Partial drain and refill: how much water actually has to go

How do you work out how much water to replace?

Start with the number you are trying to fix and the number you want. If cyanuric acid is 100 and you want it near 50, you are replacing about half the water. If calcium hardness is 600 and you want it near 400, you are replacing about a third. The relationship is proportional and it is that simple.

Then correct for the fill water, because ours is not blank. North Texas fill water arrives with meaningful calcium and alkalinity already in it, so a partial drain aimed at hardness never gets you as far as the arithmetic suggests. Cyanuric acid is different: fill water has none, so stabilizer dilution behaves exactly as the math predicts.

Then decide whether one exchange gets you there. Sometimes the honest answer is that a single partial drain gets hardness from 700 to 500 and that is genuinely as good as it is going to get this year. That is still worth doing, and it is worth knowing in advance rather than being disappointed by.

Get a full test panel before and after. Without the before, you cannot size it. Without the after, you do not know what you bought.

How far down can the water safely go?

Less far than most people assume, and the limit is set by your soil and your shell, not by your patience. Water is what holds a pool shell down in the ground. Take enough of it out while the surrounding soil is saturated and the ground can push the shell up.

For most corrections you do not need to go deep. Replacing a third to a half of the volume in a typical residential pool means taking the level down a few feet, which is a very different proposition from an empty shell.

Check the hydrostatic relief valve in the main drain first. That is the plug that lets groundwater push into the pool rather than under it. On older pools it is frequently seized, plugged with debris, or plastered over during a resurface, and finding that out during a drain is the wrong time.

Pick your weather. Dry ground and cool air. Not the week after four inches of rain, and not the middle of July, when a plaster surface exposed above the waterline in direct sun can craze in an afternoon.

What is a no drain exchange, and when does it help?

It is a technique for swapping water without ever dropping the level much. Fresh water goes in at the surface at one end while a submersible pump removes water from the deep end at a similar rate, exploiting the fact that the incoming water and the pool water do not mix instantly.

The advantage is that the shell stays full and supported the entire time, which removes the structural risk almost completely. The disadvantage is that it is less efficient than a straight drain and refill, because some fresh water inevitably mixes and gets pumped straight back out.

It is a good fit for pools where a drain is genuinely risky, which in this area means pools sitting low in a yard, pools with a high water table nearby, and any pool being worked on after a wet stretch. It also suits vinyl liner pools, where dropping the level risks the liner shrinking or floating.

What has to be done after the refill?

Rebalance in the right order, because you now have water that is part old and part brand new. Total alkalinity first, then pH, then calcium hardness if it is now too low, then stabilizer, then chlorine.

Expect stabilizer to need topping back up if you drained for hardness. You lowered everything, including the one you wanted to keep. Test rather than assuming, and add stabilizer slowly through the skimmer with the pump running.

Watch pH closely for the first week. Fresh fill water often arrives at a different pH and alkalinity than the pool was holding, and new water in a plaster pool can also pick up alkalinity from the surface for a while.

And if the fill source has any iron in it, which is common on well fed properties out toward Argyle and Justin, add sequestrant to the water as it comes in rather than after it is in.

What does it cost you beyond the water bill?

Time, mostly. Filling a residential pool a third of the way takes many hours on a garden hose and less on a dedicated line, and the pool is out of service while it happens.

Chemicals for the rebalance, which on a large exchange with our fill water is not trivial. Plan for alkalinity adjustment, stabilizer, and possibly hardness on a vinyl pool.

And a small amount of risk that is entirely manageable if the timing and the valve check are done properly and entirely unmanageable if they are not. That is the part worth paying somebody for on a deep exchange.