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Pressure Testing Tip: How to Check for Leaks in the Main Drain Without Plugging the Line

While plugging the main drain is the most accurate way to pressure test the main drain line, if you want to avoid getting in the pool, it is possible to identify a problem in the upper part of this plumbing by using a technique called an Air Lock Test. This test takes advantage of the fact that the water above the main drain in the deep end of the pool produces a measurable amount of pressure that acts as a “plug” to an air filled main drain line.

After pressure testing the rest of the plumbing lines to ensure they are sound, induce air into the  equipment end  of the main drain line until you see it bubble out of the main drain. Then, close the valve on your pressure tester. Assuming the main drain is under about 9′ of water, the water column above it should provide 4 psi of pressure on the trapped air within the line. A pressure drop below 4 psi is an indication of a leak somewhere in the upper section of the plumbing. If the depth of the main drain you’re testing is different than 9′, the pressure the water puts on the air lock can be calculated at .43 psi per foot of water.


If there is a leak in the line in the section above the bottom of the pool, the amount of pressure loss can be used to indicate where in the line that leak is. Pressure will drop quickly until water reaches the location of the leak, at this point even though water is escaping from the leak, air will be trapped and held at a pressure representing the difference between the leak level and the water level of the pool. So, if you see that you pressure quickly drops to 2 psi, it’s likely that there’s a leak in the main drain line somewhere between 4′ – 5′ under the water level of the pool.


If there is a leak  in the section of the line at or above the water level of the pool, you’ll see a quick drop to 0 psi.

A leak in the section of plumbing under the pool shell will not be indicated from this test due to the fact that the air is still supporting the full column of water in the pool. So, if a leak is still suspected in the main drain line, it will have to be inspected by fully pressurizing the line with a plug in the main drain.


LeakTrac Tip: Sound Intensity Can Provide Clues About Leaks Outside of Vinyl Liner

While the LeakTrac is designed to specifically find leaks in vinyl liners, listening carefully to the sounds it makes can give you clues as to other issues in the pool. Since the LeakTrac locates leaks by picking up a connection to ground through the leak, it also picks up connections to ground through metal elements in the pool such as light niches or gasket screws even when they aren’t losing water. While we recommend other tools to specifically locate the leaks in these areas, a stronger signal or noise than what is expected from the LeakTrac around these areas could be a clue to your water loss problem.

Man uses LeakTrac 2400 near skimmer

As you use the LeakTrac you will become familiar with how electricity flows through the pool and what levels of intensity to expect around common ground connections such as skimmers. If you notice that the LeakTrac is giving you a stronger signal than normal at a skimmer or stair gasket, the LeakTrac could very well be reading a leak on top of the normal screw ground connection that should be investigated further. While the LeakTrac’s primary purpose will always be finding holes in vinyl liners, understanding how the science behind the equipment works and paying attention to the signals it’s giving you can increase the value you get from the tool.

Case Study: Listening Devices Pinpoint Even the Deepest Leaks


Equipment Used:

XLT30, Pressure Testing Kit , Leakalyzer


The customer noticed that a lot of sand was being blown into the pool and that there were bubbles in the air filter, so a problematic skimmer line was isolated and shut off.  However, even after closing and plugging that problematic skimmer line there were still sand and bubbles being blown back into the pool.

Sand in pol from leaking skimmer return line.


When we arrived to the pool, the skimmer line was plugged and a Leakalyzer test with the pump off showed no water loss. That made us suspect that at least the portion of the plumbing below water level was good. We then pressure tested the other two skimmer lines to confirm the whole line was good and they held pressure. The main drain was eliminated as the source of a problem without getting in the water through our Leakalyzer test that covered the plumbing below the water line and an air lock test that included the line that was above water level. Eliminating all of these other areas left us to examine the problematic skimmer line as the only potential problem. Upon further inspection, the valve used to shut off the line wasn’t holding, which explains the continued bubbles and sand in the pool even after the line was taken out of use.

To determine the specific location of the leak within the faulty skimmer line, we induced air into water saturated soil to listen with the XLT30. At first the bubbling and gurgling sound of a leak was faint, but just by turning the volume up on the XLT30 we were able to identify a distinct noise where the leak was. It was fairly easy tonarrow the leak location down to a 6’ diameter, but in order to get down to within 2’ of the leak we needed to use the advanced filtering capabilities of the XLT30. Once the high filter was turned on, there was a clear spot with the most distinct noise. As sound waves travel they get more and more muffled, so the sound will be crispest and most distinct right over the leak. The sound did fade a little as time went on, but re-saturating the soil by inducing more water into the line revitalized the sound.

The customer estimated that the skimmer line was 18” to 2’ deep, but when we cut the concrete and dug down to that level there wasn’t any moisture or pipe. To confirm we were in the right place we turned the pressure back on and still heard the leak so kept digging and finally found the broken pipe at just over 4’ deep. Before we replaced the cracked fitting we cut the pipe and pressure tested both ways to assure the rest of the line was good.

Digging in concrete deck for plumbing leak

Learnings (are there any key takeaways or learnings from how this leak was found?):

  • Combining a Leakalyzer test with an air lock test of the main drain can confirm that line is solid without getting into the pool.
  • If you notice a once strong leak noise fading, you may just need to add more water to re-saturate the soil.
  • Trust your equipment! If you’re confident you heard a leak in a specific place, you may have to dig farther than expected to find it, but it will be there.

Understanding the Winter Blow-out Process

What happens underground in the winterizing process?  By setting up a clear PVC pipe system we can visualize the process and some of the challenges to winterizing pipes.   This demonstration shows us that effectively utilizing the proper equipment and  technique for blowing out plumbing lines is crucial for consistent, successful winterization. It is important to understand the three main phases of the process:

    • Water purge phase
    • Line clear phase
    • Sealing/securing phase

In the water purge phase, the water that fills the lines is “pushed” out of the pipe and replaced with air.  Several factors are important to accomplish this part of the process effectively.  First of all, the air source you use must provide enough air pressure to counter the back pressure of the water in the pool.  When blowing out return lines this is a pretty low threshold, however when purging water from the main drain line a minimum of 4 psi of pressure must be achieved to push against the 8 feet of water in many deep ends.  The second important factor during the purge phase is the amount of water flow that can be pushed through your blow-out plugs.  The wider these plugs open under the appropriate pressure the more water you will be able to clear from the line at a fast rate.

The line clearing phase happens after most of the water has been purged from the lines but may still remain in low parts of the plumbing.  Your ability to generate a high air flow rate through the purged lines is the critical factor in successfully blowing this remaining water from these locations.  Choose an air source that delivers at least 100 cubic feet per minute (cfm) of air flow – most compressors only produce 2-5 cfm so you will need a blower.  And again, use blow-out plugs that open wide at low back pressure to assure that the blower’s power can be realized as flow through the pipes.  On return lines with more than one return branching off the main line, strong air flow to the farthest branch can be achieved by blocking the flow of cleared branches earlier in the run.

Once the lines have been cleared of water they must remain so over the course of the winter, so the plugs must effectively seal to prevent water from re-entering the plumbing from the pool.  It’s still a good idea to pour antifreeze into the cleared lines just in case undiscovered cracked fittings, damaged threads, or badly installed plugs allow some water back in.   After pouring antifreeze into the cleared line, turn the blower back on just long enough to see it blown into the pool through your blow-out plugs.  The remaining anti-freeze will settle into low spots in the plumbing  . . . just the spots that would be prone to problems if water got in.

Anderson’s Big Blue Blower and Winter Duck Plugs deliver the performance you need to assure each phase of the blow-out process is done effectively.  The blower produces enough pressure “umph” to blow-out even the deepest main drain and provides an exceptional amount of air flow to clear all low spots in plumbing.  Winter Duck Plugs open wide at low back pressures to clear lines quickly during the water purge phase and enable high air flow during the line clearing phase.  Additionally, they can be easily blocked during this clearing phase with simple office binder clips to facilitate flow to all branches of the return system.  Most importantly, over the past 10+ years, Winter Duck Plugs have proven themselves dependable at keeping water out of lines and protecting thousands of pools from winter freeze damage.

For a comparison of how  Winter Duck Plugs compare to other blow-out plugs on the market see the results of our testing here.

Popular Leak Detection Seminar to be Offered at International Show and Northeast Show


International Show: Thursday, November 1 from 9:15 AM to 10:30 AM

Northeast Pool and Spa Show: Thursday, January 29 from 8:30 AM to 11:45 AM

Leak Detection Seminar at Northeast Pool and Spa Show

The basics of swimming pool leak detection will be covered at two convention venues this trade show season.  Lance Anderson and Brad Madison will teach “Finding Leaks – Turn Headaches into Profits” at the 2018 International Pool, Spa, and Patio Show in Las Vegas this October.  Then, in January at the 2019 Northeast Pool and Spa Show in Atlantic City, the presentation will be given again with the addition of a panel of experts who provide another layer of in-field experience to the topic.

These can’t miss courses cover the basic procedures, techniques and tools that will turn pesky leak problems into profit making opportunities. By focusing on the underlying principles of leak detection, the seminar provides an unbiased explanation of the systematic, 3-step process that should be followed on every leak job to assure efficiency. Special attention is payed to the following topics:

  • What time-saving pieces of information are important to gather before going to the job site
  • The “Pressure Testing Principles” that explain the behavior of air and water in tested plumbing lines
  • How to pinpoint underground plumbing leaks with sonic and helium detection methods
  • How to pinpoint structural leaks in concrete, fiberglass, acrylic, and vinyl

While the seminar is geared toward those who are looking to get started with leak detection, even seasoned professionals will be able to pick up a few tips. This is also a great course to send new employees to for comprehensive training!

Attendee testimonials from previous presentations:

“This seminar made my trip worthwhile. Excellent!”

“Lance Anderson presents an excellent message – well prepared, thoughtful, and precise – his passion for the subject is appreciated.”

“An informative, well-organized approach. Excellent presentation based on research and experience.”

“The whole class will make my job easier.”

“This is the best presentation I have attended at this show in years. Well organized and thought out. Great material for a difficult problem.”

“After 25+ years in pool sales and service, I still learn something at these seminars. Keep them coming.”

Leakalyzer Tip: Portable Shade Cover

Is the hot summer sun getting in the way of your successful Leakalyzer tests? Leakalyzers perform best when their temperature remains stable throughout the duration of the test.  When exposed to the changes in radiant heat as the sun goes in and out of the clouds, the deck plate can expand and contract leading to confusing readings. We recommend setting up the Leakalyzer in a shady place whenever possible, but Mark Spatz of Florida Leak Patrol came up with a smart solution to carry shade with you wherever you go!

Mark fashioned his Leakalyzer “dog house” out of a large plastic tub that was cut to cover the deck plate and sensor capsule when set up at the pool.   The sensor rod extends up through a hole in the top.  A bucket can be placed over this rod to protect it from sunlight or the occasional unexpected stray shower.  One edge of the tub extends into the water to provide additional protection from water turbulence when the pool system is running.  Mark says he doesn’t use the cover everytime he pulls out his Leakalyzer, but that it does make his tests more accurate in certain situations.

How to Gather Good Information for Efficient Leak Jobs

An efficient leak detection job starts well before you open the gate to your customer’s back yard.  You can save a great deal of time by gathering and processing information about the suspected leak before the job is even scheduled. Make a practice of purposefully communicating with the pool owner to accomplish the following 3 objectives as the first part of your leak detection process.


1. Make sure there really is a leak

Sometimes customers are concerned about water loss that may be the result of something besides a leak.  Evaporation, splash-out, or even miscommunications about when a backwash was performed can result in panicked calls to a leak professional.  It’s hard to charge for a leak job if there isn’t really a problem (even after you’ve spent a lot of time looking for the non-existent leak), so it’s important to ask enough questions upfront to eliminate the possibility that the symptom they describe is anything but a leak. It’s also a good idea to schedule jobs far enough out to give the customer time to do a Bucket Test for themselves to confirm the water loss they are observing is not just due to evaporation.

Example Questions to Ask:
  • Why do you think you have a leak?
  • When did the problem start?
  • How much loss are you experiencing per day?
  • When is the last time you backwashed the pool?


 2. Collect key facts that allow you to start processing the problem before you get to the pool

Leak detection is as much mental as it is pohysical .  Your observations and tests at the pool are just one source of clues that can lead you to solving the elusive problem.  By asking strategic questions of the pool owner before you get to the job, you can start thinking about and solving the problem even while you are driving. Knowing when the pool was built, who built it, how it is used, and when it started leaking can provide clues to where the leak will be, especially as you start building a mental database of previous leaks you’ve found.  Other information about the nature of the problem, like whether it leaks more with the pump on or off, will also help to establish what part of the pool may be the most suspect.

Example Questions to Ask:
  • How far down have you let the water level go?
  • Does the pool have any unique features such as an attached spa, waterfall, or in-floor cleaning system?
  • When was the pool built and who built it?
  • Who typically uses the pool and how do they use it?
  • Have there been any unusual events associated with the pool recently?
  • Have you recently had repairs or construction work done in or around the pool?


3. Clearly establish what the expectations are for the condition of, and access to the pool

Generally the tests you do on site will require that the pool be filled to its normal operating level. Make sure the customer knows this.  You may also have to get in the pool for inspection and repair, so it’s a good idea to request the pool be clean and warm if possible.  Of course, you can’t work on a pool that you can’t get to, so make sure the customer provides information on how to access the pool in case they aren’t there.  It’s easy to assume that these simple and common sense issues will be obvious, but a few wasted trips to a pool reinforce the fact that it’s better to err on the side of over-communication.

Example Questions to Ask:
  • Is the pool filled to its normal level?
  • Do I need any keys or codes to access the pool and equipment?
  • Is the pool clean?
  • Will you be home when I’m working on the pool? If not, how can I reach you during that time?


For more in depth discussion of the information gathering step and how it fits into the rest of the leak detection process check out the slide shows available in our resource center.

Case Study: Air Escaping From a Leak is Loud

Type of Pool:

Vinyl Liner

Equipment Used:

Leakalyzer, LeakTrac 2400, Pressure Testing Equipment, Dye


This job started as a request for us to run a vinyl liner scan with the LeakTrac because the customer had a leak that they suspected was in the liner in a swim ledge. So, when we arrived at the pool we did a scan of the liner, which showed the suspected swim ledge wasn’t actually a problem. The LeakTrac did locate a small leak on the vinyl stairs, but this leak didn’t draw enough dye to be the source of significant water loss. We quickly patched this leak with a Leakmaster Peel and Stick Patch before moving on to do a full leak detection.

Image of Pool


With a quick pressure test we found a skimmer line was not holding pressure. We then switched from inducing pressure with water to inducing pressure with air and heard a loud bubbling and gurgling sound  right in the skimmer bowl without even using a listening device.  The leak turned out to be a broken pipe just below the skimmer bowl.

To verify the rest of the line was good we dropped a 550 inflatable plug with an extension hose in past the leak and found the rest of the line held pressure.

A repair was scheduled for a later date, but before leaving the pool we plugged the skimmer line and ran a Leakalyzer test to confirm all leaks had been found.


  1. Pressure testing is a quick way to isolate a pipe leak
  2. Air escaping from leak in pipe makes a great noise for sonic location
  3. The LeakTrac not only can be used to find leaks, but also to eliminate the liner as the location of a leak if no leaks are found


Why is the Light Tester (LCDT) currently sold with an opaque dome?

We introduced the Light Cover for use with our LeakTrac – Vinyl Liner Leak Detector more than 20 years ago.  The purpose of the cover is to insulate the light from being a distracting electrical connection from inside the pool to ground.   As many of you know, skilled artisans in the leak detection business have also used the domes from these covers (and other types of covers, bowls and plugs) with dye to detect the presence of a leak in an isolated area, and they have been doing so for many years.

Red Rhino Leak Detection of West Palm Beach FL, recently acquired patents for a method and device for locating leaks in swimming pool lights using a transparent dome, dye, and a suction cup to hold the dome over the light, (#9464959, #9128002).  They filed a lawsuit against us, alleging that the manufacture, sale and use of our Light Cover dome for dye testing infringes on one of their patents.  We feel strongly that Red Rhino’s patents are invalid because the claimed technology has been known and used in the industry since well before Red Rhino filed its patent applications, and we also feel strongly that our LCDT product does not infringe for multiple reasons.  We shared these beliefs with Red Rhino even before they sued us.

Nevertheless, we also respect the intellectual property of others.  Thus, until the suit is resolved, we have elected to replace the clear dome of the LCDT product with an opaque dome.  The patent at issue specifically requires that the housing (or dome) of the device be “transparent whereby the flow of dye in the hollow housing is observable by a user of the device.”  With the opaque dome, the user of the LCDT product cannot observe the flow of dye in the hollow housing (dome).  Thus, the modified LCDT product does not meet this requirement of the patent claim.

This modification does not change the utility of our LCDT product.  To detect the presence of a leak in the area isolated by the dome of the LCDT Light Cover, we instruct the user to administer dye outside an opening in the dome.  If the dye is sucked through the opening in the dome the user knows there is a leak somewhere in the isolated area that will warrant further inspection and ultimately a repair.  If dye is not sucked through the opening the user can eliminate it as a possible leak source.  Thus, it does not matter whether the dome is transparent or opaque for the purposes of detecting the presence of a leak.  In fact, except in the most severe leak cases, spending time watching the movement of dye inside the dome will provide no additional benefit.

Clear domes are of benefit for other leak detecting purposes however, and we do still sell transparent domes in other configurations for these applications.

Pressure Testing Myths Busted

Pressure testing it is a topic where a number of myths and misunderstandings have developed in the swimming pool industry. Below, we explain the truth behind 4 common misconceptions that should help you stay focused on efforts that will lead to leak detection success and profits.

MYTH #1: Pressure Testing is Unnecessary for Leak Detection

While you may be able to find common leaks without pressure testing, if you want to leave the pool assured that you have found all of the leaks . . . and you want to give the customer that assurance, pressure testing is critical.

There are other ways of finding easy to reach leaks in plumbing, but from the standpoint of conclusively determining if the entire line is leak free, nothing beats a properly performed pressure test.  Furthermore, despite advancements in cameras and other probes that have applications in some situations, the most common and dependable way of pinpointing leaks underground involves the use of a listening device that picks up the sound of pressurized air escaping from a leak into water saturated soil.

MYTH #2:  Air Can Be Used Interchangeably With Water for Pressure Testing

Air and water behave very differently in pressure testing situations.  Understanding three pressure testing principles that address these differences is important as you determine when to use air and when to use water to build pressure.

  1. Air compresses under pressure, water does not
  2. Air stays at the top of the pipe, water stays at the bottom
  3. Air escapes from leaks faster than water does

Our pressure testing slide show provides helpful diagrams and more explanation of how these principles affect your test results.  As a general rule it is best to use water when testing to determine if the line is leaking since it does not compress under pressure so it will show a loss in pressure quickly, even with a small loss in volume.  On the other hand, air trapped in a line can expand as water volume is lost from the leak, slowing (and sometimes completely masking) a drop in pressure. Additionally, if plugs happen to pop out under pressure they will come out with much less force if the line is pressurized with only water . . . trapped air will propel a popped plug like a cannon ball!

The main benefit of using air is that it makes a much better noise escaping from the pipe into water saturated soil than water does.  So, once you have identified a leaking section of plumbing with a water test, switch to air to produce a good noise that can be picked up by your listening device.

Myth #3: Air and Water Can Be “Mixed” in a Pressure Tester and Will Stay “Mixed” Inside the Pipe

We’ve debunked this myth by testing it on a plumbing system made of clear PVC.  Regardless of how they are transmitted into the pipe, water stays low and air stays high.  They do not stay mixed up just because they are put under pressure.  This is simple physics and anyone who says otherwise is selling you hocus-pocus.

Adding both air and water to a line is not the first step you should take when trying to make a good leak noise.  While it is indeed important for the soil to be full of water outside of the pipe where air will blow out through the leak, no noise is made unless air actually gets to the leak.  Since water stays at the bottom of the pipe and air at the top, if the leak is in the low end of the plumbing no air will get to it if you are adding both air and water.  So, all of the water above the leak level must be purged from the line before this happens. Generally this is best accomplished by removing a low plug and blowing air from the high end.  Once you see bubbles, replace the plug, set your air source regulator to maintain no higher than 5 psi, and begin listening.  Larger leaks and/or leaks in soil that drains quickly may necessitate adding water while air is going into the line.  To avoid noises inside the pipe, do this with a separate pressure tester from the low end of the plumbing.  The best results happen when air and water do not mix inside the plumbing . . . just at the leak!

MYTH #4: Certain Kinds of Gas Make Better Noises Than Others for Leak Location

Any gas escaping from a leak will make bubbling/gurgling /hissing/spotting sounds in the full range of sonic frequencies that can be picked up with a listening device.  Sometimes, leak detectors will use nitrogen tanks to deliver the gas into the line quietly (without the conflicting sound of a compressor).  However, these tanks are used not because of any special characteristic of the gas (in fact, the air we breathe is made up of 78% nitrogen) but because of its ease of availability and inexpensiveness in relation to other gasses.  SCUBA tanks adapted with an adjustable regulator can also be used if you can get them filled.  A small compressor works just fine for inducing air pressure, especially if you use a 50 foot hose that allows you  to position the compressor some distance from where you are listening.

In some situations where soil or leak conditions make it difficult to create a noise helium gas is used to find underground plumbing leaks.  In these situations a Helium Detector picks up the presence of the gas as it makes its way to the soil’s surface.  Helium is not being used because it makes a better noise, but because it can be detected by this detector.

If you have any questions about pressure testing or swimming pool leak detection, consult the Resource Center of our website or give us a call.

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