If the expansion vessel has been checked via the Schrader valve and the pressure is too low, it will require recharging or re-pressurising to the correct pressure. This can be done using a car foot pump or similar device.
Switch off the boiler and remove the fuse as an extra precaution, to prevent it being switched back on by accident. Ensure that there is no pressure on the central heating side of the expansion vessel. If there is pressure it can be released by opening a bleed valve on a radiator or a drain off valve (be aware that water will escape when either are opened)
Connect a suitable air pump to the Schrader valve to increase the pressure in the pressure vessel. You can use a car foot pump or similar device to do this. You should now check your boilers operating manual to see what pressure the vessel requires recharging to.
Slowly increase the pressure via the Schrader valve until you get the desired pressure as detailed in your manual. Then remove the air pump from the valve and screw on a valve cap.
You now need to check and re-pressurise the central heating side of the expansion vessel. You may also have to bleed the radiators and top up the pressure on the central heating side again.
