Sanitising sealed mains-fed water coolers can be difficult but not impossible. The main issue is how to get the descaling and disinfectant liquids into the unit. Typical units are the direct chill or spiral chill type. Frequency for sanitising and filter changes is every 6 months.
The filter housing can be disconnected from the microbore tubing and a small device known as a venturi doser put in its place. The doser is filled with the recommended amount of descaler and the hot tap actuated. This will draw descaling acid solution into the tank. After descaling, flush with at least 4 litres of water and test with pH paper until neutral.
The spiral chill pathways can be descaled and then disinfected using the same technique with respectively phosphoric acid (for example) and hydrogen peroxide. The chemical is drawn from the doser by actuating the chill tap. When the chemical begins to run from the tap (you can check this with pH paper or peroxide test strips), turn the tap off and wait for ten minutes. Then flush with water until the test strips are clear.
This is not a detailed procedure and coolers vary considerably in design, however it will serve as a guideline. Always check with your cooler supplier if in doubt.