Product review (5 stars): Easy install
The HVAC company did a winter tuneup on our furance.  They mentioned our Aprilaire 550 humidifer wasn't working and quoted $230 to repair it (including a new solonoid).  I bought this solenoid and a maintence kit (pad, hose, etc.) on Amazon all for $70 including shipping.  Simple 30 minute install and the humidifer is working like new again.  You can't go wrong with this product if you have an Aprilaire 550.