Choosing A Circulator For Solar Hot Water Systems

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by Chuck Marken

Some things—the coldest beer or the biggest slice of pizza— are easy to pick, but selecting the right pump for a solar hot water (SHW) system isn't that simple. Pump choices are numerous—DC or AC powered; bronze, stainless steel, iron, or plastic; high, medium, or low head. But picking the right pump doesn't have to be a painful experience. Here's some sound advice on how to best match a pump to your SHW system for years of trouble-free service and high performance.

Low-Head Pumps

Circulator

Grundfos 15-18 SU pump. The "S" stands for stainless steel and the "U" is for union (union set shown, extra cost).

El-Sid 10 PV, DC pump. Although it may run on a 10-watt PV module, this pump is usually coupled with a 20-watt module to make sure it starts in all applications.

Grundfos 15-18 SU pump. The "S" stands for stainless steel and the "U" is for union (union set shown, extra cost).

El-Sid 10 PV, DC pump. Although it may run on a 10-watt PV module, this pump is usually coupled with a 20-watt module to make sure it starts in all applications.

Pumps used in solar heating systems are called hot water circulators. They move fluid through the solar collectors and/or heat exchanger to where the heated fluid can be stored or used. A circulation pump is made up of a motor, impeller, and impeller housing. The motor spins the impeller in the housing and, through centrifugal force, moves liquid through a plumbing circuit.

Circulation pumps must be primed or wet when they start, as they are not designed to suck liquid into the impeller. Unlike positive-displacement pumps, which can lift a fluid from below the pump, circulation pumps must have the impeller housing filled with the circulating fluid at all times. They are used in closed plumbing loops that are always entirely filled, or in systems with the pump situated lower than a tank's water level.

Common circulation pumps have maximum service temperatures of about 140°F, but almost all hot water circulators are rated above 200°F. Hot water circulators are a must for virtually all active-type solar water heating systems.

Selecting a pump is not difficult—your solar hot water system design will dictate which pumps are suitable, with alternatives falling into three application criteria:

• Pump material

• Pump head and flow rate

Pump Motor

Pump Motor

Capacitor Cover (AC pumps only)

Typical flange pump pieces.

Capacitor Cover (AC pumps only)

Taco 006-B4, domestic hot water pump. Note the 3/4-inch copper solder connections.

Typical flange pump pieces.

The Right Materials

Oxygen is good for us, but bad for iron pumps. Oxygen creates a corrosion problem in cast iron pumps, just as steel or iron rusts (oxidizes) when exposed to water and air. The less expensive circulation pumps are made with an iron impeller housing. They are usable in closed-loop systems where little or no oxygenated water exists.

But in open and potable water loops, an iron pump will corrode, impeding the flow or stopping it completely, often within a few months. Domestic hot water loops need pumps with a bronze, stainless steel, or plastic impeller housing and impeller. These corrosion-resistant materials are also recommended for any drainback system that does not use distilled water as the collector loop fluid.

The most common domestic hot water (DHW) pumps are bronze or stainless steel but plastic housing DHW pumps also can last for decades. The cost of bronze pumps has increased quite a bit in the last few years with the increase in copper prices. This has made stainless steel pumps more attractive.

Head & Flow

Depending on their application, pumps must overcome two different types of head—atmospheric and friction. Atmospheric head is the difference in height between the natural level of the liquid when the pump is off, and the height to which the pump needs to push the liquid when the system is operating. The pump must develop enough pressure to push the circulating fluid to the top of the loop or, in the case of a drainback solar water heater, to the top of the collectors. If the pump falls short,

Grundfos AC Pump Performance Curves

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