My Pool Is Only Losing Water When My Pump Is Running

Okay, picture it… You walk out to your pool and notice that the water level is lower than normal. So, you play around with the filter and valves and add more water. A few days go by and you notice the water level is low again. Suddenly, it hits you, “My pool is only losing water when my pump is running.”


As the pool expert you are, you know that it’s not ideal not to have your pump running. Though, what you don’t know is why your pool is only losing water when the pump is running.

Let’s dig a little further…

My Pool is Only Losing Water When My Pump Is Running

Whether it is from evaporation or spillovers, every swimming pool loses water. However, if you’re losing water quickly, it’s something you should address sooner than later.

If you’re reading this article, I will assume you went through the process of elimination and understand that your pool only leaks when you run the pump.

Still, that doesn’t necessarily mean the leak is at your pump. However, it does mean you have a pressure side leak and not a suction- side leak.

Pressure-side Return Leaks

Generally, if you’re losing water when your pump is running, you have a pressure-side return leak. When you’re running your pump, the plumbing after the pump is under pressure. Often times, this can lead to small leaks and water sprays through your line (from the filter to your return jets.)

Image result for pressure side return leak in pool

The good news? You can eliminate any type of air leaks before the pump and move your focus to after the pump. The bad news? Now, you have to find the leak. Where do you start?

Check All the Connections

The first areas you want to check for any leaks are the connections, unions, elbows, and your pump to filter hoses. Normal wear and tear, overtightening, and physical damage to unions can contribute to water leaks.

Make sure to check your pump, filter, chlorinator, and heater, as well. If your pool heater is leaking, you want to act fast. A leaking heater can impact the internal parts, which may or may not be covered under the manufacture’s warranty.

For above-ground pools, don’t forget to inspect the pump to filter hoses. In many instances, the coupling, unions, or hose is under large amounts of pressure and will spring a leak. Also, over time, hoses can become brittle and under pressure, spring a leak.

Filter Multiport Valve

Image result for pool multiport valve leak"

The multiport valve is important because it helps to control the flow of your pool water. As a result, a lot can go wrong. Check the in and out ports on the valve as well as the area around the cover.

Another common source of a leak is the backwash hose. Check the end of the backwash hose for any water running when it shouldn’t be.

click here to find your replacement pool filter valve parts

Internal leaks may also occur in a defective filter multiport valve. This might cause you to lose clean water down the waste line or backwash line. Another culprit inside the valve is your spider gasket. Damaged or compromised spider gaskets can certainly cause a water leak.

For more information, we recommend reading:

Return Jet/ Return Jet Fittings

Image result for pool return jet leak repair"

Other areas to check are your returns and return jet fittings. Often times, when installing a new liner, a leak can form around one of the fittings. Although it isn’t supposed to happen, installers can make mistakes while installing a new liner, making it difficult to identify.

We also recommend walking around the perimeter of your pool to check for wet spots or small puddles of water. Locating a soft or wet spot in your yard might help to identify the source and location of your leak.

Pressure Test Your Return Lines

Unfortunately, some leaks aren’t easily noticeable. If you cannot find any leaks, then you may need to do a pressure test. If you are unfamiliar with pressure testing your return lines, we recommend hiring a local pool guy.

Still, you can perform a pressure test yourself with a few winterizing plugs, a small compressor, and a little plumbing to connect a Schrader valve and gauge to the pipe leaving your filter. You can find pressure testing kits at your local hardware and supply store.

A pressure test of the return lines will show you if there is a leak in one of the pipes that carry water back to the pool. However, the test will not give an exact location where. Basically, it ranges from where the pipes go underground at the equipment and anywhere between where they end at the pool.

Can’t Find The Leak?

What happens if you cannot locate the leak? Call a professional because that’s what they’re there for, right? Seriously, in some instances, it’s best to seek the help of a professional. Leak detection companies can perform a series of tests that will help determine where the leak is and help fix the leak.

If after reading this article you think you may have a suction-side leak, we recommend reading our article HERE.

Or, if you have any more questions about pressure- side return leaks, please feel free to leave a comment or reach out to us via our chat system on our website.

click here to find your replacement pool filter valve parts

35 thoughts on “My Pool Is Only Losing Water When My Pump Is Running

  1. We have an 18’X18′ inground square pool, about 5 feet deep all around. Vinyl liner, which was replaced in 2019. This season, since we opened the pool, we noticed water behind the liner, which we have been troubleshooting for awhile. Had it sucked out a couple weeks ago, and it came right back. Anyway, developed a more serious problem just today. Ran the pool pump on filter and after about an hour the water level had dropped dramatically, nearly to the bottom of the skimmers. Shut it off, put plugs in the jets and skimmers and returns. Filled it up a bit and put tape to mark the level, for a 24 hour leak check. If it holds like this the liner is not the problem and instead the leak is in the pressure side somewhere I guess….

  2. I recently had my pool replastered and since that time (about 2 weeks ago), 3 of my 6 pool jest have massive bubbles and I am losing water. The water loss is only when my pool is running. Any thoughts?

  3. Our pool is covered with a mesh safety cover all winter. We put a new liner in two years ago. When we opened the pool in May the water was to the top of the skimmer line, (telling me the pool holds water just fine all winter.) I lost water obviously when I vacuumed on waste and backwashed to get it cleaned and clear. Once clear, I put a solar cover on. I think I have lost 1/4” of water in the five days since it has been covered. The weather has not been extremely warm and the pool temp is only 70 degrees. Pump has been running 24 hours a day. We seem to typically add water to our pool about every 7 days due to what we always thought was evaporation thru the summer months. But I don’t think that evaporation would be the case at the moment. Now becoming concerned that there is a leak in the lines. Just purchased a dye test kit. Any other suggestions?

  4. My pool is losing water when the pump is running. However, when the vacuum hose is connected the pool doesnt lose any water

          1. Your pump housing is either cracked or you’ve blown a gasket. Either way, we’d need to know the make and model of the pump to see what parts are available for it.

    1. we are having the same issue and not a single person can figure it out. We have had people come in and said there are leaks here and there but this is serious because it only loses water when we turn the pump on.

  5. The water level of the pool goes down when we run the waterfall wide open. What could cause this to happen??

  6. Pool is inground, Vinyl, 24×39 Kidney Shape, 3 1/2-8ft deep. I went away for a weekend, in January, and someone was looking after my house while I was gone.. They checked on things Sunday about Noon and all was fine, but when I came home about 8am on Monday, the Pool Level was just below the Skimmer Boxes, and had shown to drop about 6-8 inches. There are no signs in the Yard or surrounding area where the water would have gone. Does anyone have any ideas to help me with this? FYI we run our pool Year Round

  7. I have wet spots all around the yard. Unable to find any irrigation leaks. Pool isn’t losing any water with the auto filler off. What am I missing?

  8. Yesterday my pool pump got replaced and we have been running it quite a bit to get the algae out of the stagnant water.

    I notice that the water has dropped significantly but there is no obvious leaks near the motor/pipes.

    What can be the problem I wonder?

  9. Hi. Our pool is losing water ONLY when the pump is running and connected to the return line. We have bypassed the return line and are running an above ground return. The water level is above the return jets in the pool and we aren’t losing any water. We do not have the plugs in. I’m thinking this is a leak only when the return lines are under pressure. Thoughts?

      1. Your pump housing is either cracked or you’ve blown a gasket. Either way, we’d need to know the make and model of the pump to see what parts are available for it.

  10. Hello. it sounds like you have a leak on one of the outlets from the pump to the pool. isolate as best as you can from the outlet valves going to your pool to see which area is the problem. Another sure tell sign is a dip in your pavers or concrete. That water is going somewhere so the sooner you find out the better. You dont want a sink hole or your pool wall to cave in. Once you know which set of pvc piping is leaking, you will have to cut that portion off the existing piping, cap it, and re-plumb back together at the filter to get you by. To fix properly you would need to tear out a small portion of concrete, repair the leak or replace the PVC, and re concrete. I had a similar problem with a bottom drain line. I was losing an inch in 2 days and losing positive suction at the pump , which caused air to get in the system. Isolated which area was the problem, selected only skimmer suction, and capped the bottom drain. That was fun! Not! Good luck!

  11. You can help isolate a return line leak by using the shutoff valves. Shutoff the shallow return line and run the filter measuring water loss. Then repeat by shutting off the deep return line and measuring water loss. This will determine if the leak is at the deep or shallow return. The leak most likely is located at a joint of the line. A large crack in the sidewalk may also help locate the general area of the leak. A leak detection company will charge around $300 to find the leak .

    1. The check valve that prevents the water from flowing back to the lower level of water in the pool, is leaking.
      My pool does the same thing. The jacuzzi ‘s water level is about 6″ higher than the pool water , so when the pump shuts off, the water from the jacuzzi siphons back thru the check valve and into the pool. Water will seek it’s own level, and will flow from the higher level to the lower level,… Till they are the same level in both the pool and the jacuzzi.
      You can turn the “Ortega Valve” that feed the jacuzzi, off each time, when the pool pump shuts off. But this is a nuisance. I just let the water from the jacuzzi flow into the pool, and when the pump starts each day … It re- fills the jacuzzi, and the system operates as usual…. You can change out the internal workings of the check valve (spring & the disc ) , or replace the entire check-valve assembly….

  12. Is it really that bad that my German Shepherd swims whenever she wants in our swimming pool? Does the hair make the pump work harder?

  13. Doughboy 16×32 . with a deep end.
    Ours is definitely leaking from the outside of the return. This pool has been with us since last century– have certainly replaced the liner a few times and it is intact. My husband has reinforced the area with a semi-truck mud flap piece of rubber(!),but it’s time to do some more damage control. It has been determined that whoever (me) added a cheapo fountain accessory to the return is at fault and the threads are all stripped and the entire return thing needs replacing. (My son concurs)
    WHICH PARTS DO I NEED TO ORDERr? The filter and pump are relatively new. We’re experiencing the same stay at home thing as everyone else, but I have the added feature of 4 grandchildren who live in our bubble that expect to be able to swim during 2020 summer months.

    1. Hello Debbie. Sounds like you need to do some PVC pipe work and maybe some return port replacements. I cannot get a visual on the mud flap reinforced area you refer to though. if it is leaking from one pipes going to the ports, you should be able to isolate which one. Also, if it has been leaking awhile, it may be creating a loss of earth under your pool. You will most likely see a dip in the ground or concrete somewhere. That size pool should have at least 4 inlets and a selector valve for each set. I have a 16×32 kidney pool and mine has 4 with a selector for the step area or the deep end or both. You should get enough circulation with just 2 as long as you have your chemicals right. Hope this helps! Good Luck!

    1. I have a 1982 Koehler super spa with 11/2 hp booster pump and a 1/6 hp circulating pump my problem is the circulating pump after not using the spa for only a few hours will not pick up the water. There is air bubbles in the strainer basket and the water goes down then the circulating pump won’t pick up the water. There is know visible water leaks. This has never happen before. I made sure the strainer top was light. What do you think could be wrong. I need some help. I am a retired plumber and know most things about problems but not this.

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