Aprilaire Model 56 Humidistat Circuit Diagram


Old 12-19-20, 03:42 PM
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Aprilaire Model 56 Humidistat Circuit Diagram

We have an Aprilaire 700 humidifier controlled by a Model 56 Humidistat. Humidifier didnít work this year. In troubleshooting, I realized that the humidistat is powered by 23VAC from the furnace and send 14VDC to energize a relay in the humidifier which enables a fan and the water valve (through a transformer reducing the 120 to 23 VAC). In troubleshooting I found that the 14 VDC is no exiting the Humidistat. However, the 14VDC relay in the Humidistat is being energized when the call for humidity occurs (audible click occurs). I was amazed by the sophistication of the Humidistat. There appears to be a digitizer for the thermistor, a PROM to run a program to combine the temp and humidity measurements, a 14VDC power conversion, solid state switching via MOSFETS, timer for measuring amount of time the water pad is used, and heaven knows what else. Bottom line is I would like to see the Circuit details of the Humidistat. Any body have a circuit diagram of the Model 56 Humidistat? Thanks!
Natosha voted this post useful.
Old 12-19-20, 05:51 PM
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Welcome to the forums.

There is no DC voltage involved. Strictly 24vAC in the control circuit.

However..... there are two 700 humidifiers. You need to know which one you have. They both plug into 120v to control the internal fan. The old pre 2007 almond colored case uses an internal 24v transformer and requires the two input wires to be just shorted for operation. This may use DC switching.

The newer gray colored unit requires 24vAC to be applied to the input wires to close an internal relay for operation.

The HH terminals on the humidistat are just switch contacts. No voltage comes out of them.
This is a fairly old humidistat. They've come out with two or three upgrades since that model. There are no schematics floating around as these are proprietary designs and owned by Aprilaire. Install pdf manual below.
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Aprilaire 56-install.pdf (1.29 MB, 11 views)

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