How to Calculate Pump Size for an Outdoor Wood Stove

Measure the total length of piping that is used in the closed pipe system of the wood stove boiler. Also note the diameter of the piping.

Inventory the number and configuration of all the fittings used along the pipeline. Add up the resistance of each fitting according to the tables provided by the fitting manufacturer, based on the diameter of the piping.

Note the viscosity rating for the fluid in the closed system.

Identify the necessary flow rate for the length of your system by consulting the flow rate chart that came with the boiler system.

Calculate the head pressure necessary for your boiler system using the following formula: feet of pipe times resistance per foot plus resistance of the total fittings times 0.04 (conversion to head ft.).

Calculate the gallons per minute (gpm) for the system. Use a piping chart to determine this value. Most systems use 3/4-inch pipe that carries about 40,000 btu/h at a flow of 4 gpm at 27 inches per second. If you are using a glycol mix in your pipes, an adjustment in flow needs to be calculated. For a 30 percent glycol fluid, multiply the gpm by 1.187; for 50 percent, multiply by 1.349.

Locate the head ft. and gpm values on a pump sizing chart. This will give you the size of pump required for your system. Most units will fall between 3 and 10 head ft. and would require only a low-flow pump.