How Do Thermostats Work?

How do thermostats work?

Put Another Log On The Fire

That’s more than a line from a clever song with lyrics by Shel Silverstein.

Come on baby you can fill my pipe and then go fetch my slippers
And boil me up another pot of tea
Then put another log on the fire babe
And come and tell me why you’re leaving me

Aren’t these Early Bird blogs supposed to help readers with electrical projects? Why are we writing blogs about Ben Franklin and Shel Silverstein songs?! Well, ‘another log on the fire’ is how we used to turn up the heat in our homes. This blog is about how thermostats turn up the heat. That’s why we started with a song about a log. That makes… sense, right? Sure it does!

Before we get to repairing or replacing your thermostat, let’s make sure you know a little bit about thermostats and how they work.

What is a thermostat?

The word ‘thermostat’ combines two ancient Greek words: thermo (heat) and statos (related to status quo or “the condition that currently exists”). With mechanical thermostats, you set the temperature by moving a lever. This rotates the thermometer coil and mercury switch, tipping them to the left. In this position, the coil competes the circuit supplying an electrical current to the thermometer through the mercury switch. That current turns on the heater and fan or the boiler to heat the water for the radiator (or “heatiator,” as my 3 year old used to say).

Once the room gets to the temperature you want, the coil gradually unwinds until it tips the mercury switch back to the right. When the switch moves back to the right position, the electrical circuit breaks and the current stops providing power.

Now, of course, you want the room to stay at that temperature you want, but temperatures naturally change. So, when it starts to cool down, the thermostat turns the heat back on. If it gets too hot, it turns the heat off. Simply put, a thermostat makes sure the temperature (heat) stays at the condition that currently exists.

How does a thermostat do that?

Remember that thermometer coil? They’re made out of two different metals, laminated together. When the temperature changes, these two metals expand or contract at different rates. If it gets warmer, the metal on the inside expands faster. If it gets colder, the metal on the outside of the coiled strip shrinks faster. Depending on whether it’s hot or cold, the coil expands or shrinks. As it expands and shrinks, it also turns the mercury switch on or off.

What is a digital thermostat?

A digital thermostat also works by supplying power to heating devices, but the way it does that is a little different. Digital thermostats have also have several advantages over mechanical, mercury thermostats. For instance:

  • They’re much more accurate.
  • Most have a backlight, making them easier to read.
  • They have no mercury so they’re safer and better for the planet by reducing the mercury that ends up in landfills.
  • They have no moving parts so you don’t need to re-calibrate them.
  • Because they’re so precise, accurate, and programmable, Energy Star estimates you can save up to $200 each year.

Let’s get into that programmable thing. In the winter, you can program your thermostat to turn up the heat before you get up. Your house will be warm as you get ready for work! When you leave, the thermostat can also turn the heat back down. That way, you won’t have to waste money heating an empty house. The thermostat will even turn the heat back up again just before you get home. You could also program the thermostat to turn your heat down while you sleep. Digital thermostats help you safe money and heat your home more efficiently at the same time.

So everyone should have a digital thermostat?

Well, maybe. Digital thermostats are more expensive to buy and install than mechanical versions. Some people also aren’t comfortable with digital products.

And… we’ve gone too long again.

Now that you know a little about how thermostats work, we’ll get to “How To Repair Or Replace Your Thermostat” next time. If you need any electrical help in the meantime, feel free to give EarlyBird a call any time.

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