What sort of impeller do i need?

i have a incabinet boiler connected to a tap on the cabinet. when i want cold water, the flow is perfect, but when i switch to hot water to flow will stop midway to filling my cup. i know there is air trapped in the pipe to make this happen. but i was told the shape of the impeller on the pump was doing this. i have no idea how to stop these air bubble as they are driving me mad. if you think yu can help please do so. also there is no release for steam as it is not apripiate for this boiler