how can i fix the water constantly dripping from thepipe outside the back d?

halstead boiler, back from holiday put on the central heating,pressure gage went up to 3, found a rad not working checked the thermostate on the rad and it had gone so took it off and put on the heating again and the pressure went down to normal 1 to 1.5 and the rad worked, but the water constantly drips from the pipe out side the back door, how can i stop this water from flowing?