24 VAC Transformer Failure

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Old 01-22-16, 09:08 AM
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24 VAC Transformer Failure

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).

Two years ago (see link below), 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 (probably 15 years old). 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

See planning and wiring here: http://www.doityourself.com/forum/hu...est-hz432.html
 
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Old 01-22-16, 09:37 AM
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You don't say exactly which WR relay you used, but if it's a common one like 90-290Q, then it draws .125 amp nominally. Plus the .7 amps for your humidifier solenoid is .825 amps which is about 19.8 volt amps (ignoring power factor). This means your 20va transformer was essentially running at full rated load or above.

I suggest you replace the failed transformer with one rated 40 or 50 VA. That will give you margin and assure it won't fail prematurely.
 
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Old 01-24-16, 02:31 PM
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Well, now I may be getting beyond my Do It Yourself status, but here is an update:

Yes, Carbide Tipped, the relay is the 90-290Q.

So, I purchased a White Rodgers 90-T40F1 24 VAC transformer to obtain the 40 VA. I wired it up and at first the valve on the humidifier opened up and water was flowing, but then (about 20 seconds later) it stopped. About another 20 seconds after that, the transformer popped! So, something else is going on here.

Suggestions?

One thing I tested was the primary voltage and I noticed it was a bit higher than 120, like 130. I just have a cheap volt meter, so I have to guess a bit, but compared to the AC power going into the furnace (I have an outlet on the same junction box), the needle definitely goes a bit higher...

More of the story is also: I wired the new transformer differently than the old (this is why I tested the primary AC after the fact). The old transformer was mounted on the outlet box and drew the power from there (white) but the common from inside the furnace (a blue wire tied into the a temperature sensor, which I assumed was to only run the humidifier when there was heat in the furnace). The new transformer was a foot mount, so I decided to just wire it inside the furnace housing, still using the same source for common (the blue) but I drew the power from the white coming into the furnace and feeding the main transformer for the furnace as well as an exhaust fan.

My gut tells me there is something wrong on the secondary side, and all the stuff noted above is irrelevant, but I wanted to include it because perhaps it is the primary side, now.

If the secondary side, how do I isolate it?
 

Last edited by MCF_GRAP; 01-24-16 at 03:00 PM.
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Old 01-24-16, 03:51 PM
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I drew the power from the white coming into the furnace and feeding the main transformer for the furnace as well as an exhaust fan.
I don't know if you are confused but you are confusing me.

If you have a nest controlling your humidifier. You want an ALWAYS LIVE powered 24v transformer. The nest is doing the switching..... not the furnace.

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I looked at the link you left. You're showing two nest stat's controlling the relay that controls the humidifier. That should work ok but I'm not sure how the nest reacts to that. In any event.... that wiring method will not hurt the transformer or the humidifier. If you lost a transformer then you may have a short to ground or a solenoid going bad. Install a 3A fuse on the transformer's 24v side to protect it.
 
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Old 01-24-16, 07:43 PM
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New data point: I just figured out that my Multimeter will measure Ohms (and about the calibration screw and the adjusting dial -- yeaah). So, I disconnected the 24 VAC leads to the solenoid on the old Aprilaire 400 humidifier and connected the solenoid to the meter. The reading is Zero Ohms.

Does this sound like the culprit to burn up a 24VAC 40VA transformer? Does it make sense the solenoid would open for 20 seconds, then close, then about 20 seconds later pop the transformer?


Pete, sorry to create confusion, I will clean up my earlier post. What I was trying to convey was that I changed the primary source and was now tapping into the 120v lead (white) that also services the high efficiency exhaust fan as well as an electric air cleaner. While I thought I was getting an odd voltage, I now think the meter wasn't calibrated.

I agree, the common (black) doesn't need to be switched with the furnace fan now that the Nest controls the humidifier.

You bring up a good point, too, that if the first Nest is calling for humidity, the wire leading to the * on the second Nest will pick up the signal. It hasn't seemed to create any issues, but will think about that further as I get things back running.

Thank you for your time!
 
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Old 01-24-16, 08:26 PM
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I've never actually measured the resistance of an AA solenoid. I would expect to measure 10-20 ohms. 0 ohms is a dead short and will burn the transformer up.
 
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