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When it’s cold, we walk over and turn up the thermostat, and when it is hot, we walk over and turn on the air conditioner to cool us down. But, how does that little box on the wall actually control the temperature of our homes or offices?

A mechanical thermostat is actually a very simple device. It is basically a thermometer attached to a switch that turns on your heater, whether your heat source is natural gas or electricity. If you remove the cover of your thermostat, you will see the inner workings and get a better idea of how a thermostat works.

The top layer holds a mercury switch and a thermometer coil. The mercury switch is just a small vial filled with the liquid metal, mercury. Within this vial are three wires: one at the bottom of the vial; one at the left of the vial; and one at the right of the vial. As the temperature rises or cools, the vial of mercury gets tipped to the right or the left making the corresponding wire come into contact with the wire that runs along the bottom. If the mercury gets tipped to the left, a connection is made that creates a current that energizes a relay, which starts the heater and circulation fan. As the room heats up, the vial levels off and once that is in balance, the connection is broken causing the heater to turn off. If the mercury switch is tilted to the right, another relay causes the air conditioner to turn on.

What tips the vial in either direction is the thermometer coil that rests against the vial of mercury. The thermometer coil is constructed of a bi-metallic strip made of two different types of metal, usually copper and iron, which are bonded together. Because the different metals respond to heat at different levels, this strip contracts and expands causing the coil to curl up or uncurl as the temperature changes. This curling or uncurling motion tilts the mercury vial, which then signals the heating source to kick off or on. When you adjust the temperature knob on your thermostat, you are actually adjusting the tightness of the coil.

Beneath the top layer of your thermostat, you will see the circuit board, which houses the wires that actually lead to the circulation fan and heat source. The circuit board is connected to the mercury switch via a metal screw and wire, which “reads” the switch and turns on the appropriate heating or cooling device.

Newer on the market are digital thermostats. These thermostats differ from the mechanical thermostats in that they use a thermistor, a resistor whose electrical resistance changes with temperature. The microcontroller in a digital thermostat measures the resistance and converts that number to a temperature reading. Digital thermostats can save energy because they can be programmed to turn the heat or air conditioning off or on at preset times throughout the day. For example, you can set the air conditioning to come on an hour before you come home from work, or have the heater remain off during the hours while you work and then turn on an hour before you get back so your home is warm and cozy when you open the door.

While low-voltage thermostats are very reliable and rarely defective, thermostat-related problems, such as faulty wires or loose connections, may cause a failure in your heating or cooling system. The problems are easy to identify, and just as easy to fix by making the appropriate repair or by replacing a faulty thermostat.

Steps

Step One

Turn off the power to heating and/or cooling systems at the main service panel or system shutoff switch.

Step Two

Remove the thermostat. On some models, remove the cover plate and then unscrew the thermostat body from a wired base plate secured to the wall. Other models plug into the base plate without screws.

Step Three

Remove any dust from the thermostat and base plate with a soft artist’s brush.

Step Four

Check for broken, frayed or corroded wires and loose wire connections. Tighten any loose connections. Use wire cutters/strippers to cut damaged wires and strip off about 1/2 inch (12 mm) of insulation, then reconnect the wires.

Step Five

Restore the power to test the thermostat. Consult the manual for your thermostat and disconnect the power wire (usually red), then touch it to the terminal for the heat (the terminal marked as W with white wire from heater’s transformer).

Step Six

Similarly bypass the thermostat to check its cooling function by touching the disconnected power wire (red) to the terminal for the cooling system (typically yellow).

Step Seven

If one or both systems fail to start, turn off their power, then identify (see step 8 ) and replace any faulty wires between the thermostat and the low-voltage transformer(s) at the heating or air-conditioning units.

Step Eight

One way to test thermostat wires–on your heating system, for example–is to disconnect the wires from the thermostat and transformer, wire them together at one end, and test for continuity at the other end by clipping a continuity tester to one wire and probing the other wire. If the tester fails to light, replace the wires.

Step Nine

Check for loose low-voltage wire connections at the transformer or loose line-voltage connections to the transformer .

Step Ten

If both systems activate, replace the faulty thermostat (see below).

 

Replace a thermostat

Steps

Step One

Choose a thermostat suitable for your particular system. Read the thermostat labeling and/or consult with a knowledgeable salesperson or HVAC (heating, ventilating and air-conditioning) service professional.

Step Two

Shut off the power to your heating and/or cooling systems at the main service panel or system shutoff switch.

Step Three

Remove the old thermostat as described above.

Step Four

Disconnect all wires from the base plate one at a time and tape a label on each that identifies its terminal connections (R, Y, W and so forth).

Step Five

If the new thermostat base does not cover the old mounting holes, either patch them and touch up the paint or install the cover ring that the new unit may provide.

Step Six

Mount the new thermostat to the wall, leveling it according to the manufacturer’s instructions. Typically, you can align notches with a level or plumb line on the wall, or place a small level on leveling posts at the base or its edge.

Step Seven

Use the wiring chart provided in the installation instructions to connect the labeled wires to the appropriate terminals, removing the tape as you make the connections.

Step Eight

If applicable, install batteries and program the thermostat.

Step Nine

Attach the thermostat to the base and restore power.

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