Sizing Domestic Water Heater Systems - Two Sample Calculations

Sizing Domestic Water Heater Systems – Two Sample Calculations

Using Method 1

A new apartment building will have 16 three-bedroom units. Each unit will have a clothes washer, kitchen sink, dishwasher, two lavatories, and two tubs with showers. The owner has requested a storage tank system. Assume that each apartment will house a maximum of 5 people. The owner is marketing the building to middle-income households.

16 kitchen sinks x 10 = 160

16 dishwashers  x 15 = 240

32 lavatories          x 2 = 64

32 tubs/showers x 20 = 640


Subtotal                      1104

Demand factor          x 0.30


Total demand               331gph (minimum recovery per hour)

Storage factor          x 1.25


Storage                       414 gallons (minimum recommended storage capacity)

Based on information and experience, the engineer must determine how much storage is most suitable to the application. Remember that in selecting storage tank equipment, the calculated storage volumes are “usable” storage volumes. With the stratification typical inside a storage tank, only approximately 70% of calculated total volume is usable. For this example, the actual minimum storage tank size is 414/0.70= 591 gallons.

Using Method 2

1. Determine occupant demographic classification: The apartment is being marketed to middle-income people, so a low-demand occupant classification is selected.

2. Determine peak usage and maximum storage volume times: Using the 30/3 recommendation from the ASPE Manual (http://aspe.org/), a peak usage of 30 minutes and a maximum 3-hour storage volume are selected.

3. Determine peak demand and maximum storage volume: Using the Manual, the 30-minute peak demand is determined to be 1.7 gal/person. The maximum 3-hourusage (storage) is 6.1 gallons.

4. Perform calculations: 80 people x 1.7 gallon per person = 136 gph (this is the minimum recovery per hour); 80 people x 6.1 gallon per person = 488 gallons (this is the recommended storage volume).

5. Perform storage tank volume calculation: 488/0.70 = 697 gallons

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