Specifications

37
Copyright© by AOS 2015. All rights reserved.
Sizing and Performance
Determining Eciency
To determine how much hot water the heater can produce
and how fast it can produce it, recovery capacity, recovery
eciency, tank draw eciency and standby eciency will
have to be considered.
Recovery Capacity
Recovery capacity is the number of gallons of incoming
cold water per hour the water heater can heat to a specic
temperature. The dierence between the incoming cold
water temperature and the hot water produced is called
the temperature rise.
The following formula is used to calculate the recovery
capacity of gas water heaters. Input is measured in BTUs.
A BTU (Brish Thermal Unit) is the amount of energy
required to heat one pound of water by 1°F.
Input is the rated input of the water heater, as shown on
the rang plate. In this example it is 40,000 BTUs.
Eciency is the water heater’s recovery eciency; 75% in
this example. The weight of one gallon of water is 8.25
pounds. A 90°F temperature rise is used in this example:
The following formula is used to calculate the recovery
capacity of electric water heaters. Electric water heaters
are considered to be 100% ecient when calculang
recovery capacity.
KW represents kilowas. The number of BTUs in one KW is
3412. The weight of one gallon of water is 8.25 pounds.
The top example shows that 4.5KW at a 90°F rise will heat
20.7 gallons per hour. The boom example shows that
5.5KW at a 90°F rise will heat 25.3 gallons of water per
hour.
When comparing the results of the previous examples, you
might noce that the gas water heater is capable of
heang water about twice as fast as the electric model.
Gas water heaters have much faster recovery rates due to
their higher BTU input. Higher BTU inputs and higher
waages decrease recovery me
Calculang Recovery Capacity
in Gallons per Hour for
Gas Water Heaters
Recovery =
(Input x Eciency)
(8.25 x Rise
Example: A 40,000 BTU gas water heater
that has a 75% recovery eciency with a
90 degree rise.
Recovery =
(40,000 x .75)
=
30,000
(8.25 x 90)
742.5
= 40.4 gallons per hour
Calculang Recovery Capacity
in gallons per Hour for
Electric Water Heaters
Recovery =
(KW x 3412)
(8.25 x Rise)
Example 1: A 4.5KW electric water heater
at 100% eciency with a 90 degree rise.
Recovery =
(4.5 x 3412)
=
15,354
(8.25 x 90) 742.5
= 20.7 gallons per hour
Example 2: A 5.5KW electric water heater
at 100% eciency with a 90 degree rise.
Recovery =
(5.5 x 3412)
=
18,766
(8.25 x 90) 742.5
= 25.3 gallons per hour