Venting and chimneys

The rules start with a simple idea. The Illinois Mechanical Code requires every fuel-burning appliance to send its exhaust to a vent or chimney, with a few exceptions, and the chimney or vent must be designed for the type of appliance. Venting systems must be built to create a flow strong enough to carry all the exhaust outside.

Older and newer appliances do this in different ways. Atmospheric (naturally drafted) furnaces use the heat in the flue to draw in combustion air and to lift the exhaust up and out of the chimney. These Category 1 appliances are the oldest and least efficient kind, with exhaust hotter than 140°F and a flue under negative pressure compared with the room around them. Direct-vent sealed-combustion furnaces are Category 4: their vent is under positive pressure, the exhaust is under 140°F, and the water in it condenses and drains away. Because that exhaust is cool, the pipe can be made of PVC plastic. That is why the vent has to match the appliance.

Source: UpCodes (code text) · U.S. Department of Energy

Backdrafting is exhaust going the wrong way. For a gas water heater, it means the exhaust is not going up the flue pipe to the outside but is coming out of the opening above the tank into the indoor air. More generally, backdrafting happens when exhaust, including carbon monoxide, is drawn back into the house by negative pressure.

Other equipment in the house can cause it. Combustion backdrafting occurs when more air is exhausted to the outside than is brought in. That can disrupt the normal airflow of an atmospheric (naturally drafted) gas water heater or furnace and pull carbon monoxide into living areas. Exhaust fans and clothes dryers that pull air out of the home can depressurize it and make an atmospheric water heater backdraft. Very powerful kitchen hood fans, rated as high as 1200 cfm, can easily backdraft a furnace, water heater or fireplace that is not sealed-combustion.

Tight homes face this more. Naturally drafted appliances are not recommended inside the home, especially in air-tight, high-efficiency homes, where exhaust fans or fireplaces can overcome the natural pull of the flue. Adding a big fan or sealing up an older house can change how your gas appliances vent.

Source: U.S. Department of Energy · U.S. EPA

Spillage can be checked with simple tools and with gauges. On an atmospheric water heater, a flanged hood sits at the base of the flue pipe above the tank. In one test, a flame or a stick of incense is held next to that hood's opening while the water heater is heating, and if the flame or smoke is pulled in, the water heater is venting properly. If the flame or smoke is pushed away from the hood, the water heater is backdrafting, which means carbon monoxide is also spilling into your home.

Technicians also measure. Federal guidance says that if the pressure pull in the vent is weaker than the set limit, or positive, the appliance is prone to long periods of spillage or backdrafting when exhaust devices run. If carbon monoxide in the exhaust is above acceptable levels, the first step is to check that the vent system is pulling enough. If the fixes do not bring CO down to acceptable levels, the guidance says to refer the heating system for replacement as a health and safety measure.

Source: U.S. Department of Energy

Many older Chicago homes vent gas appliances into a brick chimney. Blocked, leaking or damaged chimneys or flues release harmful combustion gases and particles and even fatal levels of carbon monoxide. Chimneys and flues should be clear at all times so toxic gases, especially carbon monoxide, can escape.

Chimneys that once served oil or coal need extra care. Chimney inspections are especially important where oil or coal heating has been converted to gas, because built-up soot and debris could block the escape of carbon monoxide and send it into the home. The CPSC says homeowners converting from oil to gas should have the chimney inspected at the time of the conversion and then every year. Chimneys should be checked and cleaned every year until no more soot is found, and a chimney sweep should be called if the fall inspection finds soot on the chimney walls.

Changing an appliance can change the chimney's job. Under the Illinois Mechanical Code, an appliance that vents by its own natural pull must not be connected into a vent, chimney or connector on the outlet side of a mechanical flue exhauster. When a new sealed-combustion water heater leaves an unused opening in the chimney, that hole should be sealed. When you replace a furnace or water heater, ask how the other appliances on that chimney will vent.

Source: U.S. EPA · U.S. Consumer Product Safety Commission · UpCodes (code text) · U.S. Department of Energy

Some appliance designs take the chimney out of the picture. A direct-vent, sealed-combustion appliance brings in combustion air from outdoors through one sealed pipe and sends its exhaust outdoors through a second. For appliances inside your living space, it is the safest and usually most efficient type, and it removes the possibility of backdrafting.

Water heaters come in safer types too. Atmospheric (naturally drafted) water heaters are still sold, but power-vented models, which use a fan to push exhaust out the flue, and direct-vent models are safer and more efficient. A high-efficiency power-vented gas water heater can also be used to avoid backdrafting. If a vent problem keeps coming back, a technician can explain whether a different type of appliance fits your home.

Source: U.S. Department of Energy

Some signs of a venting problem are visible. Melted plastic washers on the pipes and soot on top of a water heater tank are signs that it has backdrafted in the past and may still be backdrafting. Look for discoloration at an appliance's burner, burner door or vent area. Check vents and chimneys regularly for poor connections, visible cracks, rust or stains.

The way the pipe runs matters too. A flue pipe that is disconnected lets carbon monoxide and other exhaust escape into the home and needs repair as soon as possible. If a flue pipe slopes down instead of up, exhaust is likely to back up and spill into the house instead of leaving. Blocked vents can disrupt airflow and let exhaust containing carbon monoxide into the home. If you notice any of these, have the appliance and its vent checked before you keep using it.

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