Heating pipes that have air in them need to to be bled out. The process is called bleeding the pipes, and it is a common occurrence with virtually every hot water type of heating. Heating pipes with zone valves and hot water radiators will all, at some point, get air bubbles in the pipes. These bubbles act like little water dams and will not allow the heated water to pass. When hot water no longer passes through heating pipes, bleeding them is the only way to restore the flow.
Turn off your boiler and allow the system to cool for at least one hour.
Locate the bleeder valve on your heating pipes. If this is a multiple story house, home or apartment, you'll want to begin on the top floor at the farthest bleeder valve from the boiler. If this is a single floor abode, begin at the last bleeder valve before the pipe returns to the boiler.
Place your bucket or cup beneath the bleeder valve to catch any water that might come squirting out.
Turn the bleeder valve open by either attaching an adjustable wrench to the nut, or on some heating pipes like radiators, use the finger twist turn knob or key. Turn it in a counterclockwise direction to open it up.
Listen for a hiss of air to escape when you turn the valve open. If no air escapes and all you get is a stream of water, close the valve and proceed to the next bleeder valve down the line.
Bleed every valve until there is no air remaining in the heating pipes, turn your boiler back on and the heat will flow.