Solar Thermal Hydronic Combisystems, like all hydronic systems, owe their very existence to the quiet, humble yet reliable centrifugal circulator pump. These pumps are at the heart of every successful thermal hydronic system, and have been engineered to perform with impressive longevity and performance over years of service. Some of these hydronic circulators have performed so well for so long, with such satisfying results that it is hard to imagine that they could get much better. But, there has been a quiet revolution with enormous gains in efficiency. Circulator pumps are not just getting a little better, their energy performance is being doubled, and without sacrificing the quiet reliability on which we have come to depend. 

PSC conventional motors

The most familiar small to medium-sized hydronic circulators consist of a compact pump-package that includes a pump-body (volute) with an impeller inside, driven by a rotor that is powered by the stator of an electric motor. This compact design is known as a “wet rotor circulator” because the rotor, impeller and shaft bearings are all immersed in the system fluid which provides all the cooling and lubrication needed for the moving parts.

Most existing wet rotor circulators installed to date have Permanent-Split-Capacitor (PSC) motor designs. These are characterized by a large "starter capacitor" wired into "starter windings" in the pump motor. The windings are coils of wire that are strategically placed around the circumference of the motor to provide a rotating series of magnetic fields that ‘pull’ the rotor around which spins the impeller. The capacitor and starter windings are added to create an electrical phase-shift large enough to generate enough torque to start the rotor spinning even when it is at rest. Once the rotor is spinning normally, the capacitor and starter windings are not needed but consume some electricity anyway simply because they are a permanent part of the motor circuitry.

ECM high-efficiency motors

In recent years, the manufacturers of wet-rotor circulators have been offering some models with high efficiency motors known as Electronically Commuted Motors (ECM).  These are permanent magnet motors characterized by a high-efficiency programmable brushless DC motor using a permanent-magnet-rotor with built-in inverter and microprocessor. The old capacitor and starter windings are gone, replaced with precise digital control.  

A microprocessor is the brains of this motor, converting AC power to DC to operate the internal electronics with the ability to adjust the power frequency (which controls the speed or RPM) and the amount of current (Amps) delivered to the motor. With a microprocessor-based motor controller, higher electrical efficiencies can now be maintained across the entire operating range, saving considerable electrical energy compared to previous motor technologies. ECM wet-rotor circulators are now widely available in the U.S. from well know manufacturers including Taco, Wilo, ITT, and Grundfos. These motors can generate about four times more starting torque than the capacitor-start system, which should help eliminate certain stuck-rotor failure situations. The new ECM circulators are packaged with the same common pump flanges and flange spacing as older models, so they can be substituted without any serious plumbing modification. For example, Two Interchangeable Circulators.

We began offering ECM circulators to our solar and hydronic combisystem installers a few years ago. The early installations typically involved remote locations or off-grid installations where locally-generated electricity was precious, and electric efficiency was a primary concern. More recently, it seems that about half the circulators we put into our new or retrofit standard combisystems are ECM. Here is a quick comparison of two circulators that we often see in the field that are virtually interchangeable with one another and, so far, seem to be equally reliable.

The UPS15-58F conventional circulator

Figure 83-1 shows the pump curve and some specifications for the conventional Grundfos UPS15-58 PSC circulator. The convenient size of this 3-speed pump has made it very popular for a variety of jobs in many of our Standard Combisystem installations.  It has been used for radiant floors, baseboards, indirect water heaters, heat exchangers, boilers and countless other hydronic circulation jobs. In many of our systems, we have been able to specify a single pump like this for every heat source and every heat load in the combisystem. The job may have half a dozen different circulators installed, but all the same make and model, with the speed switch is set to match the flow requirement of each plumbing loop. This kind of redundancy allows for pump swapping in an emergency if necessary.

Figure 83-1 shows that the maximum flow rate for this pump (e.g. around a primary loop) is 17 gallons per minute. You can also see that the maximum power input is rated at 87 watts. To get a feeling for the comparative energy consumption of this pump, divide the watts by the maximum flow (87/17=5.1 watts per GPM max at high speed). This number can be used to compare pumps on the basis of watts per unit of flow under similar conditions of maximum flow.

The ALPHA15-55F ECM circulator

Figure 83-2 shows the pump curve and similar specifications for the high efficiency Grundfos ALPHA15-55 ECM circulator. It also provides three speeds and operates in virtually the same range of flow and head as the UPS15-58 above. It has exactly the same pump flange arrangement and so fits easily into the same space as the UPS15-58. So, our installers have been using this model as a direct substitute in recent installations. When we do the same comparative energy calculation for this pump using the maximum power input divided by the maximum flow, we get 45 watts / 21.5 GPM = 2.1 watts per GPM (which is much lower than 5.1 noted above). While this may be ballpark number, it clearly establishes which circulator has the greater potential for high performance. The ALPHA pump can be expected to move the same amount of fluid while using less than half the electricity (41 percent in this example). This is, in fact what we see in the field in terms of electrical savings.

Enhanced ECM control features

When the energy consumption of a circulator is cut in half, the resulting lower parasitic energy, higher coefficients of performance (COP) and lower carbon emissions are all good reasons to use ECM pump motors. But, because of the built-in microprocessor capabilities, there are also additional benefits to be considered. Figure 83-3 shows how the other control functions built into the ALPHA circulator can be used to optimize energy performance. The fixed speed control settings allow low speed pumping power to be reduced as low as 5 watts. There are also three speed settings of constant pressure operation so, for example, the motor can adjust the flow to match the number of zone valves that are open. There is also an “AutoAdapt” function and all of these settings can be chosen by pressing the button on the back of the pump motor and stepping through a menu.  It is important for the installer to read and understand the instructions and then choose the right setting for each circulator when first installed. When power is shut off, the user-settings are not lost, and are recalled when power is restored. An LED digital display is included on the ALPHA that can be set to display the Watts or the estimated GPM flow rate, which is valuable feedback to verify that the settings have been chosen correctly.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than ten thousand square feet. The focus is on pressurized glycol/hydronic systems since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement. 

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed Mechanical Contractor in New Mexico. He is the Chief Technical Officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com.