NAT'L INST OF STAND & TECH

A111D7 ElDfiSD


National Bureau of Standards JUL 2 8 1953

8IG5I

TA4I0

.U48
National Bureau of Standards

Library, N. W. Bldg.

AUG 1 9 1952

Wet Venting of Plumbing Fixtures

United States Department of Commerce

National Bureau of Standards

Building Materials and Structures Report BMS119
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UNITED STATES DEPARTiMENT OF COMMERCE • Cliaiics Sawyer, i^icrelary NATIONAL BUREAU OF STANDARDS • F. U. Condon /Jimlor

Wet Venting of Plumbing Fixtures

by John L. French, Herbert N. Eaton, and Robert S. Wyly

Building Materials and Structures Report BMS119

Issued December 1, 1950

For sale by the Superintendent of Documents, U. S. Government Printing Office. Washington 25, D. C. Price 20 cents
Foreword

This report on the wet venting of plumbing fixtures is the second of a

series of three National Bureau of Standards reports giving the results of an

investigation of the problems involved in the proper venting of single plumbing fixtures or a relatively small group of such fixtures, the first report being

BMS118, Stack V'enting of Plumbing Fixtures, published in January 1950.

This investigation was sponsored by the Housing and Home Finance Agency

as a part of the continued efforts of that Agency to secure rational and economical design of housing.

A fixture is said to be wet vented when its vent serves also to carry the dis- charge from fixtures connecting into the drainage system at a higher level. The use of wet venting reduces the number of individual vent pipes required

hj a plumbing drainage system as contrasted with the number required by the

conventional back-vented system and hence reduces the cost of the venting

system. This fact has led to an increasing tendency among code-writing

authorities to permit the wet venting of plurhbing fixtures under certain cir- cumstances. One of the objects of this investigation was to determine under

what circumstances it is legitimate to use this form of venting in order to afford a sound basis for code provisions relating to this matter. Drainage pipes of various materials are referred to in this report as forming parts of one or more of the test systems constructed for use in the investigation. These materials were used merely because of their availability at the

time when the systems were constructed, and the fact that they were so used

should not be construed as constituting an approval or recommendation of

those materials in preference to any other matei'ials that might have been used.

The research forming the basis for this report was first undertaken for the

National Housing Agency in connection with the Veterans Emergency Housing Program of that Agency and was continued and completed under the

Housing Research Program of the Office of the Administrator, Housing and

Home Finance Agency, as part of the research program of that Agency under

its statutor}^ authority.

E. U. Condon, Director.
Wet Venting of Plumbing Fixtures

This report gives the results of laboratory tests of various wet-vented single and twostory plumbing drainage systems. A suggested test or design loading for such systems is developed, and a permissible trap-seal loss for the fixtures of wet-vented systems is proposed.

Conclusions regarding the linfits under which wet-vented fixtures located on the upper floor of a system will operate satisfactorily are given in a form sufficiently simple for inclu.sion in

John L. French, Herbert N. Eaton, and Robert S. Wyly

plumbing codes.

CONTENTS

Page

Foreword II

I. Introduction 2

II. Scope of the investigation 2

III. Previous investigations of the performance of wet-vented fixtures 3

IV. Nature of the phenomena involved in wet venting 4

\. Tests on a single-story wet-vented system with lavatory and combination fixtures connected to the wet vent 5

1. Description of the test system 5

2. Test procedure 7

3. Test results 8

a. Tub not discharged 8

b. Tub discharged 11

4. Interpretation of test results 12

a. Loads on the drainage system from a single group of bathroom fixtures and a combination fixture 13

b. Permissible trap-seal losses 16

c. Permissibility of venting a bathtub trap with a wet vent to which a

lavatory and a combination fixture are connected 16

d. Summary 17

VI. Factors affecting the performance of wet-vented systems 17

1. Length of horizontal branch 17

2. Slope of horizontal branch 18

3. Diameter of horizontal branch . 18

4. Tub drain length and slope 19

5. Trap dimensions - 20

6. Effect on the trap-seal losses of a wet-vented bathtub of increasing the diameters of the lavatory and combination fixture drains 21

7. Diameter of bathtub drain 23

VII. Tests on a single-story wet-vented system with only a lavatory connected to the wet vent 24

VIII. Tests on two-story, duplex, wet-vented system 25 IX. Wet-vented fixtures on the lower floors of multi-story buildings 27 X. Conclusions 27

XI. References 27

1
I. Introduction

Plumbing fixtures ordinarily discharge into the

building drainage system through traps located

between the fixture and the fixture drain. The

function of a fixture trap (see fig. 3) is to provide

a water seal between the drainage system and the

interior of the building in order that sewer gas in

objectionable quantities will not flow from the

drainage s^ystem into the building. The discharge

of fixtures creates pressure fluctuations in the

drainage system, which, if excessive, wifl reduce

the water seals in the fixture traps and hence may

render them unable to perform their function. In

order to control the pressures in the drauiage

system so that the water seals of fixture traps will not be reduced excessively, vent pipes connecting

various points hi the drainage system with the

atmosphere are ordinarily installed. The term

pressure as used in this report refers to pressures

either above or below atmospheric pressure. The pressures in a draiaage system that may

afl'ect the water seals of fixture traps adversely

may be classified conveniently into two groups

pressures caused by the discharge of the fixtures

connected to the fixture trap in question and

pressures caused by the discharge of other fixtures

in the system. The trap-seal losses caused by

the first group are said to be due to self-siphonage,

and, in order to control the trap-seal losses due to

this cause, it is necessary to have the vent located

relatively near the fixture trap. For this reason

it is customary to provide some type of vent adequate for the protection of each fixture trap of a plumbing drainage system. However, in this connection the argument has been advanced that

adequate protection of a fixture trap b}^ a vent

does not necessarily imply that each fixture trap

requires a separate vent, and it has been suggested

that various tA^es of group venting, in which a

single vent serves more than one fixture, provide

adequate protection to fixture traps and provide an acceptable method of decreasing the cost of the

plumbing system.

The venting of groups of fixtures by a single

vent may take two basic forms. These are shown

in figure 1. In the arrangement of figure 1, A,

two fixtures connected at the same level to a vertical drain use the same vent. The vent in this

case carries only air and is sometimes termed a

dual or common vent. In figure 1, B, the upper

fixture is vented m the conventional manner. Its vent carries only air and is the only type of vent

permitted by many plumbing codes. However,

the lower fixture in figure 1, B, does not have an

individual vent that carries only air. Its vent is frequently termed a wet vent because it carries the

discharge of the upper fixture, as well as whatever

- DU»L VENT

A B FiGUEE 1. Basic group-venting arrangements.

air may travel along the pipe to relieve pressures

at the lower fixture drain.

The use of wet vents has the general effect of

reducing the number of individual vent pipes

required by the plumbing drainage system as compared with the conventional back-vented system,

and for this reason results in a reduction in cost

of the venting system. Because of the possible

economies involved, there has been an increasing

tendency among code-wiiting authorities to permit

the wet venting of plumbing fixtures under certain

circumstances. However, there has been no unanimity among these authorities as to the adequacy

of wet venting, and many codes prohibit the use of wet vents altogether. For this reason the Uniform Plumbing Code Committee believed, in view

of the economic advantages involved, that an

experimental investigation of the merits of wet

venting of plumbing fixtures was fully justified,

and the Housing and Home Finance Agency sponsored such an investigation at the National Bureau

of Standards. The purpose of this report is to present the results of that investigation.

II. Scope of Investigation

A great many variations in the basic wet-vented

arrangement of figure 1, B, are possible. Some

arrangements that are permitted by various

plumbing codes are shown in figure 2. In this connection, it will be observed that the arrangement shown in figure 2, F, is of the type commonly

referred to as a stack-vented installation. Current

terminology in the plumbing industry loosely

describes the group of fixtures in figure 2, F, as

stack-vented, while in reality onty the topmost

fixture is stack-vented, and the thi-ee lower fixtures are wet-vented in the sense that the vent to the

lower fixtures —in this case the stack —also carries

waste from the upper fixtures. A report on stack

venting has already been published \ and hence

it will not be considered further here.

' Figures in brackets refer to references at the end of this report.

2 Wet Venting of Plmnhing Fixtures
The rosults of laboratory tests on installations

containing the type of wet-venting shown in figures 2, A, 2, B, and 2, E, will be presented in

this report.

COMBINATION FIXTURE

D E F Figure 2. Wet-venting arrangements.

Since the function of the venting system is to prevent the occurrence of excessive seal losses

in the fixture traps of the system, the efficiency

of a venting system can be measured in terms of the trap-seal losses caused by the discharge of a

particular group of fixtures. In this connection

a trap-seal loss of 1 inch is sometimes [1, 2] considered the dividing line between satisfactory and

unsatisfactory performance. Although this cri- terion of satisfactory trap performance, which has

been based solely on personal judgment, is arbitrary and is undoubtedly overly conservative in

all installations in which the stack, building drain,

and venting systems are not loaded to capacity, it

is nevertheless adequate for the purpose of this investigation and has been used throughout this

report as tlic line of (Iciiiiii-ciU ion bclwccii satis- factory and unsatisfactory trap pcrformarH-c.

The specific problem of this investigation tlius becomes one of determining the limitations on the

lengths, slopes, and diameters of the drains in the

installations of figures 2, A, 2, B, and 2, E, re- quired to prevent the o(;currenc(^ of ti'ajj-seal losses in excess of 1 inch, when a reasonable test loading consisting of the simultaneous discharge of a particular group of fixtures is ap])lied lo I lie system.

Although wet-vented fixtures may be used satis- factorily under certain conditions on all floors of a plumbing drainage system, the experimental in- vestigation reported here was confined to wetvented fixtures on the top floor of a building.

The terms trap weir, trap-seal loss, and depth

of trap seal will be used frequently in this report,

and their significance is shown in figure 3. TRAP WEIR

Figure 3. Definition sketch of trap.

ft=trap-.seal loss. i=trap seal.

III. Previous Investigations of the Performance of Wet-Vented Fixtures

In 1924 Roy B. Hunter [2] reported in detail the

results of tests with certain types of wet-vented

systems. Tests with the fixture arrangement

shown in figure 4, A, located on the first fioor of a two-story system and with a 1 /4-inch-diameter

wet vent led Hunter to draw the conclusions (1) that each small waste connecting independently

to the stack must be vented separately, (2) that

considering the slight probability of all the fixtures

of the group above discharging at the same time as the basin or bathtub in the lower group, the water

closet may be left w^ithout a separate vent with

comparative safety, and (3) that with heavier

discharges from above than described the water

closet should be separately vented.

Hunter's tests of the same arrangement on the

top floor resulted in no seal losses in any of the

fixture traps, and he concluded that the wet venting of a bathtub by the drain from a lavatory

such as shown in figiire 4, A, was also satisfactory on the top floor of a building.

Hunter also investigated the problem of wet

venting on the upper floor as applied to two

bathrooms back-to-back, with the vertical dram

of the two lavatories serving as a wet vent for the

two bathtubs. The test installation is illustrated

in figure 4, B. Hunter's conclusion was that,

for duplex installations of the type shown, the

wet vent was not sufficient to prevent a material

trap-seal reduction in one of the bath tub traps

when the other bathtub was discharged and that a back vent to the two tubs as shown in figure

4, B, was required to prevent this from occurring.

In this connection, it may he pointed out that,

Building Materials and Structures Report BMS119 3
SINK

A B

TOP OR SECOND FLOOR TOP FLOOR Figure 4. Hunter's test system.

since the observed trap-seal loss occurred when

neither of the lavatories was discharged, the

reduction in trap seal observed was not caused by

any lack of efficiency of the wet vent from the

lavatories. The specific test merely indicated

that the discharge of one bathtub into an unvented

vertical drain resulted in a reduction in pressure

in the vertical drain sufficient to cause a substantial seal loss in any trap connected to the

vertical section of the drain in question.

Hunter, in a later publication [3], although not

reporting t*st data, indicates that the wet venting

of bathroom fixtures back-to-back is satisfactory,

provided the bathtub draiiis between the wet vent

and the bathtub traps are laid on a uniform slope

and otherwise comply with the restrictions neces- sary to prevent self-siphon age.

It will be observed that Hunter's test mstallation shown in figure 4, A, is for all practical

purposes identical with figure 2, A. Experimental

data on the other two test mstallations of this investigation, figures 2, B, and 2, E, have not been

published heretofore, but wet-venting arrangements identical with either one or both of these

installations have been described in at least two

publications [3, 4] as representing safe practice.

IV. Nature of the Phenomena Involved in Wet Venting

The physical phenomena occurring in wet

venting are complicated, and the tests of this project were not sufficiently extensive, and

neither was it their primary purpose, to delmeate

clearly in detail the processes involved. Nevertheless, many of the tests were made with

transparent plastic tubing and fittings, and a general qualitative, although by no means complete, description of the functioning of a wet vent was made possible thereby.

In the tests made with transparent tubing on

the test installation shown ui figure 2, B, it was

observed that, if the flow through the wet vent was started before or at the end of the period of

tub discharge, the tub drain remained substantially

filled with water, and small quantities of air bubbled from the wet vent through the tub waste

and formed a pocket in the drain at the trap, with

the air-water interface forming, of course, a horizontal plane.

As discharge continued through the wet vent and

air bubbles continued to move up the tub drain

from the air pocket over the trap, a gradual lowering of the water surface occurred. At the end of the period of flow from the wet vent, a group of large air bubbles moved rapidly upstream toward

the trap, immediately relieving the vacuum over

the ti-ap weii' and allowing the water seal in the

trap to return to equilibrium.

If, prior to the end of flow through the wet vent,

sufficient air had been admitted to the tub drain

to I'educe the depth of water over the trap weir

to a small value or to zero, a larger trap-seal loss was obtained than if only a small pocket of air formed and the depth of water over the trap weij- was relatively large. This was due to the fact

that, if the depth of water over the trap weir was

relatively large at the time when flow through the

wet vent ceased, the trap was refilled, at least in

part, by flow from the tub drain into the tub trap,

and only a small, if any, trap-seal reduction

resulted. On the other hand, if the pocket of air was large enough so that the water level in the tub

drain was at or below the tub-trap weir, there was

not in general any flow of water from the drain

into the trap, and large trap-seal losses would

result under certain conditions. In these tests, in which the tub was discharged

while flow from the wet vent was occurring, the

amount of the tub trap-seal loss thus depended

upon the size of the aii- pocket m the tub drain and more speciflcally upon the height of the water

surface in the tub drain with respect to the trap

weir. The depth of water over the trap weir was

observed to depend on a number of factors, some

of them being relatively indefinite.

First, and as would be expected, the water

surface in the tub drain could be made to remain

above the trap weir with small or no resulting trapseal losses by making the tub drain sufficiently

short or by laying it on a sufficiently low slope. In

addition, the amount of air that bubbled back into

the tub drain was observed to be highly variable,

depending on a number of factors, such as the. rate and duration of the discharge through the wet vent,

and the diameters of the wet vent and the hoi'i- zontal branch. Figure 5 shows the. marked efl'ect on the trap-seal loss of the duration of flow through

the wet vent. The data in the figm-e were obtained

from tests made on the arrangement of figure 2, A,

with the horizontal bi'anch, tub drain, and wet

vent made of IJo-inch-diameter pipe. The tub

drain was 6 feet long and had a slope of Vi inch per

foot. In figure 5 the rapid increase of trap-seal losses with increase in the duration of flow through the

4 Wet Venting of Plutnb/ng Fixtiwes
0.8

0.6

0.4

0,2

(0

-UlXof-<n

v> -AVf RAG E 0 Fl VE F UNS

-a

-J

-nl

SEA

1 NC te: CON TINU OUS FL( W 0 F

< 10 GPM . TH 10U( H W ET V ENT

0 I Z 3 4 5 TIME FLOW CONTINUES THROUGH WET VENT

AFTER END OF TUB DISCHARGE - MINUTES

Figure 5. Effect of duration of flow in wet vent on tub

tiap-seal losses.

wet vent during the first minute of flow reflects the growth of the air pocket and the consequent

lowering of the water sm-face in the tub drain with

respect to the tub-trap weir and the smaUer flow

of water into the trap from the drain when flow

through the wet vent ceases. The comparatively

flat portion of the cmwe above a duration of approximately 1 minute reflects the fact that

sufficient air had been admitted to the tub drain

from the wet vent to lower the water surface in the

drain to a level at or below the level of the trad

weir, so that little or no reflU of the trap from the

drain was occurring.

The physical phenomena occurring when flow

through the wet vent is taking place at the end of the period of discharge from the tub are fm-ther

complicated by the fact that under certain circumstances air is drawn into the tub drain through the

tub trap. This may occiii' either at llic end of tlic period of flow from the tub or at tlie end of the

flow from the wet vent. The flow of air thi'ough

the tub trap at the end of the period of discluugc

of th(> tub appears to be princi])ally (hie to the

entrainment of air by the flowing water during

the formation of the vortex wliicl) occurs in the

tub when the depth of water over the outlet orifice has become sufficiently small, but it is also un- doubtedly due partly at least to the inertia forces

in the tub drain operating at the end of the period

of tub flow and causing the water surface, in the

down leg of the trap to recede below the dip of the

trap, with consequent flow of air through tlx* tub

trap and into the drain. The air drawn thi'ough

the tub trap in this manner has the eft'ect, of course, of increasing the size of the air pocket over

the trap and hence of decreasing the depth of water

over the trap weir. In this way it may cause

increased trap-seal losses. From the foregoing description of the process

by which a wet-vented tub trap loses a portion of

its seal when the tub and a fixture on the wet vent

are discharged, it is apparent that the phenomenon

consists of such a complex flow of air and water

that no simple analysis of the complete prot)lem is possible.

The trap of a wet-vented fixture may lose trap

seal by the discharge of one or more fixtures on

the wet vent alone, as well as when the wetvented fixture is discharged in conjunction with

flow in the wet vent. Indeed, for sufficiently

short tub-drain lengths and sufficient!}^ low drain

slopes, and for small wet-vent diameters, trap-seal

losses will be materially greater for certain fixture

discharges through the wet vent when the tub is not discharged than when it is discharged.

Apparently trap-seal reduction of the tub trap,

for loadings not inchidmg the discharge of the tub.

is a process of entramment or aspiration of air from the tub drain by water flowing through the

wet vent, which in turn, of course, causes a partial vacuum to be formed in the tub drain with conse- quent loss of trap seal.

V. Tests on a Single- Story Wet-Vented System With Lavatory and Combination

Fixture Connected to the Wet Vent

1. Description of the Test System

The system tested in this phase of the investigation is shown in figure 6. The 3-inch-diameter

vertical stack was made of transparent methacrylate plastic tubing and fittings. Some of the

plastic fittings used are shown in figure 7. The

internal dimensions of these fittings were made

identical, to within a small tolerance, with those

of the correspondmg soil-pipe fittings. The use

of these fittings, together with transparent stack

and drains, was indispensable in obtauiing an

insight into the complicated physical phenomena

occurring in the wet-vented systems. Although

many tests were made with the transparent traps,

drains, and fittmgs, the large majority were made

with the wet vent, fixture drains, and horizontal

branch of conventional metal pipe and fittings.

The building sewer was made of 4-inch-diameter

fibre conduit laid on a slope of 's inch per foot and was connected to an 8-inch-diameter clay-tile street sewer. The rate of flow in the street sewer could

be varied up to 300 gallons per minute.

The types of buiicling-drain materials used in these tests, consistmg of methacrylate tubing,

fibre conduit, clay tile, and metal pipe, were

selected only because of their availability to the

laboratory, except in the case of the methacrylate

Bidldiiig Materials and Structures Report BMSIW
3-INCH DIAMETER STACK

I— COMBINATION FIXTURE

J 2 AND 2-INCH DIAMETER WET VENT

AND FIXTURE DRAINS

BATHTUB

2- INCH DIAMETER HORIZONTAL BRANCH

3- INCH DIAMETER TRANSPARENT TUBING

8- INCH DIAMETER STREET SEWER

4-INCH DIAMETER FIBER CONDUIT

1) zz_ o

70-0"

Figure 6. Single-story wet-vented system.

Figure 7. Typical drainage fittings made of transparent

plastic material. Upper left m-inch sanitary tee. Upper right 1 H- by 2-inch sanitary tee. Lower left IJ^- by 3-inch-long-tiirn T-Y. Lower right 2-inch-long-turn T-Y.

tubing, which was selected because such tubing is transparent. The purpose of the tests reported

here was not to investigate the possible application

or the relative merits of any of these materials

for use in the plumbing drainage system.

The fixtures were of current manufacture and

were selected to give a loading on the test system

that would be representative of those found on

similar systems in service. The lavatory used was 20 by 24 inches in size with a l}4-inch-diameter

outlet orifice. The lavatory was connected to the

wet vent by a 1^4-inch-diameter trap and drain.

The water closet on the test system was of the

tank-supply, siphon-jet type, with a volume of

discharge of 8 gallons. The bathtubs were of standard design, with both l%- and 2-inch-diameter

outlet orifices. The combination fixture used on the system was the typical fixture with sink and laundry-tray

compartments. The trap was located directly

under the outlet orifice of the sink compartment.

The tray compartment was 17K inches wide by

18% inches long by 13 inches deep. The sink

compartment was of the same dimensions except

that the depth was 8 inches. The widths and

lengths are for the top of the combination fixture. The bottom dimensions of both the sink and tray

6 Wet Venting of Plumhing Fixtures
compartments were slightly smaller. The tray

compartment was eqnipped with a metal drain

plug with a cross-bar strainer and rubber stopper.

The diameter of the tray outlet orifice was V%2

inches. The sink compartment was equipped

with the removable basket-type strainer shown

in figure 8. The average rates of flow from the fixtures usetl in the investigation are shown in table 1. The

rates of flow given are average rates obtained by

measuring the volume of water in the fixture and

then observing the time required for the fixture to empty. In the case of the combined rate of

discharge from the sink and tray compartments

of the combination fixture, the problem was

REMOVABLE BASKET STRAINER

18- ^"x^" SLOTTED HOLES IN STRAINER

slightly moi'e complicated, inasiiiucli as ilic sini-c compartment empties more quickly than the tray

compartment. In this cast' both compartments

were filled with water, and the time required for the sink compartment to empty was noted. At

th(> end of the pei iod of discharge from the sink,

the height of the water level in the tray compartment was obs(>i'ved. In this manner the total volume of water discharged fi'om the combiiitition

fixture during the period of flow from the sink was obtained, and the average rate of flow was,

of course, this volume divided by the time required

for the sink compartment to empty.

Table 1. Average rata of flow of fixtures used in wet-venting

tests

Fixture

Tray_

Sink..

Lavatory Water closet

.

Bathtub

Sinlv and tray.

Sink, tray, and

lavatory.

Special conditions

'Grid strainer Basket strainer in. Basket strainer

. removed

IVi-in. trap and

waste.

Grid strainer Basket strainer in. Basket strainer removed

Grid strainer Basket strainer in. Basket strainer removed--

Average rate of flow

Plastic system.

IM-in. tub and sink wastes

gpm

2.3.1

33.8

10.6

26.

12. 5

37.1

Plastic system.

2-in. tub and sink wastes

gpm

23.0

40.2

10.3

26.1 23.7

4.3.8

Metal

system.

VA-'m. tub and sink wastes

gpm

23.2

25.2

18.0

32.0

K). 6

26.

12.9

.34.2

31.

3.5. 0 44.8

41.7

SECTION A-A

45-r? HOLES IN BOTTOM OF STRAINER

Figure 8. Removable basket-type strainer.

The rates of flow from the sink and tray varied

appreciably, depending on the length and slope of

the fixture drain. The rates shown are for a drain 16 inches long on a /4-inch-per-foot slope.

The rate of discharge for the tub given in table 1 is for an initial depth of water of 6 inches.

2. Test Procedure

The tests were made by discharging certain of

the fixtures connected to the system and then

observing the trap-seal losses or other pertinent

behavior of all the traps. For reasons to be pre- sented shortly, the flow in the wet vent was

obtained by two methods: first by filling the fix- tures connected to the wet vent and then pulling

their plugs, and second by allowing water to flow

into the fixtures continuously from their faucets,

with the fixture plugs removed. For convenience

the first method of fixture discharge, in which the

fixture was filled and the plug then removed, will be termed throughout the remainder of this report

as plug discharge.

In the plug discharge of the lavatory, the fixture was filled to the overflow level. In the case of

Building Matenals and Structures Report BMS119

904427—50- - -2
the sink and tray compartments, the fixtures were

filled to a level approximately one inch below the

rim. The tub was filled to a depth of six inches

in those tests requiring the discharge of this fixture. In the preliminary tests it was found, for those

test loadings which included the discharge of the

bathtub, that the maximum tub trap-seal losses were obtained when the fixtures on the wet vent were discharged from 3 to 5 seconds prior to the

end of flow from the tub, and this sequence of fixture discharge was used in all subsequent tests.

3. Test Results

In the test system shown in figure 6 it will be

noted that the watercloset is stack-vented and

the lavatory and combination fixture are backvented, and that these fixtures can be subject only

to trap-seal losses attributable to self-siphonage

effects or to the failure of the stack and back vents

to perform their function properly. Only the

bathtub trap of the system shown in figure 6 is wet-vented, and the tests applied to this system

were designed to make apparent the conditions

under which the wet vent protected the bathtub

trap adequately from excessive seal losses. It is convenient to divide the tests applied to the system into two groups—those in which the

tub was discharged as well as other fixtures, and

those in which the tub was not discharged. The

test results of these two general test groups will be

presented separately.

a. Tub Not Discharged

In tables 2 and 3 are given test data on the sys- tem shown in figure 6 for the discharge of various

combinations of the fixtures on the wet vent. The

data in table 2 were obtained from tests made

with the cast brass trap shown in figure 9 on the

bathtub, and table 3 includes similar data obtained with the tubing trap shown in figure 10. The tabulated trap-seal losses in these tables

represent the losses observed in 10 consecutive

test runs made under identical conditions.

It will be observed from these tables that in general, for the higher rates of flow, greater seal losses are produced in the tub trap when the lavatory and combination fixture are connected to the

wet vent by a short-turn fitting than when this connection is made by means of a long-turn fitting.

On the other hand, it will be observed also that in

general, for the lower rates of flow, the reverse is true and that the difference in trap-seal losses obtained with the two flttings is not substantial.

It is concluded, therefore, that, while the type of

fitting connecting the fixtures to the wet vent

apparently exerts some influence on the wetventing phenomena, the effect is not marked and

can be considered to have no practical importance.

Figure 9. l]2-i?tch cast brass trap.

Figure 10. iy>-irich drawn brass tubing trap.

At low rates of flow it is reasonable to expect

that the wet vent will not be completely fille.d at any point and that consequently it will be able to perform its venting function fairly well when either

type of drainage fitting is used. Probably the

greater vertical component of the entrance velocity

into the wet vent occurring with the use of a long- turn fitting will tend to create a greater aspirating

effect when the flow enters the horizontal branch,

thus possibly causing trap-seal losses to be slightly

greater than if a short-turn fitting were used for low rates of flow. When the rate of flow into the

wet vent is large, it appears that the use of a short- turn fitting results in more or less complete filling of

8 Wet Ven ting of Plumbing Fixtures
Table 2. Tub Irup-scal losses—cast Irap—liil) druiii on '/>-inch-ij(:r-fool. slope —litl/ not dtscharf/ed—plug discharge

Fixtures disc'harjjod

Sink (basket strainei- in normal position)

Sink (basket strainer out)

Sink, lavatory (basket stvain<'r in normal position)

Sink, lavatory (basket strainei- out). _

Sink, liay, lavatory (basket strainer in normal position)

Sink, tray, lavatory (basket strainer out) _

Type of fittinR con- necting lavatory and

combination fixlure

to wet vent

Long turn. ..do Short turn

.. do Long turn. .do.....

Short turn

.. do Long turn. ...do Short turn

.. do Long turn.

. do

Short turn

...do Long turn. ..do Short turn

.. do Long turn

.

. do

Short turn

...do

Rate of flow through wet vein

\\ el \ en t diameler

Tub trap-seal loss

Till) drain

4 ft long Tub drain

5 ft long Tub drain6 ft long

max avg max avg max avg

(linn in. in. in. in. in. in. 18 0 0. 50 0. 38 0. .50 II. 13 18 0 0 II 18 0 0.38 0. 33 . 38 30 . 25 . 20

18 0

0£. 0 0 0 II 32 0 134 . 50 50 . .50 . 35

'.\2 0 2 0

0' 0 0

:!2 0 IM . 50 . 45 ..38 35 . 25 . 23 32 Q 2 . 0

0' 0 0 28 fi . 38 . 38 .38 38 .50 . 43

28 6 2 0 0 28 6 1X2 fi3 43 50 38 38 2.5 28 R 2

0' 0' 0" 0'

42. 6 1X2 1.25 .82 I. 00 93 . 75 .07

42 fi 2 0

0'

1. .12 .02

42, B 1X2 1.75 1.25 1.75 40 1.62 1. 19 42 6 2 0 0 .25 .08

41 7 .88

"".'75

.75 70 . 75 .70

41 7 2 0 0

41 7 ""i.'56" '"i.'20 1.25 "i 02 .75 .1)5 41 7 2 0 0 . 12 .02

45 6 m ""i.'88 ""i.'52 1. 12 1 05 1.00 . 85

45 6 2 0 0 .25 . 15

45. 6 IK """l.88 i.48 1.75 1.57 1. 25 1.00

45 6 2 .25 12 .38 ..32

Table 3. Tub trap-seal losses—tubing trap—tub drain on yi-inch-per-foot slope—tub not discharged—plug discharge

Fixtures discharged

Sink (basket strainer in normal position)

Sink (basket strainer out)...

Sink, lavatory (basket strainer in normal position)

Sink, lavatory (basket strainer out)

Sink, tray, lavatory (basket strainer in normal position)

Sink, tray, lavatory (basket strainer out)

Type of fitting con- necting lavatory and

combination fixture to wet vent

) Long turn, -.do

Short turn

...do (Long turn.

! .- do

1 Short turn

1 ...do Long turn. ....do Short turn

.--.do Long turn. ...do Short turn

, .. do (Long turn. ...do Short turn

....do Long turn. -...do Short turn

..-.do

Rate cf flow through wet vent

gpm

18.

18.

18. 18.

.32. 32.

32.

32.

28.

28.

28.

28.

42.

42.

42.

42.

41. 41.

41.

41.

45. 45.

45.

45.

Wet vent diameter

IK

2IK

2IK

2IK

2IK

2IK

2IK

2IK

2IKIK

2IK

2IK

2

Tub trap-seal loss

Tub drain

4 ft long

0. 38

.25

01.00

0. 33

".'22'

.65

Tub drain

5 ft long

0. 38

.50

1. 38

0. 25

.43

.38

1.18

1.05

Tub drain

6 ft long

vn. 0.50

0

. 12

0.38

0

.50 0

.38

0.25

0

.88

0

1.12

0

.75

0

1.00

01.00

01.12

0

in.0.35

0

. 12

0.23

01.33

0.28

0

.17

0

.57

0

.85

0

.57

0

. 75 0

. 78

01.00 0

the wet vent at the entrance point, whereas this condition exists to a lesser extent when a longturn fitting is used. Hence it might be expected

that the wet vent would not function as effectively

with the short-turn fitting during periods of high

discharge, possibly resulting in slightly greater

trap-seal losses in the wet-vented fixture trap

when a short-turn fitting is used on the wet vent. The slope of the bathtub drains on. the test set- ups from which the data in tables 2 and 3 were

obtained was }^ inch per foot. In table 4 the

data for tub drains on a l4-incli slope are comparet

with similar data for a ,V>-inch slope from table 3. It will be observed that the ,'2-inch slope yields in

general slightly larger maximum trap-seal losses than does the )4-inch slope and appi ecial)ly greater

average trap-seal losses. As is noted in table 4, the length of the tub

drain used was 6 feet. For reasons to be dis

cussed later, it would be expected that the differ- ence between trap-seal losses for the ^^-inch and

)4-inch slopes would increase as the length of

Building Materials and Structures Report BMS119 9
Table 4. Effect of slope of bathtub drain^ on the trap-seal

losses of a wet-vented bathtub

[plug discharge]

Fixtures discharged Rate of discharge

Tub trap-seal losses

H-inch slope J^-inch slope

max avg max avg

Sink and lavatory with basket

strainer in normal position - . Sink and lavatory with basket

strainer out. . Sink, tray, and lavatory with basket

strainer in normal position

gpm

28.6

42.6

41. 7

in.

0. 38

1.00

.88

in.

0. 22

.37

. 50

in.

0. 25

1.12

1.00

in.

0.17

.85

.75

1 The length and diameter of the bathtub drain were 6 feet and IV2 inches'

respectively. The diameter of the wet vent was IH inches. The bathtub

trap used was the tubing trap shown in figure 10. Tlie combination fixture drain and the lavatory drain were connected to the wet vent by means of a double short-turn drainage fitting.

drain is decreased and would decrease as the

length of the drain is increased. It may be con- cluded from the data in table 4, nevertheless,

that the trap-seal losses given in tables 2 and 3

will not be exceeded in general, for the lengths

of drains tested, by those obtained with tub

drain slopes of less than % inch per foot. A close examination of the data in tables 2 and

3 will indicate (1) that the trap-seal losses for the tub drain 6 feet long are in general somewhat

less than those for the drain 5 feet long, and (2)

that the trap-seal losses for the 4-foot-long drain

are substantially equal to those for the 5-foot-long

drain, with a tendency for them to be slightly

less. In figure 11 the maximum trap-seal losses

for the drains 4 and 5 feet long, with the short- turn fitting, have been plotted. From this figure

and the data in tables 2, 3, and 4, it is apparent

that (1) tub trap-seal losses increase as the volume

rate of flow through the wet vent is increased,

(2) the tubing trap and the east trap yield substantially the same trap-seal losses when the

losses are appreciably under 1 inch, whereas the

cast trap yields the greater seal losses when the

losses are in the neighborhood of 1 inch or greater.

In this connection it is obvious, of course, that

the difference in behavior of the two traps is attributable to differences in the internal dimensions of the two traps (3) substantially greater

trap-seal losses are produced by the l}^-inchdiameter wet vent than by the 2-inch diameter

wet vent, (4) tub trap-seal losses do not increase

indefinitely with increase in length of tub drain,

but, on the contrary, appear to decrease after a

length of 5 feet on a slope of ji inch per foot is exceeded, and (5) the tub trap-seal losses shown

in tables 2 and 3 will not be exceeded by those

obtained with drains of the same length but on

a slope of less than ^2 inch per foot. In view of conclusions 4 and 5, it is evident

that the data in figure 11 will serve to indicate

the maximrun trap-seal losses to be expected

from any installation having a tub drain 4 feet

or more in length and laid on a slope of ); inch

per foot or less. As will be shown later, it would

not be expected that a decrease in the length below 4 feet or an increase in slope above % inch

per foot would in any way cause increased trapseal losses over those indicated in tables 2 and 3 and figure 11. It will be observed in tables 2 and 3 that the

trap-seal losses for several possible combinations

of fixture discharges have not been listed. In

particular, the trap-seal losses for the discharge

of the lavatory, tray, or water closet by themselves,

or the discharge of the water closet in any com20 25 30 35 40 45

FLOW THROUGH WET VENT- 0PM. Figure 11. Trap-seal losses of wet-vented bathtubs—tub not discharged.

bination with the other fixtures have not been

listed. On none of the tests was a trap-seal loss caused

by the discharge of the lavatory alone. Preliminary tests showed that the losses resulting from

the discharge of the ti-ay either alone or with the

sink were not significant!}' different from the

results obtained by the discharge of the sink, and

the trap-seal losses from these fixture discharges

may be approximated with sufficient accuracy

from figure 11 by using the discharge rates in

table 1. Preliminary tests were also made to determine

the effect on tub trap-seal losses of the discharge

of the water closet in combination with other

fixtures on the system. These data are given in table 5. It will be noted that, for the 2-inch-diameter wet vent, the simultaneous discharge of

sink, tray, and lavatory gave practically the same

trap-seal loss as occurred when the water closet was also discharged. It is apparent, therefore,

that the test loading need not include the water

closet when a 2-inch diameter wet vent is used.

In the case of the l}^-inch-diameter wet vent,

however, it is obvious from table 5 that the dis- charge of the water closet causes increased trapseal losses. It will be observed, nevertheless,

that the tub trap-seal loss obtained with the dis- charge of the sink, tray, and water closet is less than that obtained with the dischai'ge of the

10 Wet Venting of Plumhing Fixtures
sink, tray, and lavatory, and hence it may be

concluded that, for a given number of fixtures

in the test loading, a greater trap-seal reduction

is obtained when the fixtures of the test loading

consist only of those on the wet vent rather than

including the water closet.

Table 5. Effect of adding water closet to test loading—tub

not discharged^

Tub trap-seal reduction

Fixtures discharged

l).2-inch-diameter wet vent2-inch-diameter wet vent

Maxi- mum of

5 tests

Average

of 5 tests

Maxi- mum of 5 tests

Average

of 5 tests

Sink, lavatory - Sink, tray, lavatory

Sink, tray, water closet . ...

in.

1. 12

1. 12 .75 1.38

in.

0. 85 1.00

.60 1.20

in.0.50

.62

in.0..35 .60

Sink, tray, lavatory, water closet. .62 .55

' In these tests the length, diameter, and slope of the bathtub drain were

6 feet, 1 V-i inches, and K' inch per foot, respectively. The fitting connecting

the lavatory and combination fixture drains to the wet vent was a double

H.2-inch sanitary tee. The bathtub trap used was the tubing trap shown in

figure 10. The basket-type strainer of the sink compartment of the combination fixture was removed.

For this reason, and since, as will be shown

presently, a safe and conservative test or design

loading consists of the discharge of any two, or at most three, of the five fixtures on the system, the

great majority of the tests were made with the

discharge of various combinations of the three

fixtures on the wet vent and did not include the

discharge of the water closet. As has been stated previously, the foregoing

test results are for loadings which do not include

the discharge of the bathtub.

In the following section the results of tests in

which the bathtub was among the fixtures dis- charged will be given.

b. Tub Discharged

In tables 6 and 7 are given experimental data

for test loadings that included the discharge of the

bathtub. The fixtures on the wet vent were dis- charged 3 to 5 seconds prior to the end of tub flow. The piping arrangement was as shown in figure 6. It is apparent from tables 6 and 7 that trap-seal

losses of the wet-vented tub, for the loading now

under consideration, increase rapidly with an in- crease in the length of the tub drain.

The data in tables 6 and 7 are for a tub-drain

slope of ^2 inch per foot. Additional tests were made with a tub drain on a /4-inch-per-foot slope

and with a length of 6 feet. No trap-seal losses were obtained in these tests with any loading.

In view of this fact and the data in tables 6 and 7 for a tub drain of similar length but on a ji-inchper-foot slope it is apparent that the seal losses of a wet-vented tub trap increase rapidly with an

increase in slope.

The pi'ccise manner in w liich I hi' Icnglh nml

slope of a tub (h'ain afreet Ihe wel venting processes is not known. However, as has been staled

in section IV, it was observed in the tests with tli(! transpai'ent tub trap and (h'ain tiiat, immediately

after the flow of water from the tub c(!ased, the tub

drain was initially nearly full of water with ordy

a small air pocket in the drain adjacent to the tub

trap, and the air-water interface was, of course, horizontal. Under these circumstances it miglit

be expected that the principal effect of the length

and slope of the tub drain on the wet-venting process would be exerted through their effect on the

vertical drop in the tub drain between the ti'ap

weir and the wet vent; i. e., on the product SJ^,

where and ^3 are, respectively, the slope and

length of the tub drain, which might be expected

to be the controlling factor through which the

slope and length of the bathtub drain influence

the magnitude of the seal losses of a wet-vented

bathtub.

In figure 12 tub trap-seal losses for a constant

rate of flow of 15 gallons per minute through a

TEST SET-UP

COMBINATION

1 FIXTURE / , /A ^lo - IN. DOUBLE

N. DIAM.^C-^ SHORT TURN

TUB T UB Df AIN

<

THAP SLOPt a CAST 2 "*/ft • TUBING 2 '"/ft n f.»^T i

>

T USING . IN.//FT . LAV./ -A-li-IN. DIAM.

\ L —

J e-IN DIAM. /

- 5 X 2-IN. SANITARY TEE I

THESE DATA REPRESENT THE MAXIMUM

SEAL LOSS OBSERVED IN 5 TESTS MADE

UNDER IDENTICAL CONDITIONS

0 I 2 S 4

Sj £j- INCHES Figure 12. Tub trap-seal losses for a constant flow of 15 gpm through the wet vent.

1/2-incli wet vent have been plotted against the

factor Szls. It will be observed, for the range of tub drain lengths and slopes tested, that approximately at least, the effect of tub drain length and

slope can be considered to affect the trap-seal

losses of a wet-vented tub onl}' through their eft'ect on the factor S3I3. Data from tables 6 and 7 for the discharge of the

bathtub and all the fixtures on the wet vent have

been plotted against S3I3 in figm-e 13. Where the

trap-seal loss was greater for one type of fitting

connecting the lavatory and combination fixtm-e

drain to the wet vent than for the other, the larger

trap-seal loss has been plotted. The data in figure

13 are for the basket type strainer in its normal

position in the sink compartment of the combination fixture. Building Materials and Structures Report BMS119 11
2.0

V)UJXo

z

T 1.5

(/> o<

bJ(A

I <

sc. l-m

z>II 2-INCH OLA

.5 |_ WET VENT

3.0 CHES

Figure 13. Tub trap-seal losses for loading which includes

the discharge of tub.

Table 6. Tub trap-seal losses—cast trap—tub drain on

y2-inch-per-foot slope—tub discharged—maximum losses observed in 10 consecutive tests

Fixtures discharged

Sink (basket strainer in normal position).

Sink (basket strainer

out).

Sink and lavatory (bas- ket strainer in normal

position).

Sink and lavatory (bas- ket strainer out).

Sink, tray, and lavatory

(basket strainer in normal position).

Sink, tray, and lavatory

(basket strainer out). Tub

Type of fitting connecting lava

tory and combination fixture to wet vent

Rate of flow through wet

vent

gpm

18.0

18.0 18.0

18.0 32.0

32.0

32. 0

.32.0 28.6 28.6

28.6

28.6

42.6

42.6

42.6

42.6

41. 7 41.7

41. 7 41.7 45.6

45.6

45.6

45.6

0

Wetvent

diam-:

eter

2m2m2IM

2

2

2

2IH

2

2

21^

2

2m

Tub trap-seal losses

Tub

drain

4 feet long

0. 50

.62

.50

1.0

Tub

drain

5 feet long

m.

0. 62

0.50

0.50

. 25

0

. 50

'.'12' 0.50

0.62

. 75

. 25

Tub

drain

6 feet long

111. 1.00

. 50

.88

.62

1..38 01.25

.25 1.62

. 50

1.62 .50

1, 25

. 50

1.00

.38 1.62

01.00

. 50 1.00

.62 1.00

. 62 0

In table 8 are given data showing the effect of the addition of the water closet to the test loading when the bathtub is among the fixtures dis- charged. It is apparent that when the tub is discharged greater trap-seal losses are obtained for a test loading consisting of a given number of fixtures when all the fixtures discharged except the

tub consist of those connected to the wet vent and

do not include the water closet. For the conditions under which they were obtained, the data presented in the two foregoing

sections may be used to determine the adequacy

of venting a bathtub trap through a wet vent

carrying the discharge of a lavatory and combination fixture.

Table 7. Tub trap-seal losses—tubing trap—tub drain on

Yi-inch-per-foot slope—tub discharged—maximum losses observed in 10 consecutive tests

Fixtures discharged

Sink (basket strainer

in normal position).

Sink (basket strainer

out).

Sink and lavatory (bas- ket strainer in normal

position).

Sink and lavatory (bas- ket strainer out).

Sink, tray, and lava- tory (basket strainer

in normal position).

Sink, tray, and lava- tory (basket strainer

out). Tub

Type of fitting connecting lava- tory and combination fixture to wet vent

Rate of flow through wet

vent

gpm

18.0 18.0

18.0 18.0

.32.0

.32.0 32.0

32.0

28.6

28.6 28.6

28.6 42.6

42.6 42.6

42.6

41.7

41.7

41.7

41.7

45.6

45.6

45.6

45.6

0

Wetvent diameter

2

2

2

2iM

2

2

2lA

2lA

2lA

2lA

2lAlA

Tub trap-seal losses

Tub

drain

4 feet long

.38

.25

.50

"'.'56" 0.25

'".'25' 0

0

Tub

dram

5 feet long

.25

.38

Tub

drain

6 feet long

m.

0. 75

. 25

.25

. 25 .62

.25

. 50 .25

.75

.25

. 50

.50

.50

.25 1.00

.50 1.00

.38

.75 .50

.88

.38 1.00

.50 0

4. Interpretation of Test Results

In determining the adequacy of a drainage sys- tem with a particular type of venting system, it

is necessary first to determine by experiment the

trap-seal losses which occur in the system under

various loading conditions. This has been done

for a single-story, wet-vented system, and the

results are shown in figures 11 and 13, and in tables

6 and 7. It is also necessary to establish a criterion of satisfactory trap performance: that is, to establish a dividing line between trap-seal losses that

may be considered satisfactory and those that may

be considered sufficiently large to impair the ability

of the trap to prevent the entrance into the

dweUing of sewer gas in objectionable amounts.

Table 8. Effect of adding water closet to test loading—tub discharged

Fixtures discharged

Tub trap-seal reduction

1

1 2-in.-diam. wet

vent

2-in.-diam wet. vent

Maxi- mum of

6 runs

Average

of 5 runs

Maxi- mum if 5 runs

Average

of 5 runs

in. m. in. in. Sink and lavatory. _ 1.00 0.63 0.62 0.37 Sink, tray, and lavatory. 1.00 . 70 . 75 .45 Sink, trav, and water closet- ... .75 . 70 Sink, tray, lavatory, and water

1.50 1.20 .75 .30

12 Wet Tenting of PluniTjing Fivtufes
In these tests the U'ligth, diameter, and slope of the bathtub drain were 6 feet, II2 inches, and '2 hu'h per foot, respectively. The fitting- connecting

the lavatory and combination fixture drain to th(> wet vent was a double 1 '2-inch sanitary T. The

bathtub trap used was the tubing trap of figure 10. The basket-type strainer of the sink compartment

of the combination fixture was removed.

Finally, it is necessary to establish a criterion of what constitutes a reasonable test loading. That

is, it is necessary to select a portion of those fixtures on the system, the use of which can be assumed to occur simultaneously with reasonable frecpiency,

to serve as a guide in determining whether a trapseal loss caused by the discharge of a particular

group of fixtures is a sound basis for rejecting or accepting the wet venting of plumbing fixtures as a

satisfactory method of venting.

To summarize, figures 11 and 13 and tables 6 and 7 present trap-seal losses of a wet-vented

bathtub under certain conditions. In order to apply these data to the problem of determining

if wet venting of plumbing fixtures is permissible,

it is necessary to determine (1) what trap-seal

losses are permissible, and (2) what combinations

of fixture discharges are likely to occur simultaneously with reasonable frequency. The latter problem will be considered first .

a. Loads on the Drainage System From a Single Group

of Bathroom Fixtures and a Combination Fixture

Unfortunately there are few or no data available on which to base a decision as to what combination

or sequence of fixture discharges might be considered to constitute a reasonable design or test load

for a single-family plumbing drainage system. It might be argued that the laboratory test loading

for any single-family pliunbing system should consist of either the simultaneous discharge of all the

fixtures on the system or of that combination of fixture discharges which produces the most adverse

results. As Hunter pointed out in BH13 [5, page

160] "For lai'ge installations the matter of coincident discharge constituting a maximum fair test might be determined from tables of probable

coincident discharge . . . For small installations

consisting of two or three bathroom units and other

small fixtures the arbitrary method of selection

. . . adopted by the plumbing committee is believed more applicable for the types of buildings

under consideration." Here Hunter was referring

to one and two-story single and double-family

dwellings.

Of course, as he points out, the selection of a

lair test loading is arbitrary. Nevertheless, by

considering the probabilities of occurrences of various combinations of discharges of the fixtures

in small dwellings, it would seem that a logical

approach to the problem can be achieved. It should also be remembered that the dift'erent parts

of the system will have dift'erent design loads asso- ciated with them. For example, the design load

Building Materials and Stnictii res Report BMS119

for the building drain will be different llian the

design load U)v a wel vent in the system.

The cril crion of simultaneous dischai-ge of all I he

fixtures as a means of estal)lishing the design load

would obviously lead to safe and conservative

design of the drainag(> system. However, it is not

sufficient that tlie ])lurrd)ing di'ainage and venting

svstems be designed in such a manner that tliey

will operate satisfactorily. It is essential that, at the same time, they be designed so as to avoid

economic waste. It is a fundamental consideration

that the over-all cost be kept to a minimum, consistent with satisfactory operation of the system.

Our immediate problem, in its broader aspect is, as the heading of this section indicates, to determine what constitutes a reasonable design load for the drainage system imposed by a single group of bathroom fixtures and a combination fixture. In

its more restricted aspect it is to determine what

should be taken as the design load associated with

that part of the system which includes the wet

vent. Once this design load has been established,

then the diameter of the wet vent should be so chosen that no excessive reduction in trap seal will occur when this design load is imposed on the

system. We shall consider the more restricted

problem in what follows. The criterion of satis- factory performance will be the effect of the dis- charges of fixtures on the seal of this bathtub trap.

If we wish to use the probability of occurrence

of various combinations of discharges as the cri- terion for determining the design load, it will be

necessary to consider at some length the assumption that must be made—that the various fixtures are discharged completely at random. It is obvious

that this assumption does not fit the facts perfectly. The kitchen sink would undoubtedly be

used most frequently before and after meals.

With regard to the bathtub, the common habits

of families would indicate that this fixture would

be used most frequently early in the morning and

late in the evening. The laundry tray would

probably be used most frequently during the middle of the morning or afternoon. One code [3] has recognized this fact of the probable use of the

kitchen and laundry fixtures during periods of minimum use of the water closet, tub, and lavatory, and permits these fixtures to be neglected

in sizing the soil drains of the system.

In like manner it is undoubtedly an error to consider that the water closet and the lavatory

discharge independently at random in small sys- tems, as in normal usage there is in g(>neral a

definite sequence of use of these two fixtures. The duration of the dischai'ge from the water

closet is short, being for most such fixtures between 8 and 16 seconds. In the usual sequence

of use of the water closet and the lavatory hy a single individual, it appears obvious that the flow

from the water closet will have ceased before the

lavatory has been filled, the hands washed, and

1^5
the plug pulled, and hence the assumption of

completely random discharge of the water closet and lavatory will yield probabilities of coincident

discharge larger than what will actually occur in

service and, therefore, will yield safe and conservative estimates of the relative frequencies with

which these two fixtures will be in coincident

discharge.

In the above discussion it has been assumed that

the lavatory will be used by inserting the plug or otherwise closing the outlet oiifice, filling the fix- ture, and discharging it by removing the plug from

the outlet orifice. However, quite often the lavatory is used by merely drawing the water continuously from the tap without closing the outlet orifice. If the lavatory is used in this manner, the

probability of coincident discharges with the

water closet may possibly be increased, but, on

the other hand, the rate of discharge from the

lavatory will ordinarily be only a fraction of the

10 to 13 gallons per minute discharged when the

fixture is filled and the plug removed, and its effect on the trap-seal losses of wet-vented fixtures will be negligible.

For the above reasons it would seem that a consideration of the probable frequency of occurrence

of particular combinations of fixture discharges

may prove fruitful, even though it is apparent that

the fixtures in a single-family dwelling are not

used completely at random.

In order to facilitate the discussion we assume

that the fixtures on the single-story system in

question do discharge at random, and we justify

this somewhat questionable assumption on the

grounds that the lack of randomness pointed out

above is such that the error made is on the safe

side. That is, the error made by assuming the

fixtures to discharge completely at random will result in the computed probabilities of coincident

discharge of a group of fixtures being higher than

the actual probabilities, and hence the error made

will be on the safe side, insofar as the problem of determining a I'easonable test or design load is concerned.

We shall now compute the probabilities of occur- rence of the various combinations of discharges

which may occur, on the assumption that the dis- charge of each kind of fixture occurs on the aver- age with the frequency indicated in table 9 and

that each discharge has the duration specified.

Table 9. Assumed condit ions of use of fixtures

Fixture

Interval between

consecutive

discharges,

T, seconds

Duration

of discharge,

t, seconds

Probability, t/T

Sink 300 15 15/300 0. 0500 Tray 600 40 40/600 .0667 Lavatory . 180 9 9/180 . 0.500 Bathtub 900 60 60/900 .0667 Water closet . 300 9 9/300 . 0300

The probabilities given in table 9 are those of finding the fixture in question discharging, without

any regard to whether or not any of the other

fixtures are also found discharging. Table 10

lists the probabilities of finding any particular

fixture discharging and not discharging and will serve as the basis for computing the probabilities

of finding various combinations of fixtures dis- charging, the latter probabilities being given in table 11. Table 10. Probabilities of finding individual fixtures discharging and not discharging

Fixture

Probability of finding fixture dischargingProbability of not finding fix- ture discharging

Sink 0. 0500

.0667

0. 9500 Tray .9333 Lavatory.-- .0500 .0667

. 0300

. 9500

.9333 .9700 Bathtub Water closet -

The following examples will illustrate the use

of table 10 in computing the probabilities given

in table 11.

(a) Probability that only the sink will be found

operating:

Probability that the sink will be found operating 0. 0500

Probability that the tray will be found not . 9333

operating

Probability that the lavatory will be found not

operating . 9500

Probability that the bathtub will be found not

operating . 9333

Probability that the water closet will be found not

operating . 9700

The desired probability is found by multiplying

together the five probalDilities in the list above

and is 0.0401.

(b) Probability that all Jive fixtures will be found

operating:

Probability that the sink will be found operating. 0. 0500

Probability that the tray will be found operating. . 0667

Probability that the lavatory will be found

operating . 0500

Probability that the bathtub will be found

operating . 0667

Probability that the water closet will be found

operating . 0300

Again the desired probability is obtained by

multiplying together the five probabilities listed above and is 0.00000033. The values given in

table 11 have been computed in the manner

described above.

As indicated in table 9, it has been assumed

in the computation of table 11 that the sink,

tray, lavatory, water closet, and bathtub discharge

at random once during a time interval of 5, 10,

3, 5, and 15 minutes, respectively, on the average.

These average time intervals between consecutive

uses of these fixtures correspond in general to those

used by Hunter [3, 6] for public waskrooms,

which were based in part on limited field observa14 Wet Venting of Plumhing Fixtures
tions. The use here of Hunter's data on public,

washrooms for computing tlie probable loads on

a single-family plumbing system is not intended

to imply that the freciucncy of use of the fixtures

in the two installations is the same. On the contrary, it is obvious from our general knowledge

of family household habits that the frequency

of use of the fixtm-es in a private dwelling is in general much less than for public washrooms.

Table 11. Probability of finding the following combinations

of fixtures in a single-story, wet-vented system discharging

at any arbitrarily chosen instant of observation

Fixtures discharging

None.

Sink only » Tray only » Lavatory only <>_.. Bathtub only Water closet only. Any one fixture

Sink and tray only " Sink and lavatory only « Sink and bathtub only Sink and water closet only Lavatory and tray only « Lavatory and water closet only. Lavatory and bathtub only Water closet and tray only Water closet and tub only Tray and tub only Any two fixtures..

Sink, tray, and lavatory only "... Sink, tray, water closet only

Sink, tray, tub only

Sink, lavatory, water closet only.

Sink, lavatory, tub only . Sink, water closet, tub only Tray, lavatory, tub only Tray, water closet, tub only

Lavatory, water closet, tub only. Any three fixtures...

Sink, tray, lavatory, tub only

Sink, tray, lavatory, water closet only.

Sink, tray, water closet, tub only

Sink, lavatory, water closet, tub only. Tray, lavatory, water closet, tub only. Any four fixtures...

All five fixtures.

Probability

Individual

.0401 .0545

.0401 .0546

.0236

. 00287 .002U

.00287

.00124

.00287

.00124

. 00287

. 00168

. 00168

.000151

. 000089

. 000205

. 000065

.000151

. 000089

. 000205

.000120

. 0000108

. 0000047

. 0000063

. 0000047

. 0000063

Sum

. 2128

.001253

. 0000328

. 00000033

a Individual fixtures or combinations of fixtures on wet vent.

Hunter's data on public washrooms were used in

computing the probabilities in table 11 merely

because tlaey serve adequately in the absence of

similar data for the private home, to furnish an

upper limit for determining the design or test loading for the wet-vented drainage system of a

single-family dwelling, and it is recognized that a design load computed in this manner will be overly

conservative.

The specific problem of this section of this

report is to determine what constitutes a reasonable test or design load for the single-story, wetvented drainage system. Approaching it from the

point of view of the |)r()l)al)ility of occiinciici^ of different com])inations of (Hscharges from tlu; fixtures on tlie system, we note t^hat table 1 1 shows

the probability of the simiiltiineous discharge of all three fixtures on the wcl vent to be af)out 1 in 6,600. Thus, under the conditions of use of fixtures

assumed, the probal)ility of all three fixtures on

the wet vent discharging simultaneously is very

remote, and it appears reasonable to take tlie dis- charge of only two of the fixtures on. tlie wet vent

as the design load.

Figure 14 shows the trap-seal losses (from tables

2, 3, 5, 6, and 7) for a l}^-inch wet vent and for various combinations of fixtures plotted against

probability of occurrence. Data are plotted for two cases: the first when

the tub is not discharged, and the second when the

tub is discharged. In no instance does the trapseal loss of the bathtub exceed 1 inch when not more than two fixtures are discharged simultaneously. The corresponding probability is about

1/300. For any three fixtures discharging simultaneously the only cases in which the trap-seal loss exceeded 1 inch was when the cast trap was used

and the tub was discharged. Here the trap-seal

loss was about 1.6 inches, and the probability of occurrence is about 1/6700.

There is no corresponding plot given for the

trap-seal losses when a 2-inch wet vent is used,

but the trap-seal losses were all less than. 1 inch

when this diameter of wet vent was used. Even

with the three fixtures on the wet vent and the

tub discharged simultaneously, the trap-seal loss did not exceed 0.5 inch.

Our conclusion, from the preceding discussion

is that the design or test loading for a single-story

wet-vented system, as far as the diameters of the

wet vent and its associated drains are concerned,

may be taken as the discharge of two of the fixtures on the system. Whichever pair of fixtures dis- charged gives the worst conditions should be taken

as the test loading. This statement applies to a system having one water closet, one lavatory, one

bathtub, one sink, and one tray.

The above conclusion regarding the use of the

discharge of any two fixtures as a design load for the single-story, wet-vented system has been

influenced by the fact that loads in. excess of the one chosen do not result in any serious ill eft'ects to the system. That is, trap seals are not completely broken and flooding of fixtures or blowing

of traps does not occur. If such ill efl'ects did occur for loads in excess of the design load selected

above, it is possible that a proper consideration

of the magnitude, frequency, and relative serious- ness of these ill efl'ects would have been a compelling basis for increasing the design load above

that given in the preceding paragraph.

Building Materials and Structures Report BMS119 15
O I- ID

<o en

I 5

1,0

05

-A'

10- 10" OO

PROBABILITY OF OCCURRENCE Figure 14. Prohability of occurrence of various simultaneous discharges.

IH-inch wet vent. Drain 6 feet long on all tests. Slope li inch per foot.

Tub not discharged

Vo

A

Long-turn fitting ... Short-turn fitting... ... Long-turn fitting Short-turn fitting... ..

Cast trap. .Do. Tubing trap. Do.

Tub discharged

T

Long-turn fitting. ... Short-turn fitting Long-turn fitting ... . Short-turn fitting..Cast trap. Do. Tubing trap. Do.

S = sinlc, T = tray. L = lavator.v, B = bathtub, C = water closet.

b. Permissible Trap- Seal Losses For the purposes of this investigation it is sufficient to adopt the criterion of satisfactory

trap operation, first stated in "RecoinmencU>d

Minimum Requirements for Pkimbin.g in Dwellings and Similar Buildings." [2], that, for traps

having a 2-inch depth of seal, the seal loss shall not exceed 1 inch. In applying this criterion of satisfactory trap performance, we shall use the

maximum trap-seal loss observed in 10 consecutive

tests made under identical conditions.

c. Permissibility of Venting a Bathtub Trap With a Wet Vent to Which a Lavoratory and a Combination

Fixture are Connected

The application of the test loading and the

permissible trap-seal loss discussed above to the

data given in figures 11 and 13 and in tables 6 and 7 make it possible to draw certain conclusions

regarding the permissibility of venting a bathtub

trap with a wet vent to which a lavatory and a combination fixture are connected.

Consider first the case in which the tub is not one of the two fixtures making up the test loading.

From table 1 we see that the maximum rate of flow from any two of the three fixtures (considering

the sink and tray compartments of the combination fixture to be separate fixtures) is 35.8 gallons

per minute, if we except the case of the basket

strainer withdrawn (sink with grid strainer and

lavatory with ll^-inch trap and waste discharging).

Reference to figure 1 1 will show that, for this rate of flow through the wet vent, trap-seal losses will be

appreciably below the adopted permissible trapseal losses' for both the l^-inch- and the 2-inchdiameter wet vents for the 4- and 5-foot-long tub

drains tested. Inasmuch as the data in figure 11 and tables 2 and 3 indicate that trap-seal losses do not increase as the length of the drain, increases,

this suggests that the test loading under consideration may not impose any limitation on the

maximum permissible length of the drain of a wet-vented tub.

The strict application of the adopted test loading and the permissible trap-seal loss to the

determination of permissible lengths and slopes

of the bathtub drain for a loading that includes

the discharge of the bathtub is materially handicapped by the fact that such application, as can

be seen from figure 13, would lead to severe

extrapolation of the test results, with permissible

lengths of tub drains far in excess of those actuall}^ tested.

16 Wet Venting of Phonhing Fixtures
For example, iu llie case of tul) drniiis on a '4-inrh-per-foot slope, drains 6 feet long were the

longest tested, while an application of the adopted

trap-seal loss to the data in figure 13 would lead

to a permissible drain length for a )4-incii-per-foot

slope of more than 1 1 feet for the 1 Ji-inch wet

vent and to something in excess of 11 feet for the

2-incli wet vent. Similar, though smaller, extra- polation would be necessary for tub drains on a 'o-hich slope. Such extrapolation of limited test data of this nature is, of course, not warranted,

and it is proposed that, until such time as more

experimental data are available for tub drains on

lower slopes, a permissible design value of 83(3= 1.5 inches be adopted for the l!2-ineh wet vent and a value of iS'3/3=2.0 inches l)e adopted for the 2-inchdiameter wet vent. In computing values of

S3I3, Sz should be expressed m mches per inch or

feet per foot, and /a should be expressed in inches.

The permissible values of the factor proposed

above lead to the simple and obvious design cri- terion for a wet-vented bathtub drain—that the

value of SJz/di, where di is the diameter of the

wet vent, shall not exceed unity. Figure 15 shows

permissible lengths of tub drains, measured from

5 0.5 o

u.

a:UJ

0.

(/) ^ 0.4 o

4

a 0.3 m

3

UJ 0.2

n.o

-I

J

\ WET VENT

.12-IN CH Wl :t VE^IT

2 5 4 5 6 7 8 PERMISSIBLE LENGTH OF TUB DRAIN -FEET

FinuRE 1.5. DeHign data for wet-vented tub drains.

the trap weir to the wet vent, for various slopes

of the tub drain, computed from the criterion,

In applying this general criterion it should be

noted that shoidd be expressed in inches per

inch or feet per foot, not in'inches per foot, as is

the ciistoniai v wa\' of e.\|jressiiig (haiii slopes, and

that /.J and r/, should l)e ex])resse(l in the siinie units—inches, feet, centimeters, etc.

It should also be noted, while we are discussing

the effect of a test loading which includes tlie dis- charge of the tub together with only one fixture on the wet vent, that figure 13 shows the results

of the discharge of all the fixtures on the wet vent

in addition to the dischai'ge of the tul). Figure 13 was used merely to illustrate the fact that even

with a design load substantially in excess of the one adopted, extreme extiapolation of the test data is necessary to obtain a tub drain sufficiently

long to result in trap-seal losses of 1 mch. Shice

the application of the design load adopted pre- viously will cause smaller trap-seal losses, for a given slope and length of tub drain, than is shown by the data in figure 13, it is apparent that

this procedure will result in the introduction of an appreciable safety factor into the conclusions.

The foregoing conclusions regarding the per- missible lengths of wet-vented tub drains are based on tests of a system m which the diameter

of the horizontal branch was 2 inches and the

diameter of the tub drain was VA inches, and in which both a lavatory and a combination fixture

drained into the wet vent through a double fit- ting. The wet-vented bathroom and kitchen

fixtures were located on the topmost branch interval of the stack. The diameters of the combination fixture and lavatory drains were 1?:; and IV4 mches, respectively. In practice many of these

dimensions vary, and other factors affect the effi- ciency of a wet-vented drainage system. These

factors will be duscussed in section VI.

d. Summary

1. A loading consisting of the simultaneous

discharge of the two fixtures on the system which

cause the largest trap-seal losses has been adopted

as the design or test loadmg of the S5^stem.

2. A trap-seal loss of one inch has been adopted

as the limiting value for distinguishing between

satisfactory and unsatisfactory performance.

3. The application of the design load and the

permissible trap-seal loss given above indicate

that both I'l- and 2-inch-diameter wet vents will be adequate for the system undei' consideration,

provided that the value of the quantity, 83/3 V/,. does Jiot exceed unity. S3 and /3 in this factor

are, respectively, the slope and the length of the

tub drain, and cIi is the diameter of the wet vent,

all expressed in consistent imits, as described

previously.

VI. Factors Affecting the Performance of Wet-Vented Systems

1. Length of Horizontal Branch

As indicated in figure 6, the drain connecting

the wet vent and tub drain to the stack is com- monlv called a horizontal branch. For most of the

test results presented so far. the horizontal

branches were 4 feet in length. However, in practice this branch may be either shorter or longer

than 4 feet, and it is desirable to determine the

eft'ect on the trap-seal losses previously reported

Bvil(]'ni(/ Mntei-ialx and Sf nictn fes Report BMS119 17
of changing this lengtli. Test results relating to

this question are given in figui'e 16. These tests were not made on a complete system but were made with the test setup shown in the figure. The

water was not introduced to the wet vent from

conventional fixtures but thi-ough a valved connection to the city water supply. The length of

horizontal branch was varied from 1.3 to 8.25 feet. As can be seen from the figure, tub trap-seal losses appear to be independent of the length of the

horizont al branch

.

A number of tests were also made to determine

the effect of length of the horizontal branch on

trap-seal reductions in the complete system shown

in figure 6. These tests consisted in changing the

length of the horizontal branch from 4 to 2 feet. In no case was there a significant difference noted.

Hence it may be concluded that the test data

presented in this report are valid for any length

of horizontal branch, at least within the range of lengths tested.

TEST SET-UP

WATER INTRODUCED HERE

2-INCH DIAMETER

2-INCH LONG TURN T-Y

TEST SET-UP

10 15 20 25 30

1- FLOW THROUGH WET VENT IN GPM Figure 16. Effect of length of horizontal branch on tub

trap-seal reduction.

Slope of tub waste was H inch per foot. Data shown represent the maxi- mum of five tests under identical conditions. Cast-brass trap shown in

figure 10 used on tub. Tub discharged in tests.

Symbol Dimension

J In feet O 1.3 A 4.25

6. 41 • 8. 25

3-lNCH LUCITE STACK

COMBINATION FIXTURE

FITTING A

TUB WASTE

^-INCH DIAMETER 6-FEET LONG 1

2 NCH SLOPE

0.5 -

- 0

uf 2 .0

1.5

0.5 -

20 25 30 35 40 45 50 Figure 17. Effect of slope of horizontal branch on tub

trap-seal reduction.

A. 2-inch wet vent. B, IV2 -inch wet vent. Plug discharge of fixtures. Metal pipe fittings were used on lavatory,

sink, and tub wastes, wet vent, and 2-inch horizontal branch. Cast-brass

trap shown in figure 10 used on tub. Tub not discharged. Above data

represent the maximum seal reduction obtained in 10 tests made under

identical conditions.

Slope of horizontal branch inchper-foot Fitting A Position of basket type strainer

ViA

V

0

T•

Short-turn. . In normal position in sink. Do.

Pulled out of sink. Do.

Long-turn

Short-turn. . . . Long-tui'n.. . .

S = sink, T = tray, L=lavatory.

be observed that there is no consistent effect of

variation of slope of the horizontal branch.

2. Slope of Horizontal Branch

Although the usual slope of the horizontal

branch in a wet-vented system is ji inch per foot,

it is occasionally necessary or desirable to increase

this slope. For this reason the slope of the horizontal branch was varied in some of the tests. The results of these tests for several modifications

of the test setup are given in figure 17, and it will

3. Diameter of Horizontal Branch

The experimental data so far presented in this investigation have been, for the most part, obtained with the system shown in figure 6 with a 2-inch-diameter horizontal branch. Tests were

not made with this system using a 1/2-inch horizontal branch because the general horizontalbranch loading tables of most codes do not permit

18 Wet Venting of PlumMng Fixtures
a load on a 1/2-inch horizontal branch appi'oaching

that shown in figure 6. However, t(>sts to be

described shortly were made with a 1/^-inch horizontal branch on a single-story, wet-vented system

in which only a lavatory discharged into the wet

vent. As indicated in table 12, an 0.88-inch trapseal loss was obtained when the tub and lavatory were discharged. Inasmuch as no trap-seal loss was obtained on the system shown in figure 6 with

the same fixture discharge, it can be concluded

that a decrease in the diameter of the horizontal

branch from 2 inches to Iji inches will cause

increased trap-seal losses.

T.^BLE 12. Trap-seal losses of wet-vented bathtub'^

[Only lavatory connected to wet vent]

Fixtures discharged

Tub trap-seal loss Water

closet trap-seal

loss Tub drain

7 ft long, H-inch

slope

Tub drain

B ft long, Ji-inch slope

max avg max avg max avg

Lavatory, tub

Lavatory, water closet, tub , Lavatory, sink, trav, tub_. . ... .. Lavatory, sink, tray .. Lavatory, sink, tray, water eloset..

in. 0

n0

. 12

. 19

in. 0

(10

. 11 .13

in.

0. 88

.88 .75

. 12

. 12

in.

0. 48 .48

.08

.09

. 12

in. 00

.75

.76

0

in. 0

0

.62

.64

0

" The cast trap shown in figui'e 9 was used on the tub.

Wliether the increase in trap-seal losses would

be sufficient in the case of the system shown in figure 6 to warrant the prohibition of l^-inchdiameter horizontal branches is problematical.

There are, however, no experimental data available at present justifying the use of l)2-inch

horizontal branches on such a system.

4. Tub Drain Length and Slope

It has been concluded from the data in tables

2, 3, and 4 that, for the loadings that do not in- clude the discharge of the tub, the data in figure

11 will serve to indicate the maximum trap-seal

losses to be expected from installations with a tub

drain 4 feet or more in length on a }^-inch-per-foot

slope or less. In this connection, it will be recalled that, on

the tests with the transparent drains, a pool of

water was observed in the tub drain under certain

conditions, and when the pool was sufficiently

long to extend back to the tub trap and to cover

the trap weir to an appreciable depth, refill of the trap occurred when flow from the wet vent

ceased, resulting in a smaller trap-seal loss than

would have been the case had the drain been

sufficiently long and on so steep a slope that the

pool of water did not extend to the trap weir.

Since, when the test loading does not include

the discharge of the tub, the length and slope of

the tub drain can have no conceivable effect on

the depth of water in the tub drain immediately

upstream from the wet vent, it would be exper-ted

tliat, for sufliciently shorter drains lliaii those

tested or for sufficiently lower slopes, the trapseal losses would be smaller than tfiose shown in figure 11 or in tables 2 and 3. In this connection,

it appears obvious that the direct cause of trapseal losses in. the particular tests under consideration must be the existence of a partial vacuum in the tub drain. If this vacuum is assumed not to decrease during the discharge of tlie fixtures on

the wet vent, it is obvious that the air pocket in the tub drain must extend along the entire upper

surface of the drain, and hence the maximum

possible distance that the pool of water can ex- tend back into the tub drain is d^/S^, where and

S3 are the diameter and slope of tlie tub drain,

respectively.

If the above conception of the phenomenon is assumed to be correct, refill of the trap from the

drain cannot occur when the length 4 of the tub

drain is greater than d^/Si] or, stated differently,

when SzUldz is greater than unity and it would be

expected that, for some value of S-jl^/d^ less than

1, trap-seal losses for loadings not including the

discharge of the bathtub would be negligible for

all rates of discharge down the wet vent.

If it be assumed, as has actually been observed,

that the effect of trail discharge of the fixtures on

the wet vent causes a decrease in the vacuum in the tub drain, near the end of the period of dis- charge, at least partial refill of the trap may occur

for values of Sds/d^ greater than unity.

A careful examination of the data in tables 2 and

3 in the light of the above discussion will indicate

that, while the data in these tables, with regard

to the inffuence of the length of drain on trap-seal

losses, are somewhat erratic and nonconclusive,

there appears to be a definite tendency for the

increases in trap-seal losses that are apparent as the tub-drain length is decreased from 6 to 5 feet to level off, and for the trap-seal losses to decrease

as the drain is further shortened to 4 feet. Hence

it is to be inferred that, for the IJr inch-diameter

tub drain on a inch-per-foot slope, the effect of

refill into the trap from the standing pool of water

in the drain begins to make itself apparent when

the length of drain is approximately 4 feet, and

that any further appreciable decrease in length of tub drain will result in a decrease of trap-seal

losses. Hence it would be expected that the data

in figure 11 and tables 2 and 3 will not be ex- ceeded as the length of the drain is decreased below

4 feet, and, therefore, that the data in tables 2 and 3 and figure 1 1 will not be exceeded by the trapseal losses obtained with a tub drain of any length.

The above considerations of the eff ect of refill of

the trap from the tub drain leave unexplained the

decrease in the trap-seal losses listed in tables 2 and 3 for the drain 6 feet in length as compared

with the 5-foot drain. In this connection it is believed that this decrease in trap-seal losses is connected with an effect which must exert an

Building Materials mul Structures Report BMS119 19
increasing influence on trap-seal losses as the tub

drain is lengthened. The pressure reduction in the tub drain is created b}^ the water flowing from

the wet vent. For a given pressure reduction in

the tub drain, the volume of air to be evacuated

from a drain 6 feet long will be greater than for a drain 5 feet in length, and hence the time required

for the pressure reduction in the tub drain to reach equilibrium will be longer for the 6-foot

drain, assuming that the rate of evacuation is the same in the two cases. If the duration of the

discharge through the wet vent is greater than the

time required for the pressure reduction in the

tub drain to reach equilibrium, this phenomenon

will have no effect on the trap-seal losses as the

drain is increased in length. However, for a given duration of flow in the wet vent, it is obvious

that, as the di-ain length is increased, eventually a point will be reached at which the time required

to evacuate sufficient air from the tub drain so that equilibrium exists will be greater than the

duration of flow from the fixtures on the wet vent. Wlien this point is reached, it would be expected

that any further increase in tub drain length would

cause decreased trap-seal losses. It is believed

that the phenomenon described is the cause of the

decrease in trap-seal losses observed for the drain

6 feet in length as compared to the 5-foot drain.

Inasmuch as the trap-seal losses for the 4- and

5-foot lengths of tub drain on a };^-m-per-foot slope

are substantially equal, it has been concluded

that, as the length of dram on a )^-inch slope is increased beyond appro>imately 4 feet, refill from

the drain into the trap does not occur. Since the

only possible effect, for the loading that does not

include tlie discharge of the tub, of increasing the

slope of the tub drain is to decrease trap refill, it foflows that the trap-seal losses in tables 2 and 3 and figure 1 1 will not be exceeded when the slope

of the tub drain is increased above Yo inch per foot. In view of the above discussion regarding the

effect of tub drain length and slope, it may be concluded, for the loading which does not include the

discharge of the tub, that the trap-seal losses of

tables 2 and 3 and figure 11 will not be exceeded

with a tub drain of any length or slope.

In the case of loadings which include the dis- charge of the bathtub, it appears equally clear that the water surface in the tub drain will never be lower than would be the case for the loading

which does not include the discharge of the tub,

and hence it would be expected, as for the pre- vious loading, that there exists some minimum

value of Ss/s/f/a for which refill of the trap from the

drain is such that all loadings, irrespective of whether the loading includes the discharge of the

tub, will produce negligible trap-seal losses. It

is interesting in this connection to note that

Hunter [3] has proposed, in effect, that the slope,

diameter, and length of the unvented portion of any fixture drain, whether wet, stack, or backvented, be limited so that the value of tSVs/r/s shall not exceed unity.

5. Trap Dimensions

There are two methods by which a tub trapseal loss due to negative pressure in the fixture

drain may occur. First, if the negative pressure, expressed in inches of water, in the tub drain

created by the flow down the wet vent is less than the depth of trap seal; then, assuming that

the tub drain is sufficiently long and on a suffi- ciently high slope to prevent refill of the trap

from the drain, water will be drawn from the

trap until equilibrium occurs. When the flow

in the wet vent ceases, the seal reduction in the

tub trap will be one-half the magnitude of the

negative pressure head in the tub drain and is independent of the depth of the trap seal and of

any other dimension of the trap, such as its diameter. However, if the flow down the wet

vent is large enough to cause a pressure reduction

in the tub drain greater than the depth of the tub

trap seal, air will be bubbled from the atmosphere through the trap into the tub drain. This

process pumps water out of the tub trap and into

the drain and hence results in relatively large

trap-seal losses. Consequently it is apparent, for a given pres- sure reduction in the tub drain, that, if the depth

of the tub trap seal is greater than the pressure

reduction in the drain, a decrease in depth of trap seal will have no effect on the tub trap-seal

losses until a point is reached at which the depth

of trap seal is equal to the pressure reduction

in the drain. Any further reduction in depth of

trap seal will cause a rapid increase in trap-seal

losses. Hence the effect of increasing the depth

of tub trap seal from a value less than the pressure

reduction in the drain to a value greater will be,

in general, to decrease trap-seal losses substantially. And the effect of increasing the depth

of tub trap seal, when it is at all times greater

than the negative pressure in the tub drain, will be nil. In the tests reported here, the depth of tub

trap seal was 2 inches or more, and the adopted

criterion of satisfactory performance was a trapseal reduction of 1 inch. A trap-seal reduction of

1 inch corresponds to a negative pressure in the

tub drain of 2 inches. Hence, the particular

tests on which the conclusions as to the adequacy

of wet venting have been based were made with

loadings which produced negative pressures in

the tub drain equal to or less than the depth of trap seal. It is apparent, therefore, that the

conclusions expressed in this report for traps with

depths of seal of 2 inches would not be changed

by increasing the depth of trap seal. From the discussion above on the effect of change in tub trap-seal depths on tub trap-seal

losses, it is evident that a change in tub-trap

20 Wet Venting of PhimMng Fixtures
diameter coiikl have no effecl on tul) trap-seal

losses, as long as these losses ai-(> less than 1 inch,

which, as has been stated before, has been adopt(Hi

in this report as the criterion of satisfactoj'y trap

performance. Foi- this reason it is apparent

that the conclusions expressed regarding wet

venting apply with equal force to tub-trap diameters of any size. For seal losses greater than approximately 1 inch in a trap with a depth of seal of 2 inches,

air will be drawn tlii'ough the trap, as noted previously, and, when this condition arises, the diameter of the tub trap undoubtedly has some effect on the amount of trap-seal reduction. This

problem was not investigated, but it would be

expected that a decrease in tub-trap diameter

would cause greater trap-seal losses. From the preceding discussion on the effect of changing the tub trap dimensions, it would be

expected that, for trap-seal losses of approximately

1 inch or less, the cast and tubing traps shown in figures 9 and 10 would give the same results,

whereas for the larger trap-seal losses, the cast

trap, owing to its smaller depth of trap seal and its smaller diameter, would give the greater trap-seal

losses. Figures 11, 12, and 13 show this to be the

case.

6. Effect on the Trap- Seal Losses of a WetVented Bathtub of Increasing the Diameters of the Lavatory and Combination

Fixture Drains

Consid(>r first the loading which does not include

the discharge of the bathtub. The tub trap-seal

loss, h, under the assumed conditions will depend

only on the pressure, p, in the tub drain. This

pressure, however, depends on a number of characteristics of the system—the length l-i, slope

Sz, and diameter (I3 of the tub drain, the volume

rate of fiow through the wet vent, the pressure

Pi at the base of the wet vent, the diameter di of the wet vent, and possibly the velocity Vi at the

base of the wet vent, and the diameter ^4, the

slope S-2, and the length L of the horizontal branch.

In turn, pi and i\ depend on certain additional

characteristics of the system, such as the diameters

f/4 and f/5 of the combination fixture drain and the

lavatory drain, respectively, and the length /, of the wet vent. In addition they also depend on

the acceleration of gravity. The roughness of the

pipe walls will also have some slight effect, but this

will not be important because the lengths involved

are short. Equation 1 is the formal expression of the

statements made in the preceding paragraph.

The pressures, p and pi, and the velocity do not

appear in the right member of the equation, since

they depend, to a first approximation at least, only on quantities that are shown in the right member.

h^MO, (U, (k, (h, (h, U, I2, h, S2, S,, (I). (1)

This e(|uatioii contains 12 physical or geometrical (juantities that can i)e expressed in teims of two physical dimensions lenglli and

time—and it follows from tlie theory of diniensional analysis thai it can be i-ewritten in terms of 12 — 2 = ]0 (liniensi()nless variables. Th<' exact form of some of these var'iables is in pai-t optional

with us, and in these cases we use oui- physical

intuition oi' our past experience to form the

particular combinations that will be useful. We

rewrite eq 1 as follows:

A r Q (/, (/, f/| f/, (U (l\ o

~|

bn\!<l(h <k di dr, li I2 h J

We are concerned here only with the eU'ect on

the trap-seal losses of changes in c/4 and r/-. Aside

from this, the geometrical characteristics of the

system will remain constant, and hence we can

eliminate from consideration the variables, c/|/c/2, dxllu d-ilk, di/(:,, S2, and Si. Equation 2 then

simplifies to

d'Jgd, (h d,]

h/di = 4)

One further simplification can be made, owing

to the fact that we do not need to consider di and dr, simultaneously, but only one at a time,

depending on which fixture is producing the flow,

Q. Hence we shall eliminate one of the two

diameter ratios and arbitrarily refer to the one under consideration as di. Equation 3 then

becomes

A_ f [ Q_ , A" (4)

which is the final form of the equation we shall use to study the effect of changing the fixture

drain diameter. This equation is identical with

eq 3 of the earlier paper on stack venting [1]. In the paper referred to [1], it was shown that an increase in d^, which amounts to decreasing

the ratio djdi, has the effect of decreasing the

trap-seal loss h, or the relative trap-seal loss h/di of stack-vented fixtures. Since a change in djdi

can, for a given 0, affect the trap-seal losses only

through the resulting eff'ect on the velocities and

pressures in the wet vent or stack, and since the

basic phenomena governing the velocities and

pressures in a vertical stack and in a wet vent are

essentially the same, it would be expected that a decrease in di/di, as was the case with stackvented fixtures, would cause, for a given value of

0/d^^gdi, a decrease in the trap-seal losses of wetvented fixtures. However, the value of Qld\^,gdi

does not remain constant as d^ is increased, owing

to the fact that an increase in d^ causes an in- crease in Q, the discharge of the fixtures con- nected to the wet vent ; and the data shown in

figure 11 indicate that an increase in Q wiU

Buihl'uKj MateriuJs. and Structures Report BMS119 21
ordinarily be accompanied by an increase in h, the trap-seal loss of wet-vented fixtures. Therefore, in the absence of specific data on the effect of an increase in fixture drain diameters, it can

only be concluded that, if such an increase in

fixture drain diameters causes an increase in trapseal losses, the increase will be due_to an increase

in the value of the term, QJdl-yjgdx, and will not

exceed those shown by the data in figure 11 by

using the increased Q resulting from an increase

in di as the argument.

The data in figure 11 and table 2 for the cast trap have been replotted in figure 18, using the

dimensionless variables developed above. It will be observed from table 2 that some of the data

plotted in figures 11 and 18 are for flow from the

combination fixture alone, while other data

plotted from these figures are for flows from both

the lavatory and combination flxture. It will be

observed from both figures 11 and 18 that, for

1.2

1 1 1 1 r

o'—O h CD—I ' " 1 05 10 15 20 25 30 Q

df \/d| g Figure 18. Dimensionless plot of wet venting data for test loadings not including the bathtub.

Symbol

Diameter of wet vent

inches d,dt

dxdi

dl di

d. Nominal Actual

do

o

2

l.fil

2. 07

0. 78 1.001.00

1. 50

1. 16 1.29 1.29

1.29

the range of flows tested, there is little apparent

difference in the test results between flows introduced through the iM-inch-diameter lavatory

drain and the l^-inch combination fixture drain.

These limited experimental data on the effect of increasing the diameter of the fixture drains

introducing water to the wet vent thus are in

agreement with the conclusion expressed above

that the trap-seal losses caused by increasing the

diameter of the lavatory drain to lli inches or increasing the diameter of the combination fix- ture drain to 2 inches will not exceed those

shown in figures 11 and 18 for the proper value

of 0 or pidisj'gd,. It will be observed in figure 18 that, for the

range of variables tested, the data for the 2-inchdiameter wet vent approximate fairly closely the

data for the 1/9 inch wet vent. Inasmuch as the

design or test loads developed in section V-4-a

are such that no extrapolation of these data for either the IK or 2-inch-diameter wet vent is necessar3^ the curve in figure 18 may be safel}!-

used as a design curve for either or l}2-inchdiameter lavatory drains or with 1)2- or 2-inchdiameter sink or combination fixture drains. The

use of a l}2-inch-diameter drain will increase the

rate of discharge from the lavatory by approximately 3 gallons per minute. From table 1 the

design loading proposed in section V-4-a becomes

38.8 gallons per minute (13.6 + 25.2) for a lavatory

flow rate of 13.6 gallons per minute, which yields a value for 0/d!l^/gdi of 2.32 for 1}^ -inch-diameter

wet vent, and by reference to figure 18, it may be

concluded that a value of h/di of 0.5, or a trap-seal

loss of 0.80 inch for a nominal 1 Jo-inch wet vent,

would not be exceeded. This trap-seal loss is less than the permissible value of 1 inch adopted in

this report and hence is satisfactory.

In table 13 are given rates of discharge for a combination fixture with a drain 2 inches in diameter and 2 feet long on a slope of inch per foot. Assuming that the lavatory drain is 1^2 inches in

diameter and that consequently the lavatory has

a rate of discharge of 13.6 gallons per minute, it is apparent from table 13 that the greatest discharge

obtainable for any two fixtures on the wet vent is 41.2 gallons per minute, which yields a value of

Q/dl^/gdi of 1.30 for a 2-inch-diameter wet vent.

T.\BLE 13. Rates of discharge of combination fixture with drain 2 inches in diameter "

Fixtures discharged Eate of flow

Sink—basket strainer.. . . Sink—flat strainer

Qpm

18.5 27.6

22.3

30. .3 36.0

Tray... Sink and tray—basket strainer.. Sink and trav—flat strainer . . .

n The fixture drain was 2 feet in length and had a

slope of U inch per foot.

22 Wet Venting of Plumhing Fixtures
Reference to figure 18 shows that such a vahic of Qldi\gdi will yield a trap-seal loss substantially

below 1 inch.

It may be concluded, therefore, that the in- crease in diameter of the lavatory drain to l}^

inches or the increase in the diameter of the combination fixture drain to 2 inches will not yield

trap-seal losses in excess of the 1-inch seal loss adopted as the dividing line between satisfactory

and unsatisfactory trap performance for these

tests. The above discussion of the effect of increasing

the diameter of the lavatory or combination

fixture drain has been confined to a test loading

which does not include the discharge of the bathtub. It is obvious, however, that the inclusion of the discharge of the tub in the test loading would

in no way alter the above conclusions.

The data in table 6 for the loading, including

the discharge of the tub, have been replotted in figure 19, using the dimensionless variables developed above. The length of the tub drains used was 4 feet with the 1 ^2-inch-diameter wet vent and

5 feet with the 2-inch-diameter wet vent. The

corresponding values of SJ^/di are 1.24 and 1.21,

respectively, {Sj^Y: inch per foot, and f/i = 1.61 and 2.07 inches, respectively). Again, as the

only possible effect of increasing di and rf, will be

to increase the value of Q/dl^gdi and to increase

the pressure in the tub drain for a given value of

Q/di-\lgdi, it is concluded that the increase in

h/di because of an increase in d^ and ds will be due

to the resulting increase in the value of the term,

Qld\-\jgdx, and will not exceed the value of hjdi

obtained from the data in figure 19 by using the

increased value of Q/d'i\(idi as the argument.

7. Diameter of Bathtub Drain

The great majority of the tests of this investigation were macle w^ith 1 K-inch-diameter bathtub

drains. However, as 2-inch-diameter tub drains

are installed occasionally, a few of the tests were made with the larger diameter bathtub drain.

The results of some of these tests are shown in

table 14. It will be observed that the eft'ect of increasing the diameter of the tub drain is to decrease the tub trap-seal losses.

Table 14. Effect of tub drain diameter on trap-seal losses

losses of a wet-vented bathtub

Tub trap-seal loss Flow through

wet-vent U2-inch- 2-inch- diameter diameter tub drain tub drain

Gpm In. In. n.9 0.0 0.0

15.3 .38 .0 27.2 .38 .0

d|9

Fir.uRE 19. Dimensionless plot of wet venting data for

test loadings including the discharge of the bathtub.

Symbol

Diameter

of wet

vent

rfi dz di di

di

d.

S3Z3

di

Inches O 0.78 1.00 1. 16 1. 29 1.24

2 1.00 1.50 1.29 1. 29 1.21

The data in table 14 were obtained from a sys- tem with a Uo-inch-diameter wet vent, lavatory,

and sink drain, and a 2-inch-diameter horizontal

branch. The tub drain was 4 feet long and was

on a slope of K inch per foot. In obtaining the data given in table 14, only

the diameter of the bathtub drain was increasecl. In figure 20 are shown similar data for another

system in which both the bathtub and the combination fixture drain diameters were increased from

20 30 40 50 FLOW THROUGH WET VENT, GPM

60

Figure 20. Effect of tub and combination fixture drain

diameters on trap-seal losses.

A. li-fj-inch-diami'ter drains on tub and combination fixture, B. 2-inch- diameter drains on tub and combination fixture.

Buihihifj Materials and Structures Report BMS119 23
1% to 2 inches. It wiW again be observed that the

increase in tub-ch-ain-diameter decreases trapseal losses. Under these circumstances it is to be concluded

that the data presented previously for l}2-inch

bathtub drain diameters can be applied with at

least equal safety to installations in which the

diameter of the bathtub drain is 2 inches.

VII. Tests on a Single- Story Wet-Vented System With Only a Lavatory Connected

to the Wet Vent

The test system for this series of tests is shown

in figure 21. The stack and 18 feet of buildingdrain were made of 3-inch-diameter transparent

tubing. The remainder of the system was made

of conventional pipe materials. The slope of the

building drain was }i inch per foot. In table 15 are given tub and water-closet trapseal losses for a tub drain 7 feet long laid on a

/2-inch-per-foot slope. It will be noted from figure

21 that the water closet is stack-vented, so that

in reality the bathroom group of fixtures is partially stack-vented and partially wet-vented. For

this reason, water-closet trap-seal losses occur, and

these have been listed in table 15. When the tub was among the fixtures discharged, the other fix- tures making up the test loading were discharged

in such a manner that their flow began 3 seconds

prior to the end of the period of tub flow. In the

case of the lavatory, the only fixture on the wet

vent, the timing of its discharge with respect to the end of the tub discharge was found to influence

greatly the amount of trap-seal loss obtained. For

example, when the tub and lavatory discharges

endecl simultaneously, the average seal loss of 10

tests was 0.02 inch, whereas with the sequence of

discharges noted above the average trap-seal loss was 0.48 inch.

It will be noted from table 15 that none of the

trap-seal losses observed were in excess of the

adopted permissible trap-seal loss, and that hence

the wet-vented system in figure 21 may be considered to be adequately vented for the lengths

and slopes of drains tested here—that is, for tub

drain lengths and slopes up to a maximum length

of 6 feet when laid on a /2-inch-per-foot slope, and

up to a maximum length of 7 feet when laid on a

}^-inch slope.

Under these circumstances it is apparent that

the conclusions derived from the tests made of the system shown in figure 6 may be applied with

at least equal safety to the system being considered

here, in which only the lavatory is connected to the wet vent, and the horizontal branch is IJo inches in diameter.

The tests reported in table 15 were made with

a house trap in the building drain. These tests were repeated without a house trap installed in

the building drain, and in no case was there any

appi-eciable or significant difference in the test

J FIXTU

3-IN DIAMETER STACK

COMBINATION

RE

-1^- IN. OIAMETER

-WET VENT

-1^ - DIAMETER f DIAMETER /

W C HORIZONTAL BRANCH Jg^

I" l8'-0"

3-IN, DIAMETER TRANSPARENT TUBINS

8- IN. DIAMETER STREET SEWER

4-IN DIAMETER FIBRE PIPE Si <> 70-0

<j Figure 21. Test system.

results. It may be concluded, therefore, that

any service condition, such as a house trap or a submerged house sewer, which causes an increase

in the positive pressures in the stack will not

cause a single-story wet-vented system to operate

less satisfactorily.

Table 15. Bathiub and water closet trap-seal losses for

the wet-vented system shown in figure 21

Fixtures discharged

Lavatory and tub

Lavatory, tub, and

watei closet Lavatory, sink, tray and tub

Lavatory, sink, and

tray Lavatory, sink, tray, and water closet

Trap-seal losses '

Bathtub

Tub drain

7 ft. long on \i in. per ft. slope

Tub drain

G ft. long on So in. per ft. slope

avg

Water closet

Tub drain

7 ft. long on Vi in. per ft. slope

avg

Tub drain

6 ft. long on 'j in. per ft. slope

. 57

.58

> The data represent the ma-\imuin and average trap-seal losses observed

in 5 consecutive tests made under identical conditions. The lK>-inch cast trap shown in figure 9 was used on the tub.

24 Wet Venting of Plumhing Fixtures
VIII. Tests on Two- Story, Duplex, Wet-Vented System

These tests were made on the system shown in figure 22. The greater part of the tests were made with all of the system except the building sewer made of transparent plastic tubing and

fittings. As was the case with, the other systems

tested, the transparent fittings used were identical,

3-IN. DIAMETER STACK

Figure 22. Tivo-story, duplex, wet-venled system.

within close tolerances, with conventional soil pipe fittings. The fixtures used in these tests were identical with those used on the other sys- tems tested, and the data in table 1 for rates of

flow from fixtures apply to this system as well as

to the other two systems.

In figure 23 are given tub trap-seal losses for this system for various lengths of tub drain . The

data in figure 23 are all for tub drain slopes of K

inch per foot. The lavatories were discharged so that the flow from the tubs and lavatories ended

simultaneously. The two tubs and the two lavatories were discharged, since this loading was

found to produce the greatest tub trap-seal losses. The data plotted in flgure 23 represent the maximum trap-seal loss observed in 10 consecutive

test runs made under identical conditions.

It is apparent from this flgure that, for the plugdischarge of the flxtures on this system, the trapseal losses are independent of the length of the

horizontal branch, as was the case with the other

two systems tested. It will also be noted that

the effect of the two diflerent types of fittings

used to connect the lavatory drains to the wet

vent was negligible. Of particular interest in this figure is the fact that the trap-seal losses for the \Vi- and 2-inch diameter wet vents were for

all practical purposes identical. Since the rate of

dischai'ge of two lavatories approximates the I'ate of How of a sink, it is evi(h'nt from a comparison

of these (hita with. th.()S(> of tal)les 5 and (5 that the

volume rate of How down the wet vent is not the

only varial)le connected with flow conditions in

th.e wet vent wliich aftect the wet venting phe- nomena. The discharge of the hi vatoiics was in- troduced to the wet vent througli 1 '4-inch-diameter drains, whereas the sink cUscharge was introduced through a l/2-inch,-(Uameter drain. As has

been stated previously, the velocity with which

water is introduced to a vei'tical stack has an

important efl'ect on the trap-seal losses of stackvented flxture traps below the point of water entrance, and it is to be inferred from this fact that

the entrance velocity of the water introduced into a vertical stack or wet vent would have an important effect on the pressures in the stack or wet

LENGTH OF BATHTUB DRAIN IN FEET FinuRE 23. Trap-seal losses of a two-story, duplex, wet- vented system.

Both tubs and lavatories discharged

Symbol

Length of horizontal branch Diameter of wet vent

Lavatory

vent

fitting

ft. in. • 2. 58 1. 5 Long-turn.

2. 58 1. 5 Short-turn. H 2.58 2.0 Do. 0 6.0 1. 5 Long-turn. A 6.0 1. 5 Short-turn.

6.0 2.0 Do.

vent, and hence would affect the trap-seal losses of a wet-vented flxture. In addition, the duration of flow from the lavatories is approximately 10 seconds, while the

duration of flow from a sink is approximately 15

seconds, and the data in flgure 5 have shown that

the duration of flow, under certain circumstances,

materiaUy afl'ects the trap-seal losses of wet-vented

fixtures. It is probable that these dift'erences between lavatory and sink discharge into the wet

vent account for the fact that the 2-inch-diameter

wet vent yields smaller trap-seal losses than the

Biiildiiif/ Materials <ni<i Sttact a res Report BM8119 25
1/2-incli wet vent for sink flow and the same loss for the discharge of two lavatories.

Test data for a constant rate of flow from the

lavatories are given in figure 24, a condition which

might occur when the lavatory is used by merely

drawing water from the faucets. The data for flow rates of 6 gallons per minute were obtained

by flow from one lavatory, whereas in the tests

0

' HOAOdOvD bAei 1 3 4 5 6 7 LENGTH OF BATHTUB DRAIN IN FEET Figure 24. Effect of length of bathtub drain'^ on wet venting

of bathtub trap.

Two-story duplex system. Continuous flow from lavatories.

Horizontal

distance between wet

vent and

3-inch stack Diameter

of wet

vent

Lavatory vent

fitting

Initial depth

of water in bathtubs

Rate of flow from

lavatory

2 ft. 7 in. 6 ft.

o

A

in.

V/2

2

Lonp-turn T-V

Short-turn T... Short-turn

] 2i4 in. in one tub

> and 6 in. in

1 other tub.

gpm

1 10.8 C

A

H 2

Long-turn T-Y

Short-turn T... Short-turn T...

1 2J^in, in one tub

> and 6 in. in

J other tub 1 6.0

A

Hi

2

Long-turn T-Y

Short-turn T... Short-turn

{214 in. in each

1 tub } 6.0 a

VS

VA

1)4 2

Long-turn T-Y

Short-turn T,.. Short-turn T...

[214 in. in each

I tub. } 10.8

a Slope of bathtub drain, inch per foot.

with a flow rate of 10.8 gallons per minute, 5.4 gallons per minute was flowing from each lavatory.

These particular flow rates were chosen from the

experimentally determined fact that a flow rate of approximately 5 gallons per minute is about the

maximum that can be used in a lavatory in this manner without excessive and undesirable splashing.

Preliminary tests with continuous flow from the

lavatories showed that under certain conditions a

greater reduction in trap seal was obtained when

the tubs did not end their discharge at the same

time. The most convenient method of regulating

the length of time between the end of flow in one

tub and the end of flow in the other was to fill the

tubs to different depths and then pull the rubber

stoppers simultaneously. It was found that a

sufficiently long period between tub discharges was given by filling one tub to a depth of 2}^ inches

and the other a depth of 6 inches. However, the

maximum trap-seal loss does not always occur when the tub discharges end a minute or so apart.

Therefore, tests were also made with both tubs

filled to the same depth.

It was also determined in preliminary tests that

the initial depth of water in the bathtubs had no

effect on the amount of trap-seal loss, provided

the initial depth was greater than a certain

minimum.

The initial depths of water listed in figure 24,

therefore, have no significance except to indicate

that the discharge of the tubs either ended

simultaneously or that they ended approximately one minute apart.

As has been stated, the data in figures 23 and 24

are for tub drain slopes of Yo inch per foot. Tests were also made with tub drains 7 feet long on a

/4-inch-per-foot slope. In no case, with the tub

drain slope of '4 inch were any trap-seal losses observed.

From the data in figures 23 and 24 it is evident

that the adopted permissible trap-seal loss of

1 inch will not be exceeded on the two-story,

duplex system provided the lengths of tub drains

do not exceed 6 feet for a slope of inch per foot

and do not exceed 7 feet (the maximum length

tested) for a slope of % inch per foot. It is clear that the conclusions regarding per- missible lengths of wet-vented tub drains drawn

from the test data obtained with the system

shown in figure 6 may be applied with at least equal safety to wet-vented fixtures on the top

floor of a two-story duplex system.

26 Wet Venting of Plumhing Firtures
IX, Wet-Vented Fixtures on the Lower Floors of Multistory Buildings

All the venting tests made in connection with

this project have been made either on a one-story

installation or on the top floor of a two-story

installation. There were no fixtures discharging;

into the stack on floors above the wet-vented

fixtures being tested. For this reason the test results reported here apply directly only to the

top floor of any structure, where the primary

function of fixture vents is to prevent self- siphonage. In multistory stacks pressures and vacuum are of course created in the stack at the

lower floor levels by the discharge of fixtures on the upper floors. In such cases the fixture vents will be required to relieve stack pressures

and vacua, as well as to prevent self-siphonage.

Since the effect of using wet vents instead of back vents is to reduce the number of vents

available on the lower floors for relieving stack

pressures and vacuums, it is apparent that the

indiscriminate substitution of a single wet vent

to a group of fixtures for the purpose of replacing

several back vents may lead lo undcsiiable

results on the lower floor, uidess other means, such

as the installation of a relief or yoke vent, are provided to control stack pressures and vacuums. However, in this connection it may again ]m

pointed out that Hunter [2] has shown lliat the

wet-vented group of fixtures shown in figure 4, a, may be installed safely on the first floor of a twostory building in which a single l)atlu'oom group

of flxtures, in adtlition to a kitchen sink or combination fixture, are located on the upper floor; and in a later publication [3] prepared in the

form of a code by the Subcommittee on Plumbing,

Central Housing Committee on Research, Design

and Construction, to which Hunter was technical

adviser, this type of installation, or that of figure

6, is permitted on the lower floor without a backvent for the water closet, provided the fixtures on

the upper floor do not exceed a single batlu'oom

group and a kitchen sink or combination fixture.

X. Conclusions

From the test data presented it is concluded

that the wet venting of one or two bathtub traps on the highset branch interval of systems such as those shown in figures 6, 21, and 22 is an adequate

and satisfactory method of venting, provided:

1. The slope, S3, and the length, I3, of the tub

drains and the diameter di of the wet vent are such

that the value of the quantity Ssls/di does not ex- ceed unity;

2. The diameter of the horizontal branch is not

less than I/2 inches when one lavatory connects

to the vent and not less than 2 inches when two

lavatories, or a lavatory and a kitchen sink, or a lavatory and a combination fixture connect to the

wet vent; and

3. The fixtures on the wet vent connect to this vent at the same level. In connection with conclusion 1 above, it will be

recalled that, in computing the values of SJ.s/di, S3

should be expressed in feet per foot or inches per

inch and that ^3 and di should be expressed in the same units of length.

Acknowledgment is made to the Housing and

Home Finance Agency for its support of the

investigation reported in this paper. The authors

also express their appreciation of the cooperation

of the members of the Uniform Plumbing Code

Committee sponsored by the Housing and Home

Finance Agency, and especially that of the Chairman of that Committee, Vincent T. Manas, in formulating the problem investigated and in offer- ing many practical suggestions.

The authors also express their appreciation for the many valuable practical suggestions made by

Ed Monteath, who acted as Industrial Adviser to the Bureau in connection with the investigation.

The experimental work was carried out by Marion

R. Brockman, Anthony L. Lembeck, and Victor

Brame, Jr., and the authors are indebted to these

experimenters for their careful and thorough work

XI. References

[1] John L. French, Stack venting of plumbing fixtures NBS Building Materials and Structures Report

BMS118 (1950).

[2] Recommended minimum requirements for plumbing in dwellings and similar buildings, NBS BH2 (1924).

[3] Plumbing manual, Report of the Subcommittee on

Plumbing, Central Housing Committee on Research

Design, and Construction, NBS Building Materials and Structures Report BMS66 (1940). O

Building Alateriah md Structures Report BMSI19

[4] Emergency plumbing standards for defense housing, Division of Defense Housing Coordination, Executive Office of the President, Office for Emergency

Management (1942).

[5] Recommended minimum requirements for plumbing,

NBS BH13, (1932).

[6] R. B. Hunter, Methods of estimating loads in plumbing

svstems, NBS Building Materials and Structures Report BMS65 (1940). Washington, March 23, 1950.

2T



BUILDING MATERIALS AND STRUCTURES REPORTS

[Continued fronn cover page ii] BMS32 Structural Properties of Two Brick-Coiicrete-Block Wall Constructions and a Concrete-Block Wall Construction Sponsored by the National Concrete Masonry

Association 15^. BMS33 Plastic Calking Materials 15ji BMS34 Performance Test of Floor Coverings for Use in Low-Cost Housing: Part 1 15^ BMS35 Stability of Sheathing Papers as Determined by Accelerated Aging * BMS36 Structural Properties of Wood-Frame Wall, Partition, Floor, and Roof Constructions With "Red Stripe" Lath Sponsored by The Weston Paper and Manufacturing Co lOji BMS37 Structural Properties of "Palisade Homes" Constructions for Walls, Partitions, and

Floors, Sponsored by Palisade Homes * BMS38 Structural Properties of Two "Dunstone" Wall Constructions Sponsored by the W. E. Dunn Manufacturing Co 10^ BMS39 Structural Properties of a Wall Construction of "Pfeifer Units" Sponsored by the Wisconsin Units Co 10^ BMS40 Structural Properties of a Wall Construction of "Knap Concrete Wall Units" Sponsored by Knap America, Inc 15$i BMS41 Effect of Heating and Cooling on the Permeability of Masonry Walls * BMS42 Structural Properties of Wood-Frame Wall and Partition Construction with "Celotex"

Insulating Boards Sponsored by The Celotex Corporation 15jS BMS43 Performance Test of Floor Coverings for Use in Low-Cost Housing: Part 2 15^ BMS44 Surface Treatment of Steel Prior to Painting 10^ BMS45 Air Lifiltration Through Windows 15^ BMS46 Structural Properties of "Scott-Bilt" Prefabricated Sheet-Steel Constructions for Walls, Floors, and Roofs Sponsored by The Globe-Wernicke Co 10^ BMS47 Structural Properties of Prefabricated Wood-Frame Constructions for Walls, Partitions, and Floors Sponsored by American Houses, Lie 20^ BMS48 Structural Properties of "Precision-Built" Frame Wall and Partition Constructions

Sponsored by the Homasote Co 15^ BMS49 Metallic Roofing for Low-Cost House Construction 20^ BMS50 Stability of Fiber Building Boards as Determined by Accelerated Aging lO^i BMS51 Structural Properties of "Tilecrete Type A" Floor Construction Sponsored by the

Tilecrete Co 10^ BMS52 Effect of Ceiling Insulation Upon Summer Comfort 15fS BMS53 Structural Properties of a Masonry Wall Construction of "Munlock Dry Wall Brick"

Sponsored by the Munlock Engineering Co 10^ BMS54 Effect of Soot on the Rating of an Oil-Fired Heating Boiler 10^ BMS55 Effects of Wetting and Drying on the Permeability of Masonry Walls 10^ BMS56 A Survey of Humidities in Residences 10^ BMS57 Roofing in the United States—Results of a Questionnaire * BMS58 Strength of Soft-Soldered Joints in Copper Tubing 10^ BMS59 Properties of Adhesives for Floor Coverings 15^ BMS60 Strength, Absorption, and Resistance to Laboratory Freezing and Thawing of Building

Bricks Produced in the United States 30^ BMS61 Structural Properties of Two Nonreinforced Monolithic Concrete Wall Constructions.- 100 BMS62 Structural Properties of a Precast Joist Concrete Floor Construction Sponsored by

the Portland Cement Association 100 BMS63 Moisture Condensation in Building Walls . 150 BMS64 Solar Heating of Various Surfaces 100 BMS65 Methods of Estimating Loads in Plumbing Systems 150 BMS66 Plumbing Manual 350 BMS67 Structural Properties of "Mu-Steel" Prefabricated Sheet-Steel Constructions for Walls,

Partitions, Floors, and Roofs, Sponsored by Herman A. Mugler 150 BMS68 Performance Test for Floor Coverings for Use in Low-Cost Housing: Part 3 200 BMS69 Stability of Fiber Sheathing Boards as Determined by Accelerated Aging 100 BMS7G Asphalt-Prepared Roll Roofings and Shingles 200 BMS71 Fire Tests of Wood- and Metal-Framed Partitions 200 BMS72 Structural Properties of "Precision-Built, Jr." Prefabricated Wood-Frame Wall Construction Sponsored by the Homasote Co__ 100 BMS73 Indentation Characteristics of Floor Coverings 100 BMS74 Structural and Heat-Transfer Properties of "U. S. S. Panelbilt" Prefabricated SheetSteel Constructions for Walls, Partitions, and Roofs Sponsored by the Tennessee

Coal, Iron & Railroad Co 200 BMS75 Survey of Roofing Materials in the North Central States 150 BMS76 Effect of Outdoor Exposure on the Water Permeability of Masonry Walls 150 BMS77 Properties and Performance of Fiber Tile Boards : 100 BMS78 Structural, Heat-Transfer, and Water-Permeability Properties of Five Earth-Wall

Constructions 250 BMS79 Water-Distributing Systems for Buildings 200 BMS80 Performance Test of Floor Coverings for Use in Low-Cost Housing: Part 4 150 BMS81 Field Inspectors' Check List for Building Constructions (cloth cover, 5 x 7}i inches) 200

•Out of print. [List continued on cover page iv]
BUILDING MATEBIALS AxND STRUCTURES REPORTS

[Continued from cover page iii] BMS82 Water Permeability of Walls Built of Masonry Units 25fS BMS83 Strength of Sleeve Joints in Copper Tubing Made With Various Lead-Base Solders 15^ BMS84 Survey of Roofing Materials in the South Central States 15^ BMS85 Dimensional Changes of Floor Coverings With Changes in Relative Humidity and

Temperature lOfi BMS86 Structural, Heat-Transfer, and Water-Permeability Properties of "Speedbrik" Wall

Construction Sponsored by the General Shale Products Corporation 15^ BMS87 A Method for Developing Specifications for Building Construction—Report of Subcommittee on Specifications of the Central Housing Committee on Research,

Design, and Construction 15fi BMS88 Recommended Building Code Requirements for New Dwelling Construction With

Special Reference to War Housing * BMS89 Structural Properties of "Precision-Built, Jr." (Second Construction) Prefabricated Wood-Frame Wall Construction Sponsored by the Homasote Co 15^ BMS90 Structural Properties of "PHC" Prefabricated Wood-Frame Constructions for Walls,

Floors, and Roofs Sponsored by the PHC Housing Corporation 15^ BMS91 A Glossary of Housing Terms 15^ BMS92 Fire-Resistance Classifications of Building Constructions 30fS BMS93 Accumulation of Moisture in Walls of Frame Construction During Winter Exposure-. lOji BMS94 Water Permeability and Weathering Resistance of Stucco-Faced, Gunite-Faced, and

"Knap Concrete-Unit" Walls 15jS BMS95 Tests of Cement-Water Paints and Other Waterproofings for Unit-Masonry Walls 25^ BMS96 Properties of a Porous Concrete of Cement and Uniform-Sized Gravel lOfi BMS97 Experimental Dry-Wall Construction With Fiber Insulating Board 10^ BMS98 Physical Properties of Terrazzo Aggregates 15ji BMS99 Structural and Heat-Transfer Properties of "Multiple Box-Girder Plywood Panels" for Walls, Floors, and Roofs 15fi BMSlOO Relative Shpperiness of Floor and Deck Surfaces 10^ BMSlOl Strength and Resistance to Corrosion of Ties for Cavity Walls lOfi BMS102 Painting Steel 10^ BMS103 Measurements of Heat Losses From Slab Floors 15^ BMS104 Structural Properties of Prefabricated Plywood Lightweight Constructions for Walls,

Partitions, Floors, and Roofs Sponsored by the Douglas Fir Plywood Association. _ 30{i

BMS105 Paint Manual with particular reference to Federal Specifications $1. 00

BMS106 Laboratory Observations of Condensation in Wall Specimens * BMS107 Building Code Requirements for New Dwelling Construction * BMS108 Temperature Distribution in a Test Bungalow With Various Heating Devices lOfS BMS109 Strength of Houses: Application of Engineering Principles to Structural Design $L 50 BMSllO Paints for Exterior Masonry Walls 15fi BMSlll Performance of a Coal-Fired Boiler Converted to Oil 15^ BMS112 Properties of Some Lightweight-Aggregate Concretes With and Without an Air- entraining Admixture lOjS BMS113 Fire Resistance of Structural Clay Tile Partitions 15^ BMS114 Temperature in a Test Bungalow With Some Radiant and Jacketed Space Heaters 25^ BMS115 A Study of a Baseboard Convector Heating System in a Test Bungalow 15^ BMS116 Preparation and Revision of Building Codes 15^S BMS117 Fire Resistance of Walls of Lightweight Aggregate Concrete Masonry Units 20^ BMS118 Stack Venting of Plumbing Fixtures 15ji BMS119 Wet Venting of Plumbing Fixtures 20^

•Out of print.