A circulating pump is any pump that circulates a fluid in a closed loop. Because the fluid always flows back to the pump, the pump doesn't have to do much work. Most of the effort it expends pushing water upward can be recuperated when the water flows back down again and comes back to the pump. Circulating pumps only have to be powerful enough to overcome the inertia of the water and the resistance of the pipe. Because of the low power demands, many circulator pumps are small enough to fit entirely inside their pipes.
Pump Structure
Most circulating pumps used in homes are compact centrifugal pumps. An electric motor drives an impeller, which moves the water. The impeller is a wheel with angled blades inside of it, somewhat like a turbine. The impeller spins extremely fast, forcing water that gets trapped in it to the outside, compressing it and channeling it forward in a jet. The motor is sealed in a waterproof case which raps around the impeller, and the whole thing is designed to mount inside plumbing in most household applications. Industrial circulating pumps, on the other hand, are sometimes large enough that the motor must be mounted separate from the impeller.
Hot Water Recycling
One of the most common home applications is in domestic hot water systems. When the hot water is turned on and then off again, some of it sits in the pipe and cools. Then, when the hot water is turned on again, the user must wait for the water to warm up, wasting water. Some homes use circulating pumps to cycle the hot water from the pipes back into the water heaters and into the pipes again. This wastes heating energy but saves water, making it a good choice for areas where water is in short supply.
Solar Water Heating
Another popular use of circulating pumps is in solar heating systems. In many of these systems, a circulating pump sends water into a solar collector, where it is heated by the sun's energy. That water is then pumped through a water tank, where the heat is transferred into the water. The cooler circulating water is then water pump back to the solar cell to collect more heat.