Folks often assume that air circulating in boiler heating systems ( you know, the air that is trapped in a heating fixture preventing the system from fully circulating the heated water or coolant-called an air lock ), is caused by a leak allowing air into the buried piping. Since the system is pressurized, air cannot push its way into a pipe with higher pressure than the air in the soil or gravel around the leak.
Instead, what happens is the loss of pressure from the leak causes the automatic fill system to introduce domestic water into the system to replace what has been lost through leakage, and that domestic water has air bubbles ( the same ones you see in a glass of water ) that separate during circulation and become locked within a heating fixture or baseboard register, and keeps the system from heating that room or zone.
Air is a good symptom of a system leak and one should have the system tested by a professional to confirm leakage, and to locate that leakage before significant property damage occurs. Air in a system will sometimes produce a knocking or thumping sound in the pipes.