The Carbonator Unit
The McCanns carbonator unit is common to all these approaches, so let me describe that first. It works on a somewhat different principle than my bottle method. It merges warm tap water and higher-pressure CO2 into a continuous flow of warm carbonated water. The unit consists of an Proconn electric water pump, a stainless steel pressure vessel with electronic water level control, and associated connections and check valves. The bottom half of the pressure vessel contains water; the top half is initially purged of air and thereafter contains only CO2 gas. You set the CO2 regulator to supply about 100 psi, which maintains the same pressure in the vessel. The pump boosts the tap water pressure from the utility supply (typically about 60 psi) to something higher that will inject water past a check valve and into the 100 psi vessel. An electronic level control monitors the amount of water in the vessel and turns on the pump to maintain the water level as soda is withdrawn. The soda output is removed from the bottom of the vessel via a dip tube. Connections are through 1/4 inch and 3/8 inch stainless flare fittings welded into the sides of the pressure vessel, and a brass flare fitting on the pump inlet. An overpressure relief valve on the tank ensures that if the CO2 pressure became dangerously high (such as from a stuck-open CO2 regulator) that the tank would not catastrophically explode.
The large area of interface between the gas and liquid in the pressure vessel, and the high pressure of CO2, result in rapid dissolution of CO2 into the water, even at room temperature. The equilibrium of this solution, given the high pressure of CO2, is above the target 4 volumes of CO2 despite the room temperature operation. In the improvised bottle method, we use chilling and agitation to rapidly carbonate; in the carbonator machine we use high pressure and a larger area.
To maintain the carbonation at the ultimate delivery pressure, you must lower the temperature of the liquid after it flows from the carbonator vessel but before dispensing it to atmospheric pressure. Chilling is absolutely critical to dispensing carbonated beverages. Dispensing at room temperature results in an instantaneous loss of nearly all the carbonation ("warm soda is flat soda") due to the agitation of passing through the valve. Thus one uses a "flash" chiller to lower the temperature of the flowing seltzer just before it reaches the dispenser valve.
Carbonators are inherently more efficient in their use of CO2 than the bottle or keg methods. All of the CO2 consumed is delivered in the beverage without waste. The headspace of the carbonator vessel is purged of air when the system is started, and never is opened to the atmosphere to spoil that purge or to waste the headspace gas. Filling is more or less a continuous replacement of the draw, so that CO2 is not wasted into an emptying bottle or keg.
McCanns makes several models of carbonators, but the most common seems to be the "fast-flow" model with a 2-gallon stainless pressure vessel and a capacity that can keep up with a big 6-flavor fountain. Their smaller models have (you guessed it) slower flow rates but ought to be fine for home rates of use. There are also other carbonator manufacturers like Selmix, Lancer, Cornelius, FHPC, and Servend, but McCanns seems by far to be the most popular.
Being connected to the public water supply, to be legal, the carbonator pump must incorporate a backflow preventer. Watch out for older and used models, which may be obsolete in not having this feature.
Although that sounds like a lot of extra work and complexity and it may not even work the way you expected 
