Fundamentals
of Venting and
Ventilation• Vent System Operation• Sizing, Design and Applications• Installation and Assembly• Troubleshooting© American Standard Inc. 1993

Pub. No. 34-4010-02

Contents
Subject

Page• Motive force in vents ...................................................................................................................................

2• Factors affecting operation .........................................................................................................................

3• Ventilation air requirements ........................................................................................................................

4• Vent design considerations - .......................................................................................................................
– Category I furnaces...............................................................................................................................
– Category II furnaces ..............................................................................................................................
– Category III furnaces .............................................................................................................................
– Category IV furnaces ............................................................................................................................

6
6
6
6
7

• Types of vents ..............................................................................................................................................
– Vent classifications ...............................................................................................................................
– Vent materials .......................................................................................................................................

7
7
9• Vent system design configurations ............................................................................................................
– Single appliance natural draft ..............................................................................................................
– Multiple appliance – single story common venting ............................................................................
– Manifold venting ..................................................................................................................................
– Multi-story vent systems ......................................................................................................................

9
9
9
11
11

• Sizing and Design ..........................................................................................................................................
– Category I furnaces• Single appliance w/masonry chimney ............................................................................................
• Single appliance Type B vent ..........................................................................................................
• Multiple appliance single story vents- common venting ..............................................................
• Multi-Story Vent Systems - common venting ................................................................................
• Vents in high altitutde locations ......................................................................................................
• Power venting ..................................................................................................................................
– Category II furnaces .............................................................................................................................
– Category III furnaces ............................................................................................................................
– Category IV furnaces ............................................................................................................................

13
13
14
16
17
19
19
19
20
21

• Combustion and Ventilation Air ..................................................................................................................
– Confined space .....................................................................................................................................
– Unconfined space .................................................................................................................................

22
22
23

• General Installation .....................................................................................................................................

24• Safeties and Accessories ............................................................................................................................

27• Prohibited Installations ...............................................................................................................................

28• Troubleshooting ..........................................................................................................................................

30• Appendix – Vent System Design Tables .....................................................................................................

32• Glossary of Terms ........................................................................................................................................

37

Portions of the text and illustrations in this book are used by permissionof the copyright holder, American Gas Association.
1

Venting and Ventilation
When man first began to burn fuel for warmth, he soon
learned that the smoke, created during combustion, caused
problems when it was allowed to collect at the source. His
early caves were selected and later lodges were constructedwith an escape for these unpleasant fumes. As he began
to employ natural gas as a fuel, he continued to use what
he had learned.Today, we still find that proper vent system
operation is vital to safe and efficient furnace operation.
In this manual, we will discover the things which are needed
to provide this vent system.
We will also learn that a gas furnace requires a substantial
amount of fresh air to operate properly. This is called
ventilation air. This air is used in combustion and as a replacement for the air that a modern home loses throughoutward flow due to mechanical ventilation. Although wecannot control the loss of air, we can properly diagnose
it if it becomes a problem and suggest cures.

Figure 1

The following topics are covered in this manual:Heat drives the flue gasses out of the furnace because heatalways moves from warmer areas to cooler areas. The hotflue-gases would much rather travel up the relatively coolflue pipe than to “back-track” to the much hotter main burner
flame.

• How a natural draft (gravity) vent works.
• Fan assisted vent systems.
• Power venting.
• Sizing and designing vent systems.

The final force, which results from the heat which is created,is the pressure difference found between the inside of thevent and the atmosphere around it. The combustion air
entering the furnace is at atmospheric pressure which, at
sea level, is about 14.7 PSIA (Pounds to the Square Inch
Absolute pressure). The hot flue-gases rising out of thefurnace are less dense, creating a pressure drop within
the heat exchanger. This “gap” is continually being filledby the higher pressure entering air/gas mixture. So, in effect,
the gasses entering the furnace are helping to “push” theflue gasses out of the furnace. See Figure 2.

• Supplying adequate amounts of fresh air for
ventilation.
• Vent system installation.
• Troubleshooting.
Using the guidelines in this book can help to ensure safeand reliable vent performance for both natural draft and
fan assisted furnaces.
Note: No two installations are alike! Take the time
to study each application. Plan ahead, use good
judgment, and always comply with national, state,
and local codes.
Never sacrifice safety for efficiency or easeof installation!

VENT
GASES

Motive Force in Vents
What causes a natural draft vent to operate? Heat! As weall know, heat rises.This is because heated molecules havea tendency to become agitated and to move about. Because of all this motion, the molecules “bump” into eachother creating more space in between them. See Figure
1. This makes the substance less dense - lighter. Referring
to Figure 1: As heat is applied to the air inside the balloon
it expands and becomes lighter. When enough heat is applied,the balloon will become lighter than the air around it andbegin to rise. For this reason, gas furnaces are designedwith the heat exchanger outlet located above the inlet. This
allows the heated products of combustion to travel in anatural, upward path forming a natural draft!

ATMOSPHERIC PRESSURE
HEAT
RISES

COMBUSTION AND
EXCESS AIRGAS AND PRIMARY AIR

To come back down, the balloonist in Figure 1 merely has
to stop applying heat. Soon, the air in the balloon will begin
to cool and become heavier. The balloon will then start
dropping back to earth.

Figure 22Factors Affecting Vent System Design
and Operation

Vent Height
Where vent height is concerned, taller is better. As an illustrationof this principle, refer to Figure 3. In 3A, the air leaving theheat source has only a short distance to travel before
encountering atmospheric pressure.This retards the abilityof the flue-gases to set up a good flow before encountering
resistance. Figure 3B shows a taller vent pipe which allows the flue-gases to gather a little more momentum before
exiting the vent pipe. Therefore, they have a better chanceof overcoming the atmospheric pressure at the vent outlet.
Sample 3C is even better still. Because of resistance in thepipe, gravity, and the cooling of the flue-gases, you canmake the pipe too long. There will be a point where thesefactors counter-balance the momentum of the flue-gases.
This limit is not usually reached.

There are many variables dictating how well a ventsystem will operate. They are: 1. flue-gas temperature
2. heat loss in the vent 3. vent height 4. vent system capacity
5. restrictions to flow and 6. the ambient temperature.

Flue-Gas Temperature
Flue-gas temperature affects vent operation because thehotter the gases are, the lighter they are. The
design BTUH (British Thermal Units per Hour) capacityof the vent plus the furnace input and efficiency are factors
affecting the temperature of the flue-gases. Higher BTUH
furnaces produce higher flue-gas temperatures. On theother hand, the higher the efficiency of a furnace, the lower
the flue-gas temperature. Two cases which can cause poor
venting are the oversizing of the vent or reducing the BTUH
input to an existing vent system. Either case will result inexcessive cooling of the flue gases and a reduction of thevent forces.

Vent System Capacity
The volume of flue-gases produced by a furnace directly
depends on the BTUH input of the furnace. The greater
the BTUH input of the furnace, the greater is the amountof combustion products which need to be removed. Vent
system design tables take into account, among other things,
the volume of gases which will be produced by a furnaceof a certain input.

Heat Loss
The amount of heat lost through the vent is based on two
factors. They are: the ability of the vent pipe to transfer
heat, and the temperature difference between flue-gases
and the air around the vent. Single wall metal vents conduct
heat well. Some non-metallic vent materials may absorb
large amounts of heat. Double wall metal vents are often
preferred because of their insulating qualities and the factthat the relatively thin inner walls do not absorb much heat.

If the input is greater than the capacity of the vent,thenthe vent will be unable to carry all of the flue products away.

VENT TOO SMALL

Temperature differential affects heat loss because of heatflow-rate. The greater the temperature difference is on eitherside of a substance, the faster the heat will flow throughthe substance. For example, sections of vent pipe passing
through an unconditioned space will lose heat more rapidly
than vents within a conditioned space.

ATMOSPHERICPRESSURE(1)

FLUE GAS SPILLING OUTOF DRAFT HOOD

(2)

NO DILUTION AIR – HIGHSTACK TEMPERATURES(3)

RE-CIRCULATION OF FLUE-GAS
INTO COMBUSTION AIR SUPPLY– INCOMPLETE COMBUSTION(4)POSSIBLE FORMATION OF
CARBON MONOXIDE AND
CORROSIVE COMPOUNDS

Figure 4

As a result, flue products will spill out of the draft hood
relief opening. Figure 4 illustrates some of the problems
which may result from undersized vents.

H
E
A
T

A

H
E
A
T

H
E
A
T

B

C

We have traditionally relied on the adage “ Bigger is better!”
However, if the BTUH input is substantially below the capacityof the vent, the flue-gases will cool off before reaching theend of the vent. The fan-assisted furnaces are affected bythis and the charts which apply to them have been re-written
to reflect this concern. The reduction in the drafting forces
can lead to condensation and acid formation within thevent. Acids and condensation are the greatest enemiesof the gas appliance vent.

Figure 33

Resistance to Flowlateral runs of vent pipe must have an upward pitchof one-quarter (1/4) inch of rise per every foot ofrun. This is not a great deal of slope, but it is necessary
to help ensure the upward travel of the flue-gases. In theextreme, a lateral run with a slightly downward slope canactually act as a “trap” and prohibit the flow of flue products.

Resistance in a vent pipe can come in several forms. Some
examples are vent fittings such as elbows, tees, vent caps,
and end screens. Other, more subtle, examples are wallroughness of the vent pipe, the shape of the vent pipe (round,oval, square, rectangular) and the configuration of the ventsystem. Resistance caused by fittings is unavoidable. The
vent designer can only limit the number of fittings to theminimum necessary. Resistance pertaining to vent system
configuration can be held to a minimum using good, commonsense design procedures.

Two good rules to follow when designing and installing
vent systems are: (1) avoid an excessive number ofturns and bends (Figure 5) and (2) maximize rise;minimize run.

Ambient Temperature

Fittings such as elbows and tees cause the flue gases tochange direction and result in a loss of momentum. Other
fittings such as end screens and flue baffles take up spacein the vent. This effectively reduces the area of the ventpipe and, therefore, reduces the capacity of the vent.

Two ambients must be considered. First, consider the ambient
temperature of the air around the vent pipe. As stated earlier,heat loss through the vent pipe walls can take away fromthe draft force. Heat flow rate through a substance is directly
related to the temperature difference on either side of thesubstance. The greater the temperature difference, the greater
the heat flow rate. Care must be taken when running ventpipes through unconditioned spaces such as a ventilated
attic. It is a good idea to insulate vent pipe in unconditioned
spaces.

The shape of the vent does affect operation somewhat,
but it is not as critical as the other factors mentioned. Gas
flow through a round pipe is less turbulent than througha square or rectangular pipe. However, because we aretalking about relatively low velocities, the effects of turbulence
are minimal and are usually not considered.

On the other hand, when it comes to combustion air supply,
a lower entering air temperature is better. The cooler asubstance is, the more dense it is. Being more dense alsomeans it is heavier. Cooler air gives us more “push” atthe combustion air inlet. In the case of a unit which draws
all of its combustion air from outside, the vent system will
perform better on colder days when the difference intemperature is greatest.

Of significant importance is the configuration and installationof the vent system. Most charts allow two 90° bends inthe vent as they are written. Added 90° bends in the ventsystem cause a 10% loss in vent capacity. An excessive
number of bends in a vent can overcome the drafting action
completely. Later, in the section on vent system design,
the effects of friction losses from bends is covered in moredetail. Also, since flue-gases want to rise, horizontal (lateral)
sections do not contribute to draft action. For this reason,

Satisfactory Operation
Section 7.2.1 of the National Fuel Gas Code (NFPA54/
ANSIZ223.1), under minimum safe performance, statesthat a venting system shall be designed and constructedso as to develop a positive flow adequate to remove allflue or vent gases to the outside atmosphere.

Ventilation Requirements

AIRFLOW PLENUM

Any discussion of venting must include coverage of ventilation.
Ventilation is the process of replacing the air which is required
for the furnace operation. The amount of ventilation air
supplied within the structure must equal all of the air
requirements of all of the gas-fired equipment in the building
plus any air quantities removed by range hoods, exhaust
fans, etc. Whole house ventilation fans, sometimes called
attic fans, are not considered because they are not usuallyin operation during the heating season.

3 AIR CONDITIONING

1

COIL

4

2

A natural draft furnace’s combustion air supply is divided
into four parts. Primary air is pre-mixed with the gas inside
the burner and makes up only about 20% of the air required
to completely support combustion. Secondary air completes
combustion by entering the furnace through openings near
the burners and mixing with the gas at the burner flame.

FURNACE

In order to ensure adequate air supply to the burner, most
furnace designs have air openings sized to provide theburner with more than enough air for combustion. This
extra air is called excess air and the furnace design will
determine the amount of excess air which will be used.

Figure 54

Ex: 100,000 ÷ 1000 x 10 = 1000

This excess air supply ranges, by design, from 30% to 50%of the requirements for complete combustion. Dilution
air is that air which is drawn in through the draft hood ordraft diverter relief opening. Dilution air is a “side-effect”
of the need to have a relief opening. The main purposeof the relief opening is to relieve negative pressure extremes
in the vent which may cause pilot flame outages or “lifting“ of the main burner flame. Fan assisted furnaces do notuse dilution air.

2. Multiply the result by the excess air percentage
Calc: Minimum requirement x excess air % =excess air
Ex: 1000 x .5 = 500
3. Add the excess air to the minimum requirement
Calc: Min. req. + excess = primary & secondary air

Sometimes, dilution air is necessary to lower flue-gas

Ex: 1000 + 500 = 1500
4. Double this result to account for dilution air
Calc: Primary & secondary air x 2 = total

SUPPLY
AIR

VENT
GASES

Ex: 1500 x 2 = 3000 cfh or 50 cfm(cubic feet per minute)

Vent Sizing and Design
PRODUCTSOFCOMBUSTIONAs mentioned previously, in order for a natural draft or
gravity vent to work properly, it must have heat. The properamount of heat will be available only if the vent issized properly.Too small a vent will not have the capacity
to carry all the combustion products outdoors. Too large
a vent will allow excessive dilution air to mix with the
combustion products and cool them so that a proper draft
is not created. The vent must be made of a material thatheats up quickly so that the draft will be established before
combustion products spill into the room. Large, unlined
masonry chimneys take so much heat to warm them thatthe venting may not take place properly for some time.
Single-wall vents lose heat so fast that a proper heat levelmay not be established. In new construction, double-wallvents offer a good choice as a chimney liner for gas-fired
appliances, if a lined chimney is to be provided. Before
going on to detailed sizing instructions, a brief discussionof the types of products to be vented is in order.

DILUTION AIRCOMBUSTION AND
EXCESS AIRGAS AND PRIMARY AIR

RETURN AIR

Figure 6temperatures to a less hazardous level. Figure 6 shows
the hot flue gases exiting the heat exchanger in an upward
motion. This motion creates a pressure drop in the areaof the relief opening. Because of this pressure drop, theair around the draft hood/diverter is drawn into the flue.
This is because the ambient air is at atmospheric pressure
and will flow to any area under less pressure.

Furnace Categories
In 1981 a task force from the American National Standards
Committee was formed to develop recommendations forventing high efficiency gas appliances. One of the resultsof their work is the classification of gas appliances in relation
to their venting characteristics. The two properties which
are used for classification are the static pressure within
the vent system and the temperature of the flue-gas entering
the vent.

Just how much dilution and excess air is required? Consider
a couple of facts. First of all, we realize that the air aroundthe draft hood is at the same atmospheric pressure as the
air going into the combustion air openings. Second, thedraft force at the draft hood/diverter is equal to and possibly
greater than the draft force at the combustion air openings.
These facts can lead us to conclude that the dilution air
supply is equal to or greater than the combustion and excess
air supplies combined.

By static pressure in the vent, gas appliances are classified
as positive or non-positive. A non-positive appliance vent
will operate under zero or negative static pressures. Leakage
in the vent system will flow into the pipe. A positive appliancewill produce a positive static pressure within the vent. Becauseof this, measures to seal the vent against leakage must
be taken when venting appliances classified as positive.

To determine the total ventilation air requirement:

Where flue-gas temperature is concerned, the separationof categories is determined by whether or not condensation
may occur inside the vent. A limit of 140°F (140 degrees
fahrenheit) above the flue-gas dew-point was the value
derived through testing and simulation. Appliances producing
flue products with a temperature less than about 275°F(see Table 10 in the Appendix) have a high probability of

Example: Input = 100,000 BTUH, 50% excess air
1. Obtain the minimum air required for complete
combustion. NOTE: Approx. 10 cu. ft. per 1000 BTU
Calc: BTUH input ÷ 1000 BTUH x 10 cu. ft. = cfh(cubic feet per hour)5condensation forming in their vents. Vent systems for theseappliances, found in Categories II and IV, must be constructedof a corrosion resistant material.Table 10 lists the different
categories, the temperature and pressure attributes of each,and their venting requirements.

Design Considerations for Venting the
Separate Furnace Categories.
Category I
Furnaces classified as Category I are non-positive with enteringflue-gas temperatures greater than or equal to 140°F above
their dew-point. This category includes all natural draft
appliances and most induced draft or “fan assisted”
appliances.Table 10 lists the venting requirements as being
specified in Tables 1 through 6 or the National Fuel Gas
Code (Figure 7). A word of caution: this category covers
a wide range of products. Included in this category are themedium-high efficiency furnace models with efficiencies
ranging from around 78% to just below 83% AFUE(Annual
Fuel Utilization Efficiency). These primarily “fan assisted”
furnaces produce flue products which are equal to or
just above the 140°F above dew-point limit mentioned
earlier.The title “fan assisted” refers to the gas combustion
portion of the furnace, not the indoor blower for structureindoor air.

CATEGORY II
CORROSION RESISTANT
Figure 8

the venting material must be resistant to corrosion. See
Figure 8. This is because there is a good chance thatcondensation will occur in the vent. It is also necessarythat the vent material have a low heat transfer coefficient.
There are currently some thermoplastic materials being
used with these types of furnaces and gas appliances.

Category III
Category III furnaces are forced-draft furnaces with fluegas temperatures greater than or equal to 140°F above
dew-point. Because of the high flue-gas temperatures,condensation is not expected to occur in the vent systems
for these products. Therefore, corrosion resistant vent materials
are not specifically required. A special case is created when
an induced draft furnace is to be vented horizontally.This
will be covered later.

CATEGORY I
NO SPECIAL CONSIDERATIONS
Figure 7

The vent sizing tables mentioned were originally designedon the assumption that entering flue-gas temperatures
are above 360°F and have dilution air drawn into the ventsystem through the draft diverter. With furnace models
in the efficiency range around 80% AFUE, flue-gas temperatures can be lower. With induced draft appliances, there
is no dilution air. Accurate sizing of the vent systems forthese models is critical. Later, in the section on sizing anddesign, proper venting of any Category I appliance or
combination of these appliances will be covered using thelatest release of these tables.

CATEGORY III
LEAK TIGHT
Figure 9

Category II

The vent systems for these forced-draft products are underpositive pressure and must be leak-tight. Any leaks in thesevent systems will be outward and will contaminate theatmosphere around the vent. See Figure 9.

Category II furnaces are those non-positive furnaces which
produce flue products less than or equal to 140°F above
their dew-point. Because of the lower flue-gas temperature,6

Types of Vents

Category IV
Category IV furnaces are the 90+ AFUE high efficiency models.
These forced-draft furnaces have flue-gas temperaturesless than or equal to 140 degrees F above their dew-point.
The vent systems must be corrosion resistant, see Figure
10, as the lower flue-gas temperatures will lead to condensation in the vent. Also, since they operate under positive
static pressures, the vents must be designed and installed
to be leak-proof. PVC and CPVC plastic pipes may be usedwith the 90+% furnaces, when recommended by the
manufacturer.

Vent Classifications
Vents are classified by their construction, durability andresistance to heat and corrosion. The categories include:
Chimneys, Type B vents, Type B-W vents, Type L vents,
single-wall metal pipe, and plastic pipe.
A chimney is a vent constructed of masonry, reinforced
concrete or metal. Masonry chimneys are the most common
and were originally developed for use with coal or oil fired
systems.
Special considerations are involved when using a chimney
for venting a gas appliance. Chimneys used to vent Category
I central furnaces must be either tile-lined or lined witha listed metal lining system. If a fan assisted furnace is to
be used, the chimney must be internal to the structure andhave another appliance, with natural draft, common venting
into it or be lined with aType B vent or a listed flexible metallining system sized in accordance with the appropriate tables
and notes. If one or more walls of the chimney are exposedto the outside of the structure, the chimney must be linedwith a Type B vent or a listed flexible metal lining system.
Masonry chimneys, which have been converted for gas
after being used to vent coal, oil, or wood burning equipment
must be lined with a suitable metal liner. Unlined
masonry chimneys are prohibited.

CATEGORY IV
LEAK TIGHT &CORROSION RESISTANT

Special attention must be given to the chimney’s construction
and location. Flue construction is especially important. The
flue height and area affect the ability of the chimney toprovide a good draft. Also, cracks and leaks in flue walls
are a fire hazard.

Figure 10

Location of the chimney also affects venting. Chimneys
located outside of the structure require a long warm-up
period to establish adequate draft. See Figure 11. Chimneys
on outside walls are more prone to condensation and therefore
early deterioration.

Figure 117

Type L vents are also similar to Type B except they arehigher in resistance to heat, fire, and corrosion.Type L vents
have durability and resistance characteristics equivalent
to fire-clay tile, cast iron or stainless steel. Other parts ofthe vent system which may be subject to condensationor contact with flue gases are constructed of the samematerials and may include aluminum coated steel.

Type B vents are widely used in new construction becauseof relatively low cost coupled with good performance. These
vents can be made of aluminum, steel, or clay tile. Metal
Type B vents employ double wall construction with aninsulating air space. See Figure 12. When installed withspecified clearances, excessive temperatures on surrounding
combustible materials can be avoided.

Type L vents are for use with oil and gas appliances
listed as suitable for use with this type of vent. They
can also be used as a substitute for a type B vent or used
for chimney or vent connectors. The use of type L vents
is not common, although they are frequently selected for
applications where heat and/or corrosion may be a problem.
GALVANIZED OUTER PIPE – STRONG,
DURABLE AND FIRE RESISTANT.

Single-wall metal vents have limited applications. In
some areas, local codes prohibit the use of this type ofvent. Where permitted, vent material should be of at least
20 ga. galvanized sheet steel or at least 24 B & S gauge
sheet copper or other non-combustible, corrosion resistantmaterial.

ALUMINUM INNER PIPE – CORROSIONRESISTANT, RAPID WARM-UP,
ELIMINATES CONDENSATION.
INSULATING AIR SPACE – KEEPSINNER PIPE HOT, OUTER PIPE COOL.
TIGHT FITTING JOINTS – PREVENTSESCAPE OF GASES. SIMPLE TOASSEMBLE, EASY TO DISASSEMBLE.

The applications of single-wall metal vents are limited
to runs from the appliance location directly through theroof or exterior wall. If, however, the vent is to extend backand upward through the eaves, the use of single-wallvents is not recommended. If a single-wall vent connector
is adjacent to a cold outside wall, then it should be insulated.
Another way to prevent the connector from losing too muchheat to the wall would be to space it away from the wall.
See Figure 14. Single-wall vents are not to be used for runs
which pass through any concealed portions of a structure.

Figure 12

Flexible metal lining systems may be used in lining
masonry chimneys. These liners are sized using the tables
forType B vents with the maximum rating reduced to 80%of the table rating. (Table maximum times 0.80 equals flex
maximum.) The minimum rating is not affected.

Type B-W vents are very similar to Type B vents except
they are an oval shape. They are of double-wall metal
construction and are designed specifically for venting recessed
wall heaters. See Figure 13. Type B-W vent pipe is also
often used when it is desirable to “hide” Type B vents inthe wall.

FIRESTOPSPACER

CEILINGSPACERSHEETMETALSLEEVEDO NOT SUPPORTSINGLE-WALL VENTIN DIRECT CONTACT
WITH COLD OUTSIDEWALLINSULATE
PIPE

Figure 14

Figure 13

8orINSULATE NONCOMBUSTABLESPACER

Vent Design
Configurations

Plastic Pipe
Due to the advent of higher efficiency furnaces, and therefore
lower flue-gas temperatures, many manufacturers are
specifying plastic vent pipes for their products. Plastic pipe
has the advantages of corrosion resistance and leak-tightassembly. Two basic formulations are used: On 90+ AFUE
furnaces, PVC and CPVC. For the 78 to 83% AFUE furnaces,
a UL listed high temperature plastic pipe may be used.
See the manufacturers’ installation instructions for details.

Each individual venting application has its own special
considerations. The building structure, the orientation of
the building, and the input of the gas appliances which
the vent serves are just a few of the things which the ventsystem designer must consider.There are four basic categories
of vent systems. The first category is the single appliancenatural draft vent system. Then, there are two categories
which cover the common venting of multiple appliances.
One of these configurations is called manifold venting andinvolves multiple appliances on the same floor of a structure.
The other method of common venting involves the useof individual connectors of each appliance sharing the ventand can be a single story or multi-story installation. The
last category is special, covering power venting and direct
venting.

Vent Materials
A wide variety of materials are being used for vents today. Each of these materials has its own advantages anddisadvantages.
Galvanized steel is low cost and fairly easily formed. TypeB vents using galvanized steel have a good insulating quality
and warm up quickly. This quick warm-up and insulating
ability helps to establish a good draft early in the heating
cycle. The disadvantage of galvanized steel is that it is not
as corrosion resistant as some other materials.

Single Appliance Natural Draft Vent
System

The main advantage in stainless steel is its resistance tocorrosion. The disadvantage is the higher cost.

A natural draft or gravity type vent system relies only on
the lifting force of the hot gases in the vent and does notrely on a blower or exhaust fan to assist the flow. An important
fact to keep in mind is that the fan used in a fan assisted
furnace, found in Category I and II, does not create a positive
pressure. It only overcomes the natural resistance of theheat exchanger to the passage of flue gasses. These furnaces
are still classified as Fan Assisted, Category I, in theirapplication. Typical single appliance venting systems areshown in Figure 15. The vent is the portion of the system
where the gases normally begin their final ascent. The piping
assembly that conveys the combustion gases to the ventis called the vent connector. It may have both verticaland horizontal sections where the vent is normally vertical only. Together the vent and vent connector make up
the venting system.

Masonry, clay-tile, and concrete vents have a disadvantage
of a prolonged warm-up period. Also, the mortar used insuch vents is very susceptible to deterioration whencondensation occurs in the vent. One advantage of thesetypes of vents is they retain heat longer.
PVC vents have a high corrosion resistance and can beassembled with leak proof joints. However, this material
is good only for applications where the flue-gas temperaturedoes not exceed 180°F. CPVC has a higher flue-gas temperature rating than PVC but is also limited to those applications of 90+ AFUE high efficiency furnaces. Follow
manufacturers’ instructions on application.

VENT CAP

VENT

TOTAL
LATERALLENGTH(L)

TOTAL VENTINGSYSTEM HEIGHT
(H)

(H)

(L)

VENT
CONNECTOR
VENT
CONNECTOR

DRAFT HOOD
APPLIANCE
CLEAN
OUT
TYPE B OR SINGLE-WALL VENTMASONRY CHIMNEY WITH LINER

Single appliance natural draft vents
Figure 15

Multiple Appliance Venting
Often, it is desirable to use the same vent to vent morethan one appliance. This is known as common venting.
When venting multiple appliances, the operation of the
9the connector through the ceiling and then connect it to
the common vent. See Figure 18.

unit with the smallest input is the most critical. When only
this one appliance is operating, dilution air will also be
drawn in through the draft hoods of the inactive natural
draft appliances sharing the vent. This causes excess dilution,
which leads to lower flue gas temperatures, which causes
a reduction in draft. Figure 16 shows two examples of venting
two appliances on the same floor of the building.

VENT
CAP

TOTAL VENTINGSYSTEM HEIGHT
(H)

VENT
CONNECTORAPPLIANCE
1

IF THISWON’T WORKCOMMON VENT

(H)

VENT
CONNECTOR

(R)

VENT
CONNECTOR

CONNECTOR
RISE
(R)

(R)

APPLIANCEAPPLIANCE
1
2
CLEAN
OUT

TYPE B OR SINGLE-WALL VENTLOCATE TEE
IN ATTIC FOR
MORE
CONNECTORRISE

APPLIANCE
2

MASONRY CHIMNEY WITH LINER

Multiple appliance natural draft vent(common venting – same floor)

Figure 18

Figure 16

Also, when venting two appliances, the vent connector
from the appliance with the smallest heat input should
enter the common vent at the higher point of the interconnecting tee. Vent connectors from separate appliances
should never enter the common vent on the same horizontal
plane. See Figure 19. In other words, don’t use the “Cross”
of the “Tee” to connect appliances to the vent.

When using a common vent, the vent connector designis the key element. It is a good idea to treat each connector
for each appliance as a vent in itself. Vent connector riseis critical. See Figure 17. Where overhead clearance inhibits adequate connector rise, it may be necessary to run

RECOMMENDED
PLACE
INTERCONNECTION
TEE ATHIGHEST POINTNOT RECOMMENDED

Figure 19

The inter-connecting tee should never be smaller than thesize of the largest connector, see Figure 20, and should
be as large as the common vent.
If the basic guidelines outlined in this section are followed,
then common venting can be a safe, cost effective way

Figure 17

10

Type B vent manufacturers do supply tables with theirrecommendations. In general, adequate connector riseis essential to prevent spillage. Manifold length should
be limited to 10 feet or 50 percent of the total vent height,
whichever is greater.

COMMON VENTSAME SIZEINCORRECTTEE

TEE TOOSMALLCEILING
TOO M
UCH S
LOPE

SUFFICIENTRISE

DO THIS . . .

1

NOT THIS . . .

2Figure 20

3

4

5

INSUFFICIENT
VENT CONNECTOR
RISE
WILL CAUSESPILLAGE
ON NO. 5 . . .
AND POSSIBLY
ON NO.’S 3 AND 4

CORRECTCEILINGto vent more than one appliance. If particular attention ispaid to connector rise, the connector may actually be selfventing, and the common vent may not be required to produce
any draft.

1

Keep in mind, individual vents sized for a specific appliancewill have more reliable performance. Also, common venting,with all of the necessary fittings, may actually be the more
expensive route. Consider these things before deciding
whether to vent separately vertical, horizontal or in a common
vent.

2

3

4

5

Figure 22

As shown in Figure 22, a manifold may be either horizontal
or sloped. Excessive slope does not improve venting andcan cause problems. Many codes specify a slope of l/4 inch
per foot, sloped upward from the appliance to the common
vent. Where local codes require sloped manifolds, all theappliance connectors must have the minimum acceptablerise for proper venting.

Manifold Venting
In venting a group of appliances on one level, a manifold
vent can be used. A manifold vent is a horizontal extensionof the lower end of the common vent. The manifold portion
will be attached to the vertical vent with an elbow or tee.
As shown in Figure 21, the manifold may be graduatedor of constant size. Different design procedures are used
for each type. There are no hard and fast rules of design.

(1) TAPERED MANIFOLD
EACH SECTION SIZED TO HANDLE INPUT OF
COMBINATION OF APPLIANCES ATTACHEDALL HEATERS HAVEADEQUATE VENTCONNECTOR RISEMINIMUM SLOPE OR HORIZONTAL

The designer of manifold vent systems must use careful
judgment when selecting which type of manifold to use.
There are not any “rules-of-thumb” regarding manifold
type selection. Common sense and reasoning are a ventdesigner’s greatest assets.
For example, when manifolding medium efficiency furnaces(78% to 83% AFUE), a tapered manifold will work better
than a constant size manifold. Less material means lessheat loss. Second, there is less internal area, so there isless room for dilution when any of the furnaces are inactive.
This prevents premature cooling and condensation of theflue products.

TOTAL
HEIGHT

Multi-Story Vent Systems(2) CONSTANT SIZE MANIFOLDENTIRE MANIFOLD SUFFICIENTLY LARGEFOR ALL APPLIANCES ATTACHED –SAME SIZE AS COMMON VENT

Multi-story common vent systems are similar to manifold
vents. However, each appliance may be on a different floor.
Thus, as shown in Figure 23, they enter the common ventat different levels. The connector and vent must be designed so that they perform properly with only one appliance operating or any combination of appliances inoperation. In design of multi-story vents, it is importantto separate the gas appliances from occupied spaces. In
this way, no flue-gases will enter the occupied space if thecommon vent were to become blocked at any level. In the
remote case that blockage occurs, all appliances below
the blockage will vent through the draft hood relief openings

TOTAL
HEIGHT
CAPPED TEE

Figure 21

11

of the unit just below the blockage. Combustion products
will be dumped into the space containing the appliances.
Appliances operating at levels below the blockage could
appear to be operating normally, when they may all be
spilling combustion products at a higher level.
Even when appliances are isolated from the occupied area,
air must be provided to each appliance. The air supply serves
three major functions:
1. Combustion air for burning gas (primary, secondaryand excess)2. Draft hood dilution air (when required)
3. Ventilation air to avoid heat build-up
Relying on infiltration for ventilation air is a poor practicein multi-story applications. This is because the inner-mostquarters within a structure will typically have fewer exterior doors and windows. Also, today’s construction methods
are producing much “tighter” buildings. The practice oflocating gas fired equipment in a central hallway and using
the hallway for ventilation should not be done. Figure 23
shows a practical method of locating appliances in a separate
room with ventilation air grills on the outside wall.
Sometimes it is not practical to build a separate equipment room with an adjacent outside wall. If this is the case,the vent system designer should also design a duct system
to supply outside air to the gas appliances. Several methods
for supplying outside air are discussed in the section on
ventilation air.

OUTSIDE WALL

HOT AIR OUT

OUTLET GRILLCLOSED SOLID
DOOR ORACCESS PANELNOTE —FAF

WH

OUTLET PLENUM
AND COLD AIR
RETURN BOTH
MUST BEATTACHEDTO FURNACE

INLET GRILL

FAF

WH

EACH GRILL HAS
1 SQUARE INCH
PER 4000 BTU
TOTAL HEAT INPUTCOLD AIR IN

OUTSIDE WALL AIR SUPPLY FOR SEPARATED HEATING ROOM

Figure 23

12Sizing and Design(natural draft or fan assisted) and the BTUH input of thefurnace(s) being served by the vent. Armed with this data,
the designer can now use the capacity tables to size vents
for Category I furnaces.

The section on furnace categories discussed the differencein the venting characteristics of the separate categories.

Sizing For A Single Unit Using A Masonry
ChimneyThis section on sizing and design covers first how to accurately
size and design vent systems for Category I furnaces. Alsoin this section are some recommended procedures for venting
Categories II, III, and IV.

When attaching a gas appliance to an existing masonrychimney, the size of the chimney must be checked to makesure the flow will not overload it. On the other hand, thechimney must not be too large. Use the formula þ D2 x
7 ÷ 4 = MAX (3.1416 times the diameter of the flue collar
squared times 7 divided by 4 = maximum area) to determine
the upper limit of the flow area. Fortunately, the minimum
and maximum limits are shown at the bottom of the Table
listing the Masonry chimney sizes.

Category I
Sizing vent systems for Category I furnaces is accomplishedby applying known information to capacity tables developedby the AGA (American Gas Association) or the National
Fuel Gas Code. Before referring to the capacity tables, thedesigner must do some planning and fact gathering.
The designer needs to know how and where the flue products
are going to be expelled from the structure. Is the furnace
going to be vented through a masonry chimney? Will there
be a metal vent going through the roof? Is the chimney
located on an inside wall or an outside wall?

Table 6 (See Appendix for tables) is used to determine
capacity of masonry chimneys serving a single applianceusing a single-wall connector. Single fan assisted furnaces
may not be installed in a masonry chimney without a metal
liner.Table 8 gives values for chimney capacities with singlewall vent connectors serving two or more appliances. Thesetables contain a factor to allow two elbows used in theconstruction of the vent connectors. If more are required,reduce the maximum capacity by 10% for each added elbow.
Table 9 lists circular equivalents for masonry chimney liners.

The designer should also take into consideration theorientation of the building. Will the vent terminate on theside of the building which is exposed to prevailing winterwinds? Are there any adjacent buildings which are taller
than this one? These factors can have an effect on downdraft conditions.

A fan assisted furnace may only be vented into a masonrychimney with a tile liner, provided the chimney is an internalchimney within the structure and there is another natural
draft appliance also vented into the chimney.

The designer must also measure the distance from thefurnace location to the vent or chimney location. This willbe the lateral connector length. The height of the vent orchimney must be measured. Figure 24 illustrates the points
where these measurements are made.

As an example, consider the installation of a natural draft
gas furnace with an input rating of 100,000 BTUH. The available
chimney height is 20 feet, and the lateral for the connector
is five feet. Table 6, (See Figure 25), shows that a five-inch

The last bit of information needed is type of the furnace

Capacity of Masonry Chimney with Single-Wall Vent Connectors
Serving a Single Category I Appliance
TABLE 6

Connector Diameter – D (inches)
To be used with chimney areas within the size limits at bottom

TOTAL VERTICALHEIGHT (H) = 20 FT.

4"

3"Height Lateral
H
L
(ft.)
(ft.)

TOTAL
LATERALLENGTH(L) = 10 FT.

INPUT: 150,000 BTU/HR.
DRAFT HOOD: 6 INCH DIAM.

5"

6"

Appliance Input Rating In Thousands of BTU Per HourFAN
Min Max

NAT
Max

FAN
Min Max

NAT
Max

FAN
Min Max

NAT
Max

FAN
Min Max

NAT
Max

6

2
5

NR
NR

NR
NR

28
25*

NR
NR

NR
NR

52
48

NR
NR

NR
NR

86
81

NR
NR

NR
NR

130
116

10

2
5
10

NR
NR
NR

NR
NR
NR

31
28*
25*

NR
NR
NR

NR
NR
NR

61
56
49*

NR
NR
NR

NR
NR
NR

102
95
86

NR
NR
NR

NR
NR
NR

161
147
137

15

2
5
10
15

NR
NR
NR
NR

NR
NR
NR
NR

35*
32*
27*
NR

NR
NR
NR
NR

NR
NR
NR
NR

67
61
49*
46*

NR
NR
NR
NR

NR
NR
NR
NR

113
106
96
87*

NR
NR
NR
NR

NR
NR
NR
NR

178
163
151
138

20

2
5
10
15
20

NR
NR
NR
NR
NR

NR
NR
NR
NR
NR

38*
35*
NR
NR
NR

NR
NR
NR
NR
NR

NR
NR
NR
NR
NR

73
67*
59*
NR
NR

NR
NR
NR
NR
NR

NR
NR
NR
NR
NR

123
115
105*
95*
80*

NR
NR
NR
NR
NR

NR
NR
NR
NR
NR

200
183
170
156
144*

Minimum Internal
Area of Chimney
Square Inches

12

19

28

38

Maximum Internal
Area of Chimney
Square Inches

49

88

137

198

* SEE Note 27

This table is extracted from table on page 35.

Figure 25

Figure 24

13

1. Determine the total height (H) and total lateral length(L) needed for the vent.

connector can be used and the chimney must have an internal
minimum cross-sectional area of 28 square inches. Referring to Table 9, note that a 4 x 8 flue liner has only 12.2
square inches of equivalent area. An 8 x 8 liner has 42.7
square inches area, therefore, the liner should be a nominal
8 x 8 inch size to accommodate this furnace. Similarly, thesize of masonry chimneys can be determined for two or
more appliances using Tables 6 and 9.

2. InTable 1, read down the (H) column to the proper totalheight.
* Go down total vent height column to 20 feet
(Figure 26).
3. In the (L) space to the right of the correct (H), pick theproper lateral length (use zero for straight vertical vents
which have no elbows).

Most often, the vent designer is not involved in the chimney
construction. The designer and builder of the structure takecare of that. The vent system designer’s responsibility is
to check the chimney construction for proper size, suitable lining, and to make certain that the chimney cannot
be used for any appliance which burns solid fuels. If thechimney once served a fireplace, the fireplace opening
must be permanently sealed. If the owner of the building
wants to keep the fireplace open for use, then the chimney
may not be used to vent the gas furnace. A separate ventmust be installed.

* Read down the next column (L) to a length oflateral of 10 feet.
4. Read across the selected (L) row to the column whichshows a capacity equal to or just greater than the appliancenameplate BTUH input rating under “Nat. Max.” Thereading will be in thousands of BTU’S per Hour.
* Read to the right across (L) = 10 feet row until 150 isfound. You are under the 5 inch vent size column.

Sizing A Single Appliance Type B Vent

5. 100,000 BTUH is equal to the furnace input. A 4 inch
vent only has 89,000 BTUH capacity – too small.
A 6 inch vent has a capacity of 228,000 BTUH – much
too large. Use the five inch vent.

Category I furnace vent systems are designed using thecapacity tables found the Appendix in the back of this manual.
The same tables are found in the appendices of the National
Fuel Gas Code (NFPA54/ANSIZ223.1).

Downsizing from the flue collar size

Table 1 in this manual is used to size vent systems for asingle Category I furnace usingType B vents and connectors.
Table 2 is used for sizing vents for a single furnace using
single-wall connector vent pipe material.

Even though a flue collar provided on the furnace is a six
inch diameter, a five inch vent can be used if the following
conditions are met. First, local code has to allow sizing
vents smaller than the flue collar. Second, the totalvent height must be ten feet or more. Third, if a fan
assisted furnace is used, reduce the maximum ratingby 10% (0.90 X maximum capacity). A note of caution:
Vents for equipment with a draft hood of 12 inch diameter or less should not be reduced more than one pipe size.
A 12 inch to 10 inch reduction is one pipe size. There is
one case where downsizing a natural draft furnacevent should not be done. Never attach a three inch
vent to a four inch flue collar. This does not apply
to a fan assisted furnace.

How to use the Single Appliance Type B Vent Table(Table 1).
For this example, refer to Figure 24 on Page 11 andFigure 26.

Capacity of Type B Double-Wall with Type B Double-Wall Connectors
Serving a Single Category I Appliance
TABLE 1

Vent and Connector Diameter – D (inches)
3"

Height Lateral
H
L
(ft.)
(ft.)

4"

5"

6"

FAN
Min Max

NAT
Max

FAN
Min Max

NAT
Max

FAN
Min Max

NAT
Max

FAN
Min Max

NAT
Max

FAN
Min Max

6

0
2
4
6

0
13
21
25

78
51
49
46

46
36
34
33

0
18
30
36

152
97
94
91

86
67
64
61

0
27
39
47

251
157
153
149

141
105
103
100

0
32
50
59

375
232
227
223

205
157
153
149

0
44
66
78

524
321
316
310

8

0
2
5
8

0
12
23
28

84
57
53
49

50
40
38
35

0
16
32
39

165
109
103
98

94
75
71
66

0
25
42
51

276
178
171
164

155
120
115
109

0
28
53
64

415
263
255
247

235
180
173
165

0
42
70
84

583
365
356
347

10

0
2
5
10

0
12
23
30

88
61
57
51

53
42
40
36

0
17
32
41

175
118
113
104

100
81
77
70

0
23
41
54

295
194
187
176

166
129
124
115

0
26
52
67

447
289
280
267

255
195
188
175

0
40
68
88

631
402
392
376

15

0
2
5
10
15

0
11
22
29
35

94
69
65
59
53

58
48
45
41
37

0
15
30
40
48

191
136
130
121
112

112
93
87
82
76

0
20
39
51
61

327
226
219
206
195

187
150
142
135
128

0
22
49
64
76

502
339
330
315
301

285
225
217
208
198

0
38
64
84
98

716
475
463
445
429

0
2
5
10
15
20

0
10
21
28
34
48

97
75
71
64
58
52

61
51
48
44
40
35

0
14
29
38
46
55

202
149
143
133
124
116

119
100
96
89
84
78

0
18
38
50
59
69

349
250
242
229
217
206

202
166
160
150
142
134

0
20
47
62
73
84

540
377
367
351
337
322

307
249
241
228
217
206

0
33
62
81
94
107

776
531
519
499
481
464

0
2

0
9

100
81

64
56

0
13

213
166

128
112

0
14

374
283

220
185

0
18

587
432

336
280

0
27

853
613

20

30

Values between Table values

7"

Appliance Input Rating In Thousands of BTU Per HourIn many cases, the vent system will have total vent height(H)and lateral length(L) values in between the table values.
To evaluate these situations, the fractional difference between
the next largest and the next smallest is used.
Example: Heights and laterals in between table values.
Figure 27 represents a furnace for which we are to determine
the proper vent size. The natural draft furnace input ratingis 100,000 BTUH with a 5 inch draft hood.
Procedure – (refer to Figure 28)
1. (H) = 20 feet and (L) = 13 feet.
2. Go down total vent height column to 20 feet.
3. Under 5 inch vent size, “NAT Max”
15 feet lateral shows 142,000
10 feet lateral shows 150,000

This table is extracted from table on page 32.

Figure 26

4. Since 13 is 3/5 of the way between 10 and 15, to
14

INPUT: 100,000 BTU/HR.
DRAFT HOOD: 5 INCH DIAM.

Capacity of Type B Double-Wall with Type B Double-Wall Connectors
Serving a Single Category I Appliance
TABLE 1

Vent and Connector Diameter – D (inches)
4"3"(H) = 18 FT.

Height Lateral
H
L
(ft.)
(ft.)

(L) = 13 FT.

5"

6"

7"

Appliance Input Rating In Thousands of BTU Per HourFAN
Min Max

NAT
Max

FAN
Min Max

NAT
Max

FAN
Min Max

6

0
2
4
6

0
13
21
25

78
51
49
46

46
36
34
33

0
18
30
36

152
97
94
91

86
67
64
61

0
27
39
47

251
157
153
149

141
105
103
100

0
32
50
59

375
232
227
223

205
157
153
149

0
44
66
78

524
321
316
310

8

0
2
5
8

0
12
23
28

84
57
53
49

50
40
38
35

0
16
32
39

165
109
103
98

94
75
71
66

0
25
42
51

276
178
171
164

155
120
115
109

0
28
53
64

415
263
255
247

235
180
173
165

0
42
70
84

583
365
356
347

10

0
2
5
10

0
12
23
30

88
61
57
51

53
42
40
36

0
17
32
41

175
118
113
104

100
81
77
70

0
23
41
54

295
194
187
176

166
129
124
115

0
26
52
67

447
289
280
267

255
195
188
175

0
40
68
88

631
402
392
376

15

0
2
5
10
15

0
11
22
29
35

94
69
65
59
53

58
48
45
41
37

0
15
30
40
48

191
136
130
121
112

112
93
87
82
76

0
20
39
51
61

327
226
219
206
195

187
150
142
135
128

0
22
49
64
76

502
339
330
315
301

285
225
217
208
198

0
38
64
84
98

716
475
463
445
429

20

0
2
5
10
15
20

0
10
21
28
34
48

97
75
71
64
58
52

61
51
48
44
40
35

0
14
29
38
46
55

202
149
143
133
124
116

119
100
96
89
84
78

0
18
38
50
59
69

349
250
242
229
217
206

202
166
160
150
142
134

0
20
47
62
73
84

540
377
367
351
337
322

307
249
241
228
217
206

0
33
62
81
94
107

776
531
519
499
481
464

30

0
2

0
9

100
81

64
56

0
13

213
166

128
112

0
14

374
283

220
185

0
18

587
432

336
280

0
27

853
613

FURNACEFAN
Min Max

NAT
Max

FAN
Min Max

NAT
Max

This table is extracted from table on page 35.

Figure 27

Figure 28

find the size for 13 feet of lateral, we take 3/5 of thedifference:

3. In the length of lateral “L” space to the right, find 15 feet.
On this row, look under 3" vents. There are three ratings shown. The maximum “FAN” rating shows only
58.That’s only 58,000 BTUH. Not enough. But look closely,
there is a minimum rating shown on that line also. The
minimum is 34,000 BTUH. Each fan assisted furnace
will show both a maximum and minimum rating in orderto help control condensation in the vent pipe.

8,000 x 3/5 = 4,800… subtract the result from the 150,000 for 10 feet oflateral (we lose capacity with a longer lateral):
150,000 – 4,800 = 145,200
145,200 BTUH is the table capacity for 13 feet of lateraland 20 feet height.

Look under the 5" column. The rating jumps to 217,000
but the minimum is still below our furnace input of 100,000.
We could use this size of vent. Is it a good choice?
Not if you must pay the bill for the vent. Let’s see what
the 4" vent size will handle. The largest furnace that it
could serve is 124,000 BTUH.That’s more than we need.
The minimum is 46,000 BTUH and we have more thanthat. We’re almost through.

Therefore, a 5 inch vent would handle 145,200 BTUH input
to the furnace.
Since the furnace input in this example is 100,000 BTUH,
the 5 inch vent will do the job.
The same procedure would allow you to calculate heights
which are not shown on theTables. Remember that higher
vents increase capacity.

4. To determine the size for an 13 foot lateral:
13 is 3/5 of the difference between 10 and 15.

“Fan Assisted” Vent Sizing

Go down the “H” column to 20 feet. In the “L” space,find 10 feet. On this row, look under 4" and see 133
Fan max.

Throughout this manual the distinction between the “natural
draft” and “fan assisted” furnaces has been stressed, although
we have learned they are both Category I furnaces. Now,
it is time to see what difference this design change makesin the vent sizing decision.We will examine the same example
that was just used to select the vent size for the 100,000
BTUH natural draft furnace. This time we will select a “fanassisted” model.

Find the difference between the rating for 10 and 15 feet.
133,000 – 124,000 = 9,000
Since 13 is 3/5 the difference between 10 and 15,
then we find 3/5 of 9,000.

Procedure – (Refer to Figure 28)

9,000 x 3/5 = 5,400

1. There is no change in the installation specifications. The
height is the same, 20 feet, and the lateral distance is
13 feet.

Subtract the result from the 133,000
133,000 – 5,400 = 127,600
127,600 BTUH input is the maximum capacity of a 4 inch

2. Go down the total vent height column “H” to 20.
15

vent having 20 feet of height and 13 feet of lateral. The
minimum could be figured in the same way.

(A)
18 FEETMINIMUMONE FOOT VENTTOTAL VENT CONNECTORHEIGHT
RISE

Since the furnace input in the example is 100,000 BTUH,
the 4 inch vent will work on this type of furnace. In thisexample when compared to natural draft, the result waschanged from 5 to 4 inch vent pipe. By trying other combinations of height and lateral offset, you will soon seethat the result could have been quite different.

WATER
HEATER
45,000For appliances with more than one input rate, the minimum
vent or connector (Fan Min.) capacity determined fromthe tables shall be less than the lowest appliance rating
and the maximum vent or connector capacity determinedfrom the tables shall be greater than the highest applianceinput rating.

INTERCONNECTION
TEE

4 INCHSIZE
FURNACE
100,000(B)
18 FEETMINIMUMTOTAL VENTHEIGHT

Multiple Appliance Vent Systems

TWO FEET VENTCONNECTOR RISE

Any vent system, venting two or more appliances on thesame floor into a common vent, is a multiple or combined
appliance vent system. The minimum (shortest) total heightis the vertical distance from the highest appliance draft
hood outlet to the top of the vent. This dimension is thesame regardless of the number of appliances. The ventconnector rise for any appliance is the vertical distancefrom its draft hood outlet to the point where the next connector
joins the system. The common vent is the portion of thevent system above the lowest interconnection. The sizesof the individual connectors are found from Table 4a andthe size of the common vent from Table 4b. See Figure
30.

WATER
HEATER
45,0005 INCHSIZE
FURNACE
100,000(C)
18 FEETMINIMUMTOTAL VENTHEIGHT

145,000 TOTAL INPUTCOMMON VENT
5 INCH SIZEWATERHEATER
45,000How to Size A Multiple Vent System

FURNACE
100,000

Figure 29

The Tables relating to common venting were constructedwith an assumption which you must remember.They assume
the connector leading to the vent will never exceed a length
greater than the connector diameter in inches times one
and one half feet. In other words, for each inch of connector
diameter, the length must not exceed 18 inches. A fourinch diameter connector must not be longer than six feet.
(4 x 18 = 72 inches = 6 feet)

4. Repeat Step 3 for each appliance.

Determining Size Of Common Vent
As we look at the common vent tables, there are three columns
under each heading. They are: “Fan + Fan”, “Fan + Nat”
and “Nat + Nat”. The numbers reflect the maximum input
combination of fan assisted and natural draft appliances
which may be used. If you must use an existing natural
draft water heater with a new fan assisted furnace, choose
the center column.Two natural draft products, use the righthand column, etc. If the letters “NR” appear, that choiceof vent and product are not recommended.

What happens if you go beyond this length? Does the ventquit working? NO! You must just remember to de-rate theconnector’s capacity, that’s all. Each time you add on, up
to the connector’s original length, take away 10% of the
maximum capacity. For 7 to 12 feet of 4" connector, use
only 90% of maximum capacity. For 13 to 18 feet, de-rateby 20%, as you have three times the original length. As
you have always been told, keep it as short as possible.

1. Add together the input ratings of all appliances in themultiple vent system.

First, determine size of each vent connector.

2. Enter the common ventTable 4b at the total height value
used above.

1. Determine the minimum (shortest) total height of thesystem. See Figure 29 for measuring points.

3. Continue across to the first value that equals or exceeds
total input ratings.

2. Determine the connector rise for each appliance.
3. Using Table 4a, find the vent connector size for eachappliance. Enter the table at the selected minimum totalheight. Read across to the connector rise for the first
appliance and continue across the proper connector
rise row to the appliance input rating. Read the ventconnector size at the top of the column with the correct input rating.

4. Read the size of the common vent needed at the topof this column.
5. CAUTION: Regardless of the result, the common ventmust always be as large as the largest connector. If two
or more vent connectors are the same size, the common
vent must be at least one size larger.
16

Typical Example

Use a four inch vent connector.

Example: To size a multiple appliance vent refer to Figures 29 and 30: Connect a 45,000 BTUH water heater witha 1 foot vent connector rise and 100,000 BTUH furnacewith a 2 foot vent connector rise to a common vent witha minimum total vent height of 18 feet. The water heater
iswithin5 feet of the common vent and the furnace is only four(B) Determine Furnace Vent Connector Size
See Figure 29B.

Procedure:
1. Refer to Vent Connector Table 4a.
2. There is noTotal Vent Height column for 18 feet; therefore,
work from 15 foot and 30 foot columns.
3. Read down Total Vent Height column to 15 feet; readacross 2 foot Connector rise line. 5 inch vent size shows
97,000 BTUH, or less than furnace input. Repeat step,using 30 foot column, 2 foot Connector rise line. Herethe 5 inch vent size shows 106,000 BTUH.

Capacity of Type B Double-Wall Vent with Single-Wall Connectors
Serving Two or more Category I Appliances
TABLE 4

Vent Connector Diameter – D (inches)
4"3"
Vent Connector
Height
Rise
H
L
(ft.)
(ft.)

5"

4. 18 feet is 1/5 the difference between 15 feet and 30 feet.
The difference between 97,000 and 106,000 is 9,000.
1/5 of 9,000 is 1,800. Add 1,800 to the 15 foot figure(97,000) = 98,000 BTUH.

6"

Appliance Input Rating In Thousands of BTU Per HourFAN
Min Max

NAT
Max

FAN
Min Max

NAT
Max

FAN
Min Max

NAT
Max

FAN
Min Max

NAT
Max

F
Min

6

1
2
3

NR
NR
NR

NR
NR
NR

26
31
34

NR
NR
NR

NR
NR
NR

46
55
62

NR
NR
NR

NR
NR
NR

71 NR
85 16895 174NR
182
198

102 207123 215138 222

15

1
2
3

NR
NR
NR

NR
NR
NR

29
34
39

79
83
87

87
94
100

52 11662 12170 127

138
150
160

81 17797 185
109 193

214
230
243

116 238
138 246
157 255

30

1
2
3

47
50
54

60
62
64

31
37
42

77
81
85

110
115
119

57 11367 11776 122

175
185
193

89 169106 177
120 185

278
290
300

129 226
152 236
172 244

50

1
2
3

46
49
53

69
71
72

33
40
45

75
79
83

128
132
136

60 10972 11482 119

207
215
221

96 162
113 170
128 178

336
345
353

137 217164 226
186 235

5. 98,800 BTUH is the maximum input for 18 footminimum total vent height.
6. A 5 inch connector would be the correct size for the
furnace, providing the furnace had a 5 inch or smallerdraft hood outlet.
(C) Determine Common Vent Size
See Figure 29C.

Common Vent Capacity

Procedure:

Common Vent Diameter – D (inches)
4"
Vent
HeightH(ft.)

5"

1. Refer to Common Vent Table 4b.

6"

2. There is no Total Vent Height column for 18 feet;therefore, work from 15 foot and 20 foot columns.

Combined Appliance Input Rating In Thousands of BTU Per H
FAN FAN NAT FAN FAN NAT FAN FAN NAT FAN+FAN +NAT +NAT +FAN +NAT +NAT +FAN +NAT +NAT +FAN

6

89

78

64 136113

100 200158

144 3048

98

87

71 151

126

112 218173

159 331

10

106

94

76 163

137

120 237189

174 357

15

121

108

88 189159

140 275221

200 416

20

131

118

98 208177

155 305247

223 463

30

145

132

113 236202

179 350286

257 533

50

159

145

128 268233

204 406337

296 662

3. The total input to the vent is 145,000 BTUH.
4. 15 feet maximum BTUH for 5 inch vent is 140,000
20 feet maximum BTUH for 5 inch vent is 155,000.
5. 18 feet is 3/5 the difference between 15 feet and 20 feet.
The difference between 140,000 and 155,000 is 15,000.
3/5 of 15,000 is 9,000. 140,000 + 9,000 = 149,000 BTUH,
which is greater than total input to common vent.

This table is extracted from table on page 34.

Figure 30

6. Therefore, the common vent can be 5 inch diameter
pipe. See Figure 30.

feet away.

Multi-Story Vent Systems

The three-step procedure for working this example is asfollows:

Multi-story vent systems of any height can be sizedusing the multiple appliance vent Tables 4a and 4b. Thesetables are best suited to appliances separated fromoccupied areas such as boilers on porches or furnaces andwater heaters in closets. The system will function satisfactorily
when any one or all appliances are operating.

(A) Determine Water Heater Vent Connector Size
See Figure 29A.

Procedure:
1. Use the multiple appliance Vent Connector Table 4a.

When designing a multi-story vent system, the following
principles should be applied:

2. Read down Total Vent Height column to 15 feet. Readacross 1 foot Connector Rise line to BTUH rating equal
to or higher than water heater input rating. The figure
shows 52,000 BTUH and is in the column for a 4 inch
connector.

1. The overall system should be divided into smaller, ventsystems for each level, each using a total vent height
for each level as shown in Figure 31 on page 18.
Principles of design of multistory vents using vent connector and common vent design table.

3. Since 52,000 BTUH is in excess of the water heater input
rating, it is not necessary to find the maximum input
rating for an 18 foot minimum total vent height.

2. Each vent connector from the appliance to the verti17

cal common vent should be designed from the vent connector table, as in multiple appliance vent systems.

4. The vent connector from the first floor or the lowest
appliance to the common vent is designed as if
terminating at the first tee or interconnection. The next
lowest appliance is considered to have a combined ventwhich terminates at the second interconnection. The
same principle continues on to the highest connecting
appliance with the top floor appliance having a totalvent height measured to the outlet of the common vent.

3. For sizing of the vertical common vent, the common
vent table is used. The vertical common vent for eachsystem must be sized large enough to accommodate
the accumulated total input rating of all appliances discharging into it, but should never be smaller in diameter
than the largest section below it.

APPROVED CAP

USE INDIVIDUAL
VENT FOR TOP
FLOOR APPLIANCE
IF CONNECTORREQUIREMENT FOR
RISE, OR TOTALHEIGHT CANNOT
BE MET.

TOTALHEIGHT
FOR TOPFLOORAPPLIANCEUSE TOTAL HEIGHT FOR TOP
FLOOR APPLIANCE AND
COMBINED INPUT OF ALL
APPLIANCES ON COMMON VENT.

Precautions in Designing Multi-Story
Vent Systems

THIRD INTERCONNECTION TEE*

USE VENTCONNECTOR TABLETOP FLOORAPPLIANCE

5. The multi-story system has no limit in height aslong as the common vent is sized to accommodate thetotal input rating.

4

1. The common vent portion of the system must be
vertical with no offsets to use the values in the Tables
without de-rating.

USE TOTAL HEIGHT FOR
THIRD FLOOR APPLIANCE, AND
COMBINED INPUT OF THREEAPPLIANCES (IF TOP FLOORAPPLIANCES IS NOT CONNECTED
MEASURE TOTALHEIGHT TO VENT TOP).

RISE

TOTAL
HEIGHT
FOR THIRDFLOORAPPLIANCE

2. Common vent height must always be computed as the
distance from the outlet of the draft hood to the lowest
part of the opening from the next interconnection above.

SECOND INTERCONNECTION TEE*
USE VENTCONNECTOR TABLE
THIRD FLOORAPPLIANCERISE

3

USE VENTCONNECTOR TABLE

USE TOTAL HEIGHT FORSECOND FLOOR APPLIANCE,AND COMBINED HEAT INPUTOF TWO APPLIANCES.

TOTAL
HEIGHT
FOR SECONDFLOORAPPLIANCE

3. If the vent connector rise is inadequate, increase
vent connector size, always making use of maximum
available vent connector rise.

FIRST INTERCONNECTION TEE*

RISE

SECOND FLOORAPPLIANCE2

FIRST FLOORAPPLIANCE

1

DESIGN VENT CONNECTOR FOR FIRSTFLOOR APPLIANCE AS AN INDIVIDUAL
VENT OF THIS TOTAL HEIGHT, FOR
INPUT OF FIRST FLOOR APPLIANCE.

4. Be sure that the ventilation air supply to each applianceis adequate for proper operation. A furnace must be
enclosed in a separate room from the living area anda permanent provision for outside air supply is necessary.

* EACH INTERCONNECTION TEE IS SAME SIZE AS
PORTION OF COMMON VENT DIRECTLY ABOVE.

5. If an air shaft or chase is used for installation of thecommon vent, be sure that sufficient space is provided
for fittings, clearance to combustibles and access forproper assembly.

Figure 31

Common Vent Calculations – Four Floors
Appliance

Total Input
to Common
Vent

Available
Connector
Rise

Total
Vent
HeightConnector
Size

Common
Vent
Size

1
2
3
4

90,000180,000
270,000
360,000

10’
1’
1’
1’

10’
10’
10’
6’

5”
6”
6”
6”

5”
7”
8”
10”

Table A

Common Vent Calculations – Separate Top Floor
Appliance

Total Input
to Common
Vent

Available
Connector
Rise

Total
Vent
HeightConnector
Size

Common
Vent
Size

1
2
3
4

90,000180,000
270,000
90,000

10’
1’
1’
8’

10’
10’
16’
6’

5”
6”
6”
5”

5”
7”
6”
5”

Table B18

6. The capacity tables apply only when the entire system
is constructed of listed double-wallType B Vent materials.
Example: Figure 31 shows a typical example of a multistory vent system.
1. Assume Figure 31 represents a 4-story apartment.

A

2. Each appliance has a 90,000 BTUH input rating.
3. Each appliance has a 5 inch draft hood.
4. Total vent height is 10 feet for each of the three
lower floors.

B

5. Total vent height is 6 feet for the top floor.
C

6. Use Table 4b for sizing common vent.

D

Table A shows the calculation for venting all four floors
into the common vent.
However, if the furnace on the top floor is vented separately,Table B shows the results of increasing the minimum
total vent height of the third floor appliance to 16 feet anddecreasing total input to common vent to 270,000 BTUH.
Table B indicates the economics of venting the top floor
separately. This eliminates the larger sizes of vent pipe
and the use of extra, costly fittings.

E

Vents in High Altitude Locations

Figure 32

Vent systems should be designed on the basis of the sea
level input rating. For multiple appliance inputs, design
the vent using the greatest possible sea level input rating.

depends on the strength of the assist and the size of thevent. Exact sizing information for power assists can beobtained from the manufacturer.

When a gas appliance is derated due to altitude, the volume
flow of fuel gas required will be less than at sea level. However,the volume of air required for furnace operation is not thatmuch less. This is because the air becomes less dense as
the altitude increases. Because of the lower density, a higher
ratio of air to gas is required to supply enough oxygen.

Category II
As mentioned earlier, Category II furnaces are classified
as non-positive with flue-gas temperatures less than orequal to 140°F above their dew-point. Extreme care must
be taken when designing vents for these furnaces. There
are several special considerations involved.

Power Venting
If the steps outlined in the previous sections are followed
carefully, the vent will perform well. Sometimes, however,there are factors which cannot be overcome, such as extremely
high prevailing winter winds. Also, especially in retro-fit
applications, sometimes it isn’t convenient or even possible
to redesign an improperly sized vent. Sometimes the furnace
may be poorly located, requiring several elbows. Powerventing is a good solution to many venting problems.

Flue-Gas Temperature
The temperature of the flue-gases from a Category II furnace
can be expected to be somewhere around 275°F or less.
Existing vent sizing tables are based on the assumptionthat the flue-gases entering the vent are at a temperatureof 360°F or higher. Because Category II furnaces produce
lower temperature flue-products, existing tables may not
apply.The volume of gases being produced by these products
will be less than in lower efficiency models; therefore, thevent pipe size could possibly be smaller than indicatedby existing sizing tables.

A power assisted vent system has a fan, blower, or booster,see Figure 32, located within the vent system. The power
assist (draft inducer) can be located anywhere in the vent,but locating it at the vent outlet is preferable. If the furnace is “Fan Assisted”, a barometric relief damper must
be installed at the appliance flue outlet to prevent the power
vent from pulling the products of combustion through theheat exchanger. With a negative vent pressure, any leaksin the vent will be inward. The air around the vent will bepulled into the vent rather than the flue-gases being forcedout of the vent.

Until research yields specific tables for these types of products,
the furnace manufacturer’s instructions are the only basis
available for designing vents for Category II products.

Condensation
Because of the lower temperature flue-gases produced,
there is a good chance condensation will occur inside
the vent. As a result, the National Fuel Gas Codesuggests the use of a corrosive resistant vent material
such as stainless steel or aluminum coated stainless steel.

A power assist installed in a vent can increase the capacityof the vent up to four times. The amount of capacity increase19ner. Again, the recommended sealing method is with hightemperature silicone rubber. In some cases, foil-backed
tape may be used. Check the manufacturers instructions
and local codes.

While the piping may be more expensive initially, it willbe less expensive than several repairs on a less corrosive
resistant material.
It is also recommended that a “drip Tee” be placed in thevent connector at a location near the furnace. If proper slope
is maintained on the lateral sections of the vent, condensate
will drain into the “drip Tee” and can be drained via a hose
to a nearby drain approved by local authorities.

Sizing
Because these furnaces are forced draft, sizing of the ventpipe is not usually left up to the vent designer. The furnace manufacturer normally includes information which
will be used to determine the vent design. Usually, the ventpipe will be the same diameter as the flue connector/adapterof the unit. The design information will cover the maximum amount of lateral run which is allowed when a certain
number of elbows are used.

“Drip Tee’s” or “drain Tee’s” are available through manyvent pipe manufacturers.

Power Venting
The designer of vents serving Category II furnaces should
probably consider the use of some kind of power assist
in the vent. Because of the lowered flue-gas temperatures,the draft forces produced by these furnaces are also reduced.
There is no room for error with these furnaces. A vent systemof a “marginal” size and design will often operate improperly.

Termination
Positive pressure furnaces such as Category III and Category
IV furnaces are suitable for termination through a wall.
The vent should not, however, terminate through a wall
which is directly exposed to the prevailing winter wind.
If the wind is strong enough, it may overcome the draft
produced by the forced draft blower or pulse combustion
system. Also, the terminal should not be located where
there is no wind. “Dead corners” allow the vented fluegases to accumulate.

If power venting is chosen, there are a couple of recommended
procedures. First, the draft inducer should be located ator near the vent outlet. Installing it at the outlet causes thevent to operate under negative pressure. In this way, any
leaks in the vent will be inward. The air around the vent
will be pulled in rather than the flue-gases being pushed
out. Secondly, no matter where the power assist is located,the vent system should be sealed. Seams and joints canbe sealed either using a high temperature, silicone-rubber
sealant or with pressure-sensitive, foil-backed tape.

When venting through a side wall, care must be taken to
ensure that the flue-products cannot reenter the building.
It is also necessary to locate the vent terminal above thenormal level of snow or ice accumulation. Be wary of thepossibility of snow drifts on the leeward side of a building.

Generally, good common sense practices will go a long
way toward successful venting of Category II furnaces.
If the designer will maintain proper slope, limit the numberof elbows, insulate the vent in cold spaces, the additional
steps outlined in this section can help to ensure proper
vent performance.

DOOR
WINDOW
OR
OTHER AIR
INLET
4'Category III
Category III furnaces are distinguished by positive ventpressure and flue-gas temperatures greater than or equal
to 140°F above their dew-point. Getting the flue-products
to the outside of the building is really no great problem.
The positive pressure classification means that the combustion
by-products will be “pushed” out of the vent. Furnace
manufacturers accomplish this through the use of blowers
attached to the heat exchanger outlet or by the use of pulse
combustion systems. A special case is created when 78
to 83% AFUE induced draft furnaces are vented horizontally.

4'

VENT
CAP

10'
LESS THAN 10'

1'

3'
DOOR
WINDOW
OR
OTHER AIRINLET

The higher flue-gas temperature minimizes the danger ofharmful condensation occurring inside the vent. Therefore,
there is no need for a special corrosion resistant vent pipematerial. However, if UL listed high temperature plastic
is used in the vent, a drain Tee must be installed due tothe high heat loss and subsequent condensation. This will
occur when venting 78 to 83% AFUE induced draft furnaces
horizontally.

7'

SIDEWALKThe cap must be a least 7' above grade, if it is adjacent topublic walkways.
Avoid areas where ataining or condensate drippage may be
a problem.
The vent/wind cap must terminate at least 3' above any forced air
inlet into the building that is within 10' of the cap. The cap mustalso terminate 4' below, 4' horizontal from or 1' above any door,
window or other air inlet into building.

The prime consideration when venting Category III furnaces is the positive vent pressures. In order to prevent
leakage of flue products into the indoor atmosphere, thevent system should be assembled in a leak-proof man-

Diagram of sidewall terminal location for condensing model
furnaces.

Figure 33

20

Consult local authorities or the owner of the building on
matters such as snow accumulations.

than or equal to 140°F above their dew-point. These aresuper high efficiency models which condense water vapor
from the flue products in order to recapture heat. Venting these furnaces is very similar to venting Category III
products with a few small differences.

Figure 33 illustrates clearances for side-wall terminals used

Vent Pipe
The vent pipe for these products must be corrosion resistant. Although the furnace condenses water vapor outof the flue gasses, not all of the water vapor is condensed.
Many manufacturers are recommending the use of PVC
or CPVC plastic pipe. This pipe may be used because of
the low flue-gas temperatures. If metal pipe is desired,
Allegheny Steel type AL29-4C stainless steel pipe is recommended. Type AL29-4C stainless steel is a high quality
metal formulated specifically for resistance to acids.

Condensate
Category IV furnaces usually include, in the design, somemethod of removing and draining the condensed water
vapor. Because of this, it should not be necessary to provide
any kind of “drip Tee” or “drain Tee” in the vent. However, it is vital that all vent pipe slope toward the furnace.
Any condensate, which may form inside the vent, will runback to the furnace. The furnace condensate disposal system
can then take care of it. Also, it is important that the ventpipe be installed with the male ends of the pipe and fittings
toward the furnace. This prevents the possibility of thecondensate collecting at the “lip” formed where the pipe
is inserted into couplings and fittings.

Venting Through Combustible Walls
Pitch – 1/4 Inch Per Ft.
Clearance (0" acceptable for
PVC vent pipe) (1" acceptable
for type 29-4C stainless steel vent pipe).

Vent Design

Venting Through Non-Combustible Walls
Pitch – 1/4 Inch Per Ft.

Figure 34

near entrances into a building.
Figure 34 is a diagram showing two methods of sidewall termination above the expected level of snow
accumulation.

Category IV
Category IV furnaces - “90+” AFUE models - are characterized
by positive vent pressure and flue-gas temperatures less21

found in bleaches, detergents, cleaning solutions, solvents,
spray can propellants, cements, adhesives, and halogenatedrefrigerants. It is evident that it would be almost impossible
to eliminate all of these contaminants from the atmosphere.
Therefore, supplying fresh, outside combustion air should
be considered for every installation.

Since these furnaces incorporate some kind of forced draft
system, the pipe sizing is done by the manufacturer. The
vent designer must stay within the limitations of lateral
lengths and number of elbows which are contained in the
furnace installation instructions.

Combustion and
Ventilation Air

Confined Spaces
The National Fuel Gas Code contains recommendations
to supply outside air anytime the room containing theequipment falls under the heading of being a confined space.
A confined space is a room whose volume is less than 50
cubic feet per 1000 BTUH of total input to all appliances
installed in the room. For example, a room contains a gas
water heater with 40,000 BTUH input and a furnace with
80,000 BTUH input for a total of 120,000 BTUH. At 50 cubicfeet per 1000 BTU input, 6000 cubic feet (120 x 50) is required
to preclude the space being classified as a confined space.
If the equipment room was 12' x 20' x 8' (1920 cu. ft.) for
instance, such a room would be considered a “confined
space.

As previously mentioned, gas furnaces require a substantial
amount of fresh air for proper operation. In the past, thisair would make its way to the furnace by way of infiltration.
Older construction methods allowed a considerable amountof air leakage around doors, windows, etc. With today’s
increasing energy efficiency awareness, buildings are being
built “tighter”. Caulking, weatherstripping, and otherwise
sealing a structure has been the first major step towardmaking it more energy efficient. These practices reduce
the likelihood of enough air infiltrating into the building.
The following guidelines present design considerations
and procedures for ensuring an adequate air supply to thegas furnace.

For the installation depicted in Figure 36 to be valid, therooms on the other side of the openings must constitutea total volume which meets the requirements for an
unconfined space. For instance, in the previous written
example, the equipment room had a volume of 1920 cubicfeet. The total input to the appliances within the room was
120,000 BTUH. 6,000 cubic feet or more of volume is needed
to constitute an unconfined space. So, in Figure 36, theroom connecting through the openings would have to makeup the balance of the required 6,000 cubic feet.

The method of supplying air to the furnace is just as importantas supplying enough air. Makeup air for exhaust fans should
not come from the same space in which the furnace is located.
For example: If the furnace is located in a ventilated basement
or crawl space, the building’s makeup air supply shouldcome from elsewhere. Otherwise, the furnace and the makeup
air system may have to compete for the air in the basement.
This may cause “starvation” of the furnace. A negative
pressure condition may even arise in the basement. The
furnace draft will be affected resulting in spillage of combustion
products at the furnace draft hood.

6,000 – 1,920 = 4,080
4,080 cubic feet of free space would be needed the otherside of the ventilation air openings. That might be a roomwith eight foot ceilings which measured 510 square feet.
Also, this 510 square foot area cannot, by any means, be
reduced. If, for example, a hall and adjacent room are being
used for this 510 square feet then there cannot be any doors
or curtains between them.

Since the makeup air supply comes from the basement,in this example, flue products may also be pulled into thebuilding’s conditioned air supply. The scenario could“snowball” with incomplete combustion occurring, condensation in the vent, and corrosion.
For the same reasons, air returning for re-heating should
never be pulled from the same space as the furnace combustion air supply. Installations where the furnace is centrally
located, as in a hall closet, require the return air to be ductedto the furnace from a location outside of the closet. It isalso a good idea, in such installations, to provide a methodof bringing fresh, outside air directly to the furnace.

Figures 36, 37, 38, and 39 illustrate four acceptable methods

CONFINED SPACEAIR FROM INSIDE BUILDINGAs a matter of fact, recent studies indicate it is always agood idea to supply fresh, outside air for combustion on
any installation. There is a very good reason for this.
Contaminated combustion air can cause serious corrosion problems within both the furnace and the vent system.
Some states have now included the requirement for outside
air in their state codes.

CONFINEDSPACEPERMANENTOPENINGS

Elements such as chlorides and fluorides produce highly
corrosive acids when burned and then condensed. If thewater vapor in the flue products condenses with theseelements in the combustion air, hydrochloric acid orhydrofluoric acid may be a result. These chlorides and fluorides
can be found in abundance in any building. They can be

Figure 36

22

shown in Figures 36 through 39, one opening should belocated within 12 inches of the floor and the other within
12 inches of the ceiling. Even for short appliances, the space
between the louvered openings should never be less than
3 1/2 feet. If connected with ducts, the cross-section areaof the duct shall not be less than the free area of the louvered
opening.

CONFINED SPACEAIR FROM OUTDOORS

Unconfined Spaces
CONFINED
SPACE

Unconfined spaces will normally have enough natural
infiltration of air. If the construction is unusually tight, airfrom outdoors will be needed. The minimum dimensionof rectangular air ducts shall be not less than 3 inches.
Screening to cover the openings to the outside should not
be smaller than 1/4 inch mesh and shall not be capableof closure. Insect screening and louvers reduce effective
area.

OUTLET
OUTDOORS
AIR DUCTS
INLET

Figure 37

In calculating the free area of air openings. consideration
shall be given to the blocking effect of louvers and grilles.
If the free area through a design of louver or grille is known.
it should be used in calculating the size opening required
to provide the free area specified. If the design and freearea is not known, it may be assumed that wood louvers

for supplying combustion and ventilation air to gas appliances
located in a confined space. Of the two ventilating air openings

CONFINED SPACEAIR FROM VENTILATED ATTIC/CRAWL SPACEATTIC LOUVERSTO OUTDOORSOUTLET
AIR DUCTCONFINEDSPACE

ALTERNATE
INLET AIRINLET
AIR DUCT

Figure 38

CONFINED SPACEAIR FROM VENTILATED ATTIC
ATTIC LOUVERSTO OUTDOORSOUTLET
AIR DUCTCONFINED
SPACE

INLET
AIR DUCT

Figure 39

23the roof flashing. Its purpose is to keep out snow and rain.
The storm collar should fit tightly around the vent pipe
and above the roof flashing. The area where the storm
collar wraps around the vent pipe should be caulked withplastic cement or high temperature RTV silicone.

will have a 20 to 25 percent free area and metal louvers
and grilles will have a 60 to 75 percent free area. Louvers
and grilles shall be fixed in the open position or interlockedwith the equipment so that they are opened automaticallyduring equipment operation. Ref: National Fuel Gas Code
Section 5.3.5. The return air, circulating for reheating by
the furnace should be isolated from the air supply comingfrom the outside through these ducts.

Firestops - A firestop assembly should be installed wherethe vent passes through a floor or ceiling. The purposeof the firestop is to close up the opening made to allow
clearance for the vent pipe.This prevents a “chimney” effectfrom occurring through the opening in the event of a fire.

General Installation
Specific instructions for installing vent systems are usuallyavailable through vent and furnace manufacturers. Also,
national, state, and local codes cover many of the specifications
such as vent type, vent materials, and installation practices. In all cases, the code authority having jurisdiction
will have the final say over any installation. This section
covers some general guidelines which have been “time”
tested. There are also some additional procedures whichhave proved reliable in the field. Many of these additional
measures have been adopted by some manufacturers andare included in their installation instructions.

Vent Fittings
Figure 40 depicts different fittings which are used in a ventsystem.The use of many of these fittings is self explanatory.
It is recommended that Underwriter’s Laboratories listed
vents and fittings be used. Some of the fittings require
a little explanation.

Figure 41

Figures 41 and 42 show a typical firestop and ceiling support.
Either one can serve as a firestop. Either can also be used
to center the pipe to allow specified clearance from combustibles. When installing a sheet metal firestop such as
in Figure 41, the firestop should be attached to the joists

Roof flashing - The roof flashing should fit tightly overthe hole in the roof. It should also mesh with the roof shingles.
Notice there are two kinds of roof flashings illustrated.
Storm collar - The storm collar is a counter flashing for

5
9

2

4
10

6

3

1

11
7
8

1.
2.
3.
4.

Round Pipe
Tall Cone Roof Flashing
Draft Hood Connector
Reducer

5.
6.
7.
8.

90° Adjustable Elbow
Cover-All Face Plate
Wall Bracket
Storm Collar

Figure 40

24

9. Standard Tee
10. Wall Thimble
11. Adjustable Roof Flashingand to framing cross members. This is to ensure that nospaces will exist which will allow the accumulation of com-

The design of a draft hood or draft diverter is the resultof tested performance. It should not be altered in any way
or assembled to the appliance in any way which does notcomply with manufacturers’ instructions. Vertical draft hoods
should not be mounted horizontally and vice versa.
Support - Support of the vent system is required to keep

MFGS. CAPSEAL AROUNDCOLLAR AND
FLASHING

CEILINGSUPPORT
Figure 42bustible materials around the vent.
Joint Connections - All joints must be secure and leaktight. If a joint leaks, air may be pulled into the vent (in anatural draft vent) and further dilute the flue gases. The
result would be a reduction in the draft force. Pipe withlocking connections should be pressed together until it
locks. Pipe without locking connections should be assembled
with sheet metal screws and then sealed.

MAINTAIN
LISTED
CLEARANCESUPPORT
IF NECESSARY

Vent Terminals - There are many different types of ventterminals, as illustrated in Figure 43. In general, the ventterminal (or vent cap) should be the same size as the vent

SUPPORT LATERALSPOSITIONFIRESTOP
MAINTAIN
LISTED
CLEARANCELOCL JOINTSVERTICAL VENTLOCKJOINTS

MAINTAIN
LISTED
CLEARANCELOCL JOINTSDRAFT HOOD
CONNECTORLOUVERSADEQUATE
AIR SUPPLYLISTEDAPPLIANCE

Figure 44

COMPLETE GAS VENTTERMINATION (ABOVE)

joint connections intact.The furnace flue collar is not designed
to provide support of vertical vents or vent connectors.

TYPES OF WIND – TESTEDGAS VENT CAPS (LEFT)

Figure 44 illustrates a well assembled vent system. Support
lateral vent connectors as near the flue collar as possible
and every three feet from there. Lateral supporting is usuallyaccomplished with metal hanging straps. Vertical supporting
can be done using the ceiling support which was shownin Figure 42.

Figure 43

to prevent restricting the vent. Also, some vent caps aredesigned primarily to keep rain and snow out of the vent.
Others are designed to also baffle wind in order to prevent back drafts.

Lateral runs - Lateral runs should be avoided, if possible.
When lateral runs are necessary, the length should be kept
as short as possible. Also, avoid an unnecessary numberof turns and bends. Remember, each 90 degree elbow inexcess of two causes a 10% reduction of the vent capacity.
To explain it differently, each 90 degree elbow causes
resistance equivalent to a lateral length of pipe 40 times
the pipe diameter. A six inch elbow is the same as 240 inches
or 20 feet of 6 inch pipe. The maximum length of lateralvent pipe can not exceed 75% of the total height of the vent.
An upward pitch of at least 1/4 inch per lateral foot should
be maintained.

Vent terminals should be located at least five feet above
the furnace flue outlet or draft hood. Also, if the vent terminal
extends five feet or more from the roof, then it should bebraced or secured with guy wires.
Draft Hoods/Draft Diverter Connections - In most cases,
the vent or vent connector will be the same size as the draft
hood/diverter flue collar. The connector should be placed
over the flue collar and attached with sheet metal screws.
Flow arrows on the connector pipe should be pointing awayfrom the furnace.
25

When venting higher efficiency furnaces, the removal ofcondensate from the vent is an area of prime concern. Figure
46 shows a vent installed with the male ends pointing toward
the furnace. Also shown is a drain “Tee” with trapped drainline. Installation with the male ends toward the furnace
allows the condensate to run more freely down the ventand into the drain. Obviously, this installation will havea slight effect on the travel of the flue-products up the ventpipe. For this reason, and also to prevent condensate leaks,
it is necessary to seal these joints.

Obstructions - Internal obstructions reduce the effective
internal area of the vent and, therefore, reduce the capacityof the vent. Where vent connectors attach to a common
vent or chimney, they should be installed flush with theinside surface at the point of entry. Accessories such asvent dampers also cause some restriction. The amountof restriction will depend on the design of the damper.
Sometimes, foreign objects such as bird’s nests, mice, ice,
snow, etc. have been known to get into a vent andblock it. The proper use of bird screens and vent caps will
combat these “uninvited guests. “In installations where condensation is likely to occur, it
is a good practice to install vent pipe with the linear seams
on top. Actually, it is a good idea for all installations. This

Clearance Requirements – Vent pipe clearance fromcombustible materials must comply with national, state,
and local codes. This clearance is usually six inches for
single-wall vent pipe and one inch for Type B double-wallvent pipe. These clearance requirements can be maintainedby the use of straps, non-combustible spacers, wall thimbles,
fire-stop spacers and ceiling supports.

Eddy currents over the top of a structure.

Plastic pipe vents, where allowed by code, are used for
furnaces which have relatively cool flue-gas temperatures.
Also, plastic pipe (PVC or CPVC) does not readily conduct
heat. Because of these two facts plastic pipe vents, where
allowed by code, can be installed with zero clearance. The
special high temperature plastic pipe, used with 78 to 83%
AFUE furnaces, has clearance requirements. See the installer’s

RIDGE

Figure 47
FURNACE

Male ends away from furnace. (Low efficiency)
Figure 45

Positive pressure zone at exposed vertical wall.

guide for the furnace you are installing.
Pipe Orientation – Most Type B vent pipe will have flow
arrows stamped or embossed on the side of the pipe. Whenventing lower efficiency (high flue gas temperatures) furnaces,
the vent should be installed with the flow arrow pointing
away from the furnace. When installed in this manner, themale end of the pipe will be pointing away from the furnace.
See Figure 45. This allows the flue-gases to “slip” throughthe joint more easily.

FURNACEPOSITIVE PRESSURE
ZONEDRAIN “T”

Figure 48

DRAIN HOSE
(TRAPPED)

will also help to prevent leakage of any condensate which
may form in the vent.
Terminal Location – The vent terminal, whether it is amasonry chimney or metal-pipe vent, should not be locatedwhere positive wind pressures occur. Positive pressurezones may be found due to eddy currents of air swirling

Male ends toward furnace. (High efficiency)
Figure 46

26side-wall termination are dissipation of flue-gases, re-entry
of flue-gases into the building, damage to surroundings,
and furnace operation. Side-wall vents should not terminate
through a wall, exposed to prevailing winter winds. Excessive
vent pressures created may adversely affect furnace operation.
Side-wall terminals should not be located in “dead corners”
where the flue-products may accumulate. Care should betaken to avoid venting onto plants. The acidic nature of
the flue-gases may be harmful to plants. Condensation
which may form at the vent outlet can cause unsightly damage
to the sides of the building. For this reason, manufacturers
specify a minimum amount of vent pipe protruding fromthe wall. Some even recommend a corrosion resistant shieldor flashing around the vent terminal.

over the top of a structure (Figure 47) or where a wall faces
the prevailing winter wind (Figure 48). Vent terminationin these areas can result in down-drafts. These down-drafts
may cause flue gas spillage and/or pilot outages.
Wind tunnel tests have yielded guidelines for chimney andTermination within 10 feet of a ridge, wall or parapet.
10 FT. OR LESS

3 FT
MINIMUM

2 FT
MINIMUMLast, but in no way least, is safety. Vent terminals should
be located high enough above pedestrian and vehicularpaths to prevent the flue-gases exhausting on passersby. Some manufacturers even prohibit installation above
pedestrian walkways.

RIDGE

The manufacturers’ installation instructions for side-walltermination should contain clearances from possible air
entry points into the building. It is important that side-wallterminals be either above or at a safe lateral distance fromdoors, windows, etc. Figure 33 diagrams the clearances
specified for venting through a wall.

Figure 49

Termination 10 feet or more from ridge, wall or parapet.

Safeties and Accessories

MORE THAN 10 FT.

Vent Safety Shut-off
10 FT.

3 FT
MINIMUM

On November 1, 1987, a national standard went into effectrequiring a vent safety shut-off to be located in the draft
hood or draft diverter of listed gas-fired equipment. The
vent safety shut-off (sometimes referred to as a “spill” switch)
is a thermal cut-out located in the draft hood/diverter. It
will close the gas valve in the event of the flue-productspillage at the draft hood/diverter. In retro-fit applications,the shut-off should be located in a position where it will
not affect the normal operation of the furnace. It must
be wired into the furnace controls so that the gas
valve will not open unless the shut-off switch is in
the closed position, (in series with the gas valvesolenoid).

2 FT
MINIMUM
RIDGE

Figure 50

Accurate vent sizing is critical to avoid nuisance trippingof the vent safety shut-off. A vent which is undersized or
restricted will cause spillage at the draft hood/diverter, causing
the shut-off to open. Over-sized vent systems are prone
to excessive heat loss within the vent. This heat loss willcause the flue-gases to lose momentum. In this case, theflue-gases can “linger” in the area of the draft hood/diverter
long enough to cause the shut-off to open. In grossly oversized
vents, the heat loss can overcome the momentum of theflue-gases completely resulting in spillage. This is anotherreason why accurate sizing of vent systems can’t be overemphasized.

gas vent termination. Figure 49 depicts proper termination
when the chimney or vent emerges through the roof
within eight feet of a ridge, wall, or parapet. Figure 50 shows
vent termination at a point eight or more feet awayfrom obstructions.
Note: In both illustrations, showing 12" or less diameter
terminations, the minimum height above the point wherethe vent comes through the roof is shown in the Gas Vent
TerminationTable. Fortunately, the one foot minimum heightcovers most requirements. (See Table)

Vent Dampers

As mentioned earlier, side-wall terminal locations for forced
draft furnaces are found in the furnace manufacturers’
installation instructions. The prime areas of concern for

The purpose of a vent damper is to reduce the cost of heatingby closing off the vent during the furnace off-cycle. This27

or ignition controls.

helps the furnace heat exchanger to retain heat which would
normally be lost through the vent. The amount of savings, if any, depends on the installation. Listed automatic
vent dampers may be installed on an existing appliance
if the appliance has a history of safe, satisfactory operationand it is equipped with a draft hood or draft diverter. Manuallyoperated vent dampers shall not be installed in the ventconnectors of a gas appliance. Electrically operated ventdampers must be connected to the furnace controls toprevent the firing of the main burners with the damperin the closed position.

Protection Against Fire Hazards
The proper installation of vents in accordance with manufacturers’ instruction is aimed at eliminating fire hazard
conditions. Type B-W vent, which is used for venting recessed
heaters, must be installed with a solid header plate to serve
as a firestop. A firestop spacer is also to be placed ateach point where the vent passes through a floor or ceiling.

Prohibited Installations

Thermally operated automatic vent dampers require heatfrom the burners to activate them. It is obvious then thatthe control sequencing mentioned above cannot apply.
Most thermal vent dampers are bi-metal vane designs which
rely on the “warping” of the bi-metal to open. Until enoughheat is applied to the thermal vent damper, the flue-products
will have to go somewhere else. For this reason, automatic
vent dampers can only be installed on gas appliances whichhave a draft hood or draft diverter. The relief opening ina draft hood/diverter provides a path for the flue-productsin the event of a down-draft or flue blockage. If the equipment
room is properly ventilated, then there should be no danger
from the spillage which may occur.

There are some installations that are prohibited by codes

PROHIBITED INSTALLATION✗✗

Draft Inducers

WATER
HEATERFURNACE

When installing a power assisted draft inducer, it is always
best to install it at the outlet of the vent. When installedin this manner, the flue-products will be “pulled” throughthe vent and discharged to the outside atmosphere. Thereis a margin of safety, so any leaks will cause ambient air
to be drawn into the vent. If the draft inducer were to beinstalled at the inlet to the vent, positive vent pressureswould mean that any leaks would be outward. Installationof a power assist at the vent inlet requires a sealed vent.

CIRCULATING AIR DUCT

Figure 51

and local authorities. There are others which represent
potentially unsafe situations. If it is possible to recognize
these types of situations, serious problems may be avoided.

The draft inducer must be wired into the furnace controls
so that the burners will not be fired unless it is operating.
This is usually accomplished through the use of a normallyopen “sail” switch. The flow established by the draft-inducer
will cause the contacts of the sail switch to close, proving the operation of the draft inducer. The sail switch would
be wired in series with the room thermostat and gas valve

Single-walled pipe is not recommended for outside vents

SINGLEWALL
VENT PIPE

Figure 52

28

The National Fuel Gas Code suggests several installation practices to avoid. It should be consulted for description of these cases, but briefly they are as follows:
5'
MINIMUMNo portion of the venting system shall extend into or passthrough any circulating air duct or plenum. By adheringto this practice, the chance of flue product leakage into
circulating air streams will be minimized. See Figure 51.
The use of uninsulated single-wall pipe is not recommended
for use outdoors in cold climates. Such use may cause
condensation and corrosion of the vent system. See

VERTICAL WALL

TYPE BVENT

8" MIN.

LOWEST DISCHARGEOPENING

LISTEDCAP

MINIMUM
HEIGHT

VENTILATEDATTIC SPACEX

12
ROOF PITCH IS X/12

THE VENT TERMINATION SHOULD NOT BELESS THAN 8 FT. FROM A VERTICAL WALL
VENT CAPS 12" OR SMALLER

12'MINIMUM

Figure 53

Figure 52.
Wind flow over roof structures often causes localized pressure
disturbances. The vent termination must be placed wherethese effects are minimized.

HEATER

Termination of the vent must be at least 1 foot above thehighest point where it passes through a roof on a flat to7/12 pitch roof and at least 2 feet above the highest partof the roof within 8 feet, provided the termination is 12
inches in diameter or less. Except for direct vent appliances,the vent should not terminate adjacent to a wall or below
eaves or parapets. See Figure 53.

TYPE B-W
VENT

Figure 54

The height of Type B vents must not be less than 5 feet invertical height above the highest connected draft hood.

29

c. Insofar as it is practical, close all building doors andwindows and all doors in between the space where theappliances remaining connected to the common ventsystem are located and the rest of the building. Turnon clothes dryers and any gas-burning appliances notconnected to the common venting system.Turn on any
exhaust fans, such as range hoods and bathroom exhausts,
so they will operate at maximum speed. Do not operatea summer exhaust fan. Close the fireplace dampers.

For Type B-W vents, the minimum height must be atleast 12 feet, from the base of the heater. See Figure 54.
If vents are less than these minimum heights, spillage
may occur.
Do not exhaust a clothes dryer into a vent or chimney.The
power exhaust system of the dryer often overloads thevent resulting in unsatisfactory operation of other appliances.
The moisture-laden air is almost certain to result in condensation when the vent is cool. Furthermore, lint may
accumulate causing blockage.

d. Follow the lighting instructions. Place the appliancesin operation. Adjust the thermostat so the appliancewill operate continuously.

Troubleshootinge. Test for spillage at the draft hood relief opening after
5 minutes of main burner operation. Use the flame ofa match or candle or smoke from a cigarette, cigar or
pipe.

Problems with venting systems are not usually reportedby the building’s occupants unless they smell something.
Sometimes a “no heat” complaint due to chronic pilot flame
outages is a result of improper venting. It is important that
installers and service technicians be observant and know
the signs of poor vent performance.

f. After it has been determined that each appliance remaining attached to the common venting system properly
vents when tested as outlined, return doors, windows,
exhaust fans, fireplace dampers and any other gas-burning
appliance to their previous condition of use.

Pilot Outage or Lock Out

g. If improper venting is observed during any of the tests,
the common venting system must be corrected according
to the standards set forth in the National Fuel Gas Code,ANSI Z223.1.

More often than not, pilot outages are caused by malfunctionsin the pilot system. Dirty pilot orifices or improper thermocouple/pilot-flame orientation are some causes for chronic
pilot outages. Sometimes, however, pilot outage can betraced to the vent system. The first thing to look for is thevent cap. Make sure the cap is present and securely fastened
to the vent. The cap should also be checked to see if thedesign will reduce the effects of wind. Sometimes, changing
the vent cap to a more suitable wind-baffling design may
solve the problem.

Spillage
Spillage of flue-products from the draft hood/diverter isusually detected by an unpleasant odor. There are manyreasons for spillage to occur. One cause might be anundersized vent. If the vent is too small, it will not havethe physical capacity to move the volume of combustion
products being produced. The solution is to replace thevent with a larger diameter vent. Another solution might
be to increase the overall height of the vent. The sectionin this manual on Factors Which Affect Vent Operation
pointed out why an increase in height increases the capacity of a vent. The capacity tables found in this manualand also in the National Fuel Gas Code can be used todetermine the feasibility of this solution. If neither of thepreviously mentioned solutions are acceptable, then power
venting should be considered.

The problem could also be improper terminal location.
If the terminal is located in a high pressure area, downdrafts may blow the pilot flame out. The solution would
be to either relocate the vent terminal or to raise the ventheight to get the terminal out of the positive pressure area.
If the pilot outage problem is not very frequent, the downdrafts may not be very strong. Even in these cases, steps
to prevent down-drafts should be taken. As a possible
alternative in this situation, installation of some type ofintermittent pilot ignition system might be considered.

Vent System TestingOn the other hand, oversized vents can also result in spillage.
This is because the larger area results in a loss of velocity
or momentum. The problem is compounded because theflue-gases are given more time to cool. They also havea greater mass of metal to surrender heat to. This cooling results in a reduction of the draft force. Increasing ventheight will only aggravate this situation as the vent capacity is already too great. Two solutions to the problemof oversized vents are: install the correct size vent or installa draft inducer.

When an existing furnace is removed from a venting systemserving other appliances, the venting system is likely to
be too large to properly vent the remaining attachedappliances.
The following steps shall be followed with each appliance remaining connected to the common venting system
placed in operation, while the other appliances remaining
connected to the common system are in operation:a. Seal any unused openings in the common venting system.

Another cause of spillage can be an undersized vent cap.
Usually, if the vent itself is correctly sized, a vent cap thesame size as the vent will perform satisfactorily.The solution
is to insure that the vent cap is not highly restrictive andis at least the same size as the vent. Again, power venting

b. Visually inspect the venting system for proper size andhorizontal pitch. Determine that there is no blockage
or restriction, leakage, corrosion or other deficiencies
which could cause an unsafe condition.
30

is possible solution.
Spillage can also be caused by excessive resistance due
to poor vent system design or installation. This excess
resistance may be due to an excessive number of elbows,
exceedingly long lateral runs, or improper pitch in lateralruns. The number of elbows in a vent system should be
kept to a minimum. Vent capacity tables already take into
account two elbows.
Each 90 degree elbow, in excess of two, decreases ventcapacity by 10%. As a general rule, lateral length should
be limited to 75% of the total vent height. This rule may
not apply, however, to vent systems with several elbows.
Installations with several elbows and long horizontal runs
will perform poorly. Proper slope must be maintained inlateral runs. The pitch should be at least 1/4 inch per lateralfoot of run. In the extreme case, a downward pitched horizontal
pipe may even form a trap, possibly resulting in spillage.
Resistance to flow in a vent may also come in the formof restrictions. These restrictions may be caused by thepresence of foreign objects such as bird’s nests, rodent
nests, or insect nests, etc. Vents should be periodicallyinspected for the presence of obstructions. A screen on
the vent outlet may be necessary.
Restrictions can also come in the form of accessories suchas dampers, draft regulators or inoperative induced-draft
blowers. Faulty accessories must be replaced. Control ofthese accessories must be integrated with the combustion
system to prevent firing of the furnace in the event of amalfunction. Vent systems should be inspected for damaged
or deformed pipe. A sizeable dent in a vent pipe may also
be a restriction.
Flue-gas spillage may occur when the combustion anddilution air supplies are under negative pressures. If a structure
contains other devices which exhaust air, close attention
must be paid to the ventilation air supply. Range hoods,
bath room and kitchen exhausts are examples of such devices.
The air supply for a furnace should never come from thesame immediate area where these appliances are located.

31

32

33

34

35

36

Glossary
AFUE – The Annual Fuel Utilization Efficiency (AFUE)
is a ratio of annual output of useful energy delivered
to the heated space to the annual fuel energy input.

DENSITY – The weight of a substance per unit
volume. As applied to gases, the weight in pounds ofa cubic foot of gas at standard pressure and temperature.

ATMOSPHERIC PRESSURE – The pressure exerted on
the earth’s surface by the weight of the atmosphere above
it. At sea level this is approximately 14.7 pounds per
square inch, 760 mm of mercury.

DEW POINT – The temperature at which a vapor
starts to condense into a liquid. In undiluted fluegases it is about 140°F. In diluted gases in the vent it
is approximately 100 to 105°F.

BAFFLE – An object placed in appliance to change thedirection of, or retard, the flow of air, air-gas mixtures,or flue gases.

DILUTION AIR – Air which enters a draft hood or draft
regulator mixes with the flue gases.
DOWNDRAFT – A flow of air downward in the stackcaused by excessively high air pressures created at theoutlet of the chimney or vent cap or by negative pressure
at the inlet of the vent system.

CHIMNEY – (Also see Gas Vents) A vertical shaft
enclosing one or more flues for conveying flue gases
to the outside atmosphere.
a. Factory-Built Chimney. A listed chimney.

DRAFT HOOD/DRAFT DIVERTER – A device built into
an appliance, or made a part of the flue or ventconnector from an appliance, which is designed to(1) provide for the ready escape of the flue gases in theevent of no draft, back draft, or stoppage beyond thedraft hood; (2) prevent a back draft from entering
the appliance; and (3) neutralize the effect of stackaction of the chimney or gas vent upon the operationof the appliance.

b. Masonry Chimney. A chimney of solid masonryunits, bricks, stones, listed masonry units orreinforced concrete, lined with suitable flueliners.
c. Metal Chimney. A field-constructed chimneyof metal.
CHIMNEY FLUE – (See Flue)COMBUSTIBLE MATERIAL – As pertaining tomaterials adjacent to or in contact with heat producing appliances, vent connectors, gas vents, chimneys,steam and hot water pipes, and warm air ducts,
shall mean materials made of or surfaced with wood,compressed paper, plant fibers, or other materials
that will ignite and burn. Such material shall beconsidered combustible even though flame-proofed,
fire retardant treated, or plastered.

DRAFT HOOD CONNECTOR – A transition fitting forconnecting double-wall pipe to the single-wall flue collar
or draft hood outlet of the appliance.
DRAFT REGULATOR – A device which functions tomaintain a desired draft in the appliance by automaticallyreducing the draft to the desired value.
EXCESS AIR – Air which passes though the combustion
chamber and the appliance flues in excess of that whichis theoretically required for complete combustion.

COMBUSTION – Rapid oxidation of fuel gases
accompanied by the production of heat, or heat andlight. Complete combustion of a fuel is possible onlyin the presence of an adequate supply of oxygen.

FIRESTOP – A blockage placed in a hollow wall or
opening in any other part of a building structure to halt
upward progress of fire through the wall or opening.

COMBUSTION AIR – Air supplied to an appliancespecifically for the combustion of fuel.

FIRESTOP SPACER – A component of a vent system
which serves to secure a vertical vent and at the sametime serve as a firestop for the opening made for thevent through a floor, ceiling or wall.

COMBUSTION PRODUCTS – Constituents resulting
from the combustion of a fuel with the oxygen of theair, including the inerts but excluding excess air.

FLUE – The general term for the passageways in an
appliance and vents through which combustion products
pass to the outside atmosphere.

COMMON VENT – The portion of a vent system into
which the flue gases from two or more appliances flow.

1. Appliance Flue – The flue passageways within
an appliance.
2. Chimney Flue – The vertical conduit for conveying
combustion products delivered to it by a ventconnector to the outside atmosphere.

CONDENSATE (CONDENSATION) – The liquid
which separates from a gas (including flue gas) due
to a reduction in temperature.
CUBIC FOOT (Cu. Ft.) OF GAS – The amount of gas
which would occupy 1 cubic foot when at a temperatureof 60°F, saturated with water vapor, and under a pressure
equivalent to that of 14.73 pounds per square inch absolute.

FLUECOLLAR – That portion of an appliancedesigned for the attachment of the draft hood orvent connector.
37

FLUE EXHAUSTER – A device installed in and madea part of the vent which will provide a positive induced
draft. (Draft Inducer)

NATURAL DRAFT – The motion of the flue gases
through an appliance generated by hot flue gases risingin a chimney or vent.

FLUE GASES – Products of combustion plus excess
air in appliance flues or heat exchangers (before thedraft hood or draft regulator. (See Vent Gases).

PRIMARY AIR – The air introduced into a burner whichmixes with the gas before it reaches the port or ports.
RELIEF OPENING – The opening provided in a draft
hood to permit the ready escape to the atmosphere of
the flue products from the draft hood in the event of
no draft, back draft, or stoppage beyond the draft hood,and to permit inspiration of air into the draft hood in
the event of a strong chimney updraft.

FLUE OUTLET – The opening provided in an appliancefor the escape of the flue gases.
FLUE PRODUCTS – (See Flue Gases)
GAS VENTS – Factory-built vent piping and ventfittings listed by a nationally recognized testing agency,that are assembled and used in accordance with theterms of their listings, for conveying flue gases to theoutside atmosphere.

RETURN AIR – Air returning through ductwork to bereheated by a forced-air furnace after circulation throughthe heated space.
ROOF FLASHlNG – Metal used to waterproof roof joints
or openings where vents emerge from the roof.

a. Type B Gas Vent. A gas vent for venting gas
appliances with draft hoods and other gas appliances
listed for use with Type B Gas Vents.

SECONDARY AIR – The air externally supplied to theflame at the point of combustion.

b. Type B-W Gas Vent. A gas vent for venting listed
gas-fired vented wall furnaces.

SLIP JOINT – Joints of metal vents so fabricated thatvent sections can be secured to each other without tools
by sliding the parts together manually.

c. Type L Venting System. A venting system composedof listed factory-built components assembled inaccordance with the terms of listing for venting
appliances listed for use withType L venting systems.
They may be used also where Type B gas ventsare permitted.

SPACE, CONFINED – A space whose volume is lessthan 50 cubic feet per 1000 Btu per hour of the aggregate
input rating of all appliances installed in the space.
SPACE, UNCONFINED – A space whose volume is
not less than 50 cubic feet per 1,000 Btu per hour of
the aggregate input rating of all appliances installedin that space. Rooms communicating directly with thespace in which the appliances are installed, throughopenings not furnished with doors, are considered apart of the unconfined space.

HALOGENATED HYDROCARBON – Any number ofcompounds composed of carbon and hydrogen which
has a chlorine, fluorine or bromine constituent.
HIGH EFFICIENCY APPLIANCE – An appliance which
operates at an AFUE value above about 83 percent. These
appliances normally use a power burner, induced orforced draft blower or pulse combustion to vent the fluegases. The appliances also require special considerationof condensate. Between 83 and 90 percent AFUE condensate may occur in the vent system during cycling.
If the AFUE value is above 90 percent condensate will
occur in the appliance and vent system during cycling
and steady operation.

SPILLAGE – Combustion products flowing from theappliance air openings or draft hood relief openings
due to a malfunction of the venting system.
STORM COLLAR – A cone-shaped metal piece providing a waterproof transition from a vent pipe and aroof where the pipe emerges from the roof.

LISTED – Equipment or materials included in a list
published by a nationally recognized testing laboratorythat maintains periodic inspection of production of listed
equipment or materials and whose listing states either
that the equipment or material meets nationally recognized standards or has been tested and found suitablefor use in a specified manner.

THIMBLE – A metal device penetrating a wall with ahole for a vent pipe to pass through. The thimble securesthe pipe, seals the wall opening and insures that the
hot vent is isolated from combustible wall material. A
ventilated thimble has holes which allow air to pass freely
between the vent and the thimble thereby keeping thethimble cool.

MAKE-UP AIR – Air supplied to a space to replace air
exhausted or otherwise removed from the space.

UPDRAFT – An increase in the upward velocity andvolume of the flue gases caused by excessively low
atmospheric pressures at the outlet of the chimney orvent cap.

MULTIPLE APPLIANCE VENT – (See Vent)
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VENT – (See Gas Vents) A device, such as a pipe, toconvey flue gases from an appliance to the outdoors.
VENT CONNECTOR – That portion of the venting
system which connects the gas appliance to the gas
vent or chimney.
VENT DAMPER DEVICE, AUTOMATIC – A device
intended for installation in the venting system, in theoutlet of or downstream of the appliance draft hood,of an individual automatically operated fuel-gas burning
appliance and which is designed to automatically open
the venting system when the appliance is in operationand to automatically close off the venting system when
the appliance is in a standby or shutdown condition.
a. Electrically Operated. An automatic vent damper
device that employees electrical energy tocontrol the device.
b. Mechanically Actuated. An automatic vent damper
device dependent for operation upon the direct
application or transmission of mechanical energy
without employing any type of energy conversion.
c. Thermally Acutated. An automatic vent damper
device dependent for operation exclusively upon
the direct conversion of the thermal energy of thevent gases into mechanical energy.
VENT GASES – (Flue-Gases, Flue-Products)
Products of combustion from gas appliances plus excess
air, plus dilution air in the venting system above thedraft hood or draft regulator.
VENT TERMINAL (VENT CAP) – The fitting at the endof a vent pipe that directs the flue gases into the outside
atmosphere and keeps out rain, snow, debris and animals.
VENTING – The process of removing the gases
produced by combustion.
VENTILATION (VENTILATION AIR) – Air brought
into or allowed to enter the space occupied by anappliance to replenish air used in combustion, excess
air, and draft hood dilution air or to provide replacement of the air in a space by fresh air.
VENTING SYSTEM – The gas vent, chimney or singlewall metal pipe, and vent connector if used, assembled
to form a continuous open passageway from the gas
appliance to the outside atmosphere for the purposeof removing vent gases.
WATER COLUMN – Abbreviated as W.C. A unit
used for expressing pressure. One inch water column
equals a pressure of 0.578 ounces per square inch.
‘Y” FITTING – A tee fitting modified in shape to reduce
flow resistance.
39

Notes40

Note: This publication is general in nature and is intended for
INSTRUCTIONAL PURPOSES ONLY. It is not to be used forequipment selection, application, installation, or specificservice procedures.

P.I. (L)