UFC 3-420-01
25 October 2004
APPENDIX E
DOMESTIC WATER HEATERS FOR ARMY
BARRACKS
1. Purpose. This appendix provides basic design guidance for the sizing of
domestic water heaters for barracks buildings at Army installations.
2. Background. There have been questions regarding the sizing for domestic
water heaters for barracks. Designers have often used ASHRAE design
criteria for motels or dormitories with some over sizing. However the peak
demand for domestic hot water in barracks is significantly larger than motels
or dormitories due to the concentrated shower pattern of the occupants. This
appendix provides a uniform approach to determine domestic water heating
requirements for Army barracks, and will be used for the planning, design
and construction of new and renovated facilities.
3. Guidance.
a. General
(1) As with any domestic water heating system, storage tank
temperature should be maintained at a minimum of 60C
(140F) to reduce the potential for Legionella Pneumophila
(Legionnaire's Disease). Note that higher storage temperatures
will result in a lower recovery rate to satisfy the peak demand.
Include a cross connection with a mixing valve between the
domestic water supply and hot water supply lines (leaving the
storage tank) to limit the temperature of water distributed to
plumbing fixtures to 43C (110F). Require a strainer upstream
of the thermostatic mixing valve on both the hot- and cold-water
connections to protect the small orifice in the mixing valve from
debris that may be in either line.
(2) If a backflow preventer is installed in the domestic water main
entering a building or in the line supplying the domestic hot
water system, include provisions to accommodate thermal
expansion. In barracks this can be critical as occupants often
shower at the same time and after this peak usage event, the
hot water loop will be at a relatively low temperature. During the
subsequent recovery phase the water temperature will rise
causing the water to expand. If little hot water is being used, this
expansion will cause a pressure increase that may activate a
relief valve or rupture the storage tank. The best method to
alleviate this excess pressure is to install an expansion tank,
sized in accordance with Chapter 12 of the 1996 ASHRAE
Systems and Equipment handbook.
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