then closes the appropriate valve while air is still flowing, trapping an air column inside the line.
This technique is called an air lock. Its purpose is to prevent pool water from reentering the protected portion of the plumbing.
The condition of the valve matters as much as the bubbling. A worn or leaking valve may not maintain an effective air lock throughout winter. Some main drain systems have different manufacturer-approved winterization requirements, so the correct approach depends on the actual plumbing configuration.
Do not open a main drain valve simply to check whether the air lock is holding. Doing so can release the trapped air and allow water back into the pipe.
Warning Signs That Something May Have Been Missed
Some incomplete winterization problems leave clues, although they are not always visible after the pool has been covered.
Potential Winterization Red Flags
- A return never produced visible airflow: Its plumbing branch may not have been cleared adequately.
- Air appeared briefly and stopped: The line may have been isolated prematurely, or the airflow may have shifted to another outlet.
- A winter plug is loose or missing: Water may be able to enter a previously cleared line.
- The main drain valve was reopened after winterization: An established air lock may have been lost.
- Water remains inside exposed equipment: Drainage may be incomplete, even if the underground piping was cleared.
- An accessory circuit was overlooked: Spa plumbing, cleaner lines, and water features can retain water independently of the main circulation system.
One caution: a wet-looking fitting or occasional bubbling during closing does not independently prove the work was defective. What matters is the complete sequence of clearing, isolating, and sealing each applicable line.
What If You Did Not Watch the Pool Closing?
You can still perform a useful review without removing plugs or reconnecting equipment.
Start by counting the pool's return openings and skimmers. Compare that number with the visible winterizing plugs and any closing photographs. Next, review the equipment pad for clearly labeled valves and properly drained equipment.
Ask the person who closed the pool these specific questions:
- Did every return produce steady air before it was plugged?
- Were separate skimmer lines cleared and sealed?
- How was the main drain handled, and was an air lock established if required?
- Were spa jets, cleaner connections, and water feature lines included?
- Were the pump, filter, heater, and other components fully winterized?
A knowledgeable pool professional should be able to explain the method used for your particular plumbing arrangement. Photographs, a closing checklist, or service notes can be especially helpful if questions arise later.
Commonly Overlooked Plumbing: Spas and Water Features
Pools with attached spas, tanning ledges, waterfalls, or dedicated cleaner connections can have more plumbing branches than a homeowner realizes.
Consider an attached spa with separate jet plumbing and a spillover return. Clearing the pool's ordinary wall returns does not necessarily remove water from the spa jet loop, air blower connections, or spillover plumbing.
Likewise, a tanning ledge may contain shallow-water returns that are easy to overlook. Because some of these fittings sit relatively close to the surface, their associated plumbing deserves careful attention in freezing climates.
For complicated systems, a diagram identifying each valve and plumbing circuit is more useful than simply counting how many plugs were installed.
Can You Test the Lines After the Pool Is Closed?
Homeowners should avoid casually removing winter plugs, opening sealed valves, or applying compressed air to investigate. These actions can let water enter protected plumbing or expose equipment to unsafe pressure.
Pool components have specific pressure limitations, and some manufacturers prohibit compressed-air procedures for certain equipment. A high-pressure air compressor is not automatically appropriate for winterization. Use the equipment manufacturer's instructions and qualified professional guidance rather than assuming a universal pressure setting is safe.
If there is genuine uncertainty about whether a line was cleared, ask a pool professional to verify the winterization using the correct procedure for that system. The inspection may involve safely reestablishing airflow and resealing a particular circuit.
What If You Notice Water Loss Along With Winterization Concerns?
A falling pool water level is a separate issue from whether plumbing was successfully blown out. Water loss can result from evaporation, a plumbing leak, a fitting problem, or other conditions, and the winterizing process itself does not identify the cause.
If your pool is still accessible and conditions permit a normal evaporation comparison, the Mini Bucket Test can help compare pool water loss with normal evaporation. It is a simple first step when unexplained water loss is part of the concern, not proof of a leak or a way to determine whether an underground line contains water.
Once winterized, do not disturb plugs, air locks, or protective equipment arrangements to perform a water-loss test. Any suspected plumbing leak during winter should be assessed with the pool's closing configuration in mind.
When Should You Call a Pool Professional?
Professional verification is particularly worthwhile when a return was never observed bubbling, an important plug is missing, a main drain valve may have lost its air lock, or the pool has complicated plumbing that was not clearly documented.
It is also wise to seek help if freezing weather is approaching and you suspect water remains inside exposed equipment. Waiting until a hard freeze can leave little opportunity to correct an incomplete closing.
If your pool has already experienced sustained freezing temperatures, do not attempt to force open valves or fittings that may contain ice. Allow a qualified technician to evaluate the system without creating additional damage.
The Bottom Line: Look for Complete Clearing and Proper Sealing
The best evidence that pool lines were blown out correctly is a documented process: each required plumbing circuit was addressed, water was displaced until steady airflow appeared, and the line was sealed or air-locked using the appropriate method.
Remember that bubbles alone do not equal a completed winterization. Return branches, skimmer lines, main drains, accessory plumbing, and equipment all need attention appropriate to their design. If you cannot verify an important part of the closing, a professional inspection is safer than disturbing the winterized system yourself.
With a clear understanding of these signs, you can ask better questions, recognize potential oversights, and head into freezing weather with greater confidence in your pool's protection.