Calcified Sand in Pool Filter? Here’s Why

There is nothing more frustrating than a poorly running pool. You spend thousands of dollars every year buying pool chemicals only to go outside and see your water has turned green after a hard rain. Your immediate response is to throw some chlorine in the pool, which many of us do blindly without even testing the water.

If you’re having issues with mustard algae, cloudiness, or pressure with your sand pool filter, you may need to replace the sand. Recently, we have had issues with our pool pump keeping prime and maintaining pressure. The mustard algae has been out of control, leading us to believe something else was wrong. We weren’t expecting what we found next.

Why is There Sand in My Pool?

Dirty pool overrun with mustard algae due to a calcified pool filter.

If your pool is equipped with a sand filter, and you start to a brown residue in your pool, your first thought is that the filter is leaking sand back into the pool. That is most likely not the case. What you are seeing is mustard algae that is caused by a sand filter that is no longer properly filtering the water.

In order for sand to leak back into your pool from the filter, the internal components such as the laterals would have to be damaged. It’s understandable why anyone would think it’s sand in the pool, but you’ll notice immediately when you sweep the pool how thin the brown and yellow pollen like substance is.

This is a very common mistake made by many homeowners, and your next step would be to replace the sand in your filter or replace the complete sand filter housing due to calcification.

Replacing Sand in Your Pool Filter

After watching a few videos online, we were ready to tackle changing out the sand in our pool filter. The tutorials online showed someone draining the water, easily vacuuming out the sand in the filter with a wet/dry vac, and then adding back new sand. How hard could that be?

We removed the drain at the bottom of our pool filter, but the water was not draining. The water in our filter was stinky and dirty. After closer inspection of the inside of the filter, we noticed the sand was as hard as concrete.

Now everything was starting to make sense as to why we have had so many issues for the past few years. The sand in our pool filter had calcified due to poor pool chemical maintenance.

What Causes Sand to Calcify or Harden?

We were told by our local pool supply company that sand will calcify or harden in your filter if the PH levels in the pool water are not properly maintained. If you keep up with your chemicals properly, the sand in your filter should still be loose and free flowing many years down the road when it needs to be changed.

What To Do If Your Sand Has Calcified?

Calcified hardened sand inside Pentair sand pool filter. The sand is hard as concrete.
Calcified hardened sand inside Pentair sand pool filter. The sand is hard as concrete.

If the sand is hard in your pool filter, you will most likely need to replace the filter. When you remove the multiport valve on top, there isn’t much working room to get inside the filter to break up calcified sand.

This Bosch chipper hammer would not break up the calcified sand in our pool filter. Our only option was to replace the filter.
This Bosch chipper hammer would not break up the calcified sand in our pool filter. Our only option was to replace the filter.

Our sand was so hard that we could not break it up with a Bosch chipping hammer drill.

I Cut the Top of The Sand Pool Filter Off

Top cut off of Pentair sand pool filter showing the calcified sand inside.
Top cut off of Pentair sand pool filter showing the calcified sand inside.

I took a sawzall and cut off the top part of our sand filter tank, so I could get a better look inside. This allowed me some additional working room, so I could break up the large chunk of calcified sand, so we could remove the filter.

After chipping away for thirty minutes or so, we realized the sand wasn’t going to give way, so we yanked the filter out in one-piece with our Polaris Ranger.

How Much is the Sand for a Pool Filter?

Our pool uses the Pentair TA 100D sand pool filter, which holds 600 LBS of sand. We had to purchase (12) 50 LB bags of pool sand at $12 a bag, which cost us around $150 with tax.

How Much to Replace a Sand Pool Filter?

New Pentair TA 100D sand pool filter with new multiport valve and stand.
New Pentair TA 100D sand pool filter with new multiport valve and stand.

The Pentair TA100D sand pool filter was the big purchase, coming in at around $1,500 out the door. This is for the part only and did not include any labor to have it installed. We tackled the installation ourselves, and it was not a hard job at all.

The new Pentair TA100D sand pool filter comes with the following:

  • new tank
  • new stand
  • new multiport valve and collar
  • cover or diverter for center pipe when adding new sand
2” slip joint union
2” slip joint union

Any 2” plumbing parts we needed we got at Home Depot, but your local pool supply shop will most likely sell them as well. I installed quick release slip joint connections when plumbing the new filter, so I could remove the multiport valve in the future without cutting pipe.

Adding Sand to Your New Pool Filter

Top opening on new Pentair sand filter tank. Inside if the center pipe the multiport valve sits over along with the attached laterals at the bottom.
Top opening on new Pentair sand filter tank. Inside if the center pipe the multiport valve sits over along with the attached laterals at the bottom.

When adding sand to your new pool filter, first fill up the filter about halfway with water. This will prevent the laterals at the bottom from being crushed or damaged under the weight of dry sand. The new filter will also come with a black cover to prevent sand from going down the center pipe when pouring in sand.

The manual mentioned something about leaving around 11 inches of room at the top of your filter. After adding 600 LBS of sand to a half filled tank of water, we ended up with that exact amount of room at the top of our filter, so we left it as is.

How Often Should Sand be Changed in Your Pool Filter?

How often you change the sand in your pool filter depends on the size of your filter. A large pool filter like ours which holds 600 LBS of sand should be replaced every 5–7 years. Our pool guy said smaller sand filters might need to be changed every 1–4 years, depending on the exact size.

Backwashing a Sand Pool Filter

In general, your sand pool filter should be running around 10 PSI on the gauge when operating correctly. Once this reading goes up to 20 PSI, it is time to backwash the filter. A good rule of thumb is when your filter reads 10 PSI above your normal running PSI, it’s time to backwash.

Our Pool After Replacing the Sand Filter

Here is the final result after new paint, fresh water, and replacing our pool sand filter. The water is crystal clear and ready for summer. What a difference!

The picture above shows our pool after we drained the water, repainted the pool, and installed the new sand pool filter. The algae had taken over to the point it could not be cleaned off, so we had to repaint the surface. Daily applications of chlorine and acid would not clear the water up, which lead us back to the sand filter, which was not filtering anything at all.

The new sand filter has been running perfect, and the water is maintaining its clarity with minimal effort. Chemical maintenance is minimal when you’re not fighting against a bad filter. The learning was huge during this process, but it has better educated us on how to balance chemical levels in a pool, especially pH.

Conclusion

The reason I wrote this quick article was to help provide some guidance to anyone in the same situation we were in. We tried replacing the sand in our pool filter on a holiday weekend and did not know what to do moving forward.

I can tell you right now, if the sand in your filter has hardened up like ours did, you will not be able to break it up. Just replace the complete sand filter and get back to enjoying your favorite beverage poolside.

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