Page 150 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

130 Cha pte r T h ree

shifts. By using a water closet rate of 5 uses per day as given in Table 3.2.1, and
substituting DO for FTE, Eq. (3.2.2) simplifies to Eq. (3.3.10) such that the estimated
total annual number of water closet usages per year is

TWCU = 365 × DO × 5 for residential uses in number of uses per year (3.3.10)

In like manner, Eqs. (3.3.1) to (3.3.3) can be modified for annual lavatory, kitchen sink,
and shower usage rates in number of uses per year to

TLFU = 365 × DO × 5 for residential uses in number of uses per year (3.3.11)
TKSU = 365 × DO × 4 for residential uses in number of uses per year (3.3.12)
TSU = 365 × DO for residential uses in number of uses per year (3.3.13)

Prerinse spray valves and metered lavatory faucets are typically not applicable to
residential uses and are therefore not included. Similar to the commercial and institutional
derivation, now both the baseline and the design annual water volume usage rates for
the fixtures applicable to WE prerequisite 1 and WE credit 3 can be determined. The
applicable indoor water usage baseline (IWUB), in gallons per year, can be calculated
using the durations in Table 3.3.1 and the fixture flow rate numbers in Table 3.3.2:

IWUB = (1.6 × TWCU) + [2.2(1.0 min) × TLFU]
+ [2.2(0.25 min) × TKSU] + [2.5(8 min) × TSU]
for residences, gal/yr (3.3.14)

To calculate the applicable design indoor water usage (IWUD), the factors in Eq. (3.3.14)
are replaced with those appropriate for the alternate fixture type i used such as LFR ,
i
KSR , and SR from the design (see Table 3.3.2 for some typical values):
i i
IWUD = (WCR × TWCU) + [LFR (1.0 min) × TLFU]
i i
+ [KSR (0.25 min) × TKSU] + [SR (8 min) × TSU]
i i
for residences, gal/yr (3.3.15)
Note that the only differences between the base water usage rate and the design water
usage rate are the fixture flow rates.
Similar to the cases for commercial/institutional buildings, a reduction in potable
water used for blackwater fixtures can be due to a reduction in the flows, the alternate
use of nonpotable water in these fixtures, or a combination of both. Therefore, Eqs.
(3.3.8) and (3.3.9) can be used for WE prerequisite 1 and WE credit 3 compliance
calculations. However, there will rarely be a case in which reuse waters or rainwaters
(GW) are used in water closets for residential uses. Thus, the required WE prerequisite
1 is fulfilled when there are no reuse waters or rainwaters used for typical indoor
fixtures if the following is true:

IWUD ≤ for no reuse waters or rainwaters (GW)
IWUB . 080
used in indoor residential fi xtures (3.3.16)