Need to shut off aprilaire model 550 humidifier

Of course make sure unit is plugged in and all fuses are good or reset. Here is the owners manual for all trouble shooting: hope this helps.
http://www.aprilaireparts.com/manuals/aprilaire-550-owners-manual.pdf

Hi follow the water line from the water valve back to the cold inlet pipe should be a tap on there

Hi 550 will do 1500sqft its on the wed just google the model its all there

My Aprilair 560 turns on and off repeatedly.Making a loud banging sound.Changed my water valve worked 1 day and started agin.

Should the water flow through the water pannel al the time the blower is running?

go to
http://www.aprilaire.com/index.php?znfAction=OwnersManual

close the by pass air flow valve. shut off the water supply and finally flow the thermostat wire to the transformer or the controller and shut off or disconnect. Unit should be serviced before winter. Cell may need to be replaced

This sounds like, the 550 transformer is not connected to the blower motor power connection (in parallel)

Thanks for your question,
what you need is called an ( Ice maker kit ) any hardware store or appliance store should carry these, the copper tubing is 1/4 in, the kit comes with the service valve for turning on the water once installed, be careful, turning it on before the line is hooked will get ya soaked, it has compression fittings, make sure you dont cross thread the nuts when installing, you may have to shut the main water supply off to the house before doing this, the instructions are fairly easy to read, you can get 25ft - 100ft ice maker kits, you are going to need a tubing cutter and a cresent or box end wrench, and a big flat head screw driver, hope that help ya, lets us know, thanks again for the question,
mr.grzz

You can call 1-800-955-7075 give them the make and model #.The part # should be 4217,10x113/8 frame for $14.56 plus shipping