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If your furnace won't come on even though the house is cold, the problem could be a faulty thermostat. The thermostat measures the temperature in your home and directs your furnace to come on when the temperature falls to the level you've set. If your thermostat is broken, you don't have to shiver until the repairman can come or until you can get to the store to buy a new thermostat. You can temporarily bypass the thermostat and your house will be warm in no time.
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Turn off the breaker to the furnace. Locate the breaker and flip it to the "Off" position.
Remove the thermostat cover. On some models, you can pop off the plastic cover, while with others you'll need to unfasten a couple of screws. Set the cover aside.
Locate the red and white wires attached to two screws on the thermostat. Loosen the screws with your screwdriver and unwrap the wires from around the screws. Hold onto the wires and don't let them slip back behind the wall.
Wrap the exposed ends of the two wires together. Slip a pencil behind them to prevent them from falling behind the wall.
Return the furnace breaker into the "On" position. Your furnace should light and begin to heat.
Using this method will cause your furnace to run all the time. It will only shut off if you disconnect the wires. This wastes energy and could cause your system to overheat and quit working. You'll need to replace the thermostat as soon as possible.