Should a Pool Main Drain Be Open or Closed?

Whether a pool main drain should be open or closed as a normal part of pool maintenance is not an unreasonable question that many pool owners have and is certainly something I asked about when I first bought my house with an in-ground pool.

With so many pool valves and settings in the pool plumbing system, it is no wonder that it can be confusing for owners of in-ground pools at first.

In an ideal world, the pool main drain should be open throughout the swimming season and only closed when your pool is closed in winter. There are reasons why you may want it at least partially closed in order such as to give more suction to your pool’s skimmers and also while you are vacuuming.

Table of Contents

• What is a pool main drain?
• Where is the pool main drain valve?
• How do I know if my main drain is open or closed?
• Should skimmer and main drain be open?
• When should the main drain be closed?
• When should the main drain be partly open?
• Pool Maintenance Course
• How to empty a pool using the main drain

What is a pool main drain?

The main drain in a swimming pool is located at the bottom of the pool (the lowest point).

During normal operation, it draws in water from the bottom of the pool to be passed through the filtration system as part of the daily cleaning cycle.

It can also be used in combination with the pool pump to empty the pool completely if necessary.

One other function of the main drain is that in many pool setups it is also linked via a pipe to one of the pool skimmers. If the pool water level drops below the bottom of the skimmer opening a diverter valve in the skimmer operates and pool water is pulled from the main drain pipe so that the pump doesn’t run dry and become damaged.

See my post Why are there two holes at the bottom of a pool skimmer for more information.

Where is the pool main drain valve?

The pool main drain valve will be in the pipework on the suction side of your pool pump.

To find which it is, follow the pipework from the pump backward. You will probably come to a junction where two, three or more pipes join into one before going to the pump. If you follow these pipes backward then each should have a valve in them (probably a ball valve). One of these will be the valve that controls the main drain.

Where you have three it is common practice for the main drain’s valve to be the center port. The others would be from each of the skimmers. If you have a built-in spa you are likely to have one for this too.

To find out which is is the main drain valve, if you have two, with the pump running, close one of them and leave the other open. Then look in your skimmer/s and see if water is being drawn into them. If you can still see water flow into the pool skimmers then the open valve controls the skimmers and the close valve is the main drain (and vice versa).

If the pool is being built then ask your pool builder to label each valve. A good pool builder should do this as a matter of course.

In a few cases, in some pools there may be just one diverter valve that controls where the suction comes from which is turned either towards the skimmer/s or the main pool drain.

How do I know if my main drain is open or closed?

Once you find the valve for the pool main drain (see above if you are unsure which it is) then you can see if the pool valves open or closed. The valve will be open when the valve handle runs in the same direction as the pipe and closed when it is at 90º to the pipe.

Should skimmer and main drain be open?

Under normal circumstances, both the skimmer and the main drain diverter valves should be open. This means, that when the filtering system is running, the pool water flow will be into both.

That way the skimmer/s will pull in any debris floating on the pool surface and the main drain will also be pulling water in from the bottom. So with water flowing into both skimmers and the main drain, a good circulation should be created ensuring all of the pool water is filtered over time.

When should the main drain be closed?

There are two reasons why you may want the main drain closed:

• If you are vacuuming the pool via a skimmer and want maximum suction then closing the valve controlling the main drain will divert more suction to the skimmer/s. See my post How to vacuum a pool
• When you close your pool for the winter months (assuming you live in a climate that is cold enough to need to close the pool) then it would normally be best practice to have the main drain fully closed.

When should the main drain be partly open?

If your skimmers have insufficient water flow to adequately strain debris floating on the pool’s surface then you can partially close the valve to the main drain until the water flows into the skimmers with sufficient force.

If the valve is partially open and at 45º to the pipe, this will mean that there is now just 50% water flowing from the main drain.

How to empty a pool using the main drain

In order to drain the pool you should follow the following procedure:

• Adjust the pool valves.
  Close the valve/s that control suction from the skimmers and open the valve that controls the suction from the main drain at the bottom of the pool.
  If you didn’t do this, and left the skimmer valve open, once the level in the pool reached the bottom of the skimmer’s opening then air would be drawn into the system (possibly damaging your pump) and no further water would be expelled from the pool.
• Adjust the multiport valve.
  Turn the multiport valve of the pool filter to the “waste” position. This will make the water flow out of the system either into your sewage system or into your yard, depending on your particular setup.
  If you were to leave the multiport valve in the “filter” position then you wouldn’t drain water out of the pool as it would just be pumped straight back into the pool. The same would be true with the lever in the “recirculate” position. See my post the Pool Filter Settings Explained.
• Run out backwash hose.
  Depending on whether your pool is set up to drain into the sewerage system or into your yard then you may need to run out a backwash hose (if you have one) into your yard to wherever you want the water to drain.
• Run pool’s pump.
  Now switch on the pool’s pump and run it until the pool is empty. Make sure you monitor this so that the pump does not run dry.

You would also use this method to drain your pool below the level of the skimmer or after heavy rain.

NOTE: If you have a fiberglass pool liner then expert advice should be sought by the pool owner before emptying the pool as it is possible to damage the liner by emptying it.

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