Trapped air can significantly reduce water circulation and cause noise and uneven heating.
In fan coils
Air in the coil can restrict heat transfer and lead to low efficiency.
In underfloor heating
Large loops can trap air pockets and exhibit low or zero flow.
Automatic air vents
They help in removing air from high points, but they do not replace a proper filling and flushing procedure.
Inspection
If there is noise or low flow, check the system pressure, venting, filters, and circulator operation.