Product review (5 stars): Don't forget the water orifice & strainer
I'm an HVAC technician, and have replaced dozens of these.  Remember to order an&nbsp;<a data-hook="product-link-linked" class="a-link-normal" href="/Aprilaire-4004-In-Line-Strainer/dp/B0015S4KIY/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">Aprilaire 4004 In-Line Strainer</a>&nbsp;and the correct orifice for your humidifier model when you order this solenoid valve, because it doesn't come with them.  If you forget the orifice, don't be surprised when water starts gushing out of the humidifier, because the orifice regulates the amount of water fed to it.  The strainer goes in the long end of the solenoid fitting, and the orifice in the short end.

<a data-hook="product-link-linked" class="a-link-normal" href="/Aprilaire-4231-Yellow-Orifice/dp/B000I0UQCE/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">Aprilaire 4231 Yellow Orifice</a>: Aprilaire models 400, 500, 550, 550A, 560, 568 & 600.
<a data-hook="product-link-linked" class="a-link-normal" href="/4021-Aprilaire-Red-Orifice/dp/B002UR6MOY/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">#4021 Aprilaire Red Orifice</a>: Aprilaire models 112 & 224.
<a data-hook="product-link-linked" class="a-link-normal" href="/4232-Aprilaire-Blue-Orifice/dp/B002VBZR04/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">#4232 Aprilaire Blue Orifice</a>: Aprilaire models 350, 360, 440, 700, 760, 760A & 768.
<a data-hook="product-link-linked" class="a-link-normal" href="/4007-Aprilaire-White-Orifice/dp/B002UQZP70/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">#4007 Aprilaire White Orifice</a>: Aprilaire models 110 & 220.