Hot Water Circulator:

Most forced hot water heating systems in residential homes used something called a “circulator pump” to move the water through the pipes in the walls, radiators, and boilers. There are always dangers and mishaps that can occur when one is modifying a hot water system, so take precautions when installing your re-circulating pump.
Be very careful that you do not create air pockets in your water lines. If you put air in your lines then your pipes will begin to hum and rattle. You can bleed the system of air by opening a valve, much like bleeding brakes on a car, and allowing water to push the air out of the system. Air can actually block the water flow through your recirculation loop. Because the recirculation relies on gravity and not water pressure the air pocket will not move. Water flows very slowly through a system that uses gravity to function.