Hydronic System Flow Rate Formula and 20 Degree TD
Hydronic Systems:
Assuming cold-water with a specific gravity of 1.0.
GPM Flow rate needed: Use this to figure out flow needs.
Flow Rate = (Net Btuh Load)/((8.33lbs x 60 Min. x Delta T Goal))
Heating systems are typically designed delta T is 20 degrees with water. The equation can be simplified because a gallon of water weighs 8.33 lbs. per gallon and there is always 60 minutes in an hour x 20 for a Delta T to get 10,000 on the bottom (Technically 8.33 x 60 x 20 = 9,996 but 10,000 is close enough.
Flow Rate = (Net Btuh Load)/(10,000)
For Example:
The boiler is 125,000 BTUH. So, what do you need for a flow rate through the system to get a 20°F Temperature drop?
Flow Rate = (125,000 BTUHR)/10,000
Flow Rate = 12.5 GPM