Due to the high temperatures of hot tub water, and its resulting evaporation, the total number of dissolved solids in the water will increase over time despite regular chemical maintenance. For this reason, you must periodically drain and re-fill your hot tub water. Failure to change your water within a timely manner can cause your hot tub's filters to get dirty sooner, put extra pressure on the pump, and reduce the efficiency of your hot tub’s heater.
It can be difficult to know exactly when your hot tub water needs to be changed, but as a general rule of thumb, we say that water should be changed once every 3 months. You can also calculate when to change your hot tub water by dividing the number of Gallons of water by 3, and dividing that number by the number of daily users. For example, our Toronto Spa holds just under 300gal of water. If you have a Toronto Spa that is used daily by 2 users, you do (1/3)300/2 = 50, meaning water should be changed every 50 days.
You may also need to change your water if you continue to see foam despite the use of Foam Free, cloudy water, or if water has a persistent smell.
You may want to avoid putting sanitizer in your hot tub a couple of days before draining to ensure that the drained water isn’t particularly harsh on its surroundings. We also highly recommend using Whirlpool Rinse before draining. This chemical will help flush out your tub’s internal organs, removing harmful grease and residue.
EDIT: Before draining your Hot Tub, make sure to power it off, by switching off your Spa's GFCI.
Once the water has fully drained, use a damp cloth to wipe down your hot tub's acrylic shell. This is also a good time to clean/replace your Spa Filters. Once complete, re-fill your tub, and refer to our Initial Start-up blog post for a step-by-step guide on which chemicals to use in your freshly filled hot tub.
EDIT: For more detailed information about why Spa water should be regularly changed, check out this article by Robert Lowry of Aqua Magazine.