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Heating, cooling & air
October 2008
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Sizing calculator for storage tank heaters*
Modern appliances can reduce your hot water needs

Enter a value for each event that takes place during your household’s heaviest hour of usage. Use our standard averages or input the gpm of your specific fixtures or appliances and the number of minutes that they will be in use during this hour of use.

All averages are from Consumer Reports data except where otherwise indicated.

How to calculate gpm


Use Avg. gallons of hot water
per usage		 Times used
during first hour		 Gallons used
in first hour
Standard Custom
Shower1
(post 1992 fixture, per showerhead)		 x x
Bath2
(Adult)		 x x
Bath3
(Child)		 x x
Whirlpool tub4
 		 x x
Clothes washer5 **
(top loader)		 x
Clothes washer6
(top loader)		 x
Clothes washer7
(front loader)		 x
Clothes washer8
(front loader)		 x
Hand dishwashing9
 		 x x
Dishwashers
(new)		 x
Dishwashers
(10 years or more old)		 x
Shaving10
 		 x
Hand Washing11
 		 x
First Hour Rating
+
 
 
  • * Experts stress that a detailed consultation with an experienced plumber about your water use habits is the best way to make the most accurate estimation of your heater size. These calculations are intended to provide a basis for such consultation by illustrating how particular incidents of water usage affect your total needs.
  • ** When estimating clothes washer hot-water requirements, use only warm water wash and rinse if you never do hot washes and rinses. Otherwise, use hot-wash and rinse-water volumes.

  • 1American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) value adjusted for water temperature
  • 2ASHRAE value adjusted for water temperature
  • 3ASHRAE value adjusted for water temperature
  • 4Estimate based on 75 gallon tub x .80 (volume filled) x .80 (percentage of hot water). Check the specifications for your model to estimate hot water required.
  • 5CR estimate for hot wash and rinse
  • 6CR estimate for warm wash and rinse
  • 7CR estimate for hot wash and rinse
  • 8CR estimate for warm wash and rinse
  • 9ASHRAE value adjusted for water temperature
  • 10ASHRAE value adjusted for water temperature
  • 11ASHRAE value adjusted for water temperature
Posted: September 2008 — Consumer Reports Magazine issue: October 2008