"You had one job!"

Furnaces don't seem to have a tough life. After all, they only have to do one thing: produce warm, cozy heat all winter long. They even get the summer off for their troubles. Unfortunately, furnaces can sometimes be stubborn, and you may find that yours is giving you the cold shoulder and refusing to keep your home at the comfortable temperatures you expect.

Cold or lukewarm air coming from your vents can make for a frustrating winter night. Surprisingly, this situation is slightly different from a furnace that simply won't turn on. If you're feeling cold air, your furnace is signaling the blower to start, which helps narrow down the possible range of problems. This guide will help you understand what's happening and how you can coax your furnace back to work.

Getting Warmed Up: How Your Furnace Starts

When a thermostat in your home detects a temperature below your setpoint, it signals the furnace to request heat. However, the furnace control board needs to complete several safety checks before starting the blower motor. These checks ensure that the inducer (exhaust) blower is functioning and that a flame is present. Once completing the safety warm-up, the furnace starts the blower.

This sequence provides you with some information. If you're getting cold air from the vents, your furnace must be completing its initial start-up sequence and signaling the blower to begin. Note that this is only true if you have the fan switch on your thermostat set to "auto." If you set this switch to "on," the fan will run at all times, even if there's no warm air from the furnace.

Over the Edge: Safety Switch Shutdowns

Cold air from your vents typically means your furnace is shutting down after igniting. It may briefly ignite, turn on the blowers, and then shut off. This situation can produce cold air blowing from your vents for a few seconds or longer, leaving you shivering and wondering what your furnace suddenly has against you.

A triggered limit switch is one common cause. This switch protects your heat exchanger by shutting down the burners if its internal temperature gets too hot. Poor airflow is often the underlying cause of a triggered limit switch. You may have restricted airflow due to a clogged filter, faulty blower motor, blocked vent, or several other issues.

The pressure switch is another possible culprit for your chilly situation. This switch checks for adequate exhaust pressure to ensure byproduct gases are safely exiting the combustion chamber. A faulty switch may signal the all-clear and then trigger a safety shutdown shortly after. When this happens, the burners shut off, and you get a brisk, wintry breeze from your vents.

While these safety switches might seem frustrating, they're critical to ensuring your furnace operates safely and efficiently. If there's a problem with the switches in your furnace, it's best to contact a professional heating contractor to resolve the situation and get your furnace working again.

For more information, contact a heating contractor in your area.