Author Topic: 24 VAC Transformer Failure  (Read 2403 times)

Offline Fixer

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24 VAC Transformer Failure
« on: January 21, 2016, 12:24:02 PM »
Hello all.  I recently noticed my humidifier (old Aprilaire 440) is not working.  By testing the connections, I determined the 24 VAC transformer is dead (primary side receives power once furnace is up to heat, secondary side shows now power).

A while back (maybe a year or so), I added a White Rogers SPNO relay to allow the new Nest Thermostats to control the humidifier.  Using the * connection, I run the next to the relay with the other side of the relay going to the Common on the zone controller (Honeywell HZ432).  The white connection from the transformer bypasses the relay while the red connection is switched by the relay.

The zone controller has its own 24 VAC transformer (new) and the humidifier transformer was re-used from before Nests and Zones were installed.  The humidifier transformer draws a blue wire from the furnace on the primary side (seems to be connected to a temperature sensor) and the wire is live once the furnace is running (The white is common to the power running to the furnace), and is rated for 20 VA on the secondary.  The humidifier is rated at 0.7A.

The Humidifier transformer is quite old, but I am curious to know if the added relay (or other changes) may be causing a problem that, even if replaced with a new transformer, will create a failure again.  For instance, does the relay itself draw more power, that combined with the humidifier is too much for the transformer?

Thanks for your help

Offline Admin

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Re: 24 VAC Transformer Failure
« Reply #1 on: January 21, 2016, 12:37:02 PM »
The Nest * terminal supplies the humidifier with 24V so no additional transformer should be required.  It almost sounds like you are double powering the humidifier.  This could be why the transformer no longer works.

The Nest steals the power from R and sends it to * anytime there is a call for humidity.  You should have 2 humidifier wires.  One wire should connect to the NEST * terminal, and the other wire should connect to C on the furnace control board.  If you're using a bypass humidifier and forced air furnace, I would adjust the Nest settings so that the humidifier will activate with heat only.

Offline Fixer

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Re: 24 VAC Transformer Failure
« Reply #2 on: January 22, 2016, 10:43:35 AM »
While the wiring may be redundant, I don't think I am double powering am I?  Nest Technical Pages indicated the need for the relay switch under certain conditions.  I interpreted my configuration to need the relay.  Even if not necessary, the power off the * terminal would just connect to the common off the furnace control board and cause the relay to trip, allowing the power from the original 24 V transformer dedicated to the humidifier to complete the circuit.  At least that is my understanding of the relay.

What could be an issue, though, is if the relay (along with the humidifier) draws too much power from the transformer, which isn't what I would expect, but I am just relying on intuition...

I will look for a short in the wires as well, as that seems to be a common cause for a transformer to eventually fail.

Thanks again!

Offline Porcupinepuffer

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Re: 24 VAC Transformer Failure
« Reply #3 on: January 23, 2016, 07:53:44 AM »
You'd have to more clearly explain how the connections are wired to the relay. If done correctly, each transformer would work independently of each other with no issues of power crossing over. Can you post the VA of the main 24v transformer for the zones and the amps required for what it controls? If big enough, you may have enough available power to also work the humidifier solenoid valve with no use for a relay and simplify the wiring.

If I had a dollar for how many perfectly good working 24v transformers I throw out in the garbage every week, I'd be rich.

Offline Fixer

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Re: 24 VAC Transformer Failure
« Reply #4 on: January 24, 2016, 10:52:08 PM »
Well, sure enough, I wired up a new 24VAC transformer (40 VA instead of the old 20 VA) and fired everything up.  The humidifier clicked on and water was flowing for about 20 seconds, then stopped.  About 20 more seconds after that, the transformer popped and emitted the sweet smell of electrical burn...  Puffer, maybe I need you to throw those working 24VAC transformers my way!

So, after some more digging, I decided to hook up my multi-meter to the solenoid and discovered it was reading 0 Ohms.  From what I read, that would burn up a transformer.  Wondering if anyone feels the timing makes sense (20 seconds solenoid running, then stops, then the pop of the transformer about 20 more seconds later).  Or is there more to worry about.  I don't mind replacing a solenoid and another transformer, but at some point, I have to throw in the towel.

I think it makes sense for the humidifier to have it's own transformer as I wouldn't want to risk the furnace failing on some cold day, plus it acts as a fail safe to make sure the humidifier doesn't run when the furnace fan isn't running.

Thanks again!

Offline Porcupinepuffer

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Re: 24 VAC Transformer Failure
« Reply #5 on: January 25, 2016, 03:49:15 PM »
0 ohms is a dead short. Assuming you connected your meter correctly, the solenoid valve is the problem. That's equivalent to taking both wires and connecting the transformer directly together. Amps fly through the roof and something has to burn or blow.  :-\