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[1][2]. 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 existing pool water and refill it 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
In general, it is safer to exchange water in a pool rather than drain. We’d like to share a method done from time to time by members of TFP.[3] 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 in it 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?
Prior to exchanging the pool water to fresh, you need to determine where is the effluent (pool water you remove) going? Some municipalities have requirements. Be sure to research that. In most areas, it is easiest and best to drain to your sewer clean out at your home. If the water has salt in it, be wary of draining to your grass or plants. It may do them harm.
What pump to use?
You also will need a pump to remove the water from the pool. It is not advised to use your pool pump. It is a fairly expensive piece of equipment and if by chance it loses prime during the process you could damage it. A low power (1/3-1/2 hp) submersible utility pump is a good choice.
The rate at which it pumps is very dependent on the hose size and length you use to direct the effluent. If you use a garden hose to 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 deep end of the pool or from near the surface of the pool 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 while pumping from a top step or near the surface is recommended.
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:[4]
- Pool Water TDS (PWTDS)
- Pool Water Temperature (PWT)
- Fill Water TDS (FWTDS)
- Fill Water Temperature (FWT)
DeltaT = (PWT - (PWTDS / 165)) - (FWT - (FWTDS / 165))
If DeltaT is positive, you want to fill at the lowest point, extract at the surface, and at 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 you can fill 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 disabled 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 there are just so many things that 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 consider it read Tarp method in-place water replacement thread first.
- ↑ https://www.nbcnews.com/video/familys-pool-pops-out-of-ground-after-its-drained-55163459743
- ↑ https://www.troublefreepool.com/threads/this-is-why-you-never-drain-your-pool.236979/#post-2074211
- ↑ https://www.troublefreepool.com/threads/differential-draining-of-pool-water.179895/
- ↑ https://www.troublefreepool.com/threads/differential-draining-of-pool-water.179895/post-1988800
