Problems with air conditioners or furnaces most frequently occur because of dust in the thermostat. Clear dust from metal contact points by removing the cover and using a soft brush to sweep away any dirt that has accumulated. Use a heavy paper to clean between the points and blow it out with your own breath. Do not use a vacuum, as it may be too strong and could possibly damage delicate wires.
Some systems have liquid mercury contacts rather than contact points and will not require cleaning because they are enclosed in a hermetically sealed glass tube. The glass bulbs are fragile and may be toxic if the ampoules are broken. Although small drops of mercury will vaporize at room temperature, you should still be careful not to break the tubes. If small spills occur, take precautions not to breathe it in by sealing the room and venting to the outside.
Switch contacts should be cleaned around the top and sides of the plate with an alcohol-soaked cotton swab. They have a simple on/off regulator that can be adjusted either up or down, enabling the room temperature to be effectively regulated by the unit. Vacuuming registers and inspecting and cleaning filters all help the unit provide efficient heating and cooling.
You will need a glass tube thermometer, soft brush, level, screwdriver and tape to check the thermostat’s calibration. If you make a habit of doing this every year prior to the winter season, your unit will last. Tape a glass tube thermometer a few inches away and wait 15 minutes. When the mercury stabilizes, compare the readings. If it differs by more than a degree, remove the face plate and clean it. If it is a liquid mercury unit, use a level to make sure it is straight.