Product review (4 stars): Beware:  Designed backwards
I just replaced the original Aprilaire Solenoid that was on my humidifier with the Generlaire 4040.  I noticed that the new one had a built in metal filter screen, but it was on the "output" side of the Solenoid.  Swapping out the old valve with the new one was easy enough.  However, when I tried to test the system, I had no water flow.  I had bought 2 replacement valves (since we have 2 furnaces), so I thought I would try the other new valve, thinking the first one was just faulty.  That is when I noticed the arrow on the valve was pointed in the opposite direction as the original Aprilaire valve.  So, I put the first new valve back into the system "backwards," now with the built-in screen on the input side.  The humidifier is now working like it should.  In addition, while the first new valve had the built-in metal screen, the second one did not (nor did it come with a replacement plastic filter screen.)  While the valve does seem to be good, I can't give it 5 stars since it was designed "backwards."