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Old 11-25-2008, 11:14 PM   #1
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Aprilaire 400A Humidifier Installation

I've been procrastinating on the installation of our Aprilaire humidifier and I need to finish. I have yet to install the Outdoor temperature sensor, Model No. 50 current sensing relay, and hook up the power source.

My furnace has no accessory terminal. My first question is, "If my furnace already has a 24V transformer, can I access it or should I install the 24V transformer that came with the humidifier?" If I install the new transformer, "How the heck am I supposed to mount the thing??" The only screw that shown at the top of the first image basically points inward towards itself, butting up against the exterior of the transformer.

Any help appreciated.

Thank you.

Images attached.
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Aprilaire 400A Humidifier Installation-img_0801.jpg   Aprilaire 400A Humidifier Installation-img_0803.jpg  


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Old 11-26-2008, 12:32 AM   #2
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you can wire the tformer to the low speed wire for the fan and a neutral wire(this will however only provide humidity during a heat cycle)the current sensing relay wont be needed for this method as the trans is only "hot" when the fan is on. if you want to be able to humidify using the heat and /or fan"on" mode than wire it to the power wire that supplys power to the furnace.you will than be using the current sensing relay.(be sure to loop the wire 2 turns thru the sensing part of the relay to increase the signal sent to the relay.
as far as mounting its designed to be mounted in a 1/2 knockout, there may be an unused one on the furnace loosen the screw stick the screw side thru the knockout and tighten the screw down. you could also simply wiretie it to the wireing bundle as well.
you could also forgo the transformer and wire it to the low voltage wireing using w and c (to run with heat,or g and c to run with the fan on) in this configuration however if the humidifier shorts out it will start blowing the fuse on the board and leave you without heat until its disconnected


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Old 11-26-2008, 03:46 AM   #3
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The Aprilaire 400 doesn't do much without the furnace running in heat mode. It also doesn't work much better if you hook it up to the hot water, since it has a water sump.

The transformer will work if connected to a blower speed tap. However, it will have a short life span. The voltage spike from the motor starting isn't good for them.
Thats why they send the sensor with it.

That transformer is designed to be installed on an electrical box. If you install it through a furnace lock out. Make sure that the wires can't short to anything if they come apart.
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Old 12-13-2008, 10:03 AM   #4
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(I'm installing a Aprilaire 600a)

Just to be clear, the transformer will have a better lifespan if I connect it to an external 120V source, say at the external furnace shut off switch like where the CAUTION box points to in this pic?

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Old 12-13-2008, 10:57 AM   #5
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If your furnace is as old as that one, yes.
But, when installed on a system like that, you should have a sail switch that prevents it from running if the furnace isn't running.

On newer units, they have a HUM terminal you should use.
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