Before you remove an old pump unit, make sure you know which way the flow i.e. which end the water comes in and which ended flows out, and the user marker pen to label it accordingly. A Greenwich Plumber can fit a pump. Central heating pumps only rotate in one direction and have a flow arrow marked on the casing to help when the pump is being installed or replaced. Under the lead on the control box there is a screw connector were the electrical power is connected and fed from the central heating control. There could also be a dial or screw which is fitted so that the pump speed can be adjusted manually by you. On some units, you will notice that a metal or plastic cylinder about the size of a salt cellar is fitted onto the pump. This is a smoothing capacitor and this prevents the pump motor from causing any interference to any of the domestic electrical equipment fitted within the home such as a television and radio. This item cannot be replaced. The motor when running normally should make almost no discernible noise, although if you put your hand on it, you should be able to feel a slight vibration, ( Greenwich Plumbers fit new central heating pumps) which is normal. The large screw situated on top of the pump is almost definately the bleed screw and is fitted to allow the homeowner or a professional plumber to purge any air that may be inside. No central heating pump is able to prime itself, therefore if air was to leak in, the pump would not be able to get rid of the air until it is bled out manually. This can be done by loosening the screw slightly until the hissing sound stops and then tightening the screw back up again“