Gas furnaces

It helps to know where a furnace gets its air and where its exhaust goes. Atmospheric furnaces, also called naturally drafted furnaces, use the heat in the flue, the pipe that carries exhaust away, to draw in air for burning and to lift the exhaust up and out of the chimney. Furnaces of this kind, which codes call Category I, take their burning air from the space around them, called the combustion appliance zone. There is no barrier between their burner and that space. That is why the room around this kind of furnace matters to how safely it runs.

A direct-vent sealed-combustion furnace works differently. It brings its burning air straight from outdoors through one sealed pipe and sends its exhaust outdoors through a second sealed pipe. These furnaces are Category 4: their flue gases are under 140°F, and the water vapor made by burning condenses into liquid that leaves through a condensate drain. Because the exhaust is cool, the intake and exhaust pipes can be made of PVC plastic. So plastic pipes to the outside and a small drain line are signs you have this kind of furnace.

Source: U.S. Department of Energy

Gas furnaces use a lot of energy, so their efficiency matters. They account for about one-fifth of the energy delivered to homes and apartments in the United States. The efficiency of furnaces and boilers is measured by annual fuel utilization efficiency, or AFUE. AFUE compares the heat actually delivered to your house with the fuel supplied to the furnace. In general, it is the percentage of the heat in the fuel that becomes space heat instead of being lost. The higher the number, the more of the gas you pay for ends up as heat in your home.

The first federal standard, enacted by Congress in 1987 and in effect from 1992, set the minimum at 78 AFUE, meaning 78 units of heat out for every 100 units of energy in. Condensing furnaces have AFUEs of 90% to 98%. To earn the ENERGY STAR label, a gas furnace in the northern states, which include Illinois, must be rated 97% AFUE or greater. ENERGY STAR says a qualified furnace is 15% more efficient than a conventional one. Program criteria like these can change over time.

Size matters as well as the rating. Oversized equipment runs in short cycles and never reaches efficient operation. It may also cycle too often, which costs comfort and shortens its life. A furnace that turns on and off much more than usual is worth mentioning to a technician.

Source: U.S. Energy Information Administration · U.S. Department of Energy · ENERGY STAR

Much of a furnace's safety depends on parts you never see. The heat exchanger is the metal wall that keeps the flame and its exhaust apart from the air you breathe. A dirty burner or a cracked heat exchanger causes improper burner operation. Either can make the equipment run less safely and less efficiently. Cracked furnace heat exchangers are one source of combustion gases indoors, along with chimneys and flues that are badly installed or kept. Even new furnaces can corrode and leak combustion gases, including carbon monoxide.

Some signs you can notice yourself. The US Consumer Product Safety Commission lists problems that can point to an appliance not working properly: a furnace that cannot heat the house or runs all the time, soot on appliances and vents, an unfamiliar or burning smell, and more moisture on the inside of windows. A persistent yellow-tipped flame is generally a sign of poor adjustment and more pollution from burning. Gas flames that are pale yellow or wavy, and discoloration at the burner, its access door or the vent area, are on a gas utility's list of warning signs too. A technician also checks whether the flame is disturbed before and after the blower comes on, which may point to a leak in the heat exchanger. If you notice any of these, tell us what you see and hear.

Source: ENERGY STAR · U.S. EPA · U.S. Consumer Product Safety Commission · Nicor Gas · U.S. Department of Energy

Every gas furnace makes exhaust, and that exhaust has to leave the house. Carbon monoxide is produced any time you burn fuels such as natural gas. It is a colorless, odorless gas that interferes with the delivery of oxygen through the body. It can escape from faulty furnaces or through rusty or cracked vents. It can also be trapped inside homes by blocked chimney flues. The first symptoms of low to moderate poisoning are like the flu, but without the fever. The carbon monoxide page explains alarms and where they go, and the safety steps are at the end of this page.

Source: City of Chicago · U.S. EPA · U.S. Consumer Product Safety Commission

A few checks are yours to make. ENERGY STAR suggests you inspect, clean or change your furnace's air filter once a month. A dirty or clogged filter can sharply cut airflow, which makes the furnace run longer and adds wear on the motor and energy use. Check your thermostat settings so the system keeps you comfortable at home and saves energy while you are away. The CPSC also suggests looking at vents and chimneys regularly for poor connections, visible cracks, rust or stains.

The rest belongs to a yearly visit. Make sure your furnace gets professional maintenance each year. The CPSC says a qualified heating contractor should inspect the heating system every year, checking the furnace, its electrical and mechanical parts, the thermostat controls and the automatic safety switches. The contractor should also check flues and flue connectors for tight, secure fittings and for rust or cracks that could let toxic gases in. Heating contractors do not normally check chimneys and flues, so the CPSC says to ask them to include the venting in their inspection. A check-up also covers the starting cycle, to make sure the system starts, runs and shuts off properly.

Age is part of the picture too. Furnaces should be checked in the fall. ENERGY STAR lists a furnace more than 15 years old as one sign it may be time to think about replacing it. A technician can help you weigh repair against replacement.

Source: ENERGY STAR · U.S. Department of Energy · U.S. Consumer Product Safety Commission