How does a thermostat work?


Nest Learning Thermostat – 2nd Generation T200577

It is quite obvious that you buy a thermostat to reduce your energy bills greatly. And, most of you might be interested to see how this device works, especially in these days of escalating energy prices. Well, the working of a thermostat is amazingly straight forward and constitutes some cool technology.

The traditional mechanical thermostats and the modern digital thermostats are the two main types of thermostats available. To know the working of a thermostat, it is a good idea to look at the parts of a non-digital thermostat that is still found in most of the older homes.

1. How a thermostat works – Parts of a non-digital thermostat

Mercury Switch – The mercury switch is the integral part of a thermostat which is in the form of a glass vial with a little amount of real mercury in it. The glass vial has three wires – one wire extends to the bottom of the vial, being in contact with the mercury; another wire ends on the left side of the vial so that the mercury contacts it when the vial tilts to the left; the third wire has its ending on the right side of the vial so that the mercury can make contact between this wire and the first bottom wire when the vial tilts to the right.

Thermometers – This kind of non-digital thermostat has two thermometers. One displays the temperature and is in the cover; the other controls the heating and cooling systems and is in the top layer of the thermostat. Coiled bimetallic strips are used as thermometers.

Well, what are bimetallic strips? A bimetallic strip is a bit of metal that is made by laminating two varied types of metals together. When heated or cooled, these metals making up the strip expand and contract accordingly. When this coiled strip is heated, the metal inside expands more and the strip unwinds. A temperature-adjustment lever being connected to the center of the coil and the mercury switch being mounted at the end of the coil, when the coil winds or unwinds, it instructs or tips the mercury switch one way or the other.

On the other hand, non-digital thermostats have two switches. Small metal balls are moved by these switches and these balls tend to make contact between different traces on the circuit card embedded within the thermostat. One switch controls the mode, that is, heating or cooling and the other switch has control over the circulation fan.

Now, let’s see how these parts work collectively to make the thermostat work.

2. How a thermostat works: Inner workings

When the lever on the thermostat is moved so as to turn up the heat, the lever rotates the thermometer coil and the mercury switch, eventually tipping them to the left.

When the switch tips to the left, electric current flows through mercury in the mercury switch; this current energizes a relay that initiates the heater and the circulation fan in your house. The room gradually heats up and the thermometer coil, slowly and steadily, unwinds until it tips the mercury switch back to the right, thus breaking the circuit to turn off the heat. As the switch tips to the right, a relay initiates the air conditioner. The room cools and gradually the thermometer coil winds up until it tips the mercury switch back to the left.

Fine, it is as simple as that, right? In fact, thermostats have another cool device, a heat anticipator.  Before the air inside the thermostat reaches the set temperature, this device shuts off the heater. At times, parts of a house will reach the set temperature even before the part of the house where the thermostat is placed does. In such cases, the anticipator shuts off the heater a little earlier in order to give the heat time to reach the thermostat.

In addition, there is a kind of resistor in the form of a loop of wire above the gadget. When the heater is running, the electric current that controls the heater travels from the mercury switch to the resistive loop through the yellow wire. Until it gets to the wiper, the current travels around the loop; from there, it travels via the hub of the anticipator ring and then down the circuit board that is present on the bottom layer of the thermostat.  Similar to any other resistor, this also produces heat when electric current passes through it. The farther around the loop the wiper is positioned, the more amount of heat is produced by the resistor. This heat is sufficient to warm up the thermometer coil, making it to unwind and tip the mercury switch to the right side; consequently, the heater shuts off.

Next, let us have a look at the electrical circuits in the device.

3. How a thermostat works: Electrical Circuits

The non-digital thermostat is built for a system that has five wires and the wire terminations are labeled as:

RH – wire coming from the 24VAC transformer on the heating system

RC – wire coming from the 24VAC transformer on the air conditioning system

W – wire coming from the relay that turns on the heating system

Y – wire coming from the relay that turns on the cooling system

G – wire coming from the relay that turns on the fan

The power the thermostat uses to switch on the different relays comes from the two transformers. The power to switch on the heating system, cooling system and the fan comes from the relays.

How power flows through thermostat when the air conditioner runs

Power from the air-conditioning transformer comes into the RC terminal. A mode switch controls a ball and this ball jumps the current onto a trace that leads to the terminal present in the lower right corner of the circuit board. A screw connects this terminal to the top layer of the thermostat. It connects to the pink wire that leads to the bottom wire of the mercury switch. If the air conditioning is on, the switch gets tilted to the right and the current travels into the blue wire through the mercury. A screw connects the blue wire to a lug in the lower left corner of the circuit card. From this position, the current moves to the other branch of the mode switch through a trace on the circuit card. The ball in the mode switch jumps this current onto a trace which is connected to the terminal labeled G that energizes the fan and the terminal labeled Y that energizes the air conditioner.

This is how current flows through the thermostat when the air conditioner is running. The same applies when the heater is running, just that power flows through the terminals labeled RH and W energizing the heater.

4. How a thermostat works: Digital Thermostats

Well, these thermostats use a very simple device called a thermistor in order to measure temperature. This is a kind of resistor allowing electrical resistance changes with temperature. In a digital thermostat, the microcontroller measures the resistance and converts that number to a precise temperature reading.

Needless to say, a digital thermostat can perform a few things that cannot be done by a regular mechanical thermostat. The most useful feature of this thermostat is the programmable settings; it is unquestionably a great money and energy saving feature as you can merely turn down the heat when there is no need for it.

5. How a thermostat works: System Zoning

Now, what is system zoning?

Have you ever noticed that some rooms in your house are always colder or warmer than the others? In fact, there are many explanations for this. One reason, as heat rises, the rooms on the second or third floors become too warm and the basement rooms are generally colder. Besides, rooms that have vaulted ceilings have a tough time retaining heat whereas rooms that get long hours of sunlight don’t cool down easily. These are merely a few reasons; however, regardless of why temperature in a room is uncomfortable, there is only one definite way to even out temperature in your house. And, that is by system zoning.

System zoning is a simple process. It involves a number of thermostats being wired to a control panel that operates the dampers provided within the duct work of your forced-air system. According to the settings of the thermostat, these thermostats continually read the temperature to their particular zone, then open or close the dampers present within the duct work. System zoning is helpful for houses that have inconsistent room temperatures as well as for heating or cooling individual bedrooms depending on the favorite temperature setting. In case you have a guest room that is normally not used by anyone, you can just shut the room’s door and close the damper. It is as simple as that.

Obviously, system zoning can help save a lot of money on your energy bills. It is believed that you can save up to 30% on a typical heating and cooling bill! As seldom-used rooms like guest rooms don’t require constant heating or cooling, the process of system zoning helps you save money by running temperature-controlled air to these rooms only when need arises.

How to Select the Right Thermostat(which thermostat do i need ?- 3 steps)

As you know, a thermostat is a temperature-sensitive switch that turns on and off the heating and cooling systems in your home when the temperature attains a preset limit. When it comes to buying a thermostat, consumers are offered with a wide range of choice from simple manual units to state of the art programmable units that are highly efficient. Good news for buyers is that choosing a suitable thermostat for your home has become simple due to easy-to-understand product packaging and universal compatibility.

How to choose a thermostat(which thermostat do i need)?

You can use these three simple steps to select a thermostat for your home:

  1. Determine what heating and cooling system you have at home
  2. Choose the thermostat type
  3. Select the features and options that best cater to your lifestyle

Now, let us see each of these steps in detail.

Step 1: Determine what heating and cooling system you have at home

Single stage – Single stage heating simply means that your furnace has just one level of heat output. You might either have gas, forced air heater or an electric AC system with a separate heating system. Well, this is the commonly found system in most places. In case you already have a thermostat, there will normally be maximum six wires coming from your wall.

Multi stage – Multi stage heating means that there are two levels of heat in your house. You might perhaps have a furnace as well as another method of heat such as a solar panel. The second level of heat source is commonly considered as an emergency heat setting. So, as the low setting is sufficient to meet the household heating or cooling demands most of the time, the multi-state unit runs for prolonged periods and offers more even heat distribution.

Heat pump – This is a single system which does both heating and cooling of your home. Normally, when a heat pump is used, you don’t have a separate air conditioning system as the heat pump is capable of performing both the operations. A heat pump is employed as the emergency heat for multi stage units since it can produce heat much faster compared to any other unit and heat up your house quickly even on very cold days.

Multi stage heat pump – This system makes use of more than one system similar to the multi stage; however, it uses heat pump for emergency heating or cooling. You will have a main heating or cooling unit and you will depend on the heat pump to enable your main system to heat or cool faster and perhaps more efficiently.

Line voltage – A line voltage system uses direct electric current instead of just 24 volts used by the typical thermostat of the present day’s modern homes. This kind of thermostat can be largely seen in older homes that still use direct voltage. You can notice that the power wires are thicker and they usually run either 120 or 240 volts on direct current. These thermostats are common in older construction particularly with baseboard heaters and electric heaters. Well, these are not used with gas heaters which are generally single stage systems. Most of the line voltage systems are mechanical thermostats. Based on the voltage of the thermostat and the house, the device will use either two or four black and red wires.

Well, have you figured out the kind of heating and cooling system in your home? Your next task is to choose the type of thermostat.

Step 2:  Choose the thermostat type

Learning thermostat

These modern thermostats provide the benefit of being programmable. The best part is that you need not actually program them! These gadgets learn as you use them and can create a schedule according to your personal preferences. They can turn down automatically when you are away from home, offer you guidance on energy-efficient settings as well as provide you with monthly energy reports. In addition, these devices connect to your home Wi-Fi network; therefore, it can be monitored, set and managed remotely from a computer, tablet or Smartphone.

Remote energy management thermostat

This thermostat enables you to set, program, monitor, and control your heating and cooling system from a tablet, Smartphone, and computer. You can connect this thermostat to your home broadband network via a hub; this allows you to access the gadget over Wi-Fi from anywhere.

Programmable thermostat

If you want to save energy and reduce your energy bills, a programmable thermostat is the ideal choice. It automatically adjusts the set temperature throughout thus making the most efficient use of your system. All you need to do is just let the thermostat know your favorite temperature at different times of the day and night; the thermostat takes care of temperature control in your home.

When it comes to choosing an appropriate programmable thermostat, you need to consider the flexibility you require from day to day. Consider the following four key options:

7-day programming enables you to program each day of the week in a different manner. This is the best choice for busy families that have varied schedules every day.

5-2 programming helps you to set a standard program during the weekdays while you are at work and another separate program for the weekends when you will be spending more time at home.

5-1-1 programming lets you to set a standard program on weekdays plus two separate programs for Saturday and Sunday. If you have a structured weekend schedule, this is a great option for you.

1 – Week enables you to set one basic program for one whole week.

Digital non programmable thermostat

A digital non programmable thermostat that comes with a digital read out is the ideal choice if you prefer controlling your device settings manually; besides, this comes with the convenience of an LCD display that is easy to read.

Mechanical or manual thermostats

With this thermostat, you can control the temperature setting manually. This is the ideal option for you if you are at home often and prefer a standard fixed temperature.

Fine, now that you have known the thermostat types and chosen your desired type, you should next focus on finding the right model and options that will best fit your lifestyle.

Step 3:  Select the features and options that best cater to your lifestyle

Of course, selecting the right model depends on the features you wish to have and the style you prefer.

You might consider the following features:

  • Touch screen controls to help you modify settings right at your fingertips
  • Backlit display panel to help you see what you are doing more clearly even in dimly lit or dark backgrounds.
  • Selectable program periods enable you to set temperatures for separate time periods to suit your needs.
  • Indicator lights that alert you when the battery is low or the furnace needs a new filter.
  • Battery operation and backup for houses where there aren’t required wirings as well as for wired homes, to eliminate the need to reprogram thermostats generally after power failures.
  • Remote programming controls for convenience and comfort
  • Keyboard lock to prevent kids and others from messing up with the established settings
  • Vacation mode helps you to temporarily set your heating and cooling temperatures for maximum energy savings when you are away on a vacation and get back to normal settings when you are back, just with the press of a button.

Needless to say, by taking into account the above said factors, you will unquestionably get the perfect thermostat that matches your requirements.