Proper Gas Vent Design
August 6, 2007
The design of gas vent pipes and connectors for furnaces,
water heaters, boilers, and other fossil-fuel appliances is a serious matter.
Carbon monoxide poisoning, collapsing chimneys, fire hazards, corrosion of
vents, draft inducers, heat exchangers, and non-conformance with building codes
are all problems associated with improper gas vent installations.
If a gas vent is not properly designed, acidic condensate
from the vent gas can easily form on the inside of the vent pipe and cause
rusting and pitting. As the corrosion process continues, the vent pipe becomes
riddled with holes and ultimately fails. When this happens, vent gas containing
carbon monoxide and all the other products of combustion can leak into occupied
areas of the building.
Low-efficiency gas appliances such as older 78 percent AFUE
furnaces rarely have such a condensate problem because the flue gas exits with
a relatively high temperature, making condensation difficult to occur. However,
many furnaces and heaters have an efficiency just below 83 percent and the flue
gas exits much cooler. These are known as Category I type gas appliances, and
they are much more prone to developing unwanted condensate in vents and
connectors. Because of the many dangers related to acidic condensate, the
sizing of gas vents is extremely critical for Category I-type gas appliances.
Furnaces with efficiencies 83 percent or greater are
specially designed to handle condensation. These appliances, typically
classified as type II, III, and IV, also require proper vent sizing, but they
are much more forgiving than Category I appliances. Condensation is expected to
occur in some of these appliances, and safeguards are in place to handle it.
The document governing proper gas vent design is published
by the National Fire Protection Agency as NFPA 54: National Fuel Gas Code
[NFGC] 2006 edition. This NFGC document relies heavily on venting tables
developed by the Gas Research Institute. The corresponding international
version of this document is IFGC-06: International Fuel Gas Code 2006 Edition,
published by the International Code Council.
Many factors affect the required vent and connector sizes
for gas appliances. The appliance input Btuh rating, outlet diameter of the
appliance, number of elbows, single or multistory application, vent gas
temperature, ambient air temperature, vent wall conductivity, vent connector
type, vent thermal mass, vent pressurization (fan-assisted or natural draft),
vent height, and vent lateral length all affect vent design. The NFGC manual
provides a voluminous set of venting tables that take all these factors into
account so that both minimum and maximum allowable vent and connector sizes can
be specified for any given situation.
DESIGNING THE PROPER SYSTEMThe first step in designing a vent system entails selecting
the vent and connector material types. The vent connector is the pipe that
connects the appliance to the main vent pipe. A vent connector can be either
single-wall metal pipe or Type B, which is double-wall metal pipe with an
insulating effect. The main vent can never be made of single-wall metal pipe.
It must be either Type B pipe, tile lined masonry chimney, or a flexible metal
Selection of single-wall metal or Type B vent connectors is
mainly governed by cost and applicable restrictions. Single-wall metal vent
connectors are lower in cost than Type B connectors, but they operate at much
higher surface temperatures than do Type B connectors. Consequently, there are
many restrictions on the use of single-wall metal connectors.
For example, single-wall metal connectors cannot be used in
attics due to the fire hazard, and they must have greater clearances on all
structural components than Type B connectors. A good strategy for maximum
safety and minimum chance of violating building code requirements is to use
only Type B double-wall metal vent connectors.
If a chimney is not being used for the vent, NFGC specifies
that only Type B pipe can be used for the vent. If a chimney is involved, it
must have an appropriately sized tile liner or a flexible metal liner. Some
chimneys appear to have a tile liner, but upon close inspection, the liner may
only exist at the top of the chimney.
Venting combustion gases through a chimney without a proper
liner often results in the collapse of the chimney as condensate can dissolve
the mortar between bricks. Even if a tile liner is in place along the entire
interior of the chimney, it is also possible that it may be too large for the
appliances being vented through it. If there is any doubt as to the presence,
size, and quality of a tile liner in a chimney, a flexible metal liner of the
correct size should be used.
Once the vent connector and vent types are decided on, the
proper sizes can be read from the NFGC venting tables. In the simplest case
with only a single appliance, the information needed to size the connector and
vent includes whether the appliance venting is fan-assisted or natural draft,
the appliance input Btuh rating, appliance outlet diameter, lateral distance
from the appliance to the vent, height from the appliance to the top of the vent,
and the number of 90-degree elbows. For the given data, the venting tables list
the minimum- and maximum-size vent and connector diameters that can be used.
Sizes from the vent tables can be easily read in a
straightforward fashion. However, the size from the table must often be
adjusted according to over 30 notes and exceptions explained in the manuals.
For example, for every 90-degree elbow beyond the quantity two, the Btuh
capacity for a vent size must be reduced by 10 percent. Similarly, there are
maximum horizontal connector length limits. The Btuh capacity of a vent must be
reduced 10 percent for each multiple of a specified horizontal connector
There are many such qualifiers to the venting tables and
making sure all of them are honored is the main difficulty of vent sizing. It
is not uncommon for a vent design to need three or more adjusting factors. This
is particularly true for multiple appliance and multistory applications.
AVAILABLE COMPUTER PROGRAMSUnlike for standard HVAC calculations, there are not
many computer programs available for venting design. In fact, at this time
there are only two automated solutions available to do all the table look-up
values and all the adjustments for special conditions. Exhausto has a Web-based
application program called FanCalc2005 that can perform many of these
calculations. To access the software, the user must contact a local Exhausto
representative. More details are available at www.fancalc.com.
The other automated solution is a stand-alone Windows
program from Elite Software called GasVent. A designer can quickly enter the
information into GasVent and instantly see the minimum and maximum allowable
vent sizes with all the checks and adjustments automatically done. See details
on GasVent at www.elitesoft.com.
Both GasVent and FanCalc 2005 are very graphic and visually
intuitive with detailed help provided for every input item. On the GasVent
screen, all of the required input dimensions are clearly labeled so it is easy
to know what to enter. Calculation results are instantly displayed on the same
There is no question that gas vent sizing can be quite
technical and tedious. While software can’t remove all the risks in vent design
and installation, it can definitely help you create more accurate designs in
much less time.
Publication date: 08/06/2007