Thermostats, So Small, So Important

There are multiple thermostats in every home, they’re in use for at least half the year, and if you have air conditioning as well as heat, you’re using them year round. They’re essential to modern living and yet are overlooked and underappreciated until something goes wrong. In short, the humble thermostat is one of the least valued, but most important of all your home’s electrical devices.

In common use thermostats have been around for about 150 years, although Cornelius Drebbel did invent one in the early 1600s to control the temperature in his chicken incubator! But it was the industrial revolution – and specifically the textile mills of Britain – that brought about the creation of the first modern thermostat – a bi-metal version invented by Andrew Ure of Scotland in the 1830s. The first electric version was created by Warren Johnson of Wisconsin in 1886.

Mechanical thermostats are still in use today. In fact until quite recently, virtually every home in America had a non-digital mercury-switch thermostat.

But how do they work?

Inside the casing of a mercury-switch thermostat is a bi-metal coil. Attached to the end of the coil is a temperature lever that extends to the outside of the casing. At the center of the coil is a vial containing a small amount of mercury and three wires. One of the wires stretches across the bottom of the vial, the second ends near the left side of the vial, and the third ends near the right. If the vial is tipped one way or the other, the mercury makes contact with an unconnected wire and completes a circuit from the thermostat to the heating or air-conditioning system. So, when the temperature lever is adjusted to turn up the heat, the coil is tightened and the vial is tipped to the left. This allows the current to flow through the mercury to the full-length wire and thence to the relay switch that activates the heating system. As the temperature around the thermostat rises, the bi-metal coil slowly unwinds and the vial gradually returns to level. As soon as the mercury no longer reaches the unconnected wire, the circuit is broken and the heating system is deactivated. The same principles are applied when the lever is adjusted for a cooling temperature, except now the vial is tipped to the right until the desired temperature is reached.

Mechanical mercury-switch thermostat from the late 20th century.

Caption: Mechanical mercury-switch thermostat from the late 20th century.

The downside of the mechanical thermostat is that it cannot be programmed – it can only work to one setting for either heat or cold. But with the introduction of electronics, we can now program thermostats to work to multiple temperature settings depending on the time of day and day of the week – you can have the heat turned down when you’re asleep or out of the house, and turned up just before you wake up or come home. The digital thermostat uses thermistors, which create electrical resistance changes according to temperature. The thermostat measures the resistance and converts the measurement into a temperature reading, which will then trigger the heating (or AC) to be turned on or off.

So what’s new?

There are now thermostats that connect remotely to the heating or cooling system – no need to spend time and money fishing wires through walls from thermostats all over the house!

Electronic programmable thermostat from the turn of the 21st century.

Electronic programmable thermostat from the turn of the 21st century.

Talking thermostats that announce their programmed settings and be activated by voice commands have been on the market for some time.

Some thermostats can be controlled remotely via telephone. You simply call your home and enter a password on a touch-tone phone to access your heating system. Once connected you can control the thermostat and more on the system.

And finally (for now!) there are thermostats that interact with smart-phone technology. Thermostats such as the Nest or the Honeywell Lyric can be programmed remotely via a smart phone and will even react when the registered phone (or phones) leaves the property –the app on your phone tells the thermostat when you’ve gone out or when you’re on your way back, and the thermostat adjusts the house temperature accordingly!

Smart-phone thermostat from the 2010s, seen in four different mode settings.

Smart-phone thermostat from the 2010s, seen in four different mode settings.

If you think it may be time to replace your old thermostat with newer technology, call Mike Meserve today at 603.964.6703 or click here.

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