How A Thermostat Works

How A Home Thermostat Works

A residential heating and cooling system has 3 major components worth studying. There is the system for the conversion of gas or electricity to heat, and the second is a way of distributing that product througout the home. We are only going to focus on the third component. This is the component that controls the other systems so that the desired temperature is maintained. This is how a thermostat works.

The thermostat is a familiar device found in every home with a modern heating system. Wether it is an older mechanical lever type system or a modern digital system. These devices are found in many other devices found around your home. Your oven, microwave and iron are some common ones. Basically anyting that has a temperature setting has to have a way of determining what the current temperature is.

What is going on in side a thermostat when you increase the temperature. Well, every thermostat has an element that responds to the natural thermal expansion and contraction of material. Most will know that all materials expand as they get hotter and contract when they get colder. Lets not talk about water though:)

Thermostats Use An Bimetallic Coil

What you also may not know is that materials expand and contract at different rates. This is the amount or measurable expansion or contraction that occurs upon temperature change. An example is steel and brass. There is a equation and a material property called the Coefficient of Thermal Expansion that can be used to determine thermal deformation. The Coefficient of Thermal Expansion can be found in materials handbook.

Thermal Deformation = (Coefficient of Thermal Expansion) x (Temperature Change) x (Length)

As an example and to skip all them math a 12″ bar of steel would expand 0.004″ at a temperature change of 50 degrees. Using the same formula brass would expand 0.006″

To cut to the point the actual amount of change is so small that it would be difficult for a home thermostat to detect. In order to overcome this by bonding 2 materials together creating a bimetallic strip. The 2 materials need to change at different rates. As an example you can use Brass and Steel.

Using these 2 bonded materials when there is a temperature change the bar will not stretch but it will bend. As brass expands more than steel the brass on the bar will bend toward the steel and create a measurable change that can be adapted for a home thermostat. The effect is very subtle but can be even see with your eye were you to test this.

The amount of change depends on the length and thickenss of the strip. In a thermostat this is formed into a coil to accomodate the length and design. As temperature gets colder the coil wraps tighter unwinds as it gets hotter. This in turn will cause a strip to make contact and close the circuit signaling the furnace or A/C to start working.

The Effect of Hysteresis

hysteresis is a programming that allows the temperature to get past the set temperature before the circuit is broken. This increases efficiency by not allowing the furnace or a/c to cycle on and off as much.This is a common feature in may systems to increase the life span of a system.

Now we at least a basic understanding of how the home thermostat works in most of the systems that we come in contact with. Not exactly a truly riviting conversation piece around the dinner table but something that will come in handy when talking shop or diagnosing a furnace malfunction for repair. When searching for Bellingham HVAC services its important to get someone in Whatcom County. We partner with the top HVAC companies to deliver unbeatable service and care.

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