UFC 3-420-01
25 October 2004
Lodges, and bachelor housing units provided with individual SWH's. For
family housing, select the SWH size from Table 5 based upon the number of
bathrooms and bedrooms in the unit. For Navy lodges, use the 1 to 1.5
baths, 2 or 3-bedroom column, since entire families of 3 to 5 persons or more
occupy these single rooms for extended stays of up to 30 days. For bachelor
housing projects with individual SWH's for each unit or a pair of units, use the
number of baths and bedrooms to select from Table 5.
506.1.2 Sizing central SWH systems. Use ASHRAE Chapter 48, Table 9 to
size central SWH systems, except revise the numbers in row "9. Showers" by
multiplying by the correction factor calculated in Section 506.1.4. The
revision reflects Public Law 102-486 maximum fixture flow of 2.5 gallons per
minute (9.5 L/s), ASHRAE recommended service water storage temperature
minimum of 140F (60C), personnel safety maximum fixture delivery
temperature of 110F (43C), and the appropriate supply design cold water
temperature, which varies according to location and season. Use the Hotel
column to size central SWH systems for Navy Bachelor Housing and Lodges,
due to occupant's schedules resembling those of hotel and motel occupants
the rooms are inhabited mostly between 5 PM and 7 AM. Use Appendix E of
UFC 3-420-01, Design: Plumbing Systems for sizing the domestic water
heaters in Army Barracks.
506.1.3 Hot water mixing equation.
(Conversion Factors: 1gpm = 3.8 L/s;
degree Fahrenheit = 9/5 degree Celsius + 32)
Let: Qf = Fixture flow in gpm = 2.5 gpm per Public Law 102-486
Qs = Flow from SWH Storage Tank in gpm
Qc = Flow from Cold Water Supply in gpm
Tf = Temperature at fixture in degrees F = 110 F for personnel safety
Ts = Temperature at SWH Storage Tank in degrees F, 140 F minimum for
Legionella
Tc = Temperature at Cold Water Supply in degrees F, varies due to location
and season
Mixing Equation: (Qf x Tf) = (Qs x Ts) + (Qc x Tc)
Substituting Qc = Qf Qs gives the following:
(Qf x Tf) = (Qs x Ts) + ((Qf Qs) x Tc)
(Qf x Tf) = (Qs x Ts) + (Qf x Tc) (Qs x Tc)
(Qf x Tf) (Qf x Tc) = (Qs x Ts) (Qs x Tc)
Qf x (TfTc) = Qs x (TsTc)
Qs = Qf x [(Tf-Tc)/(Ts-Tc)]
Substituting Qf = 2.5 gpm, Tf = 110 F, Ts = 140 F, and Tc = 50 F gives the
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