Venting Gas Appliances -
Common venting. Now let's
add a complication: two appliances common-vented on a single
flue (Figure 4). The table now has to consider "vent connector
rise," and it matters whether we've got two "nat" appliances,
two "fan" appliances, or one of each.
Figure 4.When two units are commonvented, each
vent connector must be sized independently, and vent connector
capacity dictates appliance size ranges. Again, single-wall
vent connectors sharply restrict the choice, and for practical
purposes fan-assisted units will nearly always require B vent.
Given a 10-foot vent height and a 3-foot connector rise
(circled in table), a B vent connector could handle a
fan-assisted unit between 7,000 and 92,000 Btu/hr (for
4-inch-diameter pipe) or between 69,000 and 220,000 Btu/hr (for
6-inch-diameter pipe). If single-wall connectors were used, the
appliance on the 4-inch connector would have to fall between
89,000 and 91,000 Btu/hr — essentially no choice at
all. Increasing connector diameter boosts both the maximum and
the minimum, leaving the allowable range quite narrow. Many
units would be too large for a 4-inch connector and too small
for a 6-inch connector.
Of course we still have all the various vent types to choose
from: B vent or masonry chimney, B vent connector or
single-wall. As you see, chimneys don't give you much choice
compared to the B vent options.
Improving the venting arrangement can widen your equipment
options (Figure 5). If the table won't permit the appliance you
need, try to increase total vent height or connector rise by
repositioning the appliance or rerouting the vent. Consider the
options carefully: Common-venting two appliances with a single
vent often lets a vent handle more total Btus and conserves
material, but if it takes too many elbows and horizontal runs
to accomplish, you may be better off giving each unit its own
straight vent. That way, neither unit will ever have to operate
alone on a flue that is sized for two.
Figure 5.Reconfiguring the vent to gain total
height or connector height can improve the performance of a
common vent. Venting each unit separately can also improve
reliability, because it right-sizes each vent, whereas the
common vent is oversized whenever either appliance operates
alone (and requires lateral runs and elbows that degrade
You can see why a lot of hvac contractors don't use these
tables. It's a real pain in the neck. But with software
available to automate the process, there's no excuse for a
professional to ignore the rules. And most cases aren't that
complicated -- once you get the hang of the system, you can
home in on working solutions without too much trouble.
However, it's important to realize that the tables don't
account for every possibility. They don't include the effect of
elbows in the system, for example: Each table assumes no more
than two 90-degree bends, and vent capacity is reduced by 10%
for each additional 90. And all the main table numbers are
based on vents that run inside the house: Vents placed outside
the walls are more prone to failure. (There are some special
tables for masonry chimneys on exterior walls, but the
allowable unit minimums are quite large; it's always better to
reline the chimney with a listed metal liner instead.)
With experience, people learn how to make adjustments when the
tables limit their choices. Reconfiguring the vent to increase
total vent height or connector rise is one way to increase a
vent's capacity when a large appliance is called for. This may
be more economical than using a larger-diameter vent.
Do's and Don'ts of Venting
The rules are based on some fairly simple ideas: Hot air
rises, and flue gases will condense if they cool; a strong
draft is good, and condensation is bad. The important ideas are
to supply plenty of combustion air, to keep the flue warm and
dry, and not to restrict the flow of exhaust in the flue. If
you try to maximize all the good influences and minimize all
the bad influences, most systems will fall well within the
permissible range in the tables. A few basic do's and don'ts
will take you a long way toward getting good results:
* Never use unlined chimneys. Avoid masonry chimneys even with
tile liners. Reline chimneys with listed liners or B
* Avoid single-wall vent connectors if possible. If you must
use a single-wall connector, choose a corrosion-resistant
brand. Single-wall vent pipe cannot pass through unconditioned
space (attic or crawlspace) and cannot run through concealed
spaces. It must be readily accessible for inspection, cleaning,
* Keep the vent or chimney within the conditioned
* Keep appliance locations warm.
* Maximize total vent height (in all cases provide at least 5
feet of vent above the highest draft hood or flue
* Maximize vertical connector rise off the appliance (Figure
* Avoid long horizontal runs.
* Maintain correct pitch in horizontal connectors (no sag).
Support vent connectors properly.
* Avoid elbows.
* Use 45s and 60s instead of 90s -- two 45s equals one 90
* Use wyes rather than tees.
* Consider independent venting for separate appliances.
* Avoid oversizing equipment.
* Plug any unused openings in the chimney or vent.
* Observe required clearances.
* Use an approved firestop system wherever B vent passes
through a wall or floor. Never run single-wall vent pipe
through any wall or floor.
* Don't share flues with oil, coal, or wood. Mark all gas vent
systems clearly to prevent their use by any other type of
* Do not allow two vent connectors to enter a main vent or
chimney opposite each other. Instead, offset the
* Terminate vents well away from walls and well above steep
sloped roofs. Use UL-listed vent caps, not locally made or DIY
* Never connect positive-pressure vent systems to a
negative-pressure vent system.
* Isolate appliances from house indoor pressures and provide
an air supply from outdoors.
* Always test combustion products and draft after
Figure 6.When space allows, always maximize the
rise of the connector directly off the appliance —
this helps to develop a strong draft. A minimum of 1 foot is
required, but more is better.Figure 7.When elbows are needed, 45-degree or
60-degree bends are preferable to 90-degree bends. The sizing
tables assume two 90-degree elbows per appliance, and each
additional 90-degree elbow reduces vent capacity by 10%. The
two extra 90-degree bends in the left-hand example reduce the
vent’s capacity by 20%; if 45-degree bends are used
instead, the capacity reduction is only 10%.