By Bob Dwyer & Mark
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
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
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
(www.nfpa.org), 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 (www.elitesoft.com), which automates the
tedious process of cross-checking vent and appliance
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.