If you are still dropping chlorine tablets straight into the skimmer, your pool is paying for it in ways you cannot see. Every time the pump shuts off, a pocket of highly concentrated chlorine sits against the pump seals, the heater, and the metal parts of your equipment. An automatic chlorinator fixes that. It doses chlorine in a slow, steady trickle while the pump runs — and it does the remembering so you do not have to.
The catch is that "automatic chlorinator" covers three very different products, and buying the wrong one means either an install headache or a feeder that can never keep up with your pool. Here is how in-line, off-line, and floating feeders actually differ, and how to match one to your pool.
What a chlorinator does (and what it doesn't)
A tablet chlorinator holds a stack of slow-dissolving trichlor tablets and lets a controlled amount of water flow past them. You turn a dial to set how fast the water erodes the tablets, which sets your chlorine output. Fill it roughly once a week or so, adjust the dial as the weather changes, and your free chlorine stays in range instead of swinging from zero to shock and back.
What it will not do is balance the rest of your water. It does not manage pH, total alkalinity, or calcium hardness, and it is not a salt system — a salt chlorine generator makes its own chlorine from dissolved salt and needs no tablets at all. A tablet feeder is simply a smarter, safer way to deliver the chlorine you are already buying. You will still test your water and still shock occasionally after heavy rain or a crowded weekend.
The three types, side by side
Nearly every automatic chlorinator falls into one of three families. The differences come down to how it installs, how much chlorine it can hold, and how precisely it feeds.
| Type | How it installs | Capacity & control | Best for |
|---|---|---|---|
| In-line | Plumbed permanently into the return line as the last device before the water re-enters the pool. | Largest tablet capacity, most consistent feed, precise dial control. | New builds and permanent setups where you can cut into the pipe. |
| Off-line | Sits beside the equipment pad and taps in with two small saddle fittings — no cutting the main return. | Medium-to-large capacity, good control, a little more tubing to manage. | Retrofits, crowded pads, or existing plumbing you would rather not open up. |
| Floating | None — you load tablets and set it in the water. | Small capacity, rough control (you crack the vents open a bit). | Small above-ground pools, spas, and short-term or backup use. |
Floating feeders are cheap and need no tools, so they are tempting. The trade-off is that they drift. They pile chlorine against a vinyl liner, park in front of steps, and get sucked toward the skimmer, which is exactly the kind of concentrated exposure that fades liners and etches surfaces. For anything larger than a small above-ground pool, a plumbed feeder is the better long-term answer.
How to choose the right one
Four questions get you to the answer quickly:
- Can you cut into your plumbing? If yes, an in-line feeder is the cleanest, highest-capacity choice. If the pad is crowded or you would rather not open the pipe, go off-line.
- How big is the pool? Match the feeder's rated capacity to your gallons. A unit built for 5,000–50,000 gallons covers most residential pools, but a very large pool needs a feeder sized to keep up in mid-summer, not one you have to refill every three days.
- Above-ground or in-ground? Above-ground pools usually pair well with an off-line or a quality floating feeder. In-ground pools with a full pad are ideal for in-line units.
- Do you have a heater? If so, the chlorinator must come after the heater in the plumbing order, and you want a check valve between them (more on that below).
Sizing, tablets, and a safety rule that matters
Use 3 in. stabilized trichlor tablets in a pool feeder (1 in. tabs are for spas and very small feeders). Stabilized tablets carry their own cyanuric acid, which protects chlorine from the sun — helpful, but over a long season the CYA can creep up and start locking your chlorine out. If your water starts needing more and more chlorine to hold a reading, test your stabilizer and dilute if it climbs too high. Our guide on lowering cyanuric acid walks through that.
Never mix chemical types in one feeder. Loading calcium hypochlorite or any other product into a feeder that has held trichlor can trigger a violent reaction or fire. One feeder, one type of tablet — always. And a chlorinator does not replace balanced water: keep an eye on your full set of readings with our rundown of target pool chemical levels.
Installation & the check-valve detail people skip
The chlorinator is always the last piece of equipment the water passes through — filter, then heater, then chlorinator, then back to the pool. That order keeps concentrated chlorine out of the filter and away from the heater's metal core.
Here is the step most DIYers miss: install a check valve between the heater and the chlorinator. When the pump stops, gravity and pressure can push a slug of chlorine-rich water backward into the heat exchanger, where it quietly eats copper. A check valve blocks that backflow and can add years to a heater. Off-line feeders should be mounted at or just below the pipe and plumbed exactly per the saddle-fitting instructions so the feed rate stays predictable.
Five habits that quietly wreck equipment
- Tablets in the skimmer or pump basket. The single most common mistake. It corrodes the pump, heater, and any metal in the loop every time the system is off.
- Running the dial wide open. More chlorine is not better. Dial to your test results, not to "maximum."
- Skipping the check valve. Cheap part, expensive lesson if you have a heater.
- Mixing tablet types. Covered above — it is a genuine safety hazard, not just a no-no.
- Ignoring the lid o-ring. A dry, cracked o-ring leaks chlorine gas that corrodes everything nearby. Lubricate and replace it on schedule; see our o-ring guide.
PST picks
Three feeders that cover the common situations — a permanent in-line unit, a no-cut off-line option, and the classic tablet feeder for tight pads and retrofits.
Prefer to skip tablets entirely? A salt chlorine generator is worth a look — and if you are weighing the chemistry itself, our comparison of liquid chlorine vs. tablets lays out the trade-offs.
Bottom line
If you can cut the pipe, buy in-line. If you cannot or would rather not, buy off-line. Save floating feeders for small pools, spas, and backup duty. Whatever you choose, size it to your gallons, feed it one type of 3 in. tablet, protect the heater with a check valve — and stop throwing tabs in the skimmer. Do that, and your chlorine takes care of itself while your equipment lasts longer.