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By Bob Dwyer & Mark Gronley


Most furnaces, boilers, and water heaters going into homes today are not super-efficient "sealed combustion" or "direct vent" units. They are mid-efficiency "atmospheric-vented" appliances that use a regular vent or chimney, relying on natural buoyancy to carry the hot gases up the flue and out at the top.

But compared to yesterday's 65% efficient units, modern 78% to 83% efficient, naturally vented appliances keep more heat in the house and send less heat up the flue -- and that makes things tricky. The cooler the flue gas, the weaker the draft and the greater the risk of back-drafting.

Cooler exhaust also increases the risk of condensation and corrosion in the vent. Gas burner exhaust is mostly carbon dioxide and water vapor, and in modern systems, that moist mix enters the flue at a temperature not far above its dew point of 140°F. If the moisture hits a cool surface, or just takes too long to reach the top of the system, it's likely to condense -- and because the vapor contains trace amounts of oxides of nitrogen, the condensate is a weak nitric acid solution that can corrode masonry or metal (see Figure 1).



Condensation in gas flues is acidic and can attack vents and chimneys. Telltale white deposits and rust are signs of corrosion (top left), which can perforate vent pipe and allow fumes to leak (top right). Chimneys are especially vulnerable: Acidic condensate eats through tile flue liners (bottom left) and destroys brick masonry (bottom right). It takes 50,000 Btus to heat a masonry chimney above the dew point of gas appliance exhaust, while only 125 Btus are needed to warm a B vent flue.

If vents and chimneys get wet only occasionally and dry out quickly, damage isn't likely. But systems that get wet and stay wet can corrode badly, and fast. To minimize "wet time," modern vent systems have to be sized just right -- large enough to handle the exhaust but small enough to warm up quickly, so that any condensation will dry up before the equipment shuts down and the flue won't stay wet between cycles.

Material choices affect vent function, too. Masonry chimneys that take a lot of heat to warm up and uninsulated single-wall vent pipe that allows excessive heat loss have limited usefulness with modern gas equipment. For the most part, those old materials are being replaced by insulated double-wall B vent, which warms up quickly and has an aluminum inner lining to resist corrosion.

Even with modern vent materials, the rules for designing and sizing vent systems are complicated. With modern systems, the old "common sense" rules of thumb can be a recipe for failure. In fact, following the latest rule book religiously isn't always enough. To make sure the system works the way the installer intended, a trained technician should use combustion and draft testing instruments to check each installation against the manufacturer's specs.

New Rules, New Problems

In the early 1990s, after the government raised efficiency requirements to 78% for gas boilers and 80% for gas furnaces, there was a rash of complaints about back-drafting and corrosion in flues. In response, the gas industry made another rule change: It replaced the old, simple ratios of vent and chimney sizing with an elaborate new set of vent-sizing lookup tables, based on extensive research by Batelle Laboratories and the Gas Research Institute (GRI). The goal was to make sure that flues not only would draw properly, but also would stay as dry inside as possible.

The new sizing tables are published in the National Fire Protection Association (NFPA) National Fuel Gas Code (, in model building codes, and in manufacturer handbooks and instructions. They're also the basis for computer software packages such as Elite Software's Gasvent (, which automates the tedious process of cross-checking vent and appliance capacities.

The tables specify the height, lateral run, and diameter of vents and vent connectors for a whole range of situations, depending on the vent material and on the size and type of the appliance or combination of appliances being vented. And you can work the system in reverse: If you know the vent material, the vent diameter, and the horizontal and vertical layout of the vent system, the tables tell you what size appliance the vent can serve.

Unfortunately, many installers still don't understand why the new rules exist or how they work, and they also may not understand that changing venting materials can change the requirements. When we test and inspect installations in the field, we continue to see enough mistakes to know that a lot of "technicians" are still doing things "the way we've always done it" -- with predictably bad results.

Gas furnaces, boilers, and water heaters are safe and reliable when everything is done right. But builders and remodelers should be extra careful to use hvac technicians who thoroughly understand and follow the rules, and who check every installation with professional instruments.

Complex Range of Options

There's a wide range of choice in appliances and vents. But for every combination of equipment type and vent material, the basic goals and limits apply: The vent must be able to provide sufficient draft for the equipment; it must be warm and dry enough to avoid damaging corrosion; and it must be fire safe. The sizing tables and other venting rules aim to achieve that safety and durability, but in providing for the whole range of choices, the rules have become complicated.

Appliance types. Most mid-efficiency furnaces and boilers are "fan-assisted" units, which use an inducer fan to push or pull air through the combustion chamber (Figure 2). The fans help overcome resistance to airflow within the appliance and create turbulence for better combustion and heat exchange, to boost efficiency. But they don't pressurize the vent system: Once the exhaust leaves the unit, it still needs the negative pressure of buoyancy in the vent to draw it out of the house.


Figure 2.Fan-assisted Category I furnaces (left) use an inducer fan to overcome air resistance in the combustion chamber but do not pressurize the vent system. No dilution air is introduced, so the exhaust is damp and condenses readily. Draft-hood water heaters (right) rely on natural draft to bring in combustion air, and the draft hood allows dilution air to mix with flue gases, wasting some energy but creating a drier flue gas. The two types have different vent size requirements but can be common-vented on a single flue as long as sizing rules are followed.