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The Universal Hydronics Formula

Universal Hydronics Formula

It's been called the most important formula in HVAC hydronics. Here is a breakdown of the Universal Hydronics Formula.

The Universal Hydronics Formula, often called the "Hydronic Formula," is a fundamental equation used in hydronic heating and cooling systems. These systems utilize water or another liquid as a heat transfer medium to provide heating, cooling, or both to a building or space. The formula helps calculate the energy transfer in such systems and is essential for designing and optimizing their performance.

The Universal Hydronics Formula is expressed as:

Q = 500 x GPM x dT


Q is the heat transfer rate in British Thermal Units per hour (BTU/hr).

ΔT is the temperature difference between the supply and return water (in degrees Fahrenheit).

GPM is the flow rate of the heat transfer medium in gallons per minute.

500 comes from one gallon of water that weighs 8.33 pounds (the weight of water at 60F) times 60 minutes in one-hour times the specific heat of water.

It breaks down like this:

500 = 8.33 lb/gallon x 60 minutes/hour x 1.0 Btu/lb/F

Specific heat refers to the amount of heat energy required to raise the temperature of one pound of a substance by one degree Fahrenheit.

The specific heat will change if you have glycol or any solution other than 100% water.

This formula calculates the amount of heat energy transferred through the hydronic system per unit of time. It considers the temperature difference between the input and output water, the heat transfer medium's flow rate, and the medium's specific heat.

Using this formula, engineers and designers can accurately size components of a hydronic system, such as pumps, pipes, and heat exchangers, to ensure efficient heat transfer and optimal system performance. Additionally, it helps determine the heating or cooling capacity required to meet a building or space's comfort needs.

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