Please be aware of the safety hazards & control limitations when running pressure sensitive unloaders without a flow or vacuum switch for burner control. Dangerous steam explosions can occur if check-valve components in the pressure sensitive unloader fail in a way that locks pressure up stream of the unloader when the gun is shut. On some systems with limited controls, this pressure lock can keep the burner control pressure switch closed and the burner fuel solenoid open for heat demand at the wrong time.
Since pressure sensitive unloader systems are required in most multiple gun configurations, the use of a flow switch or vacuum switch can help reduce the risk of incorrect burner function and steam explosions. Flow sensitive unloaders rarely fail in this way, allowing the use of the pressure switch and temperature switch to serve for burner control. Remember to inspect pressure relief valves and burst disk safety device when servicing the heating system for either of the two unloader control types.
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