Essential Hydronic Formulas for HVAC and Plumbing Systems

Mastering these common hydronic formulas is vital for calculating flow rates, pressure drops, and system efficiency. Below are key formulas used in hydronic heating and cooling applications.

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Flow Rate Calculation

To determine the required flow rate for proper heat transfer:

GPM = BTUH / (500 × ∆T)

Where:

• GPM = Flow rate in gallons per minute

• BTUH = British Thermal Units per hour (heat load)

• 500 = Constant for water at 60°F

• ∆T = Temperature difference in °F between supply and return water

This formula helps size circulators and determine system flow requirements.

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Pump Power Formula

For calculating pump horsepower (HP):

HP = (GPM × Head) / (3960 × Efficiency)

Where:

• GPM = Flow rate (gallons per minute)

• Head = Pressure (in feet) that the pump must overcome

• 3960 = Conversion constant

• Efficiency = Pump efficiency as a decimal

This formula helps size pumps for adequate performance.

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Pressure Drop Calculation

To determine pressure loss through piping:

∆P = (L × r × v²) / (2 × D)

Where:

• ∆P = Pressure drop (psi)

• L = Pipe length (feet)

• r = Fluid density (lb/ft³)

• v = Flow velocity (feet/second)

• D = Pipe diameter (feet)

Pressure drop calculations help ensure efficient system operation.

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Heat Transfer in Water Systems

For calculating heat transfer rates:

Q = 500 × GPM × ∆T

Where:

• Q = Heat transfer rate (BTUH)

• 500 = Constant for water at 60°F

• GPM = Flow rate (gallons per minute)

• ∆T = Temperature difference in °F

This formula helps calculate heating or cooling loads in hydronic systems.

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Expansion Tank Sizing

To size an expansion tank:

ET = V × (EF / (1 - EF))

Where:

• ET = Expansion tank size (gallons)

• V = System water volume (gallons)

• EF = Expansion factor (based on water temperature range)

Proper expansion tank sizing helps prevent excessive pressure buildup.

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Boiler Efficiency Formula

For calculating boiler efficiency:

Efficiency (%) = (Output BTUH / Input BTUH) × 100

Where:

• Output BTUH = Useful heat output (BTUH)

• Input BTUH = Fuel energy input (BTUH)

Higher efficiency ensures better system performance and reduced energy costs.

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