Page 340 - Handbook of Energy Engineering Calculations

1. Find the daily DHW heating load
A typical family in the United States uses about 20 gal (9.1 L) of hot water
per person per day. Hence, a family of six will use a total of 6(20) = 120 gal
(54.6 L) per day. Since water has a specific heat of unity (1.0) and weighs
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8.34 lb/gal (1.0 kg/dm ), the daily DHW heating load is L = (120 gal/day)
(8.34 lb/gal) [1.0 Btu/(lb · °F)](140 − 50) = 90.072 Btu/day (95,026 kJ/day).
This is the 100 percent heating load.

2. Determine the average solar insolation for the collector

Use the month of January because this usually gives the minimum solar
insolation during the year, providing the maximum collector area. Using Fig.
13 for eastern long Island, where Northport is located, we find the solar
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insolation H = 580 Btu/(ft · day) [1829.3 W/(m · day)] on a horizontal
surface. (The horizontal surface insolation is often used in DHW design
because it provides conservative results.)