Calculate chilled water Btu, flow rate, or temperature difference from any two inputs, with Btu/kJ, gpm/lpm, and °F/°C conversions.
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Chilled Water Btu Formula
The chilled water heat transfer formula uses flow rate, temperature difference, and the standard water factor of 500 when flow is in gallons per minute and temperature difference is in degrees Fahrenheit.
- Btu/hr = chilled water heat transfer rate in British thermal units per hour
- GPM = chilled water flow rate in gallons per minute
- DeltaT_F = temperature difference between entering and leaving water in degrees Fahrenheit
- 500 = approximate water constant based on water weight, specific heat, and minutes per hour
If you leave the Btu field empty, the calculator multiplies flow rate by temperature difference and 500. If you leave the flow rate empty, it divides Btu by the temperature difference and 500. If you leave the temperature difference empty, it divides Btu by the flow rate and 500.
When you use liters per minute or Celsius temperature difference, the calculator first converts the values to gpm and °F difference before applying the formula.
Common Chilled Water Temperature Difference Values
Typical chilled water systems are often checked against the temperature difference across a coil, heat exchanger, or plant loop.
| Temperature Difference | Equivalent | Common Meaning |
|---|---|---|
| 6°F | 3.3°C | Low delta T, often indicates high flow or poor heat transfer |
| 10°F | 5.6°C | Common design value for many chilled water systems |
| 12°F | 6.7°C | Moderate delta T, often used in efficient system designs |
| 16°F | 8.9°C | Higher delta T, usually means lower flow is needed for the same load |
Example Calculations
Example 1: Calculate chilled water Btu/hr
You have a flow rate of 80 gpm and a chilled water temperature difference of 10°F.
The chilled water heat transfer rate is 400,000 Btu/hr.
Example 2: Calculate flow rate
You need to move 600,000 Btu/hr with a 12°F temperature difference.
The required chilled water flow rate is 100 gpm.
FAQ
Is chilled water Btu the same as Btu/hr?
For chilled water flow calculations, the result is normally a heat transfer rate in Btu/hr, not a single amount of heat in Btu. The common formula 500 × gpm × ΔT gives Btu per hour.
Why is 500 used in the chilled water formula?
The factor 500 comes from the approximate weight of water, the specific heat of water, and 60 minutes per hour. It is a simplified constant for water-based HVAC calculations. For glycol mixtures or fluids with different properties, the constant is different.
What does a low chilled water temperature difference mean?
A low temperature difference means the water is not picking up as much heat as expected as it passes through the load. This can happen because of excess flow, fouled coils, control valve issues, low airside load, or other heat transfer problems.