There are many machines and units that can be built into your kitchen either with or without a cooling system. Those without a cooling system usually provide too low a mix of carbonated water for most people therefore it is advisable to always test any unit properly before purchasing one. Otherwise the risk of disappointment may be quite high.
Machines and units with cooling systems are by far the best and most professional units to look at. The unit is hooked to the cold water mains and released through a tap so that carbonated water is continually available. As carbonation occurs in-line, all the carbonic acid stays in the water and these machines use much less acid. You can even set the level of carbonation yourself, sometimes directly on the tap.
The best feature about these machines or units is that large canisters of carbonated dioxide can be used which produces a single litre of sparkling water for an amazingly low four cents. Depending on consumption and model, a machine like this will pay for itself in less than four years. The Aromhuset flavors supplied worldwide from allfreightfree.com can be used to flavor the carbonated water.
As many people want top units, it is quite common for them to be installed with a new kitchen as the price to do so is a very affordable 1,500 euros for the luxury model. A tap or a handle for cold carbonated water also increases the value of the kitchen and some people purchase the device just for the added luxury.
These are some links for different luxury models: