A filtering system for use with a reservoir. The filter system includes a filter assembly capable of filtering particles sized 1.0 micrometers and larger and may, include a flow control device positioned to selectively provide fluid flow to the filter assembly. The flow control device may include an electrically actuated valve, such as a solenoid valve. The filter system may also include a fluid level detection mechanism operatively connected to the flow control valve. The filtering system is especially applicable to humidifier systems having a heat source