Generally, you want to completely drain and refill your hot tub every 60 to 90 days, depending on how often you use your hot tub. However, the look and smell of your hot tub water may indicate changing it sooner. So the real answer is you should be Changing Water in your hot tub when it needs it based on bather load and time.
A TDS (Total Dissolved Solids) test of your hot tub water can take a lot of the guess work out of your hot tub water quality. TDS is a measure of the combined content of all substances contained in the liquid ppm (parts per million). Tap water starts with a TDS level of 50 – 300 ppm, and each time you use your hot tub you leave behind a little TDS, from your sweat, detergents etc. You also add TDS to your hot tub every time you shock or add chemicals to your hot tub. While your water evaporates your TDS levels increase, and even when you add top up water you add more TDS.
Once your TDS level climbs to 1500 ppm over your start up TDS level you need to change your hot tub water. If you do not change your hot tub water at this point your chemicals will not preform properly, your alkalinity will run high, and in general it’s not an overly healthy body of water to sit in. You can test for TDS yourself or you can also take a sample of your hot tub water to a local spa and pool retailer and have the TSD of your water checked. Once you have an idea of your usage and your TDS levels you can set a schedule to change your hot tub water during the year.
I also like to change the hot tub water after a large group or a party has used the hot tub. A rule of thumb is water is less expensive and less corrosive than chemicals so if your water is off it is always better to start with a fresh hot tub of water. Using high amounts of chemicals can be hard on your hot tub equipment and hot tub cover and as noted above adds to your TDS. Another thing you want to consider is the time of the year and your climate. If you are in a Northern climate change your hot tub water before the snow flies.
You can use your hot tub pump to drain your hot tub, but I like to use a submersible pump and a shop vac for changing water. I find this the quickest way to drain and clean my hot tub. Always take advantage of when you drain your hot tub to clean or replace your hot tub filters.
