Twice a year, two different questions
The pool-chemical season
Pool chemicals get asked about twice a year, and it is not the same question both times. In spring: is last season's bucket still good. In autumn: how do I store what is left so that spring is not a question again.
Updated
The spring question: is last year's bucket still good
A closed bucket that spent the winter in a shed is not the same object as a closed bucket that spent the winter in a basement, and the difference is usually moisture rather than months. Solid chlorine — tablets and granular shock alike — is hygroscopic. It pulls water vapor straight out of the air, and it does not need liquid water or a leak to do it. A lid that was not fully seated, or four months of warm afternoons and cold nights in a humid shed, is enough.
What that looks like when the lid comes off, and all of it is observation only:
- Tablets fused into a single block, or grown together where they were touching.
- Granules caked, clumped or crusted instead of pouring loose.
- A strong chlorine smell coming off a container that should be close to odorless closed.
- A bucket that has swollen, bulged or distorted, or a lid that has become hard to free.
- Discoloration, or a damp look to a product that should be bone dry.
Any of those means the contents have taken on water and started reacting with themselves. This is a safety matter rather than a performance one, so this page states it and stops: chlorine products that have absorbed moisture can heat on their own, and they can release chlorine gas inside a closed space. Each product page carries the same verdict and the same full stop.
Liquids age differently. Algaecide and acid do not cake; they lose strength, separate into layers, or go to work on their own container. A jug that has bulged, gone soft, or corroded around the cap is telling you something about the age of what is inside it.
On the pool shelf
Hung fastest-first — sealed window on the tag
Test strips are the one with a real date on them
Strips are the exception on this shelf and the one product here that reliably carries a printed expiration date rather than a production code. The reagent pads are dried chemistry on paper, and they lose their calibration to humidity and light whether or not the bottle has ever been opened. That is why the date is printed, why the bottle has a desiccant in it, and why the cap is meant to go back on within a second or two.
Out-of-date strips are worse than no strips, because they fail confidently. A pad that has aged gives you a color and a number, and the number is wrong in a direction you cannot see. Everything you do next is built on it.
The autumn question: storing what is left
Everything a closed-up pool shed does in July is the opposite of what pool chemicals want. Heat, a sealed volume with nowhere for humidity to go, and a dark roof that turns a 90 °F afternoon into a 130 °F interior. These are oxidizers and acids: they gain nothing from warmth and they lose badly to moisture, and the shed is where nearly all of them spend the off-season. Heat and freeze–thaw are the real clock for this shelf more than any other on the site.
The label sentence on almost every one of them says the same thing: cool, dry, well ventilated, out of direct sunlight, in the original container with the lid closed. Read it as a ranking of where the leftovers should spend the winter — and note that for the solids, dry outranks cool. A cold damp shed is worse than a warm dry one.
Two rules that are not negotiable, and both are one-liners:
- Never combine pool chemicals with each other, and never store an open container where anything else can fall or leak into it. Products that look interchangeable are not, and several pairs on this shelf react violently.
- Never move a product out of the container it came in. The original label is the only place the product name, its strength and its storage sentence exist together, and the container itself is built for what is in it.
Beyond that, the honest answer for a lot of this shelf is that solid chlorine bought in August is a poor candidate for storing until May. The tablets keep well in a sealed pail and badly in an opened one, and the opened window on each product page is the number to plan against.
How the verdicts work · everything on the site, ordered by how fast it goes