UFC 3-420-01
25 October 2004
Including changes 1 and 2, October 2006
housing, Lodges, and bachelor housing units provided with individual SWH's.
For family housing, select the SWH size from Table
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\ 4 /1/ 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
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\ 4 /1/.
506.1.2 Sizing central SWH systems. Use ASHRAE Chapter
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Table
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\ 8 /1/ 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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