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When used at altitudes of 2000’ or more above sea level, gas-fired heater recovery capacities must be
derated 4% for each 1000’ above sea level in order to reflect actual recovery.
Recovery Capacity means hot water at the heater recovery rate minute after minute, hour after hour. If
the hot water demand period is more than 3 or 4 hours, recovery capacity usually becomes more
important than storage capacity.
Heater recovery capacity plus usable storage capacity must be sufficient to supply the amount of hot
water consumed during the peak demand period.
CAUTION: Many tables refer only to gallons per hour recovery. Be certain that the heater will
also meet your gallons per minute requirements.
Storage Capacity and Tank Efficiency
The heater tank provides a source of instant hot water, over and above the heater recovery rate. However,
the supply of hot water in the tank cannot be replenished until the recovery capacity of the heater
exceeds the demand upon the system. This is usually after the peak hot water demand period has
ended.
Obviously the actual availability and temperature drop of any system will depend upon the hot water
demand flow rate and piping concept.
The potential for hot water temperature drop during the demand period must be kept in mind by the
system designer when establishing the tank temperature. For example, while the hot water temperature
guide, page 3, lists showers at 105°F, the system temperature is actually set for 140°F. A mixing valve
would limit hot water temperature supplied to person use fixtures to 120°F. In this way the ability to
handle a 30°F drop during the demand period is built into a design. The water temperature at the end
of the demand would still be above that required by the use . . . about 110°F. Were the system
temperature designed to 105°F, the tank size would have to be about half again as large because there
Tank size is usually more important than recovery capacity when large quantities
of hot water are required in a short period of time . . . less than
3 or 4 hours.
All of the stored hot water is not available from the tank at the desired system
temperature. This is because hot water is pushed from the system by entering
cold water, resulting in temperature dilution of the water in storage.
The term usable storage is employed to indicate the quantity of water which
must be available from the tank before dilution reduces temperature to an
unusable level. Therefore, tank size should be increased by a percentage to
cover the expected loss of hot water temperature so enough usable water
will be available.
When a specific drop off characteristic for a system is unknown or tank efficiency is
not given, 70% availability within a 30° F temperature drop during the demand period
may be applied to the tank of a heater or system. For systems requiring precise
delivered temperatures, figure 60% availability from the tank.
would be no “extra” heat in the water
to “stretch” the tank contents. The
water temperature would also drop
below that required by the use. So
heating water above the needed
temperature in systems employing
tanks is common as it reduces tank
size through the added heat energy
available in the stored water.