Extract from 'Central Heating - Fault Finding & Repair'

Introduction
It is essential that when a new circulating pump has been installed or the system has been drained the circulating pump is vented. It must be vented before it is allowed to start. This will remove air and allow system water to enter. The system water will cool the pump, preventing overheating and will also lubricate the shaft and bearings.
Permanent damage may result from the pump being run dry even for a short time.

Venting
Expect water spillage so have old towels/rags available to protect floors etc and to mop up.
1.   Turn off the boiler
2.   Turn off the electrical supply to the boiler and pump, remove
      plug(s) fuses and put them in your pocket etc
3.   Remove circulating pump terminal cover
4.   Caution – using a multimeter check the voltage at the       pump terminals.
      There should be a zero reading
5.   Replace terminal cover
6.   With isolating valves open, remove the vent plug. Mop       up water spillage
7.   Fit a 3mm screwdriver through the vent plug opening and       gently turn the shaft anti-clockwise
8.   Rotate the shaft several times to remove any air
9.   Replace vent plug. Do not over tighten
10. Set the speed selector to maximum (three)
11. Replace any fuses and turn on the electrical supply
12. Set the room thermostat to maximum
13. Set the programmer to central heating ‘on’
14. Turn on the boiler
15. Adjust pump speed. There may be some noise as air is cleared
16. Reset room thermostat to original setting

Extract from 'Central Heating - Fault Finding & Repair'