As hardware and electricity prices continue to rise, it’s likely that new approaches to this traditional technology will soon become the standard.



For decades, most boilers used in North America had large internal fluid passages and exhibited very low flow resistance. When these boilers were combined with generously sized supply and return headers, the overall flow resistance of the “common piping” was low enough that several zone circulators could operate simultaneously with very little interference (see Figure 1).

Figure 1

This began to change with the arrival of modulating/condensing boilers, many of which used compact heat exchangers. Their flow resistance is much higher than a traditional cast-iron or steel fire-tube boiler. If such a boiler is connected to a header system supplying several simultaneously operating circulators, there will be significant flow “bottlenecking” by the boiler. This will lower flow through the distribution circuits and decrease heat delivery rates. It’s definitely a situation to avoid.

The traditional remedy to this situation has been the use of primary/secondary piping. The high flow resistance boiler is set up on its own secondary circuit, as shown in Figure 2.

The distribution system is configured as a parallel primary loop. The closely spaced tees in each crossover provide hydraulic separation between each of the circuits. The parallel configuration of the crossovers ensures equal supply temperature to each secondary circuit.

Figure 2

Although this configuration has proven itself in many systems, it does require additional hardware relative to a series primary/secondary system. It also requires a dedicated primary loop circulator that operates when any of the secondary load circuits are active. The cost of having this circulator is twofold: 1) the cost to purchase/install the circulator, and 2) the cost of operating the circulator over the life of the system. The latter cost is often many times the former, as the following example illustrates.

Consider a system that supplies 500,000 Btu/hr to a compact parallel primary loop at design load conditions. The common piping in the primary loop is 2-inch copper, and the five crossovers are 1-inch copper with balancing valves. Design flow in the common piping of the primary loop is 50 gpm with a relatively small corresponding head loss of 3.4 feet (1.44 psi pressure drop). Assume a wet rotor circulator with wire-to-water efficiency of 25% is used as the primary circulator in this system.

The input wattage to the circulator can be estimated as follows:

Assuming this primary circulator runs for 3,000 hours per year, and that electrical energy costs $0.10/kWhr, its first-year operating cost would be:

Assuming electricity escalates at 4% per year, the total operating cost over a 20-year design life would be:

This, combined with the installation cost of the circulator and additional piping hardware associated with the parallel primary loop, is not a trivial portion of the system’s life-cycle cost.

Figure 3

Numerous Alternatives

The schematic in Figure 3 shows one way to retain the benefits of hydraulic separation as well as equal supply temperatures to the load circuits without use of a parallel primary loop and its associated circulator.

The boilers are connected to a large-diameter vertical header within a relative compact space. Boiler piping is hydraulically separated from the vertical header using a pair of closely spaced tees. The generous diameter of the header and relatively close spacing between all supply and return connections results in a very low pressure drop between points A and B-low enough that each load circuit is effectively hydraulically separated from the others.

The header should be sized for a maximum flow velocity of 2 feet per second with all load circulators operating. Its length should also be kept as short as possible. If the header is installed vertically (as shown in Figure 3), include a float-type air vent at the top and a drain at the bottom. The latter allows sediment returned from the load circuit to accumulate at the bottom where it can be periodically flushed out. Because the load circulators are in parallel and connected to a common header, each circuit must include a check valve (internal to circulator or external) to prevent flow reversal through inactive zone circuits.

Figure 4

Another option is use of a specialized component called a hydraulic separator between the boiler and the load circuits, as shown in Figure 4. The hydraulic separator uses the same physical principles at work in the closely spaced tees of a primary/secondary piping system. The low vertical velocity inside the separator produces minimal pressure drop top to bottom as well as side to side. This results in hydraulic separation between the boiler circuits and load circuits.

Figure 5

Either of these alternatives allow the use of both fixed and variable-speed circulators on the load header side of the hydraulic separation point. Keep the headers as short as possible and size them for a flow velocity no greater than 2 feet per second under full design flow. An example of a system using both a fixed-speed circulator and variable-speed injection mixing pump on the load side of a hydraulic separator is shown in Figure 5.

Figure 6.

Hydraulic separators are a common component in European hydronic systems. Figure 6 shows a typical multiple boiler system equipped with a modular header assembly that connects to a hydraulic separator. The load headers would connect to the left side of the separator. This hydraulic separator also provides air and dirt separation functions for the system.

Figure 7.

Some hydraulic separators come equipped with load headers, as shown in Figure 7. The headers provide equal supply temperatures to all loads.

The number of companies offering hydraulic separators in North America also is growing. Figure 8 and Figure 9 show the use of a hydraulic separator in a large industrial radiant floor heating system in the Northeast.

Figure 8

Hydraulic separation also can be combined with thermal buffering using an insulated tank with appropriately placed connections, as shown in Figure 10. This concept is ideal for situations where a fixed-firing-rate boiler is used to supply a highly zoned distribution system. It also benefits situations where the turndown ratio of a modulating boiler is not sufficient to supply “microloads” without short-cycling.

Figure 9.

Back To the Future

One might look to the time when cast-iron and steel fire-tube boilers were used in the vast majority of North American hydronic systems and conclude that we’ve been relying on hydraulic separation for decades. Few of us, including the author, used the words hydraulic separation to describe the physics at work in these traditional systems. Perhaps we just assumed that circulator interference and flow bottlenecking at the boiler were “nonproblems.”

As boilers with higher flow resistance entered the market and complex multiload systems were designed around traditional piping schemes, the lack of hydraulic separation taught us some valuable lessons. The industry’s overall response was to turn to primary/secondary piping as a means of reestablishing the hydraulic separation provided in traditional systems. Although this approach works, it requires an extra circulator and additional piping components.

Figure 10

Today, the North American hydronics industry is “rediscovering” the benefits of hydraulic separation combined with traditional header-type piping applied to multiload/multitemperature systems. We’re also learning this uses less componentry and lower pumping wattage relative to primary/secondary piping. As hardware and electricity prices continue to rise, it’s likely these newer approaches will soon become the standard.