Hi!
It happened to me aswell after draining my radiators. You need to turn off your boiler and bring the pressure up to 1.5. To do that you'll need to open the two black valves under your boiler(open means they'll be in line with the pipes;they should be perpedicular on the pipes when closed) and keep them open until the water pressure gets to the level desired of 1.5. When that happens, close them both and turn your boiler back on.
Hope this helps!
Mirela, September 2010
