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Get To Know Your Furnace

Furnaces might seem complicated, but the basics are surprisingly easy to understand. Once you know them, you’ll be able to avoid breakdowns and solve simple problems yourself. Plus, you’ll understand what the heck a repair technician is saying, make smarter decisions and avoid rip-offs. That can add up to huge savings over the life of your furnace.


On some furnaces, a humidifier is mounted on the supply-air plenum to add moisture to dry air. When it stops working, it’s usually because of a clogged or burned-out solenoid (electric water valve) or bad drum motor—all fairly easy to fix. For information on repairing an existing humidifier or installing a new one, visit and search for “humidifier.”


All furnaces have a pipe that carries deadly exhaust outside by way of a chimney or through an exterior wall. Flues routed through walls can get blocked by shrubs, snow or small animals, causing the furnace to shut down.


The supply-air plenum is a large sheet metal box on top of the furnace. It distributes warmed (or cooled) air to the ducts.


If you have central air conditioning, a cooling coil cools air on its way up the supply-air plenum. Copper tubing connects the cooling coil to the compressor unit outside your home (that’s the big box with the fan in it).


A heat exchanger is the part of the furnace that heats the air. Flames warm the inside of the heat exchanger, while air passing across the outside of it gets warmed. Sometimes—due to age or lack of maintenance—heat exchangers can crack and leak deadly carbon monoxide into your house and kill you (see “Get a Carbon Monoxide Detector” on p. 33). Sometimes heat exchangers can be replaced. However, depending on how old your furnace is, replacing the entire furnace might be smarter.


Return ducts and the large return-air plenum carry air from rooms back to the furnace to be heated or cooled.


A blower is turned by a motor and forces warmed air through supply ducts. Motor bearings are usually sealed and don’t require lubrication, but even sealed bearings can fail over time and cause the motor to overheat.