The stories mentioned above are so very true. Typically, one of the most often asked questions we get from hot tub owners is asking if they will need to drain the water after they purge. The answer, of course is YES.
when they see the gunk that is released they would not want to keep that water. There are so so so very many hot tub owners who just hate to dump the water. They put off doing a purge because they are not wanting to drain the hot tub. Aside from maintaining adequate sanitizer levels on a daily basis and maintaining good water chemistry parameters the single most important maintenance detail you MUST do is purge the bio-gunk from the plumbing infrastructure every 6 months with an effective cleaner. I would bet that there at least 40-50 % of the hot tubs in America have never been subjected to a plumbing purge. If they only knew what benefits they would derive from doing so periodically. No wonder hot tub rash is so rampant in the industry.
This is why the typical hot tub owner is always griping about water quality and sanitizer decay rates going up. I blame this on a lack of information that does not come from their dealer. Many dealers and even some manufacturers, are just not comfortable telling buyers about biofilm and how it can be so disruptive to water balance and acceptable water chemistry standards. Here's an interesting tid bit of information. Canadien hot tub owners are much more diligent when it comes to purging. Other areas like the UK, France and Australia are also much more inclined to keep hot tub plumbing cleaner than US hot tub owners. There are many manufacturers that really are teaching their dealer base about biofilm Issues. Watkins (Hot Spring) and Nordic are two such manufacturers that are really on top of this. There are also many other fine manufacturers that want their dealers to know about biofilm. I could also name a few big name manufacturers that are petrified to have their dealers speak about biofilm issues to their customers. Here is the real story IMO. Purge twice a year and your maintenance will be minimal. Many people just drain the water without a purge and think this is all they need to do. Doing this is like rinsing dinner dishes under the faucet and placing the dinnerware back in the cupboard. Of course you don't do this. You scrub them or place them in the dishwasher.
Please listen to what Texas Splash, Newdude and RDspaguy and many other posters have been pontificating. Purge the Hot Tub. These people have been on the job and have seen this stuff. Believe them!
And I still gag just a little when I think of the spa that made me refuse to ever do a drain/clean/fill again. Keep in mind, this is from a guy who pulls pieces of dead rotting animals out of pools every spring. If it made me nearly quit not one but two jobs, it was bad.