Water System Design Manual, December 2009
Document source : www.doh.wa.gov
Water System Design Manual
6.4.3 Part-time Versus Full-time Residences
Water demand design data must correlate to the number of full-time or part-time equivalent
residential units in service at any time (WAC 246-290-221(1)). "Full-time" is a permanent place
of residence. "Part-time" is a vacation home, used only seasonally, such as on holidays or
weekends. The rule makes this distinction because water systems designed only for part-time
residences tend to convert gradually over time to full-time residences (due to retirement,
changing housing markets, and other factors).
Water systems designed only for part-time residences cannot be expected to provide service
levels adequate for full-time occupancy. Unless obligatory covenants or other binding
agreements prohibit full-time occupancy, engineers should consider any part-time residence a
full-time residence (one ERU) for design purposes. This concept reduces concerns associated
with part-time residents changing to "full-time" without sufficient water supply and delivery
facilities. This concept also applies to part-time versus full-time multifamily residences.
Note: When using meter records to establish the ERU quantity, be sure to account for any part-
time uses that occurred during the record-keeping period. Only services that are active during
the time of metered data collection should be used. Be sure to confirm the correlation between
meter information and the various types of service (residential versus nonresidential) when
determining the fundamental single-family residential criteria for a water system. This is
particularly important if the available data is only sufficient to estimate ADD.
6.4.4 Multifamily Residences ERUs
Multifamily residences typically use less water per living unit (dwelling) than separate single-
family residences. Water uses for multifamily residences vary from water system to water
system. They are usually specific to a given water system, but not always applicable to another
water system. When calculating ERUs, engineers should view multifamily usage data apart from
single-family usage. To determine how multifamily-metered connections contribute to the water
system's overall number of ERUs, divide the total peak-day water use for the multifamily
connection(s) by the water system-specific peak-day single-family ERU quantity.
6.4.5 Nonresidential Customer ERUs
To analyze overall demands, engineers can group nonresidential customers by type and select
average equivalency factors for each group classification (average ERU per type of
nonresidential customer). A nonresidential customer with large water demands (such as
agricultural uses, a pulp mill or plating process), should be analyzed separately and included in
the ERU totals.
To calculate ERUs for water systems that serve only nonresidential connections, engineers can
use the default estimates in Appendix D or comparable water-use data from an analogous
nonresidential water system. The ERUs would be the total maximum day demand measured or
estimated for nonresidential use, divided by the ERU quantity (gpd/ERU) for MDD (see Section
Page 48 December 2009 Water System Design Manual 6.4.3 Part-time Versus Full-time Residences Water demand design data must correlate to the number of full-time or part-time equivalent residential units in service at any time (WAC 246-290-221(1)). To determine how multifamily-metered connections contribute to the water system's overall number of ERUs, divide the total peak-day water use for the multifamily connection(s) by the water system-specific peak-day single-family ERU quantity.