It has a lot of steps and to a newbee it seems that any one error could sabatage the whole process
"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 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.
Pump from the deep end or near the surface?
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.
Adding water to the deep end while pumping from a top step or near the surface is recommended if your fill water is much colder (>20F) then the pool water.
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 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.
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, then you want to fill at the lowest point and extract at the surface and at the greatest distance from the lowest point.
If DeltaT is negative, then you want to extract at the lowest point and fill at the surface and at the greatest distance from the lowest point. <-Preferred as it is easier to do with a drain pump and you can fill inside one of the skimmers.
If |DeltaT| < 5, then there will be a lot of 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 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.
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."
