Aprilaire 700. It works on fan on mode but not otherwise. It's never on and the humidity reading is always way below the setting. It should turn on automatically to bring the humidity up, right?

Is this a newly installed unit it sounds as if there's a wiring issue. Could you show a picture of the wiring terminals. If this is not a new install try turning of your furnace fan.

Hi check out the stat is working also check water line valve is not stuck open or there is some thing stuck on the seating

If your humidifier is working properly, then the problem lies outside of the humidifier. For example how long does your furnance run when the thermostat calls for heat? If your furnace is oversized for your home, it will heat your home relatively quickly, and turn off, while your humidifier does not have a chance to work long enough for it to increase the humidity. If this is the case, you could try to set the thermostat to run the fan in manual mode instead of automatic. This will give the humidifier a chance to increase the humidity.

Hope this helps.

It sounds like it is not wired into the 24v fan controlon the furnace.

if you mean your aprilaire humidifier isn't working then open the cover and remove the 4 screws on the fan motor mount inside and check for voltage at the transformer input 120 volts and output 25.5 volts a.c., then check the control board wherever you have it located on the furnace or upstairs and make sure yhe humidity level is set high enough to make the unit run(you will hear the water solenoid click on the furnace below the aprilaire unitwhere the water line is if its working correctly),then check the humidistat on the outside of the building as they can go 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.

if it is set up right, you should be able to run your fan 100% of the time and the humidifier will come on when there is a need for humidity. you can experiment with leaving the fan on and see if this helps. it will run your electric bill up a little and also you will have a lot of waste water. that is a drawback to the bypass type humidifiers. also a factor is if the water supply is hooked up to the hot or cold side. it will humidify better on the hot side, but you tend to get more deposits that plug the solenoid over time and you will be running your water heater quite a bit. 1600 sqft may be pushing the limit of this humidifier.

When your unit is on normal the outside unit works and the inside only the fan runs. your heat pump does not remove any humidity when it operates so the installation cotractor did not wire it to operate then. you can change the wiring on the humidifier to operate all the time by wiring it to the fan (g) terminal then it should work just turn it all the way to the lowest humidity when you are in cooling mode.

clogged solonoid? clean filter and orifice