CALCULATION OF NET POSITIVE SUCTION HEAD FOR PUMPS
All GARN® wood heating units are zero pressure closed systems as opposed to:
Open system – replaces the vast majority of its contained water daily. A good example of this is a domestic water heater. Pressurized closed system – replaces little if any of its contained water on a yearly basis and operates with an internal pressure of 15 to 30 PSIG. A good example is a standard hot water boiler that is used for space heating. A zero pressure closed system does not develop internal pressure due to its unique open vent system. Such systems do replace a minor volume of contained water on a yearly basis. The designer must consider net positive suction head (NPSH) when selecting pumps for such systems. Proper selection will prevent cavitation and suction boiling that can: destroy the pump; prevent the system from attaining its rated heating capacity; or air lock the hydronic system totally. Graphs of pump performance and net positive suction head requirements are available from pump manufacturers. In all cases, the NPSHA available must be greater than the required NPSH for a specific pump. Generally, lower RPM pumps have lower NPSH requirements.
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Hydronic System Design Manual ©DECTRA CORPORATION - March 2013
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