How to Tell if My Pool Main Drain is Working? Troubleshooting Guide

Pool Main Drain

I love pools but troubleshooting problems like this can sometimes drive me crazy. Even with experience, wondering if one of my pipes is actually clogged or not working for some reason can really get into the back of my mind and start eating away at me. One of the hardest plumbing essentials to inspect is the pool main drain. The main drain provides circulation to your pool water from the deepest part of your pool and the plumbing actually runs directly under your pool liner or shell.

If you want to check if your pool main drain is working, try to identify if you have a clog, pipe burst, or a leak. You’ll need to close all other suction sidelines and valves to your pump. Inspect for bubbles around the main drain grate at the bottom of your pool, inspect the suction side to your pool pump from the main drain line, water pressure inside of your pump and filter, and check related valves connecting the main drain to the pump. This might seem like a big checklist but it’s not really so daunting after all to get some peace of mind and the knowledge if your main drain is working or not.

Keep on reading the article below, we will dive into the details on how to tell if your pool main drain is working properly.

Signs that your Main Drain is Clogged

Since the main drain is at the deepest part of your pool it may not always be obvious that you are losing suction and circulation to this part of your pool. Signs that you might have an issue with your main drain include:

  • Debris and Particles collecting at the bottom of your pool
  • New Dead Spots
  • Increased Suction from Skimmers
  • Bubbles in Filter Basket
  • Uneven Temperature of Pool Water
  • Noticeable Change in Pool Water Chemistry(using more chemicals to keep pool water balanced)

Does my Main Drain have a burst or air leak?

If your main drain is busted or leaking air from underneath the ground, this is a bit of a headache to deal with as those pool lines will need to be unearthed and replaced or repaired. Let’s not get too far ahead of ourselves though if there is an air leak it could be coming from a valve or fitting. What you would look for with an air leak in your main drain is the following:

  • Air Bubbles from your Main Drain Grate
  • Air Bubbles in your Pump Basket
  • Loss of Prime in your Pump
  • Decreased Pressure in your Pump and Filter
  • Consistent Decrease in Pool Water Level

Before calling out any service repair for your pool which can cost money that may not be necessary, let’s take a look at trying to identify if your main drain is working. If it isn’t, are we dealing with a clog or a leak?

What causes Main Drains to Clog or Leak?

The main drain can become clogged with almost any type of debris that can get through the grates in your main drain. The typical culprits will be leaves, sticks, hair, and other various particles that collect at the bottom of your pool. If your main drain as had the cover grate taken off for any reason, then something larger could have made its way into the drain unbeknownst to the pool owner. 

Leaks could result from poor winterization of the pool. Being around pools my whole life in the midwest, I can vouch for how important it is to winterize your main drain pipe correctly. This is why so many pool owners seek professional help to close their pools every Winter but you can also learn how to blow out your main drain on your own. If water had been trapped in the main drain during the winter, it’s possible that it could have created a crack in the main drain pipe from the water freezing and expanding inside the pipe. 

Air leaks could also occur around valves or fittings which is a much easier and cheaper problem to have. I discuss inspecting for this type of leak below. 

How to Isolate Suction from Only the Main Drain?

This will be the easiest and most important step before doing any other troubleshooting for the main drain. We will want to close off any water being pulled to the pump from any other line in the pool. At your equipment pad, you’ll want to close all the valves coming to your pump that are not from the main drain. That would mainly be your skimmer valves which need to be closed all the way. 

The reason we are doing this is that we want the only water being pulled from the pool to the pump will be from the main drain. This will allow us to identify if the main drain is actually working or not. You can read up about the difference between your skimmers and main drain use here.

You’ll also want to be sure that your main drain is open to 100% capacity on your valve. So turn your valve to completely open the main drain to allow as much water into the pump from the main drain. 

Check for Bubbles around the Main Drain Grate at the Deepest Part of Your Pool.

This first step of checking if the main drain is working or not is going to be identifying if there is damage around the main drain grate or the liner around the main grate. You’ll want to get your swimming suit or trunks on for this plus some goggles.

Dive down to the deepest part of your pool to closely inspect around the main drain grate. The grate will typically look like the images below. We want to look for any bubbles coming out from around the grate, liner, or even more rarely the main drain itself. If there are bubbles, coming from this area you’ll need to get a professional out to help identify the exact issue and repair needed. 

Checking for Suction into the Pool Pump from the Main Drain

Let’s take a look at your pump now which should only be pulling water from the main drain. If your pump is losing its prime or continuously spitting bubbles then we have identified an issue in your main drain. 

Watch water coming into your pump, you’ll expect the pump to have no issues as it should be pulling in plenty of water from the main drain even with your skimmer lines closed. The pump basket should be filled with water and priming perfectly. If the pump is losing prime and not getting enough water then we most likely have debris stuck in the main drain line or possibly a leak.

If the pump is spitting large bubbles as it’s priming, you’ll be able to notice that looking into your pump basket. There should be a very little amount of air in the basket. If there are large bubbles coming through then the main drain could be dealing with a few different issues. We could have a clog in the main drain, a leak in the main drain plumbing, or possible issues with an air leak somewhere in the plumbing.

Pressure Readings in Filter and Pump to Troubleshoot Main Drain

This will be a pretty quick and easy step to troubleshoot if your main drain is working or not but we will want to check the pressure gauge on the Filter and Pump. A pressure reading for your pool pump can run normally between 10-25 psi. We will want to look for any increase or decrease in the normal pressure your pump typically runs at. If you see a drop or rise, then this indicates an issue with the main drain. You could be dealing with a clog or an air leak.

During this testing, if your pump pressure remains normal but the filter pressure rises or drops below what is normal for your pool, then you may be dealing with a separate issue that isn’t related to your main drain. It could be necessary to clean your filter and run the test again with just the main drain.

Looking for an Air Leak in Main Drain Valves

If there is an air leak somewhere in your valves that connects your main drain to your pump you’ll want to identify those. An easy and simple way to check for a leak will be to use a can of Aerosol Shaving Cream. It’s a bit messy but you’ll be able to identify any air leaks around your valves or fittings in the plumbing connecting your main drain to the pump. 

Spray the shaving cream all over the areas of the valve and look for any air that is coming out. If you find any air leaking into the plumbing, then this will be a real issue that you’ll want to resolve. An air leak can really affect the suction you’re getting from your main drain to the pump.

I hope my experience has helped you identify if your main drain is working or not. If you identify the problem as a simple air leak or clog, hopefully, you’ll be able to fix this yourself and avoid any expensive service calls. If there is a major issue, it’s always best to get it resolved so your main drain is working properly to help keep your pool clean and safe.


The founder of I wanted a place to share all of the great information from my family to other pool lovers, builders, and those looking to buy a pool.

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