Wiring and Circuits

Wiring is so simple! Do you believe that? It can be. It should be.

Here's a common description of how to wire hydronic controls. Have you heard it? "Ya just grab a wire from all the stats and stick 'em on the secondary. Take a wire from the zone valves and stick 'em on the other side. Now you just connect the valves to the stats. That's all there is to it."

Wiring is so simple!

Do you believe that? It can be. It should be. But with this kind of description, it might be impossible. What's a "secondary?" "Stick 'em on the other side" of what? How would you "just connect the valves to the stats?" There's nothing incorrect about the description. But unless you already know how to do the job, the instructions don't make much sense.

Controls wiring isn't rocket science. It's not house wiring, either. I don't care how many electricians you've watched pulling wire, what they're doing isn't the same as wiring controls.

House wiring is about pulling wire and sticking the ends somewhere, just like the description up top here. Controls wiring is about making circuits. You could possibly end up with the same job done. But if you just stick the ends of the wire somewhere, it'll take a fair amount of luck to do the job right.

Think In Terms Of Circuits

The word circuit has the same root as the word circle. It's the same root as circus, too, for that matter. I remember learning in Latin class long ago that the circus was where they fed all those guys to the lions, which brings up a not-all-that-inappropriate victim image of how we sometimes feel about wiring.

When we make circuits we're making circles. Think of a rope with the ends tied together to make a circle. Now put a bug on that rope and let him run. If the bug can go all the way around in one direction (doesn't matter which direction) and end up back where he started without ever getting off the rope, that's a complete circuit.

The same goes for electricity. Imagine your little electricity bug running around that circuit. (By the way, he's running on the wires, not through them.)

In real life, the circuit probably doesn't look much like a circle. What we're really talking about here is the continuousness -- the ability to go around and around without stopping -- rather than the actual circle shape.

There are three components of a control circuit. Think back to a long-ago science or electricity class, and you'll probably recognize them -- they are power supply, switch and load.

Everything in a circuit is one of these three. Every control is either a power supply, a switch or a load. Each circuit must have at least one of each.

In your mind, make a circle of these three things -- in any order -- and connect them with wire. Now let's put real controls into the positions. In the power supply position, put a transformer. In the switch position, put a thermostat. And in the load position, put a zone valve. Simple as this seems, it's a very ordinary and real-world circuit.

Vision Into Action

Here's how it would look on the job. The transformer is in the equipment room, probably near the boiler. The zone valve is mounted in the equipment room, or in the baseboard in the living space. The thermostat of course is in the living space. Wire connects them all into a circuit.

Yeah, but what about all those other wires, and the zone valves and thermostats for the other zones?

Hold on. We'll get to that soon. But first, let's talk a bit about the transformer. There are two "sides" to a transformer. One side comes with wires attached to it. This side is called the primary. These wires get connected to electricity from the electric company. Wiring the primary is the equivalent to plugging it into a wall socket.

On the other side of the transformer are two terminal screws. This side is called the secondary. Line voltage (120V, for example) goes into the primary, and 24V come out the secondary. The sole purpose of the transformer is to step down electricity from line voltage to low voltage.

Now let's return to the zones. For each zone, there's the simple circuit of the transformer, a zone valve and a thermostat. Each zone has its own zone valve and thermostat. But we can use the same transformer for several zones. (You'll hear more about how many at another time.)

Still With Me?

Focus again on the transformer. Let's say we have three zones. A wire from the R terminal of each of the three thermostats goes to the transformer. Saying it another way, each wire connects the transformer to a separate thermostat. The wires from the three different thermostats all go to the same place on the transformer, which is one of the two terminal screws on the secondary side.

A second wire from the W terminal of each thermostat goes to its own zone valve.

The thermostat connects with one wire of the zone valve motor. It doesn't matter which. The second wire goes to the second screw terminal of the transformer. (Note that if there are four wires on the zone valve, we're talking only about the two that go to the motor. If there are three, we'd need a different thermostat entirely, and we're not talking about that situation right now.)

So, at the transformer, there should be three thermostat wires on one terminal, and three zone valve wires on the other. But that may not be what you see! You may see only one wire on each transformer terminal. Here's what's going on. It's really hard to get three wires on one terminal. Even if you can get them on, you can't be certain they'll stay there. So we attach a separate wire to the transformer terminal.

To the other end of that wire we attach the three thermostat wires. That means that four wires come together. We twist the four, screw a wire nut over the connection, and call it a pigtail.

This is where a lot of folks get mesmerized and overwhelmed -- what are all these wires?! What you see when you look at the wire nut then is four wires, one to each of the three zone valves, and one to the transformer.

We end up with three simple circuits, which happen to share a transformer -- kind of like three kids sharing the same plate of food. Technically, this is called a parallel circuit, but the job turns out the same if we think of it as three simple circuits that happen to share a power supply.

Let's Recap

Troubleshooting is where thinking in circuits really pays off. Let's say you get the job wired up and the heat won't come on in zone two. Right away instead of wondering what's wrong with the whole job, you can isolate the problem and check out the circuit that controls zone two.

Also, by thinking about the components of a circuit, you know that the power comes from the transformer, and not from anywhere else. You know that the thermostat is the switch that turns the circuit on and off. And you know that the zone valve is the load -- the motor that opens and closes the valve.

Let's recheck that original set of directions. Notice that it makes more sense now: "Ya just grab a wire from all the stats and stick 'em on the secondary. Take a wire from the zone valves and stick 'em on the other side. Now you just connect the valves to the stats. That's all there is to it."

When wiring hydronic-heating zones, whether you're using zone valves or pumps, you can get around thinking about circuits by using a zoning panel. Compared to the cost of labor and confusion, they're very inexpensive. The panel comes with the transformer. You just insert the thermostat wires into the thermostat terminals, and the valves or pumps into their terminals. That's it.

Troubleshooting is simplified, too. I call it the equivalent of a three-ring binder with subject tabs. If you put the stuff where the labels say, then you know exactly where it is when you come back to it later.

So the guy that said, "That's all there is to it," was right. It's just that you need to know a little more before that's the case. 

Would you like to learn more from Carol Fey? Check out her books and classes.