Quantcast Domestic hot water systems (DHW)

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MIL-HDBK-1003/13A
Fiberglass and plastic tanks are corrosion resistant and easily installed.
They are available in many shapes and sizes.  Although many commonly
fabricated tanks will begin to soften at temperatures above 140 deg.-160 deg.
F, there are more expensive, specially fabricated tanks available that can
withstand temperatures up to 250 deg. F.  The types of plastics needed to
store large quantities of water at high temperatures can be more expensive
than steel.
When storage tanks are to be custom made, a calculation of heat loss against
expected fuel cost inflation will almost always justify increasing insulation
around the tank to R-19, 6 inches, compared with the usual 2 inches.  HUD
Intermediate Minimum Property Standards (U.S. Dept HUD, 1977) requires that
tank losses be limited to 10% in 24 hours.  Usually R-19 insulation will
satisfy this requirement.  It is a good practice to insulate tank supports
from the ground if possible.
Costs of storage tanks vary considerably depending on the quality of
construction and the distributor.  Table 2-7 summarizes advantages and
disadvantages and Table 2-8 gives approximate comparative costs for tanks of
various materials.  All storage tanks for liquids should be located so that
if they leak, damage to the building will be prevented.  The cost of housing
the tank or burying it must be included in the total cost of the solar
heating system.  Buried tanks must be protected from ground water, and
buoyant forces resisted.  Underground tanks are not preferred, if other
options are available.  Tanks must be reasonably accessible for repairs.  In
very mild or warm climates, outdoor location may be feasible.  Tank
connections should comply with local codes with regards to backflow
preventers, safety relief valves, etc. (see Section 2.9.3).
2.3 Domestic hot water systems (DHW).  Domestic hot water systems (DHW)
(without space heating) may use lined, insulated, pressurized tanks similar
to the conventional water heater.  Appropriate temperature and pressure
relief valves must be used.  Since it is possible for solar collectors to
reach very hot temperatures, a tempering or mixing valve should be used.  A
typical two-tank installation with proper valves and connections would be as
shown in Figure 2-11 (Cole et al., 1979).
To size the collectors and storage tank it is necessary to estimate or
measure the hot water consumption of the facility or building.  For typical
family residences, 20 gal/day/person of hot water is normally consumed.  If
it is estimated the hot water consumption is larger than average, use 30
gal/day/person.  So, 80 to 120 gal/day should serve a typical four-person
family.  For estimates of other Navy facilities refer to DM-3 series,
Plumbing Systems.  Table 2-9 (Werden and Spielvogel, 1969) gives water
consumption data for different types of conventional facilities and may be
used to supplement the DM-3 data.
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