Primary Secondary Pumping

You may have heard your home heating system referred to as a “primary-secondary system”, but what does this mean?

Primary-secondary pumping is a hydronic (water-based) heating technique pioneered by Xylem RCW’s Bell & Gossett brand in the 1950s. At its simplest, it calls for two (or more) “loops” of heating water—a primary loop that exits from and returns to the boiler and a secondary loop that takes water from the primary loop and moves it around a room or similar “zone.” This arrangement needs two or more circulator pumps, but (as we’ll see) these pumps can be small, energy-saving ones.

This hydronic system was originally developed for large buildings with multiple zones and long loops of pipe. Water temperature will fall dramatically as water winds through a large system, but returning cool water is a problem for boilers—it can cause flue gas condensation and thermal shock, both of which can cause permanent damage.

Today primary-secondary pumping can be found in homes. It is an especially important for radiant heating systems, especially underfloor systems. Typically, the design specifications for underfloor heating loops require much lower water temperatures than are supplied by the boiler. However, boiler return temperatures must be kept above their minimum. Water might be exiting a secondary radiant loop at 90ºF, but the minimum return temperature of a domestic boiler might be 140ºF. In a primary-secondary system, cooler water exiting the secondary mixes with hotter primary loop water, ensuring that water returning to the boiler is warmer than the minimum.

Furthermore, modern high-efficiency condensing boilers (that recover “wasted” heat by condensing steam vented by traditional boilers) also benefit from primary-secondary pumping. These boilers have a higher resistance than traditional boilers, which potentially could restrict water flow throughout a hydronic system. However, in primary-secondary pumping, the flow in each circuit is separate, so the resistance in the primary loop will not affect flow in the secondary loop.

Aside from protecting boilers and separating flow, there are two other important advantages to primary-secondary pumping:

  1. Less energy is required to move water through the entire system (rather than one large circulator, small energy efficient circulators can be used to overcome the friction and inertia (“pressure drop”) of their respective loops).
  2. More control can be taken over zones (and each zone can operate at its own optimum temperature).

The engineering principle behind primary-secondary pumping is simple, and beautiful. As long as the “common pipe” shared by the T-junctions where the primary and secondary loops meet is no more than 12 inches apart—to eliminate any pressure drop in this pipe—water flow in the loops is kept separate.

The small common pipe comes into its own when a secondary loop’s circulator pump is off. Under these conditions, water will see the short common pipe as the path of least resistance, and it will ignore the secondary loop. If the common pipe had significant pressure drop, water might flow (unwanted) into the secondary loop. When the secondary circulator is on, some hot water can still flow across the common pipe and mix with the cooler water coming out of the return T-junction. This mixing raises the temperature of water returning to the boiler.

For more information on primary-secondary pumping, and to learn about the many variations on this technique beyond the scope of this article, contact your Xylem dealer, distributor, or installer. He or she can tell you about the components needed to make your system work, including:

Circulators—Wet rotor cast iron circulators are an excellent choice to loop water around a secondary circuit (don’t forget always to pump away from the boiler), and a must for any “green” primary-secondary hydronic heating system are Xylem RCW’s new energy efficient electronically commutated motor (ECM) wet rotor circulators.
Enhanced Heating Module—If building a new primary-secondary system, you’ll want the entire hydronic system to be well designed. Your Xylem dealer, distributor, or installer can explain how this is done, using safety and comfort components that are combined in the enhanced heating module.
Flow Control Valves—These are recommended to prevent any flow into the secondary loop induced by even the slightest pressure drop that may exist in the common pipe, or by gravity. Because gravity flow can occur within a single pipe, two valves are best, one on the supply and one on the return.
Primary-Secondary Header—One of the most useful primary-secondary components on the market, the primary-secondary header is a combination common pipe/manifold, air separator, and purge valve that allows the user to remove any debris trapped in the system.
Zone Controllers—Take complete control of your system with a digital zone controller that is weather-responsive (adjusting system water temperature to outdoor temperature) and that intelligently mixes system water for optimal performance of radiant loops and the boiler.