You could replace a thermostat with a light switch, and the heating equipment would never know the difference.
One day long ago, when I was newer in this industry than I am now, I was sitting in the well-appointed office of a large homebuilder. I was there because the heating contractor had told me that the reason he was putting in the cheapest (in both meanings of the word) thermostat was because the builder told him to. The builder had just told me that he went with whatever the heating guy recommended.
As I began my pitch, "Well in that case, we can do what's best for the homeowner..." he interrupted.
"Stop! The homeowner doesn't care what thermometer's on the wall."
Thermometer. Thermometer? A thermostat is not a thermometer. A thermostat is a switch!
That's right. A thermostat is a simple switch. When I teach, I even have the class repeat, "A thermostat is just a switch. A thermostat is just a switch." No matter how fancy the thermostat, its function is a simple turn-it-on, turn-it-off switch.
You could replace a thermostat with a light switch, and the heating equipment would never know the difference. (The people would certainly know the difference. We'll get to that in a moment.) But when you're thinking about a control circuit, you can substitute the simplest switch you can imagine into your thought process in place of the actual thermostat.
I say this because thermostats are greatly misunderstood. Some folks, such as that builder, think they're much less than they are. But other folks, such as some in our industry, think they're much more than they are. In terms of the parts of a circuit, a thermostat is not a power supply (as in "I know this stat's no good because it's not putting out any power"). Neither is a thermostat a load (as in "How much power does that stat pull?").
So as far as what a thermostat does in a circuit, it is either on, calling for heat (closed switch, letting power through so the equipment can come on), or is it off, not calling for heat (open switch, not letting power through to the equipment).
Too Hot, Too Cold
The reason we use a thermostat rather than a wall switch is to automate the temperature control and to keep the temperature consistently comfortable.
With a wall switch, someone would have to notice that heat is needed and flip the switch. By then the room would be too cold. Then the heat would run until someone realized it was too hot. The too hot, too cold situation is called temperature swing, or in every day people terms, "This place is never comfortable."
The switching happens between the screw terminals that are either on the thermostat or the wall plate that comes with it. The heating terminals are marked with the industry standard R and W.
R means "red." A considerate installer will use a red wire to connect this to one of the transformer terminals. (Experts say it doesn't really matter which.)
W means "white." The considerate installer will use a white wire to connect this terminal to the "TH" (for thermostat) terminal of the load, if the terminals are marked, as they are in a gas valve or ignition module. If they aren't marked, as in a zone valve, either one will do.
The R and W thermostat terminals are just like the two screw terminals on a light switch. The switching action happens between them, inside where you can't see it. When there's a call for heat, the thermostat closes the connection between R and W.
If a thermostat has only R and W terminals, it is "heating-only" or "two-wire." Most thermostats today have at least four terminals: R, W, Y and G. This is called "heat-cool" or "four-wire." You can always use just the two heating terminals of a four-wire stat.
The Y (yellow) terminal is for an air-conditioning compressor, and the G (green) terminal is to bring a blower up to high speed for air conditioning. A four-wire stat has three switches inside it: R-W for heating, R-Y for the compressor, and R-G for the high-speed fan.
Sometimes an electronic thermostat also has a C terminal. If the C terminal is present, it must be wired to the transformer in order to power the clock of a setback thermostat. Other terminals, such as O and B, are for accessories, and you can ignore them.
How does the thermostat switch get opened and closed? This is where automation comes in, even if it's an old-fashioned (read that as non-electronic) thermostat.
Bring out your recollection of scientific principles. What does metal do when it gets colder? Right. It contracts. And of course it does the opposite, expands, when it gets warmer. Inside the thermostat is a piece of metal, called a bimetal. It's called bimetal because it's two (bi = two, as in bicycle = two wheels) different kinds of metal sandwiched together.
These two metals move at a predictable rate when temperature changes. In inexpensive snap-action-type thermostats, the movement of the bimetal itself causes the switching action. These are inexpensive because it takes many degrees of temperature change before the heat comes on or off.
The amount of temperature change that happens before the heat turns on or off is called temperature swing. Less is better. The industry definition of comfort is that people don't notice any temperature change in the room at all. It's been found that for most people that's less than two degrees. Two degrees! That's not much. Since we're very much in the comfort business, we need to use a thermostat with a temperature swing of less than two degrees.
We can still stay within the two-degree temperature range with an electro-mechanical thermostat, provided a mercury bulb is added to the bimetal. The bimetal is coiled to about the size of a quarter. A mercury bulb is fastened on the outside end of the coil. The thermostat switch is inside the mercury bulb.
Take a look sometime -- you can see the little switch terminals in there. As the room temperature changes, the bimetal moves, and that movement causes the mercury to move as the bulb tips. A cooler temperature causes the bulb to tip so the mercury covers both terminals of the switch. That "makes" or closes the switch, which brings the heat on.
That's an explanation of how a thermostat does its switching. As you might expect, there are several other factors that enter into doing it well. Folks in the heating business have usually heard these terms. Most aren't quite sure what they mean or how to use them.
It turns out they are a very inexpensive, non-time-consuming way to deliver a top-notch heating comfort system. Some of these are cycle rate, temperature swing and the anticipator.
Would you like to learn more from Carol Fey? Check out her books and classes.