Is 50% outside of the Aprilaire humidistat operating range?

Saturday, December 10, 2011 by Aprilaire Team
Consumer question:

I have a 700 unit that my installer (just because he was lazy, I guess) installed in Manual mode. Now that heating season is here, I have noticed something odd with the humidity in my home. The humidifier panel on the furnace has been reading around 50% humidity. My humidistat up on the second floor, however, has been reading 32-38%, and it feels a bit dry at times.
I have the control knob set to 7 (max), but because the panel reads 50% humidity, the humidifier never kicks on when I'm watching it, because I guess 50% is above when it would turn on, even with the dial at 7. The unit does operate when I turn it to test mode.

I guess my main question it odd that the control panel is reading 50%, while the upstairs of the home is 15% lower? I figured the furnace would churn up the air to make these numbers more equal, and therefore be triggering the humidifier to come on, and raise the 35% number.

As I mentioned, I do not have it hooked up in Automatic mode at this time, but it seems to me that doing that would not really remedy the situation, because the panel in the basement would still be reading the "50%" humidity that it is picking up.

Any thoughts on this range of humidity readings, and is it normal? Any suggestions or concerns?

Aprilaires' response:

You are correct that 50% would be outside the operating range of the humididistat. I would try to put it into test reset to try to force it to take a new reading. There could be some discrepancies since the upstairs unit is pulling from one specific area and the duct mounted humidistat is taking an average.
If test-reset does not clear the reading we could potentially do a hard reset pulling power at R&C from the control

Comments for Is 50% outside of the Aprilaire humidistat operating range?

Sunday, January 1, 2012 by Chad:
I have the same situation as described above. I went ahead and removed the humididistat and placed it outside the return plenum. I then placed my humididistat next to the aprilaire control. They read 10% different and my movable humididistat that I keep in my room always reads lower. I don't understand this. I have done a hard reset and it does not help.

Does anyone know why the reading is always higher. Is it possible these humididistats are defective?
Thursday, December 5, 2013 by Kyle:
Just wanted to comment that I also have the same issue. I have two humidistats in my home in two different locations, but they both typically read within 3% of eachother. My Aprilaire humidifier control, however, always reads about 10% higher. This is a concern because I can't seem to get enough humidity in my home even with the control set to 7.

Could a new controller provide better results?
Thursday, December 5, 2013 by brad:
Thank you for your inquiry regarding the operation of your Aprilaire digital humidifier control. Each control is going to display the humidity in the duct the control is mounted to . Since the temperature in one HVAC system is typically different than the other, the humidity levels on the 2 Aprilaire humidifier controls will be different as well. As temperature changes the humidity levels in the air will change as well. The Aprilaire digital humidifier controls have an accuracy of +/-3% and can control humidity up to a maximum level of 45%. The accuracy of other humidity measuring devices may be more or less accurate than our controls. Whenever your Aprilaire digital humidifier control is reading above 45% it will not add humidity to the air. Please be aware our humidifiers are designed to maintain an indoor humidity level of 35% anytime the indoor temperature is 70F and the outdoor temperature is 20F or above.
Saturday, January 4, 2014 by Tommy:
i have my aprilaire 558 humidifier and tried each setting and changed the filter in my humidifier but it keeps reading low on my thermostat. the outside tempature is currently -20c. what can i do to bring it up to 30%-35% it's very dry in the house and windows freeze up which shows me that the humidifier is not at the proper setting. another thing humidifier is working and we did try putting it into test mode first to reset the system but that hasnt work PLEASE HELP!!!!! im at my wits end and tired of going up and down the stairs.
Wednesday, January 8, 2014 by brad:
Thank you for your inquiry regarding the operation of your model 558 Aprilaire humidifier. The indoor humidity our humidifier control is designed to maintain at an outdoor temperature of -20C(-4F) is approximately 22%. A level of 30-35% can cause ice on windows. To determine the actual humidity level in your home based on our humidity control please refer to page 5 of the attached manual. This manual also describes the proper humidity to maintain. If the control is left in test mode it will only operate for one minute and can never operate again until the dial is taken out of test mode.

Thank you for giving us the opportunity to be of assistance.

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