By The Zagosa Team. Published .
Reviewed or updated .
You can, in theory, start dropping bromine into a chlorinated tub, but it is one of those jobs that looks simple and turns out not to be. The two chemistries clash, the test strips go unreliable, and you end up resetting the whole lot anyway. A partial drain and refresh is the smarter route.
Why a clean switch beats a straight swap
Free chlorine and free bromine are measured by different reactions. A strip built for one gives nonsense for the other, and combined strips are forgiving rather than accurate.
More to the point, the cyanuric acid (stabiliser) you have built up under a chlorine routine does nothing for bromine. It simply sits there raising your TDS and your alkalinity demand.
The 24-hour switch routine
Drain to a quarter full, then top back up with fresh water. That single dilution drops the residual chlorine and stabiliser to a level the new bromine will not react badly against.
Set up the bromine feeder, or a starter dose of sodium bromide, as the bottle directs. Bromide in water needs an oxidiser (chlorine or non-chlorine shock) to convert into active bromine, so add a one-off small chlorine dose to kick the cycle into life.
Let it run for 12 hours with the jets on the longest filter cycle. Test free bromine in the morning and adjust to 4 to 6 ppm.
What the first week looks like
The water will feel a touch different. Bromine has a softer feel and a less sharp smell.
You may need to dose a little more often at first while the bromide reservoir in the water builds to a steady level.
After a week you should be on a normal bromine routine: a feeder topped up monthly, weekly tests, and a non-chlorine shock every fortnight.
Common questions
What if I run out of bromine and only have chlorine?
A one-off chlorine top-up in a bromine tub is fine. The chlorine simply oxidises and the bromide reservoir carries on doing its job. Just do not make a habit of it.
Is there a tablet that does both?
BCDMH (bromochlorodimethylhydantoin) tablets contain both chlorine and bromine. Some commercial tubs use them to sidestep the choice entirely, but they need a feeder rated for them.