Comparison of Three Vertical Wet Venting Systems

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Technical Note (National Research Council of Canada. Division of Building Research), 1973-08-01

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Comparison of Three Vertical Wet Venting Systems

Beach, R. K.

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NATIONAL RESEARCH COUNCIL OF CANADA

DIVISION OF BUILDING RESEARCH

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PREPARED BYR. K. Beach CHECKED BY D. G. S. APPROVED BY

No.

575

N. B. H .

PREPARED

.E.A!.!. 1 August 1973

SUBJECT COMPARISON OF THREE VERTICAL WET VENTING SYSTEMS

/

The Task Group on Venting of the Standing Committee on Plumbing Services has proposed that a system of vertical wet venting be included in Part 7 of the 1975 issue of National Building Code of Canada. Vertical wet venting is considered to be the use of the soil-ai-waste stack to provide both drainage and venting. (A definition should probably be included in Part 2 of the NBC.)

This Note has been prepared by the Division of Building Research to assist members of the Standing Committee on Plumbing Services in their review of the proposaL It sets out the facts on which the Task Group

members based their decisions and suggests limits for some details that have not been specified. Three systems are covered in detail; the British Single-Stack System, the Philadelphia Single-Stack System and the Task Group's proposal, which is referred to as the Canadian Vertical Wet-Venting System.

Vertical wet-venting systems have been or are being used in Canada and in many cities in the U. S. A. Prior to the establishment of the Ontario Plumbing Regulations a form of the English single stack-system was used in the London area in Canada, -and a type of vertical wet-venting system is permitted by the new British Columbia Plumbing Code. As this system only permits two water closets to be wet vented it is much more restrictive than either the British or Philadelphia single-stack systems and has there-fore not been included in this comparison-,

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-The British and the Philadelphia systems were developed primarily fot- apartment buildings. They have been in lJ.se in their present form for many years and may thus be considered well established. The proposed Canadian system. on the ｯｾｨ･ｲ hand. is in its preliminary stage and much research remains to be done before it will be fully developed. The present version should be considered a first step in the development of the system and consequently it is rather conservativ.e.

Each of the three systems is described in slightly different terms. so that it has been necessary to convert some requirements to a common base. The various requirements have been tabulated in Table I as they apply to a 4-in. stack. Piping diagrams for a four -storey apartment

building with back-to-back bathrooms and kitchens on each floor are shown for each system in· Figure 1. These diagrams clearly show two thingst the propos ed Canadian vertical wet venting system is more restrictive.than

either the British or Philadelphia systems; and it is considerably cheaper to install than other Canadian and American systems.

Some of the differences in the terms used are clearly shown in the first two items in Table 1. There is a difference in the fixture unit ratings (Item 1), but thes e are compensated for ｾｮ the loading tables (Item 2).

Consequently. the really significant item. Item 3. is the limit placed on the maximum number of storeys that can be served. In the Canadian system the load capacity of a vertical wet vent has been arbitrarily and tentatively set at one-half the capacity of a stack of the same size that passes through more than three storeys (col. 3 of Table 7. 4. 11. B of the NBC). The

British system does not rely on loading tables for sizing the stack. but

instead provides a table showing the maximum number of storeys from which the 4-in. stack can receive the discharge from single - and double-dwelling groups and the supplementary venting that is required when the load exceeds 10 dwelling groups.

The 5-ft length limitation on the fixture drains (Item 4) makes it impossible to load a stack to the extent permitted by the load table. and practically impossible to use a single stack to handle all the drainage from two apartments. The British permit 7-ft 6-in. fixture drains. while

Philadelphia allows 12 -ft 0 -in. It would appear. therefore. that the 5 -ft limit in the Canadian code could and shol'lld be increased to allow a single stack to handle all the drainage from adjacent apartments.

The use of branches. covered in Item 5. need not be considered at present. but it should be investigated because it will undoubtedly facilitate some installations and provide more freedom in bathroom layout. The use of S traps, Item

6.

falls into the same category.

3

-There are naturally differences between the fixtures and pipe fitt.i.ngs used in North America and Great Britain. The experience gained in Philadelphia and Detroit and research work carried out in Washington and New York clearly shows, however. that the differences do not have a significant effect on systems of the size now allowed.

There: are also differences in the depth of trap seal used. In Great Britain WC's have a 2-in. seal and other traps a 3-in. seal. In Canada, Part 7 of the National Building Code specifies a minimum seal of 1-1/2 in., but traps are. in fact. usually manufactured with seals that range between 2 and 3 in. The same situation exists in the U. S. A. The common require-ment for all plumbing systems, however, is that the venting system should limit air pressures in the drainage system to the range of plus 1 or minus

1 in. of water. As research has shown that deep seal traps are no mor.e effective in controlling pressures than normal seal traps, the difference in depth of trap seals is irrelevant.

Problems associated with fitting at the base of the stack also seem to occur in systems with separate venting. The experience in Philadelphia

shows that for the light loads that occur on a four -storey stack there are no problems.

The Canadian system follows the British practice in limiting connections immediately downstream of a WC connection (Item 8). This is neces sary to control the use of special fittings since the limitation is not included in the fitting standards.

The Canadian system deviates from the other systems in that it requires the stack vent to be sized according to the regular tables (Item 9) rather than arbitrarily requiring'the same size as the vertical wet vent. When the stack vent passes through the roof it must still comply with the requirements for terminals. In many cases the vertical wet vent will be extended full size to the outside air. The vent sizing tables. however. allow a contractor to use a smaller size if he wishes.

Although the Canadian system does not now limit the distance between the lowest fixture and the building drain. a limit Of 2 ft 6 in. should be

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TABLE I

COMPARISON OF SYSTEM REQUIREMENTS

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(1) Fixture unit ratings, residential service Water closets (WC) (a) British 2-gal flush

(b) British 3-gal flush

(c) North American 3 1/2 to 5-gal flush Basin (B) ( lavatory)

Bathtub (BT) Kitchen sink (KS)

Bathroom. group WC, BT, B

Dwelling gr oup B T, KS, 1 or 2 B, 1 or 2 WC (a) with 2 -gal flush WC's

(b) with 3 -gal flush WC's *Estimated equivalent load

(2) Maximum load on 4-in. stack (fixture units) (a) under 50ft in height

(b) 50 to 120 f't in height (c) over 120 it in height

*Equivalent lim.it without the addition of a vent stack (3) Maximum. no. of storeys served by 4-in. stack

(a) when serving groups (see text) (b) when not serving water closets

(c) when serving an otherwise unvented branch with 4 WC's on a 4 in. branch or 4 basins on a 2 -in. branch each floor (peak use)

Canadian 4 1 1 1/2 1 1/2 6 NIA

12 1/2*

250 250 250 4 load tables not permitted British 7 10 1 6 6 14 17 170* 170* 170* 5 or 10 ? 4 Philadelphia 6 1 2 4 8 NIA 19* 225 75 36 5 load tables not perm.itted

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TABLE I (continued)

(4) Maximum length of fixture drains Canadian British Philadelphia

(a) fall must not exceed size Yes No No

(b) 1 1

14m.

size 5' 5'6"

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12'

(c) 1 1/2 in. size 5' 7'6" 12'

(d) with 104 degrees WC fitting NIA 5' NIA

(e) 4 in. for WC at

114

in.

1ft

5' ? 12'

(5) (a) maximum length of branch not not 4'

permitted permitted

(b) fixtures required to connect to stack yes yes no

( 6) S traps permitted no no yes

(7) Bend at base of stack

(a) 90 degree bend or std long std

radius

(b) 2 to 45 degree bends yes yes yes

(8) Downstream of a WC connection, other c onnec tions ar e

not permitted within ( ) in. 8 8 no limit

(9 ) Size of stack vent vent stack stack

tables size si:z;e

(lO) Distance between lowest fixtures on stack and building drain

(a) stack height up to 2 storeys no 2'6" 1

limit storey

(b) stacks over 3 storeys

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A.Typical American System _{B.Philadelphia Single Stack System} FIGURE 1 DRAINAGE SYSTEMS FOR LOW-.RISE APARTMENTS

(A. and B. from "Philadelphia's Codes Allow Single Stacking, " by George Rochford. Southern Building, March 1973)

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C. Typical Canadian System D. Canadian Vertical Wet ｖ･ｮｴｾｮｧ

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E.British Single Stack System