Every pool and spa reaches a point where the water itself is the problem. No amount of chemical adjustment fixes it. The question is when you’ve actually reached that point — and whether the answer is a full drain, a partial drain, or nothing at all.
This is one of the most common questions we hear from facility managers, and it’s the subject of an ongoing discussion in the CAI Members Open Forum about how often spas should be drained. Here’s how we think about it.
Why water goes bad
Pool water doesn’t wear out all at once. It accumulates things that filtration and sanitizer can’t remove:
- Total dissolved solids (TDS) creep up with every chemical addition, every bather, and every top-off.
- Cyanuric acid (CYA) builds up from stabilized chlorine, and at high levels it locks up your free chlorine so it can’t sanitize effectively.
- Calcium hardness climbs, driving scale on tile, heaters, and equipment.
- Metals, oils, and organic residue collect over time, feeding stains and biofilm.
At some point the water is carrying so much dissolved material that your chemistry stops responding normally. That’s when draining — full or partial — is the fix.
Spas: a different schedule than pools
Spas are where draining matters most, because the math is unforgiving. A spa is a small volume of hot water with a heavy bather load, so contaminants concentrate fast.
A good rule of thumb for spas: spa gallons ÷ 3 ÷ average bathers per day = days between drains. A 500-gallon spa averaging 10 bathers a day works out to about 17 days. A lightly used spa stretches further; a busy one needs draining more often. For most commercial spas, that means a full drain every two to four months — and the busy ones more often than that.
For a spa, the decision can also be driven by what you see and test: cloudy water that won’t clear, a chlorine demand that never quits, or TDS and CYA readings that keep climbing. And don’t forget cold spas and cold plunge tubs — they need the same drain discipline as hot spas, even though the water is cold.
Pools: rarely a full drain
Pools are a different story. A full drain of a pool is a big event — downtime, refill cost, rebalancing, reheating — and most pools go years between full drains. Usually, a partial drain and dilution is the smarter move: drop the water level by a foot or two, refill with fresh water, and bring TDS and CYA back down without emptying the vessel.
A full drain makes sense when:
- The chemistry is unrecoverable — TDS more than about 1,500 ppm above your source water, or CYA above roughly 100 ppm, and dilution alone won’t get you there.
- You’re doing major work — replastering, resurfacing, tile replacement, or structural repair.
- Staining or scale won’t respond to treatment in the water.
The danger of draining wrong
This is the part people skip, and it’s the part that can cost the most. A drained pool is a vulnerable structure:
- Hydrostatic pressure. Groundwater pushes up on an empty pool shell. With a high water table, a drained pool can literally lift or pop out of the ground. Never drain a pool after heavy rain, and always know your groundwater situation first.
- Check your hydrostatic relief valves before draining. They’re the safety valve that lets groundwater in rather than letting it lift the shell — and they need to be working.
- Plaster and sun don’t mix. Exposed plaster can crack and dry out if it’s left baking in the sun. Plan the work so the pool isn’t sitting empty and exposed any longer than it has to be.
- Vinyl liners can shrink and shift when the water comes out. A drained vinyl pool needs a plan for getting water back in promptly.
Never drain a pool without a plan for what’s happening while it’s empty, and never drain one on a guess. Get the groundwater and structural picture first.
Use the downtime
If the pool is drained, use the time. Inspect the waterline tile and grout, the expansion joints, the skimmers, and the main drain covers — including VGBA compliance. A drained pool is the one chance you get to see everything, and it’s the right time to assess whether the plaster needs an acid wash or a full resurface.
A note for Georgia facilities
Here in Georgia, there’s no winter drain. We keep water in and balanced year-round — draining for the season isn’t a thing the way it is up north. That makes the drain decision purely a water-quality and maintenance question, not a calendar one.
When in doubt, test first
The drain decision should come from your test readings, not the calendar. TDS, CYA, and calcium hardness tell you whether the water is done. If the readings are in range and the water is clear and responsive, leave it alone. If they’re not — and dilution won’t fix them — it’s time to drain.
If you’re not sure where your water stands, we can test it and give you a straight answer on whether a drain is needed, and whether a partial drain will do the job.
Matthew Selman is the owner of Southern Aquatic Services, a commercial pool service company in North Georgia, and a CPO instructor who has trained more than 1,000 pool operators.