Sizing Water Supply
Piping
Determining the size of water supply
piping is the next step in the process. The size
of the pipe and fittings depends on several
interrelated factors:
• Water pressure at the meter.
• The amount of water needed by each
fixture.
• Friction loss due to length of pipe and the
number and type of fittings.
Local code regulates pipe size. A series of
tables used to factor in the variables mentioned
may be presented in the local code. Water pres-
sure at the meter can be determined by
consulting with the local water authority. The
amount of water required for each fixture is
given in fixture units and can be referenced in
a table that is part of the code. Friction loss can
be estimated by determining the approximate
length of each pipe run, from either the floor
plan or by measuring distances in the building.
On the schematic drawing (Figure 20-1),
consider the runs of pipe farthest from the
water meter first. The cold water line from
bathtub branch to hose bibb A is approximately
15′ in length. Referring to a table in the model
code notes that a hose bibb is rated at 2.5
fixture units. Another table in the code indi-
cates that 1⁄2″ diameter pipe is sufficient for this
installation. Fixture units, as well as the
approximate length and diameter of each
section of pipe, are noted on the schematic
drawing in Figure 20-1.
The model code provides a fixture unit
value of 1.5 for hot water and 2.7 for cold water
in a bathroom group of fixtures including a
water closet, lavatory, and tub with shower.
The cold water valve is larger because the
water closet does not use hot water. The hot
water pipes from the water heater to the tub
faucets are approximately 21′ long and must
provide for a load of 3.0 fixture units (1.5 × 2
bath groups). The table from the code indicates
that 1⁄2″ pipe is sufficient. Likewise, 21′ of cold
water with a fixture unit load of 7.9 (2.7 for
each bath group + 2.5 for the hose bibb) can be
supplied by 1⁄2″ pipe. Therefore, the cold water
pipe from the water heater to the bathrooms
can also be 1⁄2″.
There is an alternative to the piping layout
that would help balance the load on the hot
and cold water pipes to the bathroom. Rather
than running the pipe to the hose bibb as an
extension of the cold water pipe to the bath-
room, a separate pipe could be run from near
the meter. This would reduce the fixture unit
load on the cold water pipe to the bathrooms
by 2.5. In some instances, this may make it
possible to install smaller diameter pipe than
would otherwise be required. This type of
installation also reduces the possibility of
noticeable fluctuation in cold water pressure at
the shower head when the hose bibb is oper-
ated. Such fluctuations can cause significant
changes in the water temperature at the shower
head. Another option is to install a branch to
serve both hose bibbs ahead of the water filter.
This reduces the volume of water that runs
through the filter. Since the water is used for
watering the yard and cleaning outside,
filtering may not be necessary.
When sizing the water supply pipe for the
kitchen and laundry facilities, consider the
kitchen first. The code indicates that the fixture
unit load for a kitchen group (sink with or
without a garbage disposal and a dishwasher)
is 1.9 for hot water and 1.0 for cold water.
Given the relatively short runs of pipe, it is
Chapter 20 Installing Water Supply Piping
347
Local code: Code that regulates size and types of pipe
to be used in installation.
Kitchen group: Sink with or without a garbage
disposal and a dishwasher.
In an effort to conserve water, the following
maximum flow rates have been established in
model codes:
Lavatory faucet 2.2 gallons per minute (gpm)
Shower head 2.5 gpm
Sink faucet 2.2 gpm
Toilet 1.6 gallons per flush cycle
Code Note