Really basic electricity

Feature Story

Understanding the most complex circuits is really just a case of knowing the simplest

By George Lanthier

If there’s one thing most people agree on in this business, it’s that electricity and controls are one of the most troublesome areas for them. Then, they target low voltage and zone valves as the biggest problem. It’s so bad with some of us that we won’t use zone valves just because we don’t know what we’re doing with them.

Figure 1

Figure 2

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Figure 6

Figure 7

Figure 8

The sad part is that understanding the most complex circuits in the HVAC business is simply a matter of understanding the most simple, the basic circuit. A lot of people like to teach neutrons and magnetic fields and all kinds of other things that you just can’t see, and most people never understand these theories. What’s more, and this is only my opinion, you just don’t need to know it. What I like to teach is what I call ‘working wires.”

In my opinion, what’s important is what are the wires or conductors, the switches and the loads there for and how to make them work That’s all, no mumbo-jumbo and no theory that’s over most of our heads. I’m not crazy about over-simplifying the theories just to impress people, either; just give me the facts and leave out the BS.

Let’s start with the basic building block and go from there. The following is an excerpt from my latest book, WIRING & Fuel Burning Equipment, Volume 1, Fundamentals:

To understand all wiring circuits, you must remember a very simple thing to handle any troubleshooting, and to correct problems made by others, the basic circuit. The basic circuit consists of three components:

The power source
The switch
The loadIn Figure 1, we have those three components represented by a battery for the power source, a switch and a lamp, and representing the load, a flashlight. If the power source is present and can supply the correct voltage and amperage to operate the bulb, then when the switch is closed, the bulb will work. Before we proceed any further, let’s take a look at where this basic principle for wiring shows up over and over again.

In Figure 2, we now have replaced the battery with standard 120-volt electrical power. The bulb has been replaced by a motor and the switch is still there, giving another example of a basic circuit. In Figure 1, the power is a DC battery and in Figure 2 it could be nothing more than a standard wall outlet.

In Figure 3, we have another example of a basic circuit, but this is low-voltage, 24 volts AC. Using Figure 1 again we can see that our battery has been replaced by a transformer that converts 115 Vac to 24 Vac. Our switch remains present, but in most cases will become a room thermostat and, finally, what was the bulb in our flashlight becomes the motor in our zone valve. Keep this basic building block of all electrical circuits in mind and there’s not a circuit you can’t troubleshoot.

Next up is switches and basically all of the switches that we use in the heating service business fall into two groups:

The single-pole, single-throw (SPST) switch
The single-pole, double-throw (SPDT) switch As you will see, this is not only true of manual switches, like the service switch, but also of mechanical switches, like an aquastat and a relay. In another article we’ll discuss relays and will add another switch that may be within the design of the electrical relay called a double-pole, double-throw (DPDT) switch. However, keep one thing in mind: if you really know how a SPST switch works, then you know how all switches work.”

SPST Switches
In Figure 4, we show a schematic example of the most common switch that you will work with, the SPST switch, shown in Figure 5. The SPST switch provides for only one path of electricity to follow. In the drawing on the top, the switch is shown in the normally open position, and on the bottom, in the normally closed position.

In reality, this switch design is used in pressure controls, water controls or aquastats and warm-air controls. It can be found in many circuits in both open and closed applications and in the common wall switch shown in Figure 6.

Here’s a question for you. Is a customer switch considered normally open or normally closed? If you said normally closed, you are right. We normally want the burner to run, right? Now, where would you find a SPST switch? A high-limit, or any limit control is normally an SPST switch.

Service switches, customer (emergency) switches and most of our limit and safety switches are all SPST switches, but are they normally open or normally closed? If you said closed, you are right.

In all of these cases we would want the burner to run unless we had an unsafe condition. If we had too high a pressure or temperature, we would want our high-limit switch to open and shut off the burner. If we did not have enough water in the boiler or system, we would want our low-water cutoff switch to open and also shut off the burner.

Now where would we use a normally open switch? Well, how about the switch that turns on the blower on a warm air furnace, or what about a circulator reverse on a boiler? In both of these cases we would not want operation unless a preset temperature was established, so we would use a normally open switch.

A normally open switch is one that will not allow power to travel across its contacts until the switch is closed. A normally closed switch will allow power to travel across its contacts until the switch is opened.

When we close the contacts of a switch, through either a manual, mechanical or electrical action, we normally refer to this action as ‘making” a switch. When we open the same contacts, we refer to it as ‘breaking” a switch.

SPDT Switches
With a single-pole, double-throw switch you can direct electricity into two paths. In Figure 7, we have a schematic view of an SPDT knife switch, Figure 8. Note that in both views, one of the switches is made. That’s because the design of an SPDT switch allows us to do two things at different times. In one case, you might want to make sure that ‘something” is off and that ‘something” is on. After a certain pressure, time or temperature is reached, you want to change the operation around and flip the item that is powered off and turn the device that is off, on. Sound like double talk? Keep reading.

The common terminal in heating controls in always the ‘R” terminal; regardless of manufacturer, it’s standard. In heating controls it’s also a standard that ‘R-B” is closed until set point and ‘R-W” is open. When set point is reached, ‘R-B” opens, and ‘R-W” closes. In fact, as you will see, one of the mind-joggers for this is ‘Run-Burner” and ‘Run-Water” and we’ll show you what that means.

So, now where would we use an SPDT switch? How about the switch that is now the heart of any hydronic system controller? This switch is found in every multi-function controller today that is shipped with a boiler with an internal tankless. It may have a name like Honeywell’s L7224, L8124 or R8182, but it’s in there.

Now, in the case of multi-function controllers, it would be what is known as a triple-acting aquastat.

Keep one indisputable thing in mind about SPDT switches: they cannot pass current through both load terminals, period. So don’t get in a panic when it confuses you on a job. Stop, take a look at your switch action and figure it out.

Switch Differential
Differential is probably the most difficult thing about switches to understand. In actuality, it is not that difficult, but many of us try to make it too complicated, so here goes.

If I told you to turn on the lights in a room, you would go to a light switch and turn it on. If I told you to turn the lights off, you would reverse the process. The action that you would take in both cases is mechanical. You would turn the switch on and you would turn the switch off. Your finger on that switch handle is a mechanical action and would require your manual motor skills to perform it. In the switches around oil, coal and gas heating equipment we use magnetic, mechanical, sensible and timed actions to make a switch react. Magnetic switch actions are created through the use of electromagnetic coils in relays and by using solid-state devices.

In sensing-type switches, the actuator recognizes the presence of light, pressure or temperature. A sensible light application is similar to a cad cell on an oil burner. A sensible heat application is similar to a thermocouple or flame rod on a gas burner.

In steam systems, and any pressure application, pressure is used to push on a diaphragm or a bellows to open or close a switch.

In older-type hot water systems, temperature is used to expand the medium fluid in a sensing well and open or close a switch through hydraulic action. In newer systems it’s done through an electronic sensor called a thermistor. A thermistor is a heat-sensitive resistive device that increases or decreases resistance based on temperature.

And finally, in warm-air systems and any air-related application such as a room thermostat, heat once again is used to expand a bi-metal and cause a mechanical reaction. In timed applications, timing mechanisms and resistors are used to control switches.

Now, what about that differential? Well, in all of these cases there will have to be a difference or differential in conditions. We will either have a certain amount of pressure or temperature or we will not.

Let’s say that you have a normally closed switch. That switch is set to remain closed, to allow ‘something” to run until the temperature reaches 180F, and then it opens. Now, when does it turn that ‘something” back on? Aha, you would have to know the differential, right?

Let’s say that the differential was 10F, then it would turn that ‘something” back on again at 170F. If the differential was 80F then that ‘something” couldn’t come back on until the temperature dropped to 100F. Now, that’s a difference in temperature, and one heck of a differential.

Now another thing about differentials, they come two ways in this business:

AdjustableWith a fixed differential you cannot change the switch’s action. If you have a 10F differential, then that’s what you have, and it will take 10F of change from the set point to make something happen.

With adjustable differentials, you can set the differential and get the switch, and the controlled ‘something” to do what you want it to do.

Now, it doesn’t matter whether the switch is SPST or SPDT, it has to have a differential to work, got it?

See ya!

George Lanthier is the owner of Firedragon Enterprises, a teaching, publishing and consulting firm. His Web site can be found at

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