Thermostat transformer overheating, casing melting


  #1  
Old 12-19-13, 08:33 PM
T
Member
Thread Starter
Join Date: May 2013
Location: Canada
Posts: 13
Likes: 0
Received 0 Likes on 0 Posts
Thermostat transformer overheating, casing melting

I have a 25VAC transformer on my thermostat circuit. The transformer is mounted inside my gas fired furnace (close to the gas valve, fan control, etc.) The problem is that the plastic casing of the transformer is overheating and seriously deforming.

About a month ago the transformer became detached from its mount and fell on a sloping heat shield that is 1 to 2 inches underneath it. It toasted there and after a couple days it failed electrically. The case was badly distorted from the heat.

I did not know the cause, but replaced the transformer. But now I'm noticing that the bottom of the case of the new transformer is deforming. It must be overheating. It's still working, but I don't know how long it can last.

The transformer is a Honeywell AT20B. Input is 120 VAC, output is 28 V open circuit. The thermostat is a simple mercury contact. The system is 25 years old.

The AT20B is rated for 20 VA. I measured the AC amps at 0.45A in the secondary. That is 11 VA according to my calcs. So it seems that the transformer is not overloaded.

The other odd thing is that the deformation of the plastic case is at the bottom of the transformer, not what you'd expect if there was a short or electrical overloading. The bottom is facing the heat shield of the furnace. That suggests the heat from the furnace is causing the overheating. But the old transformer managed fine for 25 years.

I noticed that the temp range spec for the transformer is max 41C (105F).

What is happening here?

Is the overheating a symptom of something going wrong elsewhere? Should I just mount the transformer outside of the furnace space to save it from overheating? Is it maybe just that the plastic of the transformer is lower melting than before?

Any suggestions appreciated.

Thanks
 
  #2  
Old 12-19-13, 09:00 PM
T
Member
Thread Starter
Join Date: May 2013
Location: Canada
Posts: 13
Likes: 0
Received 0 Likes on 0 Posts
OK, I measured the temperature at the bottom of the transformer. It was 75 deg C. In deg F, that's VERY HOT.

I guess I should relocate the transformer to a spot outside the compartment where it now lives along with the other controls.

Can't figure out how the old transformer managed to last for so long, though. Also don't know why it eventually failed.
 
  #3  
Old 12-20-13, 08:23 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 70,912
Received 3,161 Likes on 2,840 Posts
Transformers are internally protected and will open a safety link if the transformer overheats.
Old transformers withstood a lot more abuse then their counterparts today.

Today the transformers will open faster on an overload, continuous short or possibly on a brief line surge.

I was thinking that maybe in your case it was partly heat from the transformer itself and partly heat from being so close to the heat exchanger.
 
  #4  
Old 12-20-13, 09:19 AM
T
Member
Thread Starter
Join Date: May 2013
Location: Canada
Posts: 13
Likes: 0
Received 0 Likes on 0 Posts
Transformer relocated

Okay, the transformer is now outside of the area with the gas control valve and fan control, etc, and just outside the sheet metal wall of the furnace. Everything is working well.

But now I have the 120VAC lead coming from inside the furnace running to the transformer -- and the low voltage leads going back in. Electrically that is fine. But safety wise maybe not.

So I think I will drill a hole in the side of the furnace and mount the transformer in it using its 1/2 inch conduit nipple. That way the 120V line stays inside the furnace.

Any comments on whether that would be the right way?
 
  #5  
Old 12-20-13, 10:49 AM
Houston204's Avatar
Forum Topic Moderator
Join Date: Oct 2009
Location: Texas
Posts: 7,573
Received 50 Likes on 46 Posts
I have installed quite a few 50 va transformers with that conduit mount. It is a good idea. The fact that this is required is alarming. This should probably be investigated further.

If you ever replace that transformer again, I recommend 40va.
40, 50 and 75 va are common sizes. I have never seen a 20 used in a furnace.
 
  #6  
Old 12-20-13, 11:00 AM
T
Member
Join Date: Jul 2008
Location: Kansas
Posts: 317
Likes: 0
Received 0 Likes on 0 Posts
I agree that it should be checked out further. The hot gas combustibles should be going out the vent and not coming back into the cabinet. The shield will get hot, but should be forcing draft up.
 
  #7  
Old 12-20-13, 03:25 PM
T
Member
Thread Starter
Join Date: May 2013
Location: Canada
Posts: 13
Likes: 0
Received 0 Likes on 0 Posts
Just called a local furnace company and have arranged for them to come out for a "tuneup" this coming Monday, just to make sure everything is okay. (Didn't mention the transformer problem.)

Thanks for the helpful comments.
 
  #8  
Old 12-24-13, 10:53 AM
T
Member
Thread Starter
Join Date: May 2013
Location: Canada
Posts: 13
Likes: 0
Received 0 Likes on 0 Posts
Update.

The furnace guy came out and noted rollout on the furnace. The flames lick out for a fraction of a second on ignition. After that, it burns normally. The gas pressure was too high (5.7), and he was unable to adjust it on the gas valve. Replaced the valve and successfully adjusted the gas pressure to 3.5.

That set me back $800 but he said the rollout was still occurring. He recommended flue and duct cleaning, implying that that would fix it.

I noticed though that he didn't check the flue himself to see if that might be contributing to the problem. And IMHO only by the greatest stretch could the ducts be at fault.

So after he left, I had a look myself inside the flue. (Very easy to do, he should have done it.) There is a bit of soot on the inside walls, and some traces of rust, but there is no obstruction at all. The flue is clear.

Accordingly, I think it would be a waste of money to have this company clean the flue.

Would appreciate any comments, though, in case I'm overlooking something.
 
  #9  
Old 12-24-13, 11:12 AM
T
Member
Join Date: Jul 2008
Location: Kansas
Posts: 317
Likes: 0
Received 0 Likes on 0 Posts
Any soot is bad IMO. To charge 800 for a gas valve and walk away is even worse. Probably the over firing of the old valve caused the soot and it needs a thorough cleaning of the heat exchanger and possibly the vent. Soot can combust. With more service to be done, you are well on your way to buying a new furnace.
 
  #10  
Old 12-24-13, 11:48 AM
T
Member
Thread Starter
Join Date: May 2013
Location: Canada
Posts: 13
Likes: 0
Received 0 Likes on 0 Posts
To clarify, he charged $75 for the call, $340 for furnace inspection of 2 furnaces (we have 2 back-to-back), and $386 for the valve incl thermocouple.

Instead of yet more service, maybe we should consider a new furnace.

Would like to get thru the winter with what we have, though, as long as it is safe.
 
  #11  
Old 12-24-13, 12:01 PM
T
Member
Join Date: Jul 2008
Location: Kansas
Posts: 317
Likes: 0
Received 0 Likes on 0 Posts
I can't diagnose from afar and tell you whether it will last or not. I do recommend having CO detectors though. Merry Christmas to you and yours.
 
  #12  
Old 12-24-13, 01:58 PM
T
Member
Thread Starter
Join Date: May 2013
Location: Canada
Posts: 13
Likes: 0
Received 0 Likes on 0 Posts
Thanks for your comments, and Merry Christmas to you too.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: