Did You Know, How Do Home Thermostats Work?

Home Thermostat makes your home a place of comfort and peace. It gives you the homely feelings once you return home after a busy and tiresome day out in the chaos. So, knowing how it works is a need to maximize your comfort and peace.

With this article, I will give you an idea about what home thermostat is, and how it works. At the same time, you will get a little understanding about the difference between traditional thermostats and smart thermostats.

What is a Thermostat?

A thermostat is a temperature sensing device. It turns a system or circuit on and off when it reaches a particularly set temperature. The device senses the temperature of the system. So, the system can maintain its temperature to the desired point.

Various systems use the thermostat as a device or part in their internal frameworks, those works for heating and cooling like air conditioners, HVAC systems, water heaters and the like.

How Do Home Thermostats Work?

All modern thermostats are digital thermostats. To get a clear view of the innermost mechanism of a thermostat, we have to look into a non-digital i.e. analog/mechanical (traditional) thermostat first.

The thermostat has various parts; among the major parts are- mercury switches, thermometers, two-metal strip, circuit board, circulation fan, heat anticipator, and wiper.

Mercury Switch:

A mercury switch is a glass tube. Inside it, you can find a small amount of actual mercury. Mercury is one of the 105 basic elements of the earth. It is a metallic chemical element. At standard conditions for temperature and pressure, it gets in liquid form, it flows like water.

It is a poor conductor of heat but a fair conductor of electricity.

You can find three wires inside the glass tube. The first wire runs across the bottom of the tube, and so mercury always contacts it. The second goes up to the left of the tube, so when the tube tilts to the left, mercury contacts it setting a contact between this wire and the wire in the bottom. Accordingly, the third wire goes up to the right side of the tube.

So, the tube tilts to the right and mercury contacts this wire, and sets a connection with the bottom wire.

That special and uncommon chemical property of mercury is the biggest contributor to the functions in the thermostat.

Metal Strips:

Two thermometers work in the traditional or mechanical or non-digital thermostat. One stays on the cover of the thermostat, displays the temperature. The other stays on the top layer of the thermostat, controls the phenomena of heating and cooling. These thermometers are bimetallic strips i.e. two-metal strips in the form of coil.

Herein, two pieces of metals but with different attributes bolted together make a two-metal strip.

The strip acts as a link in the electric circuit connected to your heating or cooling system. It controls the electric contact and the source of heating and cooling. The metals making the strip expand when heated and contract when cooled.

Normally while the “link remains down”, the strip passes electricity through the circuit and heating starts. As soon as the strip gets hot, one metal of the strip expands higher than the other, so the whole strip gets slightly hot.

Later it gets hotter and bends so much that it breaks up, the circuit opens and the strip unwinds. Then, when the “link becomes up”, the electricity supply switch goes off, heating goes off. Consequently, the room begins to cool. Then, what?

The room will be heated.

Before that, let’s get a preview of the circuits’ elements that cause room heating.

Room Heating:

The thermostat has a circuit board to hold all the elements of the circuitry, nuts bolts, screws, and lugs.

Herein, the circuit board has a lever to adjust temperature. The lever is connected at the center of the coil. The mercury switch remains placed at the endpoint of the coil in a way that causes the coil to wind or unwind. As a result, mercury switch moves to one way or the other way.

You will find two switches in an analog (mechanical) or non-digital thermostat. The circuit board contains small metal balls. The metal balls come in touch of the switches, make contact between dissimilar traces on the circuit board. One switch controls the state of heating or cooling, whereas the other switch controls the circulation fan.

Traditional Thermostat’s Circuit (Photo: Xpress Bro 2)

By moving the lever of the thermostat, you have to generate and increase the heat. When the lever moves, the thermometers and the mercury switch rotate and tilt to the left.

Whenever the switch tilts to the left, current flows promptly through the mercury in the mercury switch.

The flow of the current creates a relay that powers the heater and the circulation fan. Then the room starts getting heated, and the coil of the thermometer unwinds gradually, once fully unwinds, the mercury switch tilts back to the right. This breaks the circuit and turns off the heat.

Heating/Cooling Control:    

As soon as the mercury switch tilts to the right, a relay knocks the air conditioner to start. When the room cools, the coil of the thermometer winds up gradually, once fully winds up, the mercury switch tilts back to the left. In this way, heating and cooling loop, in turns, happens in the thermostat.

You can call the above wire loop one type of resistor. When the heater runs, the current controlling the heater, moves from the mercury switch, through the yellow wire towards the wire loop. It moves around the loop until it reaches the wiper.

From the wiper, it goes through the hub of the anticipatory ring. Then it moves down along the circuit board in the bottom layer of the thermostat.

To give more comfort to the occupants of the air-conditioned environment, thermostats have another device named heat anticipator. For this purpose, the heat anticipator stops the heater, so that the air inside reaches the set temperature.

Sometimes it happens, parts of the house have reached the set temperature, but the part containing the thermostat has not yet. To solve this above problem, the heat anticipator shuts the heater off a little early so that the heat reaches the thermostat.

The more distance the wiper is from the yellow wire, the more path of the resistive wire the current has to travel through. Alike any other resistor, it generates heat when current passes through it. The more distance around the loop the wiper is set, the more heat the resistor generates.

This heat warms the coil of the thermometer. And so, the coil unwinds and tilts the mercury switch to the right, and it causes the heater shut off.

Digital Thermostat's Circuit
Digital Thermostat’s Circuit

Circuit Description

Five wires make the circuit of the thermostat. We can describe the wire terminals as follows;

  • RHand RC– The RH wire indicates the path from the 24V AC transformer in the heating unit. Whereas, RC wire indicates the path from the 24VAC transformer in the air-conditioning unit. A jumper connects these wires. Both the wires have same color pink.
  • W– This wire indicates the relay path that turns the heating unit on. Its color code is white.
  • Y– This wire refers the relay path that turns the cooling unit on. It is of yellow color.
  • G– This designates the relay path that turns the fan on, and its color is yellow.
  • B– This is a common circuit wire. It is for changeover the heat pump. Its color is blue.

Color code varies a little in circuit wiring based on manufacturers. Learn about C-Wire.

The thermostat receives power from the couple of transformers. Then it switches power on various relays. Lastly, the relays switch on different components of the thermostat. Let’s see more!

The transformer in air-conditioning unit sends power to the terminal RC. The metal balls send the power into the trace. It guides the power towards the terminal in the right corner of the circuit board.

A screw connects that terminal with the thermostat’s top layer. It connects the pink wire (see the picture) that joins the bottom wire in the mercury switch. When the switch leans to the right, the current runs along the mercury into the blue wire (see the picture). A screw joins the blue wire to a lug residing in the circuit board’s lower left corner.

A trace in the circuit board, from that point, pushes the current to the other part of the state switch. A ball in the state switch pushes the current into a trace that goes to the G terminal. It boosts the fan and the Y terminal which boosts the air-conditioning unit.

Difference between Traditional Thermostat and Smart Thermostat


Simply saying traditional thermostats are non-digital and smart thermostat are digital. Digital thermostats have different internal framework than those of traditional thermostats. A digital thermostat has a device called thermistor for measuring temperature. The thermistor is a kind of resistor. It allows electrical resistance change with temperature.

A digital thermostat has a microcontroller to measure the resistance and to convert it into a number to represent the precise reading of temperature. It has a few advanced features those a non-digital thermostat does not have.

One of the advanced features is programmable settings. For example, in cold season, you can program your thermostat so that automatically turn ups heat for 1 hour or 2 hours in the morning while you become ready to go out. Likewise, you can turn the heat down until you return from your office.

Options are also available to set to turn the heat up for the evening and turn it down while you sleep. This feature is great for energy saving.

In conclusion,

Digital or smart thermostats connect to the internet. It allows you to adjust heating settings from your remote devices like Smartphones. This way, you can adjust the temperature remotely.

No doubt, your home thermostat is a useful device. It can minimize your electricity costs if you go wisely with it. It cares about the comfort and the quality of your life. Therefore, being aware of its internal functions will help you balance between your comfort and its expense.

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