Useful Plumbing Design Formulas

Tables & Tips

Domestic Water

Domestic Hot Water

• 1 BTU will raise 1 pound of water 1° F.
• 1 BTU will raise .12 gallons of water 1° F.
• 1 KW = 3413 BTU° F.

Water Heating Calculations

• Natural Gas
• BTU/Input = GPH x 8.25 x Temp.Rise x 1.0 - divided by - %Efficiency of heater
• Gal/Per/Hour = Btu/Hr Input x %Efficiency - divided by - Temp.Rise x 8.25

• Electric
• KW = GPH x 8.25 x Temp.Rise x 1.0 - divided by - 3413 Or… GPH x Temp.Rise - divided by - 414
• Gal/Per/Hour = KW x %3413 - divided by - Temp.Rise x 8.25 Or… KW x 414 - divided by - Temp.Rise

BTU CONTENT of VARIOUS FUELS

Natural Gas

• 1 psi = 28" water column.
• 1 psi = 16 ounces.
• 1psi = 2.036 inches of mercury

Storm Drainage

• 1 inch rainfall rate = .0104 gallons per minute per square foot.
• 1 cubic foot per second = 448.831 gallons per minute.
• Formula (rational method) to Determine Storm Runoff on a Site Q = CIA Q = Runoff in cubic feet per second C = Coefficient of runoff I = Intensity of rainfall in inches per hour A = Drainage area in acres (43,560 square feet per acre)

(Note: Composite value depends upon relative amount of lawn area)  

Miscellaneous

Volume of Liquid in a Tank

• Gallons = 5.875 x Diameter squared x Height {dimensions must be in feet}
• Gallons = (Radius squared x 3.14) x Length divided by 231 {dimensions must be in inches}

Tips

• Remember to provide a properly sized expansion tank in the cold water supply to a hot water heater when a check valve or backflow preventer is installed in the water service.
• The jockey pump capacity for a fire pump should be 2 to 3 percent of the fire pump capacity and be set to start at 10 psi above the start pressure setting of the fire pump.
• Natural gas is lighter than air. Propane and Butane are heavier than air. Propane and Butane normally are supplied in cylinders at a pressure of 80 to 120 psi. LPG cylinders should not be located in vaults or pits unless they are mechanically ventilated. When LPG cylinders are located in a room, vents must be located at floor level.

 

Domestic Water

2.31 feet of head = 1 psi 1 foot of head = .433 psi 28" of water column = 1 psi
1 gallon weighs 8.33 pounds 7.5 gallons occupy 1 cubic foot 1 gallon occupies 231 cubic inches

Domestic Hot Water

1 BTU will raise 1 pound of water 1° F.
1 BTU will raise .12 gallons of water 1° F.
1 KW = 3413 BTU° F.

Water Heating Calculations


Natural Gas
BTU/Input = GPH x 8.25 x Temp.Rise x 1.0 - divided by - %Efficiency of heater
Gal/Per/Hour = Btu/Hr Input x %Efficiency - divided by - Temp.Rise x 8.25
Electric
KW = GPH x 8.25 x Temp.Rise x 1.0 - divided by - 3413 Or… GPH x Temp.Rise - divided by - 414
Gal/Per/Hour = KW x %3413 - divided by - Temp.Rise x 8.25 Or… KW x 414 - divided by - Temp.Rise
BTU CONTENT of VARIOUS FUELS

Fuel Amount BTU
Coal 1pound 10,000 - 15,000
Coal 1 ton 25,000,000
Electricity 1 kilowatt 3,412
#2 Oil 1 gallon 138,500
Butane 1 pound 21,300
Butane 1 gallon 102,600
Butane 1 cubic foot 3,260
Natural Gas 1 cubic foot 1,000
Propane 1 pound 21,600
Propane 1 gallon 91,000
Propane 1 cubic foot 2,570

Natural Gas

1 psi = 28" water column.
1 psi = 16 ounces.
1psi = 2.036 inches of mercury