Sometimes you will have the impression that your furnace is staying on too long or not staying on long enough—when that happens, it is time to look at the furnace thermostat.
The Simple Process:
Remove the Face Plate
Find the thermostat control box and remove the front plate. Check to make sure there are no obvious problems like loose wires—use the compressed air to blow away accumulations of dust so you can see better.
Level the Housing
Most thermostats won’t work correctly if they are not horizontal so use your level to check. If you have to make a significant correction to the position of the thermostat you might have cured the problem so check over the next few hours to see if the furnace is working normally.
If it still isn’t working properly, try the following:
The "Longer" Scale
Return to the thermostat and find a small lever on a scale that is usually marked "longer." This is not the same as the temperature scale
Furnace Stays on Too Long
If your furnace is staying on too long move the lever one notch away from the word longer.
Furnace Switches on and off too Frequently
If your furnace is switching on and off too frequently move the lever one notch towards "longer. "
Wait a Few Hours and Check Again
Give the thermostat a few hours and see what difference you have made, If the furnace is still staying on too long, move the lever one more notch away from "longer" and towards "longer" if it is still switching on and off too frequently. Do this until the furnace is behaving correctly.
A properly adjusted furnace thermostat will keep the home temperature more stable and probably improve fuel economy.