# How to test and replace Aprilaire humidistat?



## ilyaz (Oct 29, 2013)

We have a Trane furnace/AC + Aprilaire humidifier. These were installed together about 3 years ago. We have a yearly maintenance contract with the company that installed them and so they come twice a year to check the system. Had a check up today and the tech said the humidifier is broken. More specifically, the humidistat controller is (Aprilaire 884021BS). He offer to replace it for a "reduced" price (parts + labor) of $250. Reduced? Sounds steep to me! I said I'd think about it.

If it's only the humidistat, $250 seems to be a bit steep, no?

So I am trying to figure out how hard it would be to test and, if needed, replace the humidistat myself.

1. How should I test the humidistat to verify it's indeed bad?
2. How hard is it for someone without any HVAC training who only has experience fixing small things around the house to replace it if it's indeed busted?
3. Less than 3 years for a humidistat seems too short. Should I look for a different brand of the controller, given that I don't want to replace the whole humidifier which is Aprilaire?

Thx!

UPDATE: Contacted Aprilaire and was referred to this as the replacement.


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## kok328 (Oct 29, 2013)

Turn it to max humidity and then turn the heat on.  Then inspect the humidifier for operation while the system is running.
You could always check the operation of the humidistat with a volt/ohm meter.
There are only two wires connected to the back of the stat, so if you want to change it, it should be pretty easy, just shut off the service switch on the furnace before/while doing anything.
As far as getting a different brand, Aprilaire is very common but, the humidistat can be any brand you want.


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## ilyaz (Oct 29, 2013)

So when the heat is on and the humidistat is supposed to be working, there is supposed to be voltage between the two contacts in the photo, right? How high? Thx


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## kok328 (Oct 30, 2013)

Correct.  24 Volts.


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## ilyaz (Nov 10, 2013)

This morning I finally found some time to do some "sanity testing"

I shorted the two contacts on the old humidistat with a screwdriver and soon as I did that I heard a sound coming from the humidifier that sounded like air going through a pipe, which I presume actually meant that it opened flow of water and turned it on. Make sense, right, except at that moment the furnace itself was not running, i.e. the room temperature was at the level set by the thermostat. Is that right? 

If that's correct and I just need to replace the humidistat, other than flipping the breaker turning off the furnace, should I turn off or disconnect anything else?

Thanks


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