The domestic central heating pumps are now supplied in a standard type and size, but should the old pump be different, maybe longer than its replacement you could need to fit extra pieces to allow for this difference. Before you go to purchase the new pump it is a good idea to take a note of all the details of the old one, so you get the correct one. Measure the length of the old pump, and the diameter and type of the connections. The majority of domestic pumps use 1?in BSP threaded connections. Also note the type of pump and the setting of its output regulator (domestic pumps are available with different strengths). East London Plumbers can supply and fit new appliances.Method:1). Isolate the electrical supply to the central heating system and pull out the plug.2). Sketch the electrical wiring on the old pump. A good idea is to label each wire. Then disconnect the wires with a screwdriver.3). Shut the isolating valves either side of the pump using the valve handles or an adjustable spanner. If there is no isolating valves drain the system. Place a bowl and towels under the pump ready to catch any water as you remove it. Undo the union nuts holding the pump. Turn them anticlockwise.4). Take out the old pump. An East London Plumber will remove an old pump. Place the new pump in position using the new sealing washers in the unions. Open the isolating valves (or refill the system) and check that the unions are watertight, then, re-connect the wiring.5). Switch on the electrical supply and try the new pump by turning on the central heating system at the programmer or time switch. You may also need to turn up the room thermostat. When the central heating system is running, check that the open safety vent pipe over the feed and expansion cistern does not discharge water when the pump starts or stops.6). If you have to add too much freshwater to the cistern, bleed any air out of the system in order to guard against future corrosion and to protect the new pump.