Note: The El-Sid warranty only covers pumps to temperatures up to 175°F, which could be a problem in collector loops that experience higher temperatures. aDouble the PV wattage when not using water as a heat-transfer fluid; in some cases, even circulating water will require a larger PV module to start the pump reliably. bPotable water

Note: The El-Sid warranty only covers pumps to temperatures up to 175°F, which could be a problem in collector loops that experience higher temperatures. aDouble the PV wattage when not using water as a heat-transfer fluid; in some cases, even circulating water will require a larger PV module to start the pump reliably. bPotable water

A pump's performance under various conditions is shown by its "pump curve." This performance curve is typically presented as a graph or a table, with selected flow rates given at different pump pressures. The pressure a pump exerts is usually expressed in feet (sometimes decimeters) of head. Feet of head is a more useful way of expressing the pressure in real-world circumstances and is used in most pump curves. It can also be expressed in pounds per square inch (psi), where 1 psi equals 2.31 feet of head. In graph form, the head is the vertical axis and the flow is the horizontal axis. As you can see in the example graph (opposite page), as the head decreases, the flow increases.

One of your final considerations for choosing a pump depends on whether you're planning to use AC or DC to power it. Both kinds of pumps are available, but the range of available DC pumps is much narrower than for AC. AC pumps have an unlimited energy supply if they are powered by a reliable utility grid. DC pumps can be run directly by a PV module and make a solar water heating system independent of the grid.

One way to approach the DC and AC pump choice is to examine relative system efficiencies. The efficiency of some heating systems is rated by the relationship of the amount of energy output to the energy input. If you have a system that produces a certain amount of heat with half the equivalent electrical input, the "coefficient of performance" (COP) is 2. Produce four times as much hot water as the amount of energy input from electricity and the COP is 4. We can use this same methodology in evaluating the efficiency of SHW pumps.

factor here. Solar collector loops will operate efficiently over a wide range of flow rates, but choosing too large a pump can cost more up-front and will use more energy. And an undersized pump without sufficient head in a drainback system is a disaster—the system just won't work. Collector manufacturers' recommended flow rates are usually published in their literature. If not, you can find this information in the OG-100 ratings directory (see Access).

High-Head Pumps

A Taco 009F high-head iron pump, suitable for most drainback and larger antifreeze systems.

Using a utility-powered AC pump for your solar water heating system will give you a COP between 12 and 25, and this is an excellent value compared to electric water heaters, which have a COP of 1. But the COP will never be as good as a DC PV-powered SHW system. DC hot water circulation pumps can have a higher COP than AC pumps because there is no traditional energy input if a PV module powers the system. If you use a solar-electric module to power the pump, your COP is infinite— you're not adding any input energy. The sun provides it all, and you get something for nothing after the initial investment. PV-powered systems are also immune to utility outages. This is a big plus with antifreeze systems, since the collectors can overheat on sunny days if the pump stops operating due to a power failure. An overheated collector can actuate the pressure-relief valve, which will make it necessary to recharge the system with antifreeze solution. In some cases, the overheating can be so severe that the antifreeze solution will be compromised to the point of needing replacement.

Although it seems like a no-brainer to go with a DC PV-direct power source for your solar water heater pump—not so fast. A few other factors can influence your decision about the power source:

• Some DC pumps are noisier than AC pumps, which can make an installer think twice about the placement of a DC pump.

• High-head drainback DC pumps are few and far between. Finding a reliable high-head DC hot water circulator is impossible at this time, limiting the head of a DC drainback system to about 15 feet.

• Any given PV module and SHW collector are rarely a perfect match. The PV module often will "outproduce" the collector and the pump may run early in the morning or late in the afternoon when the collector isn't producing useful heat. The result? Unwanted pump operation can actually cool the water in the solar storage tank. Until recently, no DC-powered differential controllers were available to limit this unwanted pump operation. Art Tec (see Access) recently began manufacturing a DC differential controller that optimizes pump run-time in PV-direct SHW systems.

AC hot water circulators are firmly entrenched in normal distribution in the United States and are therefore less expensive and easier to procure. A DC pump will cost more than an AC pump of the same head and category, and the PV module will add to the cost—but if it fits into your design and budget, the extra cost is well worth the expense. PV-powered DC pumps are normally the optimal choice for a solar heating system except in high-head drainback and very large antifreeze systems.

The Fine Print

Knowing how to decipher the fine print on the pump can give you valuable insight into whether or not it'll be a good match for your SHW system. For example, the "15-18 SU" model number of a Grundfos pump tells you that the impeller housing inlet is

15 millimeters and the maximum head is 18 decimeters; "S" is for stainless steel, and "U" is for union attachment.

Other manufacturers have model numbers that may also denote the power consumed or the pump construction. An "F" in a model name usually denotes a flange iron pump, which can make the pump housing easily removed and replaced. "B" stands for bronze, so a "BF" would be a bronze flange pump. Look at the Solar Pump table (previous page) to see some of the relationships between model numbers and specifications.

Common Pumps

Several pumps and manufacturers are listed in the table and Access. The models listed were included because they are readily available and most folks in the solar industry are familiar with them, but there are also others on the market. One very important point: Make sure any circulation pump you consider for a SHW system is intended for hot water—at least 200°F for most systems.

Besides that, knowing a few simple rules and the manufacturer's pump specifications is all you need to make an intelligent choice, whatever your needs. After almost thirty years installing and servicing solar hot water circulation pumps, almost all the models I've used seem very durable and long lasting. So pick your pump(s) and get into some really hot water.


Contributing editor Chuck Marken ([email protected] com) is a New Mexico licensed plumber, electrician, and heating and air conditioning contractor. He has been installing and servicing solar thermal systems since 1979. Chuck is a part-time instructor for Solar Energy International and the University of New Mexico.

Art Tec • 866-427-8832 • www.arttec.net • DC differential temperature controller

Solar Rating and Certification Corp. • www.solar-rating.org • OG-100 ratings directory

Pump Manufacturers:

Bell & Gossett • 847-966-3700 • www.bellgossett.com

Grundfos Pumps Corp. • 913-227-3400 • www.grundfos.com

March Manufacturing Inc. • 847-729-5300 • www.marchpump.com

S. A. Armstrong Ltd. • 416-755-2291 • www.armstrongpumps.com

Taco Inc. • 401-942-8000 • www.taco-hvac.com

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