Thanks to ChrisR
Ensure that you get a pump with the correct capacity and speed that is correct for your system. Most modern systems use a 3 speed pump, and most low water capacity boilers will need it running on the highest speed.
The first thing is to ensure that the boiler thermostat, controls, and power to the C H system is OFF.
Remove the terminal cover where the wires go, note which is live, neutral and earth, and remove the cable from the terminal connections.
There should be a pump valve at the inlet and outlet of the pump, turn these to the closed position and remove the bleed screw in the middle of the pump. (If you have Gate Valve type pump valves, it may be necessary to loosen the spindle nuts)
NOTE HOW THE OLD PUMP IS ORIENTED BEFORE REMOVING IT AS THE DIRECTION OF FLOW IS IMPORTANT.
The big nuts are 52mm, or 2 1/16th inch. 50mm won’t usually go on!
It can be a 10 minute job or take all day.
If you have pump valves with screwdriver slots (and maybe allen key hole in the middle of the slot) don’t expect them to work.
As soon as you touch them they usually leak a LOT of water out past the screw. Even the Gate Valve type pump valves don’t always work. Don’t over tighten them, or they’ll jam shut and break and you will need to freeze the pipes or “BUNG” the system or drain the system to below the level of the pump.
If you have the same make/model of pump to replace the existing one, you can just change the motor. Remove the 2 or 4 allen screws, and tap the motor body sideways. Sometimes I find I have to hammer a penknife blade into the join to get them apart.
Note that the two halves of a pump can be oriented 4 ways by rotation, Get the part with the pipes on right, first. It will have an arrow on the side to indicate flow direction. Then put the electrics box at the top - definitely not at the bottom.
Also make sure the rotating shaft in the pump is horizontal. If vertical, the bearing will rapidly wear out.
Check all the other joints nearby. Often, the compression pipe connections will start to weep, and always use new pump washers, having cleaned the union surfaces.
Wrap a few turns of PTFE tape around the new pump threads (in the direction that the nuts will tighten), and tighten each nut onto the pump body. This is not to seal the threads, as they shouldn’t get wet, but it will stop them seizing.
* Rewire the power input lead noting the correct terminals for LIVE, NEUTRAL and EARTH, and replace any terminal cover.
* Open both pump valves, and watch for leaks. Tighten a bit more if leaks are spotted. If the valve leaks from the spindle, tighten the nut around it
* If all is well, turn on all power, and controls to the ON position. Keep the boiler thermostat on the cool position until tested out
Refit the bleed screw in the middle of the new pump after all the air has come out.
Ensure that the pump is set to the highest speed for large systems.