Aprilaire 400A installation


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Old 02-23-21, 04:41 PM
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Aprilaire 400A installation

I installed an Aprilaire 400A on my American Std (Trane) 2 stage variable speed furnace over the weekend. The furnace is model number S9V2D120U5PSAAA and it is tied into both propane and a heat pump. The board in the furnace has both W1 and W2 terminals. I noticed when the propane burner is active terminal W2 to B/C shows voltage so I hooked the white wire going to the humidistat to W2 and the humidifier works. However, when the heat pump is running in heat mode, W2 is not showing any voltage - I also checked W1 when in heat pump mode and it is not reading any voltage either. Any ideas? I apologize in advance if this has already been answered, I found many solutions for various issues but not one for this situation. thanks scott
 
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Old 02-23-21, 04:52 PM
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Welcome to the forums.

You have a dual fuel application. W1 and W2 are furnace only connections.
I looked back at some of the other threads and didn't see that I provided a positive solution.

My recommendation is to use two relays to control the humidifier.
One relay for the heat pump and the other for the furnace.

Your furnace may have a HUM connection that could be used.
Post the model or part number off the furnace control board.

Another method just occurred to me. Use the electronic air cleaner connection.
No relays would be required.
I'll know better when I see what board you have.
 
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Old 02-23-21, 05:09 PM
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Pete, thank you for the quick reply. The board has both EAC and HUM connections which according to the manual are 110V. I only want the humidifier active during heat mode. Attached is a picture of the board.



 
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Old 02-23-21, 05:27 PM
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The EAC terminals are live when using the heat pump. They are also live using the furnace.
You could put a small 24v transformer on the EAC and that would supply switching power to the humidistat.
If you used this method.... you wouldn't use the "FAN ON" mode on the thermostat in the winter.
You'd turn the humidifier off in the summer.

If you don't like that idea you'd need two 24v relays. One connects to C and W on the furnace.
The other connects to C and Y on the thermostat or at the furnace board.
 
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Old 02-23-21, 05:58 PM
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Thanks again Pete. One more question on how to wire this - The EAC terminals power the transformer - does one wire from the transformer go to one of the H terminals on the humidistat and the W terminal is not used?
 
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Old 02-23-21, 06:58 PM
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The secondary of the transformer goes to C and W.
That would be the same C as on the furnace board. The W is on the humidistat.

The diagram below is what you'll be doing. You'll use EAC instead of HUM.
 
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Old 02-24-21, 04:10 AM
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One more question - since I already have the supplied transformer wired to the solenoid valve, could I connect the W terminal from the humidistat to the G terminal on the furnace board?
 
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Old 02-24-21, 10:33 AM
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The W wire can only go to one voltage source. G is a voltage source that becomes live when FAN ON is used or in A/C mode. It doesn't get power in heating mode. Your back to two relays then.

Look closely at the diagram. The transformer is not supplying the solenoid with power. If you have a transformer supplying power form the solenoid.... then to use my idea you would need a second transformer.
 
 

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