The water in the boiler needs regular replenishing and two methods can be used to introduce it when the boiler is under pressure. The introduction of cold water into the boiler though necessary, will also lower the steam pressure for a while. A great deal of skill is needed in operating the injector while still maintaining sufficient power for operation.

1. Mechanical feed pump. Some locomotives preheat the water prior to pumping.

2. Injectors

The principle of the injector is based on the fact that steam escaping from a nozzle has a greater velocity than that of a jet of water issuing under the same pressure from a boiler. If cold water is added to the jet of steam, it begins to condense and the velocity of the steam will increase sufficiently to overcome the pressure of water in the boiler. By this means, water can be introduced into a boiler against its internal pressure.

Some injectors used a combination of exhaust steam and live steam. A connection at the base of the blast pipe was run to the exhaust part of the injector where it heated the feed water before it passes to an auxiliary injector. The auxiliary injector used live steam to force the water to the boiler. This type was patented by JJC and RD Metcalfe in 1908 and was claimed to save up to 15% on fuel and water.

There was a type of injector, with features patented by J Gresham in 1884 and 1887, which was a "vertical restarting injector". Steam supply and feed water passed through the flange by which it was attached to the boiler. There was also a Davies and Metcalfe type patented in 1899 and 1907 which was designed to operate with feed water too hot for an ordinary injector.

Injectors are tricky instruments and require a degree of skill to "prime" them and get them working. This is normally the task of the fireman. Once the steam is turn on, the right balance of water being applied has to be found. This will only work if the steam and the water are at the correct pressure. A balance also has to be found between too little and too much water being in the boiler. Too little risks melting the fusible plug, too much risks boiler water rising to reach the regulator, known as "priming", and getting into the steam pipe leading to the cylinders.