Reasons behind draining a pool
There are several reasons a pool owner may wish to replace the pool water with fresh water. Elevated CYA, CH, high ammonia, a complete algal swamp, 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]. Also, plaster can fail if let to dry out in a hot sun.
How do you drain a pool
There are multiple ways to drain existing pool water and refill with fresh water. Below are the most popular.
Lower Water then Refill
Drain alot of your water out.......then toss in a hose and refill.
No Drain Water Exchange
In general, it is safer to exchange water in a pool rather than drain. The process to exchange entails 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).
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.
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) sump pump is a good choice. The rate at which it pumps is very dependent on what hose size and length you use to direct the effluent. If using a garden hose to a sewer cleanout, expect a flow rate of 6-9 gallons per minute.
To determine 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 characteristic.
If your fill water is much colder (>20F) then adding that water to the deep end while pumping from a top step or near the surface is recommended.
If your fill water is nearly the same temperature as the pool water, you have a saltwater pool, or have very high CH, then reverse it. Put the pump in the deep end and fill from the shallow end. If you have a skimmer in the shallow end, put the fill hose in it. If no skimmer, then use a bucket to put the water hose in and have the top of the bucket above the pool water surface. Be sure to secure the hose to the bucket.
Be sure to balance the water out and water in so the pool level stays the same. Once you have that, 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 to run the process. It is suggested to exchange 5-10% more water than needed to reduce your CYA/ CH/etc to account for errors.
Also be sure your pool pump is disabled during this process. Once started do not stop until you have exchanged the amount of water you wish.
A very good thread on the subject of water exchange is [Water Exchange]
