How gas appliances burn and vent

A burner mixes gas with air, and it needs enough air to burn cleanly. The Illinois Mechanical Code says combustion air must be supplied to appliances to prevent poor combustion, which can create several health and safety hazards. For gas-fired appliances, it points to the International Fuel Gas Code for those requirements.

Appliances get that air in one of two ways. Sealed-combustion appliances draw their air from outside, while atmospheric ones draw it from the room they sit in, the combustion appliance zone; without enough air, burning is inefficient and dangerous. For atmospheric units, Department of Energy guidance says the space around the equipment must hold 50 cubic feet per 1,000 Btu per hour of the combined input of all the burning appliances there. In a closed-in space without enough air, that zone must be vented to the outdoors or opened to a larger indoor space. This is one reason a technician looks at the room around a furnace or water heater, not only the appliance.

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

The flue is the pipe or chimney passage that takes exhaust gases out of your home. Illinois code requires vent systems to be built so they produce a positive flow strong enough to carry all the exhaust outside. Vent systems must be listed and labeled.

Older and simpler appliances rely on hot air rising. Atmospheric furnaces use the heat in the flue to pull combustion air through the furnace and lift the exhaust up and out of the chimney. This only works when the path is open. Chimneys and flues should be clear at all times so toxic gases, especially carbon monoxide, can escape.

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

Gas furnaces and water heaters are sorted into groups by how they vent. The National Fuel Gas Code names four categories, based on whether combustion is sealed or open and on the temperature in the vent pipe. The category tells an installer what kind of vent the appliance needs.

Category I appliances are the oldest and least efficient type: their flue gas is over 140°F and the flue pipe pressure is lower than the room around the appliance. They are not sealed, because nothing separates the burner from the room. Both plain atmospheric furnaces and fan-assisted furnaces that take combustion air from the room are Category I.

The other three run cooler or push their exhaust. A Category II appliance has a vent temperature below 140°F, condenses some of its flue gas and has negative pressure in the vent. A Category III appliance has a vent temperature above 140°F, hot enough to avoid condensing in the vent, and positive pressure in the vent. Direct-vent sealed-combustion furnaces are Category IV: the vent is under positive pressure, flue gases are under 140°F, and water vapor condenses and drains away.

Source: U.S. Department of Energy

When a fan pushes the exhaust, the vent itself must be built for it. Where an appliance's fan creates positive pressure in the vent, Illinois code requires a vent system designed and listed for positive pressure. Those parts must be gas tight so exhaust cannot leak into the building. An appliance that relies on rising hot air may not be connected into a vent on the outlet side of a mechanical flue exhauster.

Cooler exhaust allows different pipe. Because the flue gases are cool, the intake and exhaust pipes on a Category IV condensing furnace can be made of PVC. Illinois code requires plastic vent pipe and fittings to be installed following the appliance maker's instructions. Direct-vent appliances' vent systems must follow both the appliance and the vent maker's instructions. This is why the right vent depends on the exact appliance, not only on the fuel.

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

Condensing furnaces sit at the high-efficiency end of the range. Condensing furnaces have annual fuel utilization efficiencies (AFUE) of 90% to 98%. In these furnaces, the condensed water leaves through a condensate drain, and the rest of the flue gas leaves through a sealed exhaust pipe. That drain line is a normal part of a condensing furnace.

Many of these appliances are also sealed. A direct-vent sealed-combustion furnace, boiler or water heater brings combustion air straight from outdoors through one sealed pipe and sends the exhaust outdoors through a second sealed pipe. Because both the burning and the exhaust are sealed, the risk of backdrafting is eliminated. For appliances inside the living space, the Department of Energy calls a direct-vent sealed-combustion unit the safest, and usually most efficient, type.

Source: U.S. Department of Energy

Older homes raise their own venting questions. The Consumer Product Safety Commission says chimney inspections are especially important where oil or coal heating was converted to gas. It also notes that heating contractors do not normally check chimneys and flues, so it suggests asking them to include the venting system in their inspection. The EPA advises having furnaces, flues and chimneys inspected every year and repairing cracks or damaged parts.

Changing one appliance can change the chimney's job. PNNL's water heater checklist says that when a new sealed-combustion, direct-vented water heater leaves an unused opening in the chimney, the hole shall be sealed. A flue that is the wrong size, blocked, disconnected or leaking can make an appliance a source of carbon monoxide. The venting and chimneys page explains the warning signs and what a technician checks.

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