Draining

Reasons behind draining a pool

A pool owner may wish to replace the pool water with fresh water for several reasons, including elevated CYA, CH, high ammonia, a complete algae swamp, maintenance requirements, seasonal closing, etc.

Precautions

Draining pools entails risk. Vinyl liner pools should not be drained below, leaving 12-18” of water in the shallow end. The liner can shift and get destroyed. Fiberglass pools should not be drained unless significant precautions are taken, as they can pop out of the ground. Gunite/plaster pools can also pop out of the ground if local water levels are high. Also, plaster can fail if let to dry out in the hot sun.

Draining any above-ground pool is a risk due to the fragility of its walls and supports. For advice about draining an above-ground pool, describe your situation and post pictures in the Above Ground Pool section of the Forum.

Consider the time of year if you periodically need to drain the pool due to accumulating CH or CYA. For example, pools in the southwest typically see high CH levels every 18 months or so, ideally like to exchange water in the winter when the sun isn't so intense on their plaster.

Here is where a contractor drained a fiberglass pool before a rain storm and the pool popped out of the ground from hydrostatic pressure.

How do you drain a pool

There are multiple ways to drain and refill existing pool water with fresh water. Below are the most popular.

DE or Sand filters with MPV that drain to waste

• Turn the pump OFF
• Turn MPV to WASTE
• Turn the pump ON
• Monitor the pump and turn the pump OFF immediately if the water drops below the skimmers and the pump runs dry. Depending if you have an operational main drain, your pump may continue to drain below the skimmer level
• When water is at the desired level, turn the pump OFF
• Turn the MPV back to FILTER

Submersible Pump

Use a submersible utility pump connected to a hose.

Rent a Gas Powered Pump

You can rent a gas-powered pump with a large hose to drain the pool more rapidly.

The old siphon method

• "You know, like you used to use when you stole gas from your neighbor's car"...
• Fill a garden hose with water and then put one end of a garden hose in the pool and the other downhill somewhere.

No Drain Water Exchange

It is generally safer to exchange water in a pool than drain. We’d like to share a method done from time to time by members of TFP. The process to exchange water depends on determining a few factors:

• The temperature of your fill water versus the pool water temperature.
• Does the pool water have salt above 2000 PPM or a Calcium Hardness of 800 PPM or more?
• Essentially, is the pool water high in Total Dissolved Solids (TDS)?

Where will the effluent be discharged?

Before exchanging the pool water for fresh, you need to determine where the effluent (pool water you remove) is going. Some municipalities have requirements. Be sure to research that. It is easiest and best to drain to your home's sewer cleanout in most areas. If the water has salt in it, and most pool water does, be wary of draining it to your grass or plants. It may harm them.

What pump to use?

You will also need a pump to remove the water from the pool. It is not advisable to use your pool pump. It is an expensive piece of equipment and can be damaged if it loses its prime during the draining process. A low-power (1/3-1/2 hp) submersible utility pump is a good choice.

The rate it pumps depends on the hose size and length you use to direct the effluent. If you use a garden hose into a sewer cleanout, expect a flow rate of 6-9 gallons per minute.

Pump from the deep end or near the surface?

Whether you pump from the pool's deep end or near the pool's surface depends on your fill and pool water characteristics.

If the pool water is much colder (> 20F) than the fill water, adding water to the deep end is recommended while pumping from a top step or near the surface.

Put the pump in the deep end and fill from the shallow end if your fill water is nearly the same temperature as the pool water, you have a saltwater pool or have very high CH. Put the fill hose in the skimmer, if you have one, in the shallow end. If there is no skimmer, use a bucket to put the water hose in and have the top above the pool water surface. Be sure to secure the hose to the bucket.

Replacing water in-place process depends on several factors:

• Pool Water TDS (PWTDS)
• Pool Water Temperature (PWT)

• Fill Water TDS (FWTDS)
• Fill Water Temperature (FWT)

Note: If you don't have a way to measure TDS, you can do the following:

TDS = Calcium Hardness (CH) + Total Alkalinity(TA) + CYA + Salt. (it is more complex, but for purposes of draining, this is sufficient).

If you have a chlorine pool, measure your CH, TA, CYA and assume your salt is 1000. Add them together to determine PWTDS.

If you have a salt water pool, measure your CH, TA, CYA and Salt. Add them together to determine PWTDS.

For fill water, you can Check your city’s annual Consumer Confidence Report (CCR), available on your water utility’s website or by request. If that is not available, you can measure your fill water for CH and TA, add them together and use that for Fill Water TDS. (Typical municipal supply salt levels are 20-50, and CYA is not naturally occurring or added to municipal water supplies).

These short hand TDS calculations will work just fine. If you want complete TDS calculations, see the last section in this article.

Determining DeltaT and where to add and pump out:

DeltaT = (PWT - (PWTDS / 165)) - (FWT - (FWTDS / 165))

If DeltaT is positive, you want to fill at the lowest point, extract at the surface, and the greatest distance from the lowest point.

If DeltaT is negative, you want to extract at the lowest point, fill at the surface, and fill at the greatest distance from the lowest point. This method is preferred as it is easier to do with a drain pump and can fill it inside one of the skimmers.

If |DeltaT| < 5, there will be much mixing.

Salt is the largest component of TDS. No matter what chlorination method you use, salt is added to the pool water and can be significant after many years. Every 1000 ppm of salt is worth about 6F in temperature difference.

Balance the water flow out and in

Be sure to balance the water out and water in so the pool level stays the same.

When you have the pump you will use, take the effluent hose and fill a 5-gallon bucket while timing it. Calculate your gallons per minute (GPM) from that. Then, you can estimate how long the process will take.

To account for errors, it is suggested that you exchange 5-10% more water than needed to reduce your CYA/CH/etc.

Be sure your pool pump is turned off during this process.

Once started, do not stop until complete

Once started, do not stop until you have exchanged the amount of water you wish.

Tarp Method In-place Water Replacement

Warning—We are wary of recommending the tarp method as so many things can go wrong, and it is dangerous to have a tarp of that size floating in the pool. Someone can fall in and drown because they got caught up in a submerged tarp. Then, there’s just the sheer weight of water sitting against a tarp.

If you are considering it, read the Tarp method in-place water replacement thread first.

Complete TDS Calculations

The simple TDS calculations provide plenty of precision for the no drain water exchange. They are provided if you like the deep end.

• It is best if you test and use your actual Salt content in the pool.
• You can and should test your pool water and fill water for CH, TA, and Salt.
• If you do not use borates in your pool, assume borates in the pool and in your fill water are zero.
• You should assume that CYA is zero in your fill water, no need to test as it is not naturally occurring.

• Salt = (CH*1.1678) + (TA*1.6859) + (Borate*2.6976)

• Sodium = 23*((TA-CYA/3)*2/100 + CYA/129 + Salt/58.4 - 2*CH/100)

• TDS = CH*(40/100) + (TA - CYA/3)*2*(61/100) + CYA*(128/129) + Borate*(61.8/10.8) + Salt*(35.5/58.4) + Sodium;