The NAHB Research Center has learned that installing an undersized
water softener in a residential plumbing system can reduce the
flow of water to some of the fixtures in the home. Undersizing
can also reduce the water softeners efficiency, causing
it to deliver "hard" water.
The Research Center has also learned that, in certain circumstances,
installation of a water softener in a home with a residential
fire sprinkler system can potentially cause safety problems by
reducing flow and pressure below the sprinkler system's requirements.
The NAHB Research Center has published a guide to help builders
and plumbers avoid these problems when installing water softeners.
Water Softener Flow Capacity
The pressure and flow rate of water in home plumbing systems
are reduced when the water passes through water softeners. Softeners
are manufactured in a range of flow capacities to accommodate
the requirements of different homes. Their size must conform to
plumbing code requirements and must accommodate the house's water
Utilizing provisions of the U.S. model plumbing codes (UPC,
SBCCI, CABO, and IPC), the Research Center's guide helps builders
and plumbers to determine the proper flow capacity for water softeners
on the basis of the number and type of plumbing fixtures in the
Many water softeners have rated flow capacities of five to
eight gallons per minute. Plumbing code requirements and actual
water demand for many homes can be considerably greater. In addition,
requirements for anticipated water demand are different in various
local plumbing codes.
For example, the Uniform Plumbing Code (UPC) requires that
a house with two-and-a-half baths must have a water flow capacity
of seventeen gallons per minute, while the CABO Plumbing Code
requires that a similar size house must have a flow capacity of
eight gallons per minute. In order to comply with code requirements,
it is imperative to determine which plumbing code is used in the
city or county where the home is located.
Fire Sprinkler Systems
Safety issues can be raised by incorrect installation of water
softeners in homes with fire sprinkler systems. When possible,
water softeners should be installed downstream of the take-off
for the fire sprinkler system, to avoid impact on the sprinkler
system's flow and pressure needs.
If water softeners are installed upstream of a sprinkler system,
they can reduce downstream flow and pressure below the system's
Pressure requirements for home sprinkler systems, which are
specified in National Fire Protection Association Standard 13D,
are a flow of twenty-six gallons per minute plus pressure of about
twelve psi. Pressure loss for water passing through water softeners
can be about fifteen psi. A pressure of at least twelve psi must
remain after water has passed through the water softener.
In combined systems in which the sprinkler system piping supplies
cold water to domestic fixtures, the water softener cannot be
isolated from the sprinkler system and will reduce the flow and
pressure to the system. The size and flow of the water softener
must account for these factors.
Labeling and Certification
Prior to purchasing a water softener, builders and plumbers
should check the device's flow rate and pressure drop. These should
appear on a label affixed near the softener's water connection,
or in the manufacturer's literature.
The manufacturer should also state that the water softener
has been tested by a third-party certification agency for compliance
with the requirements of Water Quality Association Standard 100.
Purchasers can take the following additional steps:
- Ask the water softener contractor for technical literature
on the product;
- Ask the water softener contractor to document compliance
with the local plumbing code;
- Check with the local plumbing code enforcement agency for
- Check with the Water Quality Association, 630-505-0160, for
product certification status;
- Review Plumbing Code Requirements and Sizing Guide for Residential Water Softeners.