First introduced in the UK about 1876. The use of exhaust steam to assist the work of injectors allowed some fuel savings over the pure live steam injector. BR class H, J, H/J and K types of exhaust steam injectors are all basically similar. Two controls are provided in the cab, a water regulator and a steam valve. The water regulator handle has a "sector" to denote the position of the valve. The valve itself is part of the injector body mounted outside the cab - often under it. The steam valve is mounted on the boiler backplate and its housing includes the water delivery pipe from the injector, which passes through the backplate, over the firebox crown and down the boiler to deposit the feed water towards the front. Saturated steam is taken from the dome or the steam fountain to provide the live steam supply to the steam cone when the injector is turned on. Exhaust steam for the injector is supplied from the cylinder exhaust. Steam from this source is only at about 10 psi but it is combined with live steam to drive the water into the boiler against its pressure. An additional supply of superheated steam from the downstream side of the regulator is passed from the smokebox to the injector to allow closing of the regulator to be detected. The absence of superheated steam causes an automatic shuttle valve to close in the injector. This prevents the saturated steam supply from reaching the exhaust valve control and this valve closes. Since the closing of the regulator will mean no exhaust steam is available, the injector will work entirely on the saturated steam available.