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2 Failed transformers


mike_'s Avatar
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12-28-17, 10:35 AM   #1  
2 Failed transformers

Hi,

I've got a very old(20 years) Trane heat pump, located in the southeast US. Ideally, I would replace the whole thing, but I just don't have the money. I'd like to get it running again if I can.

A few days ago, heat pump stopped working, no blower in the indoor air handler, no outdoor compressor-fan.

Figured out it was the 240-24 transformer. Replaced it, understanding that the failed transformer might be a symptom, not the true cause. The old transformer did not have a fuse, and the wires and coil were burnt on the primary side. Multi-meter shows open resistance on primary 208-240 side between common and 240 line. 24 volt side reads around 1.9 Ohms.

After replacing the transformer, unit started working. Had it in heat mode, warmed the house, aux mode came on a few times. Started at 65, took it up to 71, in increments, so it took a few cycles. We usually just increase the target temperature a degree or two at a time until everyone (ie wife) is comfortable.

At some point a few?(2-3) hours later, unit stopped working. New transformer failed. I have not pulled it out yet, so I don't know if the primary side or secondary side failed yet.

Researching on the internet, I'm getting conflicting information on the possible source of the problem.

Some sites say that since the primary side failed, I have an issue with the air handler. The most common suggestion is short to ground in blower motor winding. On my unit, i really can't get to the blower easily, it is behind an electric panel, so i have not tested the blower motor yet.

Other sites say a primary side transformer fail is caused by a short somewhere on the low voltage side and I should test the 24 volt wiring between the transformer and the thermostat and test the 24 volt wires between the transformer and the outside compressor.

I took a visual look at the wiring between the transformer and the thermostat, it looked OK. No obvious visual damage.

Looking at the outside unit, I did find something with the 24v wiring. A small v shaped nick where the 24v wire was resting against the copper cylinder of the reversing valve. I can see the outer Brown jacket of the cable is compromised, i can see a little tiny bit of the internal green wire, but I cant quite see any exposed copper. Like I said, it is a very small nick, so there could be exposed wiring that I can't see.

My specific question is if that nick is the source of the short, wouldn't the unit fail right away instead of running for a few hours and then failing ?

Since the second transformer ran a few hours before failing, I was a little suspicious of the defrost cycle causing the failure. It somewhat cold(southeast cold) and pretty damp here, so my understanding is that the defrost cycle could be required. Could there be some relationship between the nick in the wire against the reversing valve cylinder that causes a short to happen when the unit goes into the defrost mode ?

I hate to even say this, because I'm not sure it related, but an hour or so before we noticed that the second new transformer failed, I though I heard a thump in the house. I asked, but nobody else in the house noticed.

Anyway if you gt this far, thanks for reading all that. Any advice or thoughts appreciated. I'm off the the attic to test the wire between the transformer and the outside unit for a short, but I will check back in.

Also, I WILL post any conclusion to this issue, not going to leave it hanging open like some other threads I've run across, even if I just end up replacing the whole system.

 
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12-28-17, 11:19 AM   #2  
Welcome to the forums.

A 24v short doesn't usually cause the primary to go bad. Excessive continuous load on the 24v line could make the transformer run warm/hot to the point where the thermal safety opened. Your old transformer sounds like it died from old age.

You can/should install your own fuse in the 24v secondary transformer line. I'd use a 3A or 4A fuse. The automotive ATC plastic type work very well.

A heat pump can defrost several times a day. It can be governed by run time or a sensor monitoring for ice buildup on the coil. A shorted wire to the reversing valve could definitely be an issue.


~ Pete ~

 
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12-28-17, 11:54 AM   #3  
Thank you PJ.

I'll add a fuse before I use the next new transformer(on order)

Looks like I can get to the blower motor enough to test for a short to ground in blower motor winding, so I'm going to attempt that now.

Using this site: Service Clinic: Why and How to Test HVAC Blower Motors | Contracting Business

 
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12-28-17, 03:53 PM   #4  
The blower motor will not cause the that transformer to blow out.
You are looking for a short in the 24vac low voltage wiring.


~ Pete ~

 
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12-29-17, 12:41 PM   #5  
I checked the blower motor before I saw PJmax's last comment, no short to ground, resistance readings looked good, spins freely, so the motor seems fine.

I was able to detect a short to ground in the thermostat wire between the air handler and the outdoor compressor. I did this by letting the nick in the control wire contact the reversing valve, I found it like that when I first opened the compressor case, the nick fits perfectly against the edge of the reversing valve, guessing it was formed over time due to vibrations..... The thing is, I found the ground short in the green wire, but the green wire is not used, ie it is not connected on the air handler side or the compressor side. I'm going to check for an internal short between the green wire and the rest of the wires next. I suspect this is the issue, but I wish I could prove it more conclusively...perhaps in moving the wire around I somehow eliminated the ground short between the green and another wire...i might just replace the whole wire

Appliance store called, 2nd new transformer is in, going to get that and also going to pilfer my deceased dads tools, I think there should be one of those clamp multi-meters that I can use to read the transformer amps.

 
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12-30-17, 11:02 AM   #6  
Pulled the new failed transformer, it's the same as the original transformer that failed. Primary side burned, open resistance, secondary side visually looks good and has resistance.

I'm wiring in the blower fan only, going to test with that and monitor the transformer current, see how that goes.

 
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12-30-17, 01:16 PM   #7  
I had an issue where I started losing transformers on a house that I was renting. It turned out to be a loose ground connection between the breaker panel and the ground stake.

 
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12-30-17, 02:34 PM   #8  
thanks Houston I'll check that out.

 
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12-30-17, 02:41 PM   #9  
That issue is losing the neutral and having the 120v leg go up in voltage.

Are we dealing with a 240VAC system and a 240v transformer here ?
Make sure you are using the 240v tap...... not the 208v tap if a 240v system.

Be sure your 3-4A protection fuse is installed on the secondary before powering it up.


~ Pete ~

 
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12-30-17, 03:16 PM   #10  
Yes, 240v system with a 208\240 transformer. Using the 240 wire, 208 wire is capped like I originally found it.

As far as the inline fuse does this look good ?

Holder:
https://www.autozone.com/electrical-...ries/32415_0_0

Fuse:
https://www.autozone.com/electrical-...=&model=&year=

 
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12-30-17, 06:04 PM   #11  
Good choices.... ATC fuse and fuseholder.


~ Pete ~

 
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12-31-17, 11:24 AM   #12  
OK, got the fuse wired inline, it's on the 24v red wire coming out of the transformer.

The red wire from the transformer connects to the R terminal in the thermostat.
The other wire on the 24v side is a brown wire that connects to the C terminal in the thermostat.

So, is the red wire the correct location for the fuse ?

Also, I put a multi-meter(Fluke 87 if that matters) inline with the red wire from the transformer. When the system is energized with just the air handler and thermostat wired in, it only draws about .15 or so amps. The transformer is rated to ~1.7 amps, but I can't find any info on what the expected load should be. When I wire in the outdoor compressor, will the only additional load on the transformer be the contact coil ? What amps does a 24v contact coil draw ? Just trying to get an idea of what a normal load should look like....


Last edited by mike_; 12-31-17 at 11:44 AM.
 
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12-31-17, 12:53 PM   #13  
The fuse can go in either transformer secondary line. Red side is ok.

1.7A = 40 watts or 40va.
.15A = 3.6 watts.

The A/C contactor draws 9-12 watts or around 1/2A.
In cooling mode there is also the reversing valve to consider. 4-5 watts.... I believe.


~ Pete ~

 
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01-01-18, 04:41 PM   #14  
An inline fuse is definitely a good idea. A 3amp fuse between the 24volt red or the common side. Power off cut one side on the low voltage side of the transformer red or common, crimp a female connector to fit a 3 or 5 amp fuse and wrap with electrical tape( thats if you dont have a fuse holder or a "popper"). Next go to the air handler where the thermostat wire is located and disconnect the wire nuts on each low voltage circuit one at a time and hook your clamp meter on, reconnect your wire nut and turn the breaker on to the air handler very shortly to avoid blowing your fuse. Read the amp draw on each of the low voltage(thermostat) wires. Find the wire that is pulling the excessive amps and trace the circuit. Hope this helps.

 
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01-01-18, 04:59 PM   #15  
BTW over half of blown transformers are the result of a short in the contactor coil. You can check continuity(ohms) of the contactor by removing one of the low voltage wires. Set your meter to ohms/resistance and check the 24 voltage coil side it should be between 12-18 ohms.

 
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01-02-18, 10:11 AM   #16  
Thanks jreed.

I checked the contact coil, it reads 10 ohms.

At this point, I have the all the wiring between the thermostat and air handler connected. I found a bunch of small nicks in the insulation of wires at the thermostat side, like somebody grabbed all them with pliers to pull on them or something and squeezed to hard. Anyway, had enough play, so i just clipped them off and got rid of the damaged sections.

At the thermostat with the air handler powered up reading a very low amp draw, less than 1 amp. Can run the blower, works fine.

Outside, on the 24 volt line, I found 2 problems. I already mentioned the wire being damaged where it was resting against the edge of the reversing valve cylinder, can see just a little flash copper through the nick.

Found a second nick in the 24v wire where it enters the compressor shell. For some reason the installer ran the wire through a hole without a rubber grommet, even though there is a hole WITH a grommet available just like 3 inches past the hole they used. Wire is nicked pretty deep where it was resting against the edge of the hole.

Running a new wire to the outside now.

Thanks

 
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01-03-18, 11:59 AM   #17  
OK, got all the 24v wires connected, heat pump ran last night and all morning, seems good.

But......no aux heat. Thermostat calls for it(aux light on thermostat illuminates), but as far as i can tell, the heat strips are not working. I hate the aux heat, my thermostat is always calling for it even when it is plenty warm outside, but yea, last night was pretty cold and it took a while for the heat pump to go from 62 to 70. Plus I worry about the aux not being avail. for the defrost cycle.

I looked, no lose wires, I took notes and pictures, pretty sure I wired in the transformer just like the old one was wired in.

Looks like there are 5 individual heating elements, all read around 17 ohm.

All 5 of the element limit switches are closed. There was one more smaller limit switch, it was not wired into any heating elements, I think it has something to do with the defrost cycle, it was closed also.

2 sequencers and the blower fan relay seem to be in play.

Looking close at the blower fan relay, I can see just a little oil looking gel leaking out of the terminal that connects to one of the sequencers via a brown wire. Going to try to figure out how to test the blower relay. Update, it looks like the sequencer controls the blower fan. Since the blower is working when the thermo is calling for aux heat, guessing that the fan relay is ok...????

 
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01-03-18, 02:09 PM   #18  
OK, not getting 24v into the sequencers with the thermostat aux light on. Going over wiring now

 
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01-03-18, 06:06 PM   #19  
Just FYI..... on a thermostat set up for a heat pump with AUX heat..... when the room temperature is 3 or more below the set point.... the AUX heat is turned on. The AUX heat runs with the heat pump to recover. When the gap closes.... the AUX shuts off and the heat pump maintains.

If your stat has an E or emergency position.... that's the AUX heat with the heat pump shut off.

This is one reason why setback with a heat pump is not usually cost effective.


~ Pete ~

 
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01-11-18, 08:25 AM   #20  
Update, since replacing the control wire to the outside unit and getting rid of the nicks in the control wires at the thermostat, the heat pump has been working for 8 days now.

The aux heat issue was just a wiring issue at the thermostat, the aux heat wire was connected to the thermostat Emergency terminal. Wife says we did this a year ago to avoid using the aux heat since most of the time it's above 45 and climbing around here when we get up in the mornings and the heat pump works fine without aux. Our thermostat (honeywell chronotherm iii) is pretty quick to call for aux even when we increase the temp by 1 degree at a time. I guess a better thermostat would have a way to read and account for the outside temp.

I found the grounding wire for the house, still planning on digging it up and checking the clamp, pretty sure it's connected to the water main. I'll also check the ground connection in the main breaker if I can find out how to do it safely.

Thanks for all the suggestions, I spent around $40(2 transformers, new wire, fuse holder and fuses) to fix it, learned the basics of how the heat pump works. Last time it broke it was a bad start-run capacitor and the company charged us 300. The heat pump was the only house "appliance" I had not worked on.

 
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01-17-18, 03:13 AM   #21  
Thanks for the follow-up.

 
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01-17-18, 04:52 AM   #22  
Be careful of that ground wire. Turn off the main breaker if you plan on interrupting this circuit.

 
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