GAMA VENTING TABLES FOR CATEGORY I CENTRAL FURNACES
This booklet contains new venting tables designed specifically for use with Category I central furnaces. These tables are unique, in that, industry wide venting tables now exist for fan-assisted combustion system central furnaces. Venting tables for these types of appliances are not contained in the National Fuel Gas Code (NFPA 54/ANSI Z223.1-1988).
All requirements contained in this booklet apply to both Category I draft-hood equipped central furnaces as well as fan-assisted combustion system central furnaces. At no time should a venting system for a listed Category II, III, or IV central furnace be sized with these tables. The National Fuel Gas Code (NFPA 54/ANSI Z223.1-1988) may also be used to size venting systems for draft-hood equipped central furnaces. However, at this time, the National Fuel Gas Code does not include alternate sizing methods for fan-assisted combustion systems. Therefore, until engineering data is developed to allow alternate sizing methods for Category I fan-assisted central furnaces, the enclosed venting tables must be used for fan-assisted combustion system central furnaces. These tables apply to venting single appliances and common venting multiple appliances, in chimneys other than masonry chimneys.
The new venting tables were developed by Battelle under contract (GRI-5088-245-1728) to the Gas Research Institute (GRI). The computer program (VENT-II) developed by Battelle Columbus generated the venting tables in this booklet and this procedure has been accepted by the American Gas Association Laboratories as an appropriate engineering methodology for determining venting requirements of Category I central furnaces.
For your information, the general venting requirements listed in this booklet are not intended to be used as complete installation instructions and represent only a partial list of venting considerations.
For venting applications that fall outside the parameters of the new venting tables, refer to the furnace manufacturer’s complete installation instructions, the specific vent manufacturer’s complete installation instructions and state and local codes.
GRI DISCLAIMER
LEGAL NOTICE: This information is, in part, a result of work performed by Battelle under the sponsorship of the Gas Research Institute (GRI). Neither GRI, members of GRI, nor any person acting on behalf of either:
A. Makes any warranty or representation, express or implied, with respect to the accuracy, completeness, or usefulness of this information, or that the use of any apparatus, method, or process disclosed may not infringe privately owned right; or
B. Assumes any liability with respect to the use of, or for damages resulting from the use of, any information, apparatus, method, or process disclosed.
VENTING REQUIREMENTS FOR CATEGORY I CENTRAL FURNACES
I. INTRODUCTION. This booklet contains the current definitions, instructions, and tables necessary to vent today’s modern Category I Gas Fired Appliance. A variety of definitions of new terms describing today’s gas appliances are included to supplement the actual venting tables which have been generated to correctly vent various combinations of Category I Appliances using Type B or single-wall metal vent connectors attached to Type B vents. Tables are also included covering similar venting material combinations when applied to common venting arrangements of two appliances.
Finally, a series of examples are presented demonstrating how the vent tables are used to size the vent connector and the vertical vent for a variety of typical applications.
II. DEFINITION OF TERMS.
“Fan Assisted Combustion System”: An appliance equipped with an integral mechanical means to either draw or force products of combustion through the combustion chamber and/or heat exchanger.
“FAN Min” refers to the minimum appliance input rating of a Category I appliance with a fan-assisted combustion system that could be attached to the vent.
“FAN Max” refers to the maximum appliance input rating of a Category I appliance with a fan-assisted combustion system that could be attached to the vent.
“NAT Max” refers to the maximum appliance input rating of a Category I appliance equipped with a draft hood that could be attached to the vent. There are no minimum appliance input ratings for draft-hood-equipped appliances.
“FAN+FAN” refers to the maximum combined input rating of two fan-assisted appliances attached to the common vent.
“FAN+NAT” refers to the maximum combined input rating of one fan-assisted appliance and one draft-hood-equipped appliance attached to the common vent.
“NAT+NAT” refers to the maximum combined input rating of two draft-hood-equipped appliances attached to the common vent.
“NR” means not recommended due to potential for condensate formation and/or pressurization of the venting system.
“NA” means not applicable due to physical or geometric constraints.
“Draft Hood”: A device built into an appliance, or made a part of the vent connector from an appliance, which is designed to (1) provide for the ready escape of the flue gases from the appliance in the event of no draft, backdraft, or stoppage beyond the draft hood; (2) prevent a backdraft from entering the appliance; and (3) neutralize the effect of stack action of the chimney or gas vent upon the operation of the appliance.
“Vent”: A passageway used to convey flue gases from gas utilization equipment, or their vent connectors, to the outside atmosphere.
“Vent Connector”: The pipe or duct which connects a fuel-gas burning appliance to a vent or chimney.
“Flue Collar”: That portion of an appliance designed for the attachment of a draft hood, vent connector, or venting system.
III. GENERAL VENTING REQUIREMENTS. All requirements contained in this document apply to both Category I draft hood equipped and fan-assisted combustion appliances. At no time should a venting system for a listed Category II, III, or IV appliance be sized with these Tables. The alternate sizing methods described in the National Fuel Gas Code (NFPA 54/ANSI Z223.1-1988) may also be used to size the venting system for a draft hood equipped appliance. At this time, alternate sizing methods have not been developed for fan-assisted appliances. Therefore, until engineering data is developed to allow alternate sizing methods for Category I fan-assisted appliances, the vent tables must be used.
1. If the vent size determined from the tables is smaller than the appliance draft hood outlet or flue collar, the smaller size may be used provided:
a. The total vent height “H” is at least 10 feet.
b. Vents for appliance draft hood outlets or flue collars 12 inches in diameter or smaller are not reduced more than one size (e.g. 12 inches to 10 inches is a one size reduction).
c. Vents for appliance draft hood or flue collars above 12 inches in diameter are not reduced more than two sizes (e.g. 24 inches to 20 inches is a two size reduction).
d. The maximum capacity listed in the tables for a fan-assisted appliance is reduced by 10% (0.90 x maximum capacity).
e. The draft hood outlet is greater than 4 inches in diameter. Do not connect a 3 inch diameter vent to a 4 inch diameter draft hood outlet. This provision does not apply to fan-assisted appliances.
2. Single appliance venting configurations with zero lateral lengths. Tables 1 & 2, are assumed to have no elbows in the vent system. For all other vent configurations with indicated lateral lengths, the vent system is assumed to have two 90° elbows. For each additional 90° elbow, or equivalent* beyond two, the maximum capacity listed in the venting table should be reduced by 10 percent (0.90 x maximum listed capacity).
3. The common venting Tables 3 & 4 were generated using a maximum vent connector length of one and one-half feet (18 inches) for each inch of connector diameter as follows:
Connector Diameter (Inches) | Maximum Connector Length (Feet) |
Connector Diameter (Inches) | Maximum Connector Length (Feet) |
3 | 4 1/2 |
4 | 6 |
5 | 7 1/2 |
6 | 9 |
7 | 10 1/2 |
8 | 12 |
9 | 13 1/2 |
10 | 15 |
12 | 18 |
14 | 21 |
16 | 24 |
18 | 27 |
20 | 30 |
22 | 33 |
24 | 36 |
The vent connector should be routed to the vent utilizing the shortest possible route. Longer connectors than these listed are possible but require an increase in size, rise, or total vent height to compensate for the additional length. Consult the appliance manufacturer for specific recommendations concerning long vent connectors.
4. If vent connectors are combined prior to entering the common vent, the maximum common vent capacity listed in the common venting tables must be reduced by 10% (0.90 x maximum common vent capacity). See Figure 7. The length of the common vent connector offset (L) should not exceed one and one-half feet (18 inches) for each inch of common vent connector diameter.
5. If the common vertical vent is offset as shown in Figure 8, the maximum common vent capacity listed in the common venting tables should be reduced by 20% (0.80 x maximum common vent capacity).
6. The flow area in the common vent should be greater than or equal to 80 percent of the sum of the flow areas of the individual vent connectors. However, the common vent diameter must always be at least as large as the largest vent connector diameter. All interconnection fittings must also be the same size as the common vent.
7. Type B gas vents shall terminate above the roof surface with a listed cap or a listed roof assembly in accordance with the terms of their respective listings and the manufacturer’s instructions. See Figures 1 and 2.
*Two 45° Elbows are equivalent to one 90° Elbow.
WIND-TESTED CAPS - (12 inches and Smaller)
Listed gas venting systems using listed vent caps 12 inches and smaller in size may terminate in accordance with the VENT TERMINATION TABLE.
VENT CAPS LARGER THAN 12 inches
Listed vent caps larger than 12 inches must be located at least 2 feet above the highest point and at least 2 feet higher than any portion of a building within a horizontal distance of 10 feet
8) No portion of the venting system can extend into, or pass through any circulating air duct or plenum.
9) All vent pipe passing through floors, walls, and ceilings must be installed with the listed clearance to combustible materials and be fire stopped according to local codes. In the absence of local codes, refer to NFGC.
10) Vent connectors serving Category I appliances shall not be connected to any portion of mechanical draft systems operating under positive pressure such as Category III or IV Venting Systems.
11) A Category I appliance must never be connected to a chimney that is servicing a solid fuel appliance. If a fireplace chimney, lined with a metal liner, is used to vent this appliance, the fireplace opening must be permanently sealed.
12) These tables are not applicable to outside chimneys or vents unless enclosed in a chase. A Type B vent passing through an unused masonry chimney is considered to be an enclosed vent system and these tables may be used.
13) A vent connector shall be supported without any dips or sags and shall slope a minimum of 1/4 inch per lineal foot of connector, back towards the appliance.
14) When two or more vent connectors enter a common gas vent, chimney flue, or single-wall metal pipe, the smaller connector shall enter at the highest level consistent with the available headroom or clearance to combustible material.
15) Vent connectors shall be firmly attached to draft hood outlets or flue collars by sheet-metal screws or other approved means, except vent connectors of listed Type B vent material which shall be assembled in accordance with the manufacturer’s instructions. Joints between sections of single wall connector piping shall be fastened by sheet-metal screws or other approved means.
16) When the vent connector used for Category I appliances must be located in or pass through a crawl space or other area which may be cold, that portion of the vent connector shall be of listed double-wall Type B vent material or material having equivalent insulation qualities.
17) The entire length of a vent connector shall be readily accessible for inspection, cleaning, and replacement.
18) When more than two appliances are connected to a common vent, and at least one of the common vented appliances is a fan assisted type, the common vent must be sized to accommodate:
1) The capacity of all the common vented appliances.
2) Have a minimum capacity lower than that of the smallest fan assisted appliance. Minimum capacity should be determined using Tables 1 and 2.
Example 1: Single Draft-Hood-Equipped Appliance
Suppose that an installer has a 120,000 Btu/hr input appliance with a 5 inch diameter draft hood outlet that needs to be vented into a 10 foot high Type B vent system. What size vent should be used assuming (a) a 5-Ft lateral single-wall metal vent connector is used with two 90° elbows, (b) a 5-Ft lateral single-wall metal vent connector is used with three 90° elbows in the vent system?
Solution
Table 2 should be used to solve this problem because single-wall metal vent connectors are being used with a Type B vent: Refer to Figure 4 of Typical Applications.
(a) Read down the first column in Table 2 until the row associated with a 10-Ft height and 5-Ft lateral is found. Read across this row until a vent capacity greater than 120,000 Btu/hr is located in the shaded columns labeled “NAT Max” for draft-hood-equipped appliances. In this case, a 5 inch diameter vent has a capacity of 122,000 Btu/hr and may be used for this application.
(b) If three 90° elbows are used in the vent system, then the maximum vent capacity listed in the tables must be reduced by 10 percent (see Note 6). This implies that the 5 inch diameter vent has an adjusted capacity of only 110,000 Btu/hr. In this case, the vent system must be increased to 6 inches in diameter. See calculations below:
122,000 x .90 = 110,000 for 5-inch Vent
From Table 2 - Select 6-inch Vent
186,000 x .90 = 167,000; This is greater than the required 120,000, therefore use a 6-inch Vent.
Example 2: Single Fan-Assisted Appliance
Suppose an installer has an 80,000 Btu/hr input fan-assisted appliance that must be installed using 10 feet of lateral connector attached to a 30-Ft high Type B vent. Two 90° elbows are needed for the installation. Can a single-wall metal vent connector be used for this application?
Solution
Table 2 refers to the use of single-wall metal vent connectors with Type B vent. In the first column find the row associated with a 30-Ft height and a 10-Ft lateral. Read across this row, looking at the “FAN Max” columns, to find that a 3 inch diameter single-wall metal connector vent is not recommended. Moving to the next larger size single wall connector (4 inches) we find that a 4 inch diameter single-wall metal connector has a recommended minimum vent capacity of 91,000 Btu/hr and a recommended maximum vent capacity of 144,000 Btu/hr. The 80,000 Btu/hr fan-assisted appliance is outside this range, so we conclude that a single-wall metal vent connector cannot be used to vent this appliance using 10 feet of lateral for the connector.
However, we see that if the 80,000 Btu/hr input appliance could be moved to within 5 feet of the vertical vent, then a 4 inch single-wall metal connector could be used to vent the appliance. Table 2 shows the acceptable range of vent capacities for a 4 inch vent with 5 feet of lateral to be between 72,000 Btu/hr and 157,000 Btu/hr.
If the appliance cannot be moved closer to the vertical vent, then Type B vent could be used as the connector material. In this case, Table 1 shows that for a 30-Ft high vent with 10 feet of lateral, the acceptable range of vent capacities for a 4 inch diameter vent attached to a fan-assisted appliance are between 37,000 Btu/hr and 150,000 Btu/hr.
Example 3: Common Venting Two Draft-Hood Appliances
Suppose a 35,000 Btu/hr water heater is to be common vented with a 150,000 Btu/hr furnace using a common vent with a total height of 30 feet. The connector rise is 2 feet for the water heater and 3 feet for the furnace. Assume single-wall metal connectors will be used with Type B vent. What size connectors and combined vent should be used in this installation?
Solution - (Table 4 applies in this example)
Table 4 should be used to size single-wall vent connectors attached to Type B vertical vent. In the vent connector capacity Table 4, find the row associated with a 30-Ft vent height. For a 2-Ft rise on the vent connector for the water heater, read the shaded columns for draft-hood-equipped appliances to find that a 3 inch diameter vent connector has a capacity of 37,000 Btu/hr.
Therefore, a 3 inch single-wall metal vent connector may be used with the water heater. For a draft-hood-equipped furnace with a 3-Ft rise, read across the appropriate row to find that a 5 inch diameter vent connector has a maximum capacity of 120,000 Btu/hr (which is too small for the furnace) and a 6 inch diameter vent connector has a maximum vent capacity of 172,000 Btu/hr. Therefore, a 6 inch diameter vent connector should be used with the 150,000 Btu/hr furnace.
For the capacity of the combined vent, the lower portion of Table 4 should be used. The combined vent capacity required is 185,000 Btu/hr. Table 4 shows that the combined vent capacity of a 6 inch diameter vent with a 30-Ft vent height is 257,000 Btu/hr. This is more than adequate to handle the 35,000 Btu/hr input water heater and the 150,000 Btu/hr input furnace.
Example 4: Common Venting a Draft Hood Water Heater with a Fan-Assisted Furnace
In this case, a 35,000 Btu/hr input draft-hood-equipped water heater with a 2 foot connector rise is to be common vented with a 100,000 Btu/hr fan-assisted furnace with a 3-Ft connector rise. The common vent consists of 30-Ft rise of Type B vent. What are the recommended vent diameters for each connector and the common vent?
Solution - (Table 4)
Water Heater Vent Connector Diameter. Let us assume the installer would like to use a single-wall metal vent connector. Using Table 4, Vent Connector Capacity, read down the Total Vent Height “H” column to 30 feet and read across the 2-Ft Connector Rise “R” row to the first Btu/hr rating in the “NAT Max” column that is equal to or greater than the water heater input rating. The table shows that a 3 inch vent connector has a maximum input rating of 37,000 Btu/hr. Since this is greater than the water heater input rating, a 3 inch vent connector is adequate. Furthermore, since the water heater is equipped with a draft hood, there are no minimum input rating restrictions.
Furnace Vent Connector Diameter. Again, let us assume the installer would like to use a single-wall metal vent connector. Using Table 4, Vent Connector Capacity, read down the Total Vent Height “H” column to 30 feet and across the 3-Ft Connector Rise “R” row. Since the furnace has a fan-assisted combustion system, find the first “FAN Max” column with a Btu/hr rating greater than the furnace input rating. The 4 inch vent connector has a maximum input rating of 119,000 Btu/hr and a minimum input rating of 85,000 Btu/hr. The 100,000 Btu/hr furnace in this example falls within this range, so a 4 inch connector is adequate. If the furnace would have had an input rating of 80,000 Btu/hr, than a Type B vent connector (see Table 4) would have to be used in order to meet the minimum capacity limit.
Common Vent Diameter. The total input to the common vent is 135,000 Btu/hr. Using Table 4, Common Vent Capacity, read down the Total Vent Height “H” column to 30 feet and across this row to find the smallest vent diameter in the “FAN+NAT” column that has a Btu/hr rating equal to or greater than 135,000 Btu/hr. The 4 inch common vent has a capacity of 132,000 Btu/hr and the 5 inch common vent has a capacity of 202,000 Btu/hr. Therefore, the 5 inch common vent should be used in this example.
Summary. In this example, the installer may use a 3 inch diameter, single-wall metal vent connector for the water heater and a 4 inch diameter, single-wall metal vent connector for the furnace. The common vent should be a 5 inch diameter Type B vent.
(1960 Code, Ch. 27.1)