Question about Aprilaire 550 Humidifier

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Looks like my humidistat calls for coil fan and for furnace heat

I have an Aprilaire 560 and recently it started to give me headaches. I think that the humidifier should work only when the coil fan is on…and that should happen only when the thermostat calls for heat on this zone. The fan is on “AUTO” and is controlled by the thermostat not by the furnace. If I change settings on my humidistat dial while the coil fan is off, I hear the “click” and the fan is starting instantly…after a while the furnace goes too and the temperature goes beyond the thermostat program. If I lower the humidity level just under the typical “click”, the furnace and the fan stops.
The only way to get some humidity without reaching 78F is to let the fan on “ON” instead of “AUTO” and to raise the level on my humidistat…this way the furnace follows the thermostat program set for 73F.
Any suggestions?

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  • wargold51 Jan 23, 2009

    Same problem.

  • thom46 Feb 07, 2009

    I had a similar problem with my 560. The humidifier was tricking the furnace computer into calling for heat. My furnace short cycled all day long, still providing heat when the stat called for it, but the fan would NEVER shut down. I thought I had a bad limit switch, then a bad stat, then after saying bad words I decided to treat it like a computer problem - disconnected all wires to the brain board and reconnected one at a time. After adding the stat wires the furnace began behaving normally. Soon as I added the aprilaire wire - same problems started up again. That's how I found the problem

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It sounds like the humidifier may be improperly wired.
It's hard to solve without checking the wiring, but it seems like the humidistat is wired so that as the dial is advanced, it incorrectly turns on the fan, or furnace, or both.  The contacts of the humidistat should be wired so that they turn on the solenoid only.  Also, the power to the humidistat, to turn on the solenoid, should come on only when the fan is on.
Hope this helps!

Posted on Jan 28, 2009

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