Aprilaire Humidifier Model 600 squeals shortly after being turned on

do you have a test mode on your humidistat? does it turn on when you put it to test?

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

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.

humidifier should only get power when indoor blower motor is running in the heat mode.

if the wire to the outdoor sensor runs close to high voltage wiring it may cause that

You should have a #50 relay hooked up to the main blower assembley to turn the 24 volts on and off with the blower to the humidistat. Alot of humidifiers sold do not tell you about the #50 relay. They assume that you are a hvac pro and you aready know about.
Good luck
HVAC Teacher

No, it is not an accurate statement.

These units have simple configurations They have a water supply similar to a ice making referigerator, an enclosure connected to your supply duct that houses a prop fan and media, a seperate humidistat and a drain for excess water. Your unit may also include fancy controls including an outside thermometer & humidistat to avoid over humidification that can cause problems during freezing weather. The humidistat looks a lot like a thermostat and is located in your living space or in the return duct. The prop fan in the box pulls air from the duct, through evaporation media and back to the duct. An electric solenoid valve releases water that pours over the media.

If the humidiy is below the humidistat seting and the AC/heater is running, the Aprilaire fan starts and the solenoid valve opens. I suspect the facier controls include the flashing warning light.

The warning light spells problem. Aprilaire's customer service should be able to provide guidance for the homeowner in the form of simple checks. I suspect your contractor is avoiding a warentee call and the trip it requires.

A word of caution. These humidifiers can be powered directly from your air conditioning unit control board from an auxiliary connection. If the control board isn't rated to carry the current required by the Aprilaire fan, valve and controls or the Aprilaire encounters and electrical problem, the board can be damaged. This can be avoided by using a relay. The AC board switches a low energy relay. 120 volt house power is connected to load side of the relay and runs the Aprilaire.

My board cooked at a location near this connection, but I can't be sure that the load from the humidifier caused it since four of our neighbors also lost their board. It failed the evening of a snow storm and required a costly emergency after hours service call. The next day a new board required a three hour drive. Fortunately a fan powered propane heater kept our pipes from freezing.

Turn the control knob to the test/reset position untill the change water panel light blinks to reset the change panel timer. Make shure the furnace is running calling for heat.Turn control knob to prior setting after resetting timer. Hope this helps.

There are 3 things to check on the humidifier. Check the metal mesh screen, is it wet after the humidistat is call? If it is dry the water solenoid my not be opening or the nozzle is plugged. The circulating fan blows air through the wet screen to add humidity. Does the fan spin when the humidistat is calling? There is an intake and exhaust on the humidifier. Check that the metal was cut out properly when the humidifier was installed to allow air to feely circulate?