Look at the overall schematic of hydronic piping schematics as a group of subassemblies, each of which accomplished a specific (but limited) task.

Years ago in high school shop class, I remember being shown the schematic of a radio. Spread out over the large sheet of paper were hundreds of neatly drawn symbols for components like resistors, capacitors and transistors. All woven together into a web so seemingly complex that it amazed me that anyone could actually have thought it all out. It looked like a giant maze. How could the electrons possibly know where to go much less how fast to go there, and how to oscillate back and forth in such complex ways?

In the days that followed, my shop teacher patiently explained the radio schematic as a collection of “stages.” He drew dividing lines through the complex web and explained how the components in one stage collectively functioned to provide power for the entire system, how another stage provided the proper oscillating frequency that tunes the radio to a given station, and still another amplified the signal strength to drive the speakers. Each subassembly received some kind of input signal, did something to that signal, then passed it along to the next stage.

He taught the class to look at the overall schematic as a group of subassemblies, each of which accomplished a specific (but limited) task. Once it sunk in, this approach made what appeared to be a hopelessly complicated system pretty easy to understand.

The same approach can be applied to hydronic piping schematics. Both for designing new systems, as well as when trying to figure out how a system designed by someone else is supposed to work.

Some Things Seldom Change: Most residential and light commercial hydronic systems have several essential subassemblies. These include a make-up water assembly, an air separator/expansion tank assembly and a purging assembly.

The make-up water train typically consists of a shut-off valve, pressure reducing valve, backflow preventer — preferably a pressure gauge — and fast-fill bypass valve. It’s such a common grouping of components that an installer could make up several ahead of time on a slow day back in the shop. Each one ready to become part of a future system. A designer can basically do the same thing with the group of symbols representing these components. The symbols can be grouped together as a single entity using their CAD system. When a new system is being drawn, a copy of the entire sub-assembly can easily be “pasted” into the appropriate location on the drawing. Whether dealing with hardware or just symbols on a drawing, this “modular” approach is simple, fast and consistent.

The air separator and expansion tank are often paired together as a subassembly and piped into the system near the boiler outlet. This allows the air separator to work on the hottest water in the system where conditions for bubble formation and capture are near optimal. When the expansion tank is connected to the bottom of the air separator — either directly threaded into it, or connected by a length of pipe — it establishes the point of no pressure change at the air separator. Hence circulators (other than secondary circulators in a primary/secondary system) should be located reasonably close to, and pumping away from, this point.

Purging subassemblies consists of a means of temporarily blocking flow at some point in a circuit, as well as a way of directing the initial mixture of air and water out through a drain valve. In a single loop system, one purging location is fine. In multiple loop or primary/secondary systems, it’s best to repeat this subassembly once on each loop.

Carving It Up: One of the easiest piping arrangements to dissect into subsystems is a primary/secondary system. An example of a fairly complex (hypothetical) primary/secondary system is shown in Figure 1. Overall it looks like a complex labyrinth of lines. How can water possibly “figure out” what it’s supposed to do in such a complex situation?

Now I realize that some of us expect water to know how to follow the arrows on piping schematics. But from what I’ve seen and heard, the water here in North America just doesn’t seem to be as smart as that over in Europe. Thinking back on some of the piping layouts I’ve seen, I would almost suggest that some imaginative (albeit unscrupulous) European marketer could sell high priced “smart water” — you know — trained to follow arrows to some folks on the other side of the Atlantic.

Seriously, the best way to know what the water will do in complex systems is to view it as a collection of several interconnected subsystems. Many of the subsystems in Figure 1 are close to being self-contained, but still lack a heat source and a central means of coordinating their individual operation. In this case the primary circuit provides the framework to tie several subsystems together. Think of it as a means of extending hot water outside the boiler, to a point where any of the secondary circuits can pick it up and carry it on to some heat emitter(s). Like a quarterback inconspicuously handing off the ball to a running back.

A secondary circuit can be as simple as a piping loop with its own circulator supplying a single heat emitter. Or, instead of being a circuit, it might be a more complex subsystem that divides flow among several independently controlled piping paths, or incorporates a mixing device to reduce water temperature to its heat emitters. For example, the three baseboard “strings” shown in Figure 1 each has its own circulator and flow-check and can be independently controlled. They are connected to supply and return trunk pipes that in turn connect into the primary circuit with closely spaced tees. This arrangement allows each baseboard circuit to receive water at the same supply temperature. A possible application would be to provide second stage supplemental heat to certain high load floor heating zones.

Moving downstream along the primary loop we encounter a “home run” subassembly supplying panel radiators and baseboards via individual tubing runs to a common manifold station. Such a subsystem might be used, for example, to supply several individually controlled rooms on the second floor of a house. Each room thermostat would activate its respective valve actuator on the manifold. The end switch of the actuator would signal the manifold circulator to run. It would also enable the boiler to fire and start the primary circulator.

Part of the home run subsystem is the differential pressure bypass valve. It’s a preferable component whenever several valve-controlled zone circuits are supplied from common trunk pipes or manifolds. Just like a make-up water assembly, the manifold, circulator, bypass valve and piping that connects them right up to the primary circuit can be thought of as a single entity. In this case, one that can be sized up or down to accommodate the necessary zone circuits.

Next along the primary circuit is a small floor heating subsystem using a three-way thermostatic mixing valve. Like the other secondary circuits, it’s connected to the primary loop with a pair of closely spaced tees. This fundamental primary/secondary sub-assembly is what prevents the pressure distribution created by the primary circulator from interfering with that created by any of the secondary circulators or vice versa.

Finally, at the tail end of the primary loop is another floor heating subsystem. It uses a variable speed injection pump as the water temperature control device.

The one subsystem that’s not connected as a true secondary circuit in Figure 1 is the indirect domestic water heater. Instead, it’s piped in parallel with the primary circuit. This subsystem eliminates the need of circulating hot boiler water through the primary circuit — which otherwise serves only space heating loads — during every call for DHW heating. It’s especially nice for eliminating extraneous heat loss during warm weather when the DHW tank is the only active load. Because the DHW circuit is parallel pumped with the primary circuit both require flow-check valves to prevent reverse flow during inactive periods.

Pondering Performance: The next time you encounter a piping schematic, look at it as a collection of subsystems. In your mind, run each of the subsystems through its paces one at a time. Think about temperatures and flows in all parts of the system when each particular subsystem is running by itself, and again when other subsystems are operating with it. Ask yourself if it’s possible — or even desirable — for all the systems to operate simultaneously. What happens when the building is recovering from a deep temperature setback? If simultaneous operation of all loads is not what you want, start planning your prioritizing strategy and controls. Think through how the system will handle a “cold start” condition of a large thermal mass if one is present. Will any particular subsystem or combination of subsystems cause the (conventional) boiler to operate with unacceptably low return water temperature? If so, how do you plan to protect it? These queries can help refine your design and eliminate — or should I say minimize — surprises in the field.