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Heat Transfer in Active Solar Heating Systems

An integral part of your solar hot water system is the hot water recirculation pump. It circulates fluid to the collectors on your roof where the fluid is heated and transferred back to your tank or heat exchanger. When choosing a solar hot water pump, four primary characteristics need to be considered: the structural material, the fluid flow rate, the pumping capacity, and the power source.
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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.

Active Closed LoopSource: 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.

In Summary

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.

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Written by: Christian Märtel, Editor
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