Series Circuit - Switches: As you know, a series is a group of similar things strung together. For example, the World Series is a string of baseball games. In a circuit, we can put switches in series or we can put loads in series. For a series circuit to work, the electricity has to get through everything in the circuit and back to where it started.
In control work, a series circuit can be either a great thing or a bad thing. It depends on whether we're putting switches in series or loads in series. Switches in series is great. In the controls biz we try to avoid loads in series - they just don't work right.
Let's look at switches in series (see Figure 1).
All of the switches must be closed before the electricity can get to the load.
Often in controls, we have switches in series. That's because switches frequently function as a safety committee. Each switch stays closed unless its safety condition is violated. Examples of safety switches are a pressure switch, a flame roll-out switch, a low water cut-off or a high limit.
Power can't get to the load unless all safety conditions are met. The thermostat can be in series with the safety switches. Just like many other committees, no action can be taken until all parties agree. That means that even though there may be a call for heat (thermostat switch closed) the burner can fire only if all the safety switches are closed.
This brings up the importance of using a wiring diagram. The wiring diagram that comes with a boiler shows all the safety switches. If you're troubleshooting and there's an open safety switch that you don't know about in the series, no amount of replacing the other safety switches or thermostat will fix the problem. The wiring diagram will show you exactly how many switches exist.
Series Circuit - LoadsWe can put loads in series, but we don't usually do that in control work. That's because loads in series don't behave right. The reason is that they have to share electricity. For example, two 60W light bulbs in series are only as bright as 30W bulbs. Figure 2 is a drawing of loads in series.
The typical electricity book and electricity class spends a lot of time on loads in series. That's where you do all that math. If you ever suspected the math and loads-in-series part of the class had nothing to do with your heating or air-conditioning work, you were absolutely right.
The classes include loads in series because it's key to electronic circuits. Many schools think the purpose of an electricity class is preparation for electronics. In electronics, much math can be done to figure out the right combination of resistors, transistors and diodes to make a circuit to get the desired magical outcome. In heating and air-conditioning control work, we use electronic devices, but we don't design itty-bitty electronic circuits. That means the only thing we need to know about loads in series is "don't do it."
Parallel CircuitsInstead, we wire loads in parallel. Notice that the components stay the same as in a series circuit. Only the wiring changes. (See Figure 3.)
Notice that a parallel circuit is merely two simple circuits that happen to share a power supply. Just ignore the shaded portion of the circuit to see that each of the two loads can get electricity from the power supply without the electricity going through another load. If these were 60W light bulbs, both would be fully bright.
Notice there's just one switch shown in this circuit. Also like the bathroom vanity lights, in this case one switch turns on all the lights.
It's easy to add switches so that each light can be switched alone. But the switch must be placed in the circuit so that it controls only that light.
In a "real life" drawing, a parallel circuit with a switch for each load looks like this.
It may simplify your zone valve wiring to think of each zone valve and thermostat combination in this way. You may refer back to my column in the December 2003 issue of PM to figure how many zone valves you can put on a transformer.