Sizing & Placement Guideline

Supply Drainage Support Specialties

DETERMINING ARRESTER

SIZE BY FIXTURE UNITS

The National Plumbing Code offers

this denition of xture unit: “A xture-

unit is a quantity in terms of which the

load producing effects on the plumbing

system of different kinds of plumbing

xtures are expressed on some

arbitrarily chosen scale.” The xture

unit values shown in Table 1 below

represent the standard ratings used

by engineers to size water distribution

systems as well as water hammer

arresters. “Public” xtures, as referred

to in Table 1 below, are xtures found in

public rest rooms, ofce buildings and

other places where each xture is open

and accessible for use at all times.

MULTIPLE FIXTURE

BRANCH LINES

On many types of applications, a single

arrester must serve multiple xtures.

In these cases, the total xture units

should be determined for all xtures

served by the branch line where the

arrester is to be placed. Once the

xture units for the branch line have

been totaled, choose the appropriate

arrester by matching xture units in the

table (below) to the arrester size with

the corresponding xture unit capacity.

If the total number of xture units has

a fraction, it should be rounded to the

next largest whole number. In addition,

if the ow pressure at the xture

exceeds 65 psig, the next largest size

water hammer arrester should be used.

ARRESTER PLACEMENT ON

MULTI-FIXTURE BRANCH LINES

Once the correct size arrester has

been determined, the nal concern is

placement of the arrester within the

system. Arrester placement depends on

the length of the branch line on which

the arrester is to be installed, which

can be divided into two cases which are

described below:

BRANCH LINES OF

20 FEET OR LESS

(See Figure 1)

Place arrester at the end of the branch

line within 6 feet of the last xture

served, as illustrated on page 15.

BRANCH LINES

OVER 20 FEET

(See Figure 2)

Calculate xture units for each 20-foot

section separately and place an arrester

at the end of each 20-foot section

(within 6 feet of the last xture served

in that section) as illustrated in Figure 2

on page 15.

The following should be used as a guide only. Always consult local plumbing codes for specic requirements before installation.

For water hammer control in commercial plumbing applications, such as water closets, urinals and lavs in public restrooms, use the

following sizing and placement guidelines based on xture units.

FIXTURE

TYPE OF SUPPLY

CONTROL

FIXTURE UNITS

PUBLIC PRIVATE

TOTAL C.W. H.W. TOTAL C.W. H.W.

Water Closet 1.66 PF Flush Valve 8 8 - 5 5 -

Water Closet 1.66 PF Flush Tank 5 5 - 2.5 2.5 -

Pedestal Urinal 1.06 PF Flush Valve 4 4 - - - -

Stall or Wall Urinal Flush Valve 4 4 - - - -

Stall or Wall Urinal Flush Tank 2 2 - - - -

Lavatory Faucet 2 1½ 1½ 1 1 1

Bathtub Faucet 4 2 3 2 1½ 1½

Shower Head Mixing Valve 4 2 3 2 1 2

Bathroom Group Flush Valve Closet - - 8 8 3

Bathroom Group Flush Valve Closet - - 6 6 3

Separate Shower Mixing Valve - - 2 1 2

Service Sink Faucet 3 3 3 - - -

Laundry Tubs (1-3) Faucet - - - 3 3 3

Combination Fixture Faucet - - - 3 3 3

Clothes Washer Solenoid Valves - - - 4 3 3

Dishwasher Solenoid Valve - - - 1.5 - 1.5

Ice Maker Solenoid Valve - - - 1 1 -

ARRESTER SIZE AA A B C D E F

FIXTURE UNITS 1-4 5-11 12-32 33-60 61-113 114-154 155-300

TABLE 1

TABLE 2

Sizing & Placement Guideline