Humidity chambers need purified water to create humidity. As the water is heated to create vapor, and hence, humidity, it leaves any impurities behind. Supplying straight tap water will leave a lot of deposits on the humidity heater, requiring more cleaning, less efficient heating, and shortening the heater's life.
On the other end of the spectrum is water that is too pure. Some electronics companies use high-purity water for cleaning, and think that they can supply this water to the chamber. The water is so pure that it is trying to refresh itself of the minerals missing in it, which is why it makes a good cleaner. Used in a chamber over time, it will etch holes into the stainless steel material the chamber is made of. A hole in a chamber is disastrous because moisture can now fill up the insulation space.
What does ESPEC recommend? We want humidity water that is in the range of 0.1 to 6 megaohms of resistivity. Megaohms is a common measurement of water's purity. Water above 10 megaohms is definitely destructive. We offer a “deionizing” water filter system for our chambers. For typical water this is enough to meet quality requirements. The filters change color over time, allowing you to check them easily.
How long do they last? That depends on your local water quality. A local water conditioning company (like Culligan) can check your water. If it is high in mineral content, an additional filter may be needed. Sometimes plumbing a chamber directly to a water source is not a viable choice. Then a water tank needs to be installed on the chamber. To avoid the need for a high-pressure pump, most water tank systems don't integrate a filter. Be sure the water tank can be filled easily because you will be doing this on a regular basis (once a week to once a month).
ESPEC recommends caution when reusing humidity water. A “recirculating” system takes spent water to refill the tank, meaning less filling and no drain needed. But you need to make sure the water stays clean and doesn't become contaminated. That means you will still need to regularly drain the tank. Of course, reusing the same water to humidify your samples raises issues about the quality of your test. That alone should be a good reason not to reuse the water.