Pump stations are available in numerous configurations and designs, and from a variety of manufacturers. Companies specializing in SHW components frequently sell pump stations—either of their own design or rebranded models from independent manufacturers. These “kits” are also available from radiant floor companies and general plumbing companies. In many cases, pump stations are available in packages with tanks designed for direct mounting, or they can be used with other tanks from different manufacturers.
Most pump stations are for use in closed-loop, active solar hot water systems—the dominant domestic system type. A heat transfer fluid (HTF) is circulated through a collector and then transfers the heat to the domestic, potable water in a storage tank. There are many variations, the most common being pressurized systems, which use a freeze-resistant fluid such as propylene glycol, and drainback systems, which are unpressurized and often use water as the HTF.
However, most pump stations have pumps that are too small to overcome the increased head typical of many drainback systems, so are best suited for pressurized, closed-loop systems. Depending on the model and system layout, it may be possible to replace the pump with a larger size or plumb a second pump in series to overcome the head and achieve the necessary flow in a drainback system. (Caution: Using two pumps in series can have serious consequences—if one pump fails, the other pump may be able to move fluid only high enough to allow it to freeze in the tubing.)
A primary distinction between pump stations is if they include an integrated heat exchanger, which is required for closed-loop systems. There are three potential locations for the exchanger: it can be built into the storage tank; it can be an independent component plumbed into the system; or it can come preassembled as part of the pump station.
Models such as the Taco Solar Pump Station, Heliodyne HPAK, PAW Solex, or Purist SPS-2-AC can be used with standard or four-port hot water tanks because the heat exchanger is built into the pump station. These types of tanks are less expensive than tanks with built-in heat exchangers and may be more readily available (see “Solar Hot Water Storage” in HP131 for more information on tanks with built-in heat exchangers).
Pump stations that include built-in heat exchangers have two pumps, one to circulate the HTF and the other to circulate the domestic water through the exchanger. These stations will have four connections: HTF in (from the collector) and out (to the collector); and domestic water in (cold) and out (heated). The size of the heat exchanger must be adequate for the system size, and models are available that can handle 300 square feet or more of collector surface area. Plate and tube-in-a-tube exchangers are most common (see “Fundamentals of Solar Heat Exchangers” in HP128 for more information). Different sizes of heat exchangers may be available in the same pump station.
Pump stations without heat exchangers are less expensive, but require a tank with a built-in exchanger or a separate, external exchanger, both of which add cost. Overall, a pump station with an integrated heat exchanger paired with a four-port tank will be less expensive then a pump station and a tank with a built-in exchanger. However, the price difference is usually not tremendous, so choice depends more on space constraints, availability, and backup heat sources.
Choices of pump stations without heat exchangers include numerous FlowCon models from PAW, some of which are rebranded and sold by other companies. Other manufacturers include companies such as Resol and Solarnetix. Solar thermal component manufacturers such as Stiebel Eltron, Buderus (Logasol), and Schuco (Solar Station) also offer models to complement their internal heat-exchanger tanks, all of which can also be used in conjunction with different brands of tanks or with separate, external heat exchangers.
Many of these pump stations also have four connections: HTF in (from the collector); HTF out (to the heat exchanger); heat-exchanger return (to the pump station); and HTF out (to the collector). The smallest pump stations, however, are only designed with two connections and are plumbed between the heat-exchanger return and the collector. Optional components for these pump stations include isolation valves and temperature gauges, which are installed between the collector’s return and the heat exchanger’s input.
Pump stations can be used in a range of applications. The most common is with a single tank, typical of residential SHW systems. While the pump station replaces the components required for basic systems, integrating a second storage tank, external back-up heat source, or space/radiant heating is nearly the same in terms of parts and labor as with a system built from scratch, and depends on the tank and other system components. Particular attention should be paid to pump station selection when installing more complicated systems—many offer preplumbed options for additional storage tanks and backup heat sources, as well as the types of controllers needed.
Some models, such as the PAW FlowCon D2F, are designed to operate two independent arrays of collectors so that different orientations can be used to heat the same system. For example, an east-facing collector favoring morning production could be combined with a west-facing collector that heats water primarily in the afternoon. Rather than requiring the HTF to flow through both collectors—one of which would be cooler than the other—two independently operated pumps allow fluid to pass only through a hot array. This could also be accomplished by plumbing two pump stations in parallel on the return from the heat exchanger, and also requires a controller with additional sensor inputs and at least two separately controlled electrical outputs for two pumps.
Paralleling two pump stations on the HTF in (return from collector) connections allows two tanks to be heated by the same array of collectors. Depending on the system controls, one tank may be prioritized or both loops may circulate simultaneously as needed, providing heat to both tanks from a single array. Again, the controller must be capable of operating two pumps and reading the temperature of two tanks. The PAW FlowCon S2F is a prepackaged, all-in-one pump station for this type of system. This design can be especially useful for large systems that are prone to summertime overheating. Storage capacity is increased with a secondary tank, which decreases the likelihood of overheating. However the system will heat the primary tank first—and reheat it as necessary—before putting heat into the second tank, which is important for reaching useful water temperatures during the winter when daily peak sun-hours decline.
Many pump stations are made of similar, high-quality components, including brass and copper fittings, and pumps and controllers from well-known, reputable manufacturers. Selecting between the various options often comes down to the specifics of the system design, price, availability, and/or whether the pump station will be mated with a tank from the same manufacturer.
Pump. Pump brands and sizes vary, but most are interchangeable with other common brands. Multispeed pumps are common, providing the ability to adjust the flow rate to the heat available. DC pumps are unlikely to be an option. Typically there will be isolation valves on either side of a pump for servicing or replacing.
Differential controller. Some pump stations include differential controllers, or they are optional, allowing installers to use a favorite model instead. Included controllers may be proprietary or third-party models, and, depending on the unit, may allow for additional sensor inputs and increased functionality for multiple tank or radiant systems. One of two types of sensors, either 10K thermistors (common in U.S.-made controllers) or PT1000 sensors (typical of European controllers), are used for sensing tank and collector temperatures. If the station is added to an existing system, the existing sensors may need to be replaced to ensure compatibility.
Controller choices and features have grown along with the rest of the industry. Many now include advanced displays and system monitoring of collector, bottom-of-tank, and upper-tank temperatures so a user can monitor system operation. Other features may include estimated energy production in Btu; HTF flow rate; and data logging. A desirable feature can run the pump at night to cool an overheated tank, such as could occur during a summer vacation or when hot water is not being used.
Valves and gauges. A pump station includes the valves and gauges required for a pressurized, closed-loop system. Fill and drain ports allow flushing, pressure testing, and charging the HTF loop.
A check valve prevents thermosiphon by allowing the HTF to flow only in one direction. Without it, dense, cool HTF in the collector will sink, causing the HTF to flow in reverse and dump heat from the tank into the collector, cooling the system.
An integrated pressure-relief valve ensures that the HTF pressure doesn’t exceed a set limit, and protects the piping and components from extreme overheating or loss of circulation.
A pressure gauge provides a visual indication of system pressure, and if it reads zero or is dramatically below normal system pressure this may indicate a loss of HTF, whether due to a leak or because the pressure-relief valve has operated.
Temperature gauges on the pump station show the HTF temperature before it flows into the heat exchanger and then after it exits, to verify that heat transfer is occurring. Some pump stations are designed to be plumbed between the heat exchanger return and the collector only; in this case, a second temperature gauge would need to be added between the return from collector line and the heat exchanger input.
Other components may include a port and fittings to connect an expansion tank for the HTF, an air separator to remove microbubbles in the HTF, and a flow-rate meter. A valve to regulate flow may also be included, though on some models flow is set with other valves in the station.
The majority of pump stations package all of these components into a streamlined, insulated case, while still allowing access and pump ventilation. Heat is kept in the system and the difficult job of neatly and effectively field-insulating these components is already taken care of.
There is some variability in the specified operating parameters of different pump stations. Be sure to check the specifications for the maximum HTF temperature and pressure. Knowing the maximum HTF flow rate is also important, especially for larger, multicollector systems or if a drainback system (and larger or second pump) is being considered. If the station has an integrated heat exchanger, it must be appropriately sized for the amount of collector surface area.
The size and type of fittings also vary. Many of the popular pump stations are imported, and use metric components. Adapters to standard pipe sizes are included or available, but finding extras locally can be difficult. Whether the station components are metric or not, the adapters are usually easy-to-connect unions or compression fittings.
By themselves, pump stations are not very large—many are smaller than a carry-on suitcase. Weight is not usually an issue, though the stations should be secured, not merely be supported by pipes. Some systems include brackets for mounting on a wall or the tank, or brackets can be easily fabricated. Wall mounts generally require longer pipe runs to the tank, but also may make future tank removal or replacement easier.
Charging the pump station with HTF may require special fittings or a kit, which can then be reused. Some pump stations incorporate a check valve in the pump inlet housing; this is unacceptable for drainback systems and can make filling or charging pressurized, antifreeze systems difficult, so be sure to read the manufacturer’s instructions thoroughly. Access to the charging ports is an important consideration for whether to mount the station on the wall or tank. Additionally, when using a pump station and a tank with an integrated heat exchanger, it is a good idea to install a drain valve at the low point where the HTF exits the exchanger, which will typically also be the lowest point in the HTF loop plumbing. This allows complete draining; otherwise, the lowest drain could be on the pump station, potentially 4 to 6 feet above the low point, which will add to the mess when the tank needs replacing.
Many installers prefer to locate the HTF pressure relief valve outside, at the collector(s). Pump stations are only suitable for installation inside, so the integrated pressure relief valve may require an additional drain line. This drain may be able to be run in the same line as the drain for the temperature and pressure-relief valve on the storage tank, but in many locales it requires its own piping.
Installers agree that building a pump stack from individual components can be time-consuming—three to four hours (or more) is typical for plumbing the copper and brass fittings, gauges, valves, and pump, and installing a controller. Often this can be done ahead of time, though space and time constraints may require that it be done at the installation site. Furthermore, the tank may be more difficult to transport with the plumbing components attached. Either way, a bunch of fittings, valves, gauges, and components are required, as well as the means to pressure-test the final product.
Because there are fewer connections required and a pump station can quickly be installed on the tank or wall, it is often easiest to perform all the work on-site. But first be sure to verify that all of the required adapters and components are included and, as with all systems, perform a pressure-check before charging it.
Casey McDonald, an installer for Solar Technologies in Santa Cruz, California, has installed many SHW pump stations. “Pumping stations both make the system look more professional and simplify installation, and the fittings are more durable than standard hardware-store quality.”
According to McDonald, servicing pump stations is easier, too. “On custom, built-on-site systems, a lot of time is spent on trying to figure out the layout of the plumbing.” He says that pump stations cost more than build-up systems, and proprietary parts, such as metric-to-NPT adapters, can be difficult to find if one is missing or damaged. “Definitely don’t assume that all the pump station fittings are tight right out of the box,” McDonald says. “Pipe dope and pipe-thread tape first can avoid chasing leaks later.”
Justin Trievel has been installing solar hot water systems for EV Solar Products, Inc. in Chino Valley, Arizona, for several years, using pump stations and building pump “stacks.” He agrees that while pump stations are more expensive than individual components, they do save a good amount of time.
However, Trievel says that there are more limitations for nonstandard installations. He usually uses unmatched brands of pump stations and tanks, prefers to hang the pump station on the tank when possible, and says he has had only a few issues with the individual components in the pump stations. He is trying different brands and models of pump stations, as well as fully integrated systems, with all of the components pre-mounted on or in a tank.
Brian Mehalic is a NABCEP-certified PV system installer, with experience designing, installing, and servicing PV, thermal, wind, and water-pumping systems. He is an instructor for Solar Energy International and develops curricula for SEI’s PV program from his home in Prescott, Arizona.