Home » Sand in Sight Glass (When Backwashing Pool Sand Filter)

Sand in Sight Glass (When Backwashing Pool Sand Filter)

If you’re struggling with sand in sight glass when backwashing your pool sand filter, this is just the article for you. Like many pool owners with a sand filtering system, a common phenomenon is getting sand in the sight glass.

But don’t stress.

Did you know, there are more pools in the U.S than there are stories in New York City. You better believe out of all these swimming pools, there are a couple (thousand) people finding sand in their sight glass when backwashing their pool sand filter.

Let’s take a look at what could be causing this, and more importantly, what you can do about it!

What is a Pool Sight Glass?

Diagram showing pool filter sight glass, backwash outlet and multiport valve.
Diagram showing pool filter sight glass, backwash outlet and multiport valve.

The sand filter sight glass is a plastic or glass “window” near the waste port on the top of the sand tank.

They are generally bubble shaped and have a thread on one end that screws into the filter housing. An O-Ring is often used to seal them.

If your sight glass is leaking, it’s likely that the gasket or O-ring needs replacement or the sight glass could be cracked.

Related Reading: Sand Coming Out When Backwashing or Rinsing (Answered!)

What Does a Sight Glass Do?

The sight glass allows you to keep an eye on the water clarity when backwashing. Often there is no way to see the water during backwashing, the sight glass is a way to see the water clarity.

For example, when backwashing, the water will start out cloudy. When the water appears clear in the sight glass, you know you’re done backwashing and it’s time to run the rinse cycle.

Backwashing your pool sand filter prevents the buildup of dirt accumulation in the sand. The reversed water flow during a backwash lifts and flushes the sand in the opposite direction as the filtering cycle. It can then expel the dirty water out through the waste port on the top.

The types of pool filters that use a sight glass include any and all sand filters that don’t have any other way to check in on the quality of the water through the tank. If you don’t have a sight glass on your filter, you can install an “inline sight glass”.

Sand In Your Sight Glass When Backwashing

During the backwashing process described above, some of the sand used to filter your pool water can get trapped in the sight glass. This is completely normal and to be expected in the process.

What Causes Sand in Sight Glass?

When back washing, the water rises up through the sand in the tank and then is directed through the multiport valve to the backwash outlet. Usually you will have a pipe or backwash hose connected to the waste (backwash) outlet which takes away the waste water from the backwashing process. You can get a backwash hose here:

Milliard Heavy Duty Backwash Hose
  • Can drain water out to the street or sewer
  • Weather and chemical resistance 
  • Made from heavy-duty vinyl

During this process, a small amount of sand, sediment and debris can be pushed out through the backwash outlet and past the sightglass. It’s during this process some sand or sediment might get stuck or trapped in the sight glass.

Related Reading: What is a Filter Backwash Sock? (Do You Need One?)

How to Remove Sand from Sight Glass

I’m sure you’re thinking to yourself “I understand how I got sand into my sight glass, now how on earth do I get it out?”.

First of all, there isn’t actually any need to remove it. It’s normal.

However, after backwashing, you’ll need to run the rinse cycle. A rinse cycle uses the same water flow as when you’re filtering, but instead of clean water going back into the pool, it’s sent out of the waste port.

Rinsing your sand filter helps to both clear the sand from the sight glass, clear out any remaining debris from your filter and evenly distribute the sand — bonus!

Sand in Sight Glass After Rinsing

On occasion, you may still notice you have sand in your sight glass after backwashing and rinsing. If it’s a small amount of sand (like a dozen or so grains), don’t worry about it.

But if there is quite a bit of sand stuck in the sight glass, you’re likely experiencing one or more of the following issues:

  1. You Have Cracked Or Broken Laterals

    Within the sand filter apparatus, there are small pieces of tubing located on the bottom known as laterals. Laterals function to keep sand inside the tank, while also allowing the newly filtered water to return back to the pool.

    If they crack or break, the flawed system now allows sand back into the pool and into the sight glass. You’ll usually get sand in the bottom of your pool too if this is the problem.
  2. You Have A Cracked Or Broken Stand Pipe

    The standpipe is a larger tube that runs vertically through the middle of your sand filter. It connects the laterals at the bottom to the valve on the top.

    Just like damage to your laterals affects your pool filtering, a cracked or broken standpipe will also cause sand to re-enter your pool and remain in the sight glass.

    And just like cracked laterals, you’ll likely get some sand in the bottom of your pool too.
  3. You’re Using The Wrong Grade Of Sand In The Filter

    When sand is too small it will get sucked through the laterals and returned into the pool. When means you’ll have sand in the sight glass and on the bottom of your pool.

    If you think this is the issue, you’ll need to replace your sand with the right sand for your filter.

    A majority of sand filters use #20 silica sand, which means the filter captures particles measuring 20 microns and larger. On the rare occasion, they may also use filterglass or zeolite.

    You can get new sand here.
ZeoSand Sand Filter Media
  • Lasts up to 5 years
  • Captures more particles than normal sand
  • Use half as much as regular sand
If you click this link, we may earn a commision at no additional cost to you.

If you have found sand in your pool, see our article here on how to remove sand from the bottom of your pool.

What are Sand Filters?

Internals of a pool sand filter.
Internal construction of a pool’s sand filter.

A little background reading for those of you who would how a pool sand filter works.

A sand filter is a tank filled with sand that connects to your swimming pool’s filtration system. The purpose of a sand filter is to purify your pool water to maintain a clean and clear water environment throughout the season.

Typically, sand filters have 6 features: filter, backwash, rinse, circulate, closed, and waste. You can test out all these features by switching the valve on the top.

The primary function of a sand filter is to filter or clean the pool water. The process is as follows:

  1. Pool water enters the filter through a pipe at the top of the tank to begin its filtration cycle.
  2. Once inside the sand filter, the pool water enters a distributor where it’s spread out evenly on the bed of sand.
  3. The pool water is washed through the sand where it’s filtered through, trapping any dirt, particles, and debris inside the sand.
  4. The pool water, free of impurities, can now exit the sand tank through the lateral pipes on the bottom that are large enough for water to pass through without any sand moving with it.
  5. The newly filtered pool water now re-enters the pool through the return outlets or jets.

With regular use, your sand filter collects more and more dirt and debris which gets deposited into the sand. This results in dirty or clogged sand. Over time, this dirty or clogged sand will impeded the flow of pool water.

This is why it’s necessary to backwash your pool sand filter.

Sand Filter Tips and Tricks

  1. You should backwash and rinse your pool filter once a week after you’ve vacuumed  the pool to avoid excessive dirt and debris from entering the filter
  2. Backwash and rinse your pool when your filter gauge rises above 80 kpa or 8-10 PSI  (the exact pressure depends on what is normal pressure for your filtering system).
  3. Don’t backwash your pool water more or longer than necessary. Frequent backwashing will discard more pool water than necessary while also reducing the sand filter’s effectiveness.
  4. Always turn off the filter pump before changing the valve settings (i.e. from filter to backwash). Neglecting to do so can damage the system.

Related Reading: How Much Sand to Put in a Sand Filter (How Far to Fill)


There you have it, you should now have a thorough understanding on why there is sand in your sight glass and how to get the sand out of the sight glass.

In short, having sand in the sight glass is normal and usually goes away when you use the rinse setting (after backwashing).

If you have excessive amounts of sand in the sight glass or notice it at the bottom of your pool, then it’s time to investigate and check if the sand filter is defective.

I’d like to hear your experiences. Do you have sand in your sight glass and is this a common occurrence?

4 thoughts on “Sand in Sight Glass (When Backwashing Pool Sand Filter)”

  1. There is no water in my site class why is that? On your sand filter do you only use the three functions like backwash rinse and filtering?

    • If water is visible in the sight glass at times other than during backwashing, it indicates a potential issue with either the multiport valve leaking or the backwash drain line not draining properly. In such cases, inspect and troubleshoot the multiport valve and the backwash drain line to identify and resolve the source of the problem.

      In a typical sand filter system, the primary functions are indeed filtering, backwashing, and rinsing. However, some sand filters may have additional functions or settings depending on the specific model and manufacturer.

      For example, some sand filters may have a “waste” function, which allows you to bypass the filter media entirely and discharge water directly out of the system. This function is useful for lowering the water level in the pool or removing large amounts of debris without sending it through the filter. You may use this if you’re vacuuming algae from the pool.

      Additionally, some advanced sand filter systems may have options for recirculating, winterizing, or other specialized functions. However, the basic functions of filtering, backwashing, and rinsing are the most common and essential for proper operation and maintenance of a sand filter.

    • Check for leaks or cracks around it. There could also be something wrong with the multiport valve. If the filter functions OK though (all functions), it’s probably nothing to be concerned about.


Leave a comment