The Aprilaire 550A humidifier was working perfect last year. At the beginning of the summer I turned off the humidistat and shut off the water. I turned them on yesterday for the winter but the humidistat does not turn on (no light at all). Which steps to follow to know if I have to change the humidistat or something else? Thank you,

Odd for both to be out. Check the water supply sometimes corrosion or scale will block the saddle piercing valve. Run it all the way into the pipe. Usually need to spin the little valve handle clockwise. Then back it all the way back to full open and reset humidifier power and furnace power. After that may want to crack open water supply connection right before the solenoid at each humidifier to make sure water is getting to them.

I believe they just updated model number descriptions. The basic design of that humidifier hasn't changed much over the years. I would go to their website and look for the one that is simiar to your model and get the humidistat for it. They are pretty basic devices as far as what the humidistats do. 24v power and common

Hello, make sure you are getting 24 volts at the transformer, if not turn off power to transformer and check resistance on the secondary side of transformer, you should have resistance if not replace transformer. If you have 24 volts at transformer than check the wiring and make sure it is tight. the last thing to check is to make sure you are getting 24 volts in and out of the humidistat. if you are not replace humidistat.

Hope this helps, check and see if humidistat is set to 35% humidity and that bypass damper on humidifier is in the open position or winter position.(humidistat is usually located on cold air return duct near furnace). Next, replace humidifier media pad, remember to replace media pad (filter) annually, you also may want to clean any lime deposit out of humidifier housing with a solution of warm water and white vinegar before installing your new media filter.Remember to turn power off at side of furnace when cleaning humidifier and replacing media. Before installing new media turn power back on to furnace, set humidistat to the on position, turn furnace thermostat above room temp., and inspect humidifier to see if water is running from top,(you may want to have a bucket under humidifier when checking for water), if you see water running, turn power back off to furnace, properly install new media filter, reconnect humidifier cover & set humidistat to 35%. If you do not see water present, check water supply valve to humidifier and make sure its turned on. If water supply valve is open, then check water supply tubing for blockage.And the last things to check is to make sure all registers through out your home are open and not blocked by any objects, also make sure furnace filter is cleaned, if none of this works refer to installation manual to see if humidifier was sized and installed properly, Good luck.

It sounds like you got it...

The solenoid will not open with only 13 volts to it. Jump or apply the full 24-27 volts to the solenoid and see if it clicks and opens and allows water to flow.

If the valve still won't open or pass water, start looking for a plugged inlet solenoid inlet screen or a faulty solenoid valve as well as a faulty humidistat.

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

turn the saddle valve in and back out. sometimes the small pin hole for the water gets plugged with something over the summer when the units aren't running and just turning the valve in and back out clears it out and allows the water to flow. check to see that you have water flow at the solenoid and if you do then it is probably the solenoid that is bad.

Humidity must be hi enough in the house or Humidistat is not working. Do you any way to check humidity in the house?
If the Humidistat is set as Automatic, try to leave it as Manual.

The humidifier should not turn on untill the furnace blower starts. Since the fan stays on. does the solenoid valve also stay on? If they both stay on you probably have a bad humidistat. If only the blower stays on then the circuit bourd in the humidifier is defective.
Good luck
HVAC Teacher