Hydronic GPM Calculator
Hydronic systems move heat with water. To carry a set heat load at a chosen temperature drop, the water has to flow at a set rate. This works out that rate in gallons per minute.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
For water, gallons per minute equals the heat in BTU per hour divided by 500 times the temperature drop in degrees Fahrenheit. The 500 comes from 60 minutes, about 8.33 pounds per gallon and one BTU per pound for each degree.
The heat load can be entered in BTU per hour or kilowatts, and it is converted first. The temperature drop is always in degrees Fahrenheit.
Worked example
A heat load of 60,000 BTU/h with a temperature drop of 20 degrees needs 6 gallons per minute of water.
Questions
Why is 500 in the formula?
It is 60 minutes times about 8.33 pounds per gallon, times one BTU per pound for each degree. It is a constant for plain water.
Does antifreeze change the number?
Yes. Glycol mixtures have different density and heat capacity, so use the mixture's own figures for those systems.
Where do I get the temperature drop?
It is set by the system design. The equipment's data sheet or the designer's plan is the place to find it.
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