Just curious: after brewing a dozen times with no chlorophenol and then you suddenly got it, how do you conclusively know it was due to not filtering your water vs. wild-yeast infection?
I dunno. I haven't had a problem with chlorophenol since then. I have actually had infections several times over the years. None of them tasted like chlorophenol. I wouldn't call that "luck" though.
Of course, these are anecdotes. I won't claim to know anything conclusively. I guess I could run experiments using chlorinated water on purpose to see what happens. However, I'm also not interested in accepting the risk of having to dump a batch if/when it goes chlorophenolic. Campden doesn't hurt anything, and is very very cheap insurance.
I have to wonder if there is something special to your process whereby the chlorine really is all offgassing before you brew. Perhaps they use a very small amount of chlorine at your water treatment plant. Or are you pouring all your water days or weeks in advance of brewing to where it has that chance to all get out? These things can help. I myself keep a 5-gallon jug of brewing water filled from my tap, to which I still do add 1/2 Campden tablet but it stays there in my basement until it's needed. I do this actually as double insurance, just in case the Campden doesn't work, hey then at least the weeks or months of sitting in the basement will take care of what the Campden didn't. That's how careful I've become about eliminating chlorine from my tap water. So I guess I am indeed a little curious how you get your water, how long it sits before use, do you preboil it, etc. Not necessarily "luck", maybe that was the wrong term. Maybe you're just careful in a different way that helps.
All I do is fill two 10-gal kettles straight from a garden hose the evening before or the morning of brewing. Usually it's the morning of. Straight from the hose, my chlorine measures between 1 to 2 ppm. Important detail: my garden hose is a drinking-safe hose called Zero-G. Don't ever do this with the green, garden-variety plastic hose. It has to be drinking-safe.
Then I fire up the burner and it takes 20-30 min to reach strike temp, i.e. 165-170 F. I immediately dump the water into a cooler and immediately mash in. I then heat the second kettle to ~180 to prep for sparging. Once in a great while I'll take a sample of the strike water before mashing in and cool it and test it, just to be safe. It always comes back with zero chlorine. But I don't do this much anymore, maybe twice a year.
Time is, admittedly, important. I've done tests on 250-mL tap-water samples heated in the microwave. Heated to 160 and then immediately cooled and tested, a little chlorine is still present. Heated to 160 and then let it stand for 5 min, no chlorine. Time is important, but the time it takes me to heat to strike/sparge temp is waaaay more than adequate.
I have been very careful and methodical about this, and I still am. I know the chlorophenol off-flavor very well and I'm as terrified of it as the next brewer. But I just don't worry about it vis-a-vis my water because I've done extensive testing to know that I don't need to. Maybe YMMV? I dunno. I tend to think that if your chlorine comes in at <2ppm, you'll be fine without filtering or treating your water (unless your city uses chloramine! In which case forget all of this, all bets are off, and I've been consistent about this).
This started because where I used to brew commercially, our water filter broke and had a stuck valve, and tap water was bypassing the charcoal filter and going straight into the HLT. We made 5 batches with this water before we realized the unit was broken. No chlorophenol. We mixed the HLT water with cold (and unfiltered!) water to maintain temp during mash-in and, again, no chlorophenol.
Make your own conclusions. I made mine 5 years ago. And my brewing is simpler for it.
You don't need to risk a batch to test this. Just fill your kettle and heat it, and then test it. Or heat a sample in the microwave. It's preposterously easy to test, all you need are some test strips.