Hydronic Formulas
Hydronic Formulas Explained

The hydronic formula is used heavily in the boiler industry. You can calculate the flow in the system or boiler, determine a Delta-T or BTUs gained or lost by applying the correct formula with accurate information.

  There are the 3 commonly used hydronic formulas;
     Delta-T=Q/(500*F) (Calculates Delt-T through boiler or system)
     f=Q/(500*Delta-T) (Calculates flow in boiler or piping)
     Q=500*Delta-T (Calculates BTUs removed or inputted)
     f=Flow Rate in GPM
     Delta-T=Temperature change in water
     500 - Constant physical properties of water

Let's look at an example to determine boiler flow in a cast iron boiler.
     Boiler DOE output = 150,000 btu's

  Using the formula to determine flow is f=Q/(500*Delta-T)
     F=21.4 gpm

The boiler flow rate should be a maximum of 15 gpm (Input divided by 10,000 = 20f Delta-T) has a flow rate over 21 gpm.

Here is the problem.
Residential cast iron boilers the industry usually try to keep the flow rate between a 20f and 40f delta-T. To do this the flow rate must be kept within a range for the boiler. To determining the flow rate is simple. Find the DOE input of the boiler. Let's use the above boiler of 150,000 btu. A 20f delta-T can be determined by dividing the DOE output by 10,000. In our example 150,000/10.000=15 gpm. To get the flow at a 40f delta-T divide the 20f in half. A 20f to 40f Delta-T for a 150,000 btu doe output boiler the flow rate should be between 7.5 and 15 gpm.
Generally speaking a flow rate below 7.5 gpm in this example, would cause the boiler to short cycle lowering the boiler efficiency. If the flow rate is above 15 gpm the boiler would struggle to get the water temperature up in the boiler. This can cause the building to take much longer to reach thermostat setpoint, and possibly condensing for a longer time during operation.
There are variables to consider when changing a boiler or installing a new system. These consist of the following.

  • Quanity of zones
  • Zoning with circulator or zone valves
  • Pipe Sizing
  • Water volume of system
  • Circulator sizing (flow)
  • Boiler Sizing
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