Quick Calculations for Hydronic Heating Systems

We receive numerous calls regarding the flow rates on hydronic systems. The following are a couple of charts/ formulas that may help eliminate many of the calls.

Maximum Flow Rates for Closed System, Hydronic Heating
Copper GPM Iron Pipe GPM
½" 1.5 ½" 2
¾" 4 ¾" 4
1" 8 1" 8
1 ¼" 14 1 ¼" 16
1 ½" 22 1 ½" 24
2" 44 2" 48

Flow rate calculations @ 20 degrees F drop = 1 GPM for each 10,000 BTUH

What does the above mean? Assume you have a 300,000 BTUH load on a closed hydronic system with two zones. First zone is 130,000 BTUH, and the second is a 170, 000 BTUH. You can determine from the Flow rate calculation that you will need 13 GPM for the first zone and 17 GPM for the second zone. Based on the flow capacity in the pipe sizing chart, you can determine that you will need a 2 copper or iron pipe connected to the boiler with a manifold to 1 ? copper/ iron pipe for zone one, and 1 ? for zone two.
IE: 300,000 BTUH (Total system) = 30 GPM = 2 copper/iron pipe required
10,000 BTUH
130,000 BTUH (zone 1) = 13 GPM = 1 ? copper/iron pipe required
10,000 BTUH
170,000 BTUH (zone 2) = 17 GPM = 1 ? copper/iron pipe required
10,000 BTUH

**Note: These calculations work well on normal residential system piping. The above do not take into consideration excessive piping restrictions and/or excessive friction losses due to unusual/long piping lengths. Exceptional system piping must be determined using piping loss/flow calculations.