Pump Housing Construction: Typically, a solar hot water pump is made of iron, brass, stainless steel, or plastic. Determining which material is best for your hot water recirculation pump depends on the type of system that is installed. Closed loop systems can use iron since oxygen cannot enter and corrode the pump. For open systems, a brass, steel, or plastic pump must be used. Brass pumps can be expensive, so stainless steel might be a better alternative. Thanks to technological improvements, plastic pumps are gaining wider acceptance and are considered as reliable as other hot water recirculation pump options.
Source: U.S. Department of Energy
Fluid Flow and Pump Capacity
The efficiency of your hot water recirculation pump depends on its flow rate and pressure capacity. Since solar collectors are located above the tank, atmospheric resistance must be overcome for the fluid to reach them. The hot water recirculation pump will only lift fluids to a certain level above the tank; therefore, it is critical to know how far above the tank the collectors are located. Furthermore, pumps are rated to provide different flow rates to ensure that there is sufficient fluid flowing to your tank. The optimal flow rate will be determined by the length and diameter of the pipe that is used.
AC or DC Power?
A hot water recirculation pump can be powered with either AC or DC voltage. An AC-powered hot water recirculation pump runs off of the power provided by the local electric utility. Some major advantages to AC pumps are that they are generally cheaper, they are rated for higher flow rates, and they have higher head ratings than DC pumps. Their primary disadvantage is that they are not as energy-efficient as DC pumps.
Alternatively, a DC-powered hot water recirculation pump is much cheaper to run since it can be powered by a solar photovoltaic panel. Drawbacks include the fact that they are noisier and have smaller lift capacities, which limit them to a maximum lift of 15 feet. All things being equal, it's best to purchase an AC-powered hot water recirculation pump unless you're off the grid.
Ultimately, your choice of which hot water recirculation pump to use will be determined by your building's layout and the type of system that is to be installed. It's important to evaluate all of the factors listed above, and perhaps consult a solar professional, before deciding which hot water recirculation pump is best for you.