Why Are There Two Holes at the Bottom of a Pool Skimmer?


I have two skimmers, one has two holes and the other only one. Why are there two holes at the bottom of a pool skimmer was one of the earliest questions I had when I first got my inground pool and started to clean it.

There are two holes at the bottom of a pool skimmer as the first one sends water to the pool pump and the second, known as an equalizer, is there to prevent the pump running dry should the pool level drop below the bottom of the skimmer by drawing water from the pool drain.

Where there are more than one skimmer in a pool, the second usually has one blanked off with a screw-in blanking plate as only one equalizer is required per pool.

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What are the two holes in a pool skimmer?

Why are there two holes at the bottom of a pool skimmer?
Two holes at the bottom of a pool skimmer

What is the purpose of the two holes in a pool skimmer? Well one hole at the bottom of the pool skimmer takes water to the pool filter, via the pool pump, to allow it to be cleaned and then returned to the pool by the return line/s.

The second hole, called an equalizer, connects to either the drain inlet at the bottom of the pool or to a separate inlet a few feet below the surface of the water in the side of the pool.

If the water level in the pool falls below the water entrance at the top of the skimmer, so no more water is running in over the weir door, or if the skimmer basket becomes clogged so water cannot pass through, then water will start to be drawn from the pool via the second hole.

If this didn’t happen then the pump would continue to operate but would start to run dry. Prolonged dry running would cause the pool pump to overheat and fail. This would mean a costly repair which no-one wants.

2nd skimmer with one hole blanked off
My 2nd skimmer with one hole blanked off

You will probably find that, if you have more than one skimmer, the there will actually be two holes in skimmer bottom but only one will have two holes open.

This is because only one is required to have two holes at the bottom of a pool skimmer open in case the water level drops to prevent the pump running dry. The other skimmer/s may actually have two holes but the one closest to the pool is likely to be closed with a blanking plate.

Pool float valve assembly

None of the above will actually happen, should the water level drop in the pool, if your pool does not have a pool float valve assembly installed in the skimmer. Many pools may be designed to use these (which is why there are two holes) but over time people either don’t put them back in after cleaning or they break (or perhaps don’t even know what they are).

A float valve assembly, also known as an equalizer assembly, sits over the the 2 holes at the bottom of the skimmer and effectively does very little until either the pool level drops to below the skimmer or if the skimmer basket becomes so full that no water can be drawn though it (so no water goes to the pump.

When either of these two things happen the float in the valve is sucked down forming a seal and then causes water to be drawn from the pool drain pipe, preventing the pump from running dry.

Hayward Float Valve Assembly
Hayward Float Valve Assembly

If you do not have one I suggest you buy one to fit your particular skimmer. The most common float valve assemblies are:

Which skimmer hole should you use to vacuum the pool?

When vacuuming your pool you should insert one end of the vacuum hose into the hole that goes to the pool pump and filter. If there are two holes at the bottom of a pool skimmer then this is always the one furthest away from the pool. The hole nearest the pool is the one that draws water from the pool drain if the water level drops.

If you were to plug the vacuum hose into the other hole at the bottom of the skimmer it would do no harm as frankly nothing would happen as there is no suction from that hole. You could try vacuuming all day and get nowhere!

Hayward skimmer plate

If you use a skimmer plate into which you attach the vacuum hose so that any debris is caught in the skimmer basket then you do not need to worry about which hole to use.

You can buy skimmer plates very cheaply should you wish to try one such as this Hayward model but they are available for all makes of skimmer.

If you have a dedicated pool vacuum port in the side of the pool then you should use this to vacuum the pool.

Should pool skimmers be on all the time?

Pool skimmers should be used at all times the pump is running in order to remove any debris such as leaves, bugs, pollen and even sun oil that may be floating on the surface of the pool. By used I mean that the valve or valves controlling the flow water from the skimmers to the pump should be open.

Depending on how strong the suction you have then you may also have the valve to the main drain at the bottom of the pool open, at least part way.

pool skimmer water level

Why might the water level in the pool drop?

For the second hole at the bottom of a pool skimmer to come into play then the water level must drop below the level of the skimmer.

There are a number of possible reasons why your pool is losing water:

  1. All pools lose water through evaporation. The rate of evaporation can be quite significant, particularly on warm and windy days with low humidity. I found it quite amazing when I discovered that a pool can lose between 2-4 inches per week due to evaporation alone.
    You can significantly reduce this water loss by using a solar pool cover. These will not only help to keep your pool water warm but also will drastically reduce the rate of evaporation since there will be little water surface exposed to the air.
  2. If you have an automatic top up system then check that the float is not stuck in the up position which will prevent water flowing into the pool to top up the level.
  3. When you backwash a pool the water is pumped out into the garden or the sewer system so the water level will drop due to that. It would be normal practice to top up with a hose (unless you have an automatic top up in 2 above). See my article “How to backwash a pool – step by step
  4. If your pool is very dirty then you may need to vacuum to waste with the filter multiport valve in the waste position. When you do the water level will lower quickly as all the vacuumed water will be going to the garden or the sewer.
  5. If none of the above seem to be the reason then unfortunately it may be that you have a leak either in your pool lining or in your pool pipework.

My top 3 pool cleaning tools

These are the pool cleaning tools I have found the most useful since I have had my pool.

Step and corner vacuum brush

This is a really useful tool for getting into the areas that a standard vacuum head simply cannot reach. Aquatix Pro Pool Step & Corner Vacuum Brush

Leaf rake net

If, like me, you get plenty of leaves at the bottom on your pool then a good leaf rake/net is a must. The Stargoods Pool Skimmer Net gets under the leaves easily.

Robotic Pool Cleaner

These are quite expensive and it was a number of years before I bit the bullet and bought one. I have never regretted it. The Dolphin Nautilus CC Plus is the most recommended on all of the pool forums.

Pool Maintenance Course

When I first bought my house with a pool I knew nothing about how to clean and maintain it. I was recommended Swim University’s Pool Care Handbook so I bought it and have never regretted it. It was probably the best money I spent that year as I have saved thousands by dong it myself.

Pool care handbook

JP in Fuerteventura

I have had hot tubs for many years and a pool for the last 7 years. I had to learn how to clean it, maintain it and fix it the hard way. Since then I have helped many friends and neighbors with their pools and now I want to share everything I have learned with you.

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