ETL 1110-3-489
3 April 98
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.
(3) Normally if a central energy plant is available it will be life cycle cost effective (an
analysis must be performed) to generate domestic hot water using distributed media as the
heat source. If there is no central plant or if the plant does not operate during the non-
heating season provide a separate domestic hot water source. If the site consists of
several barracks buildings, evaluate the life cycle cost of these alternatives:
(a) A single boiler to provide low temperature hot water, 94C (200F) to heat
exchangers in each barracks for domestic hot water generation.
(b) A single heating source for domestic hot water in each individual barracks
separately.
If the barracks building is not part of a building complex and there is no central plant or
distribution system available then a single heating source must be provided.
b. Calculations: The following information applies to all types of domestic water
heating systems for barracks.
(1) Peak Demand: Design for the case where all occupants are taking showers at
essentially the same time. Public law limits the maximum flow from of each shower head
and lavatory faucet to 0.16 Lps (2.5 gpm). It is assumed during peak demand that the
shower requires 7.5 minutes and the lavatory requires 2 minutes of full hot water flow. As
a result, the peak domestic hot water demand can be found using equation (1) below:
Td
Vp = (h)(occ)(P)
(1)
Ts
where:
Vp = volume of domestic hot water required at peak, L (gal).
h = number of shower heads, ea.
occ = number of occupants using each shower, ea.
Td = temperature of water delivered to shower valve = 43C (110F)
Ts = temperature of water in storage tank, degrees C (degrees F)
P = amount of water used per occupant during peak demand, 90 L/occupant
(23.75 gal/occupant)
2
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