Product review (2 stars): Pressure Switch issues
I'm not a huge fan of this unit.  I have a heat pump with a variable speed fan.  In the winter I  find that my unit runs as much if not more with a low fan speed than it does on high speed.  The pressure switch won't recognize the air flow in my duct system on low speed, therefore the solenoid won't engage unless my fan is on high speed.  So the unit never adequately humidifies my house.  I ended up just trashing the pressure switch and getting a friend of mine help me direct wire the solenoid so it comes on when the heat pump is in heat mode.