HVAC transformer issue

[SlowTinkerer]

Hi, I'm working on an old Carrier 3 ton gas/electric package unit at my house. When I turn the breaker on, I have no power to the control board, but when I unplug the 24V output from the transformer and plug it back in, I have 24V again. Is this a bad transformer? I have 120v at each leg of power at the contactor and going into the transformer. It's a Carrier 48GX036090 model.

 

[PJB12]

Hello SlowTinkerer,
Are you saying that after cycling the breaker, 24 volts shows up at the board, but then leaves again on its own?

First, ensure that you have proper & steady voltage getting to the transformer. Check this at terminals 11 Black and 13 Yellow (If my memory is correct for GX series) If OK...

If the secondary simply quits after some time, the transformer may have a wire in a winding that is separated. When the transformer is energized for some time, the wires warm and bad one separates, causing no voltage. When it cools, the wires touch again. This isn't terribly uncommon, especially in outdoor units.

I apologize but I no longer have my GX series diagrams to check for you. But memory says 24 volts from the transformer goes straight to the board. Memory also says power to the transformer goes straight from source to transformer. This leads me to blame the transformer.

But in case my memory is wrong-
I don't think any Carrier RTUs are wired this way, but just in case...
If the secondary stays energized until the unit starts or is called to start, investigate to find out if any components are between the transformer primary (higher voltage side) and the source power or between the transformer secondary (24 volt side) and the board. A high limit, an air flow switch or even a thermostat malfunction could be the cause of the interrupt.


But if the line voltage goes straight from power source to the transformer and the secondary goes straight from transformer to the board, my thoughts go to a transformer problem. Fortunately, aftermarket transformers fit the space provided in the 48GX- a big money saver.


Here's Hoping For An Easy Fix!
Paul

 

[PJB12]

Something of a light bulb just came on in my mind that puzzles me.
As far as I remember the 48GX series is a 3 phase unit. The default in my brain went to commercial, but you said house.

I did a quick search at Carrier and it shows 48GX as 3-phase only.

Do you have 3-phase at your house or is this running on a phase converter? Or have all the motors and controls been replaced with single phase components? (Doubtful)

If it's running on a phase converter, throw away my reply at #2 and call for in-person examination. Converters can cause interesting (and sometimes frustrating) voltage readings and behaviors.

Paul

 

[SlowTinkerer]

Something of a light bulb just came on in my mind that puzzles me.
As far as I remember the 48GX series is a 3 phase unit. The default in my brain went to commercial, but you said house.

I did a quick search at Carrier and it shows 48GX as 3-phase only.

Do you have 3-phase at your house or is this running on a phase converter? Or have all the motors and controls been replaced with single phase components? (Doubtful)

If it's running on a phase converter, throw away my reply at #2 and call for in-person examination. Converters can cause interesting (and sometimes frustrating) voltage readings and behaviors.

Paul

Mine is a residential unit, single phase 240V unit. I confirmed it on the data plate. 48gx036090300. The 3 phase one has a 500 for the last digits of the model. 300 indicates 240v single phase. Sorry for the incomplete info, I was working from memory.

The 240v power comes into the unit and goes directly to the contactor. A 120V leg (constant) comes off of the contactor and feeds the transformer, which then feeds a 24V line to the control board. I have good 240v into the unit and a good 120V going into the transformer. I just went out to the unit yesterday and could not get any 24V at all out of the transformer. This intermittent problem just became a failure. I guess I caught the transformer on its way out. I have a new one on order and should be here in a few days. We'll see from there.

Thanks for the help! It's great to have another brain on the problem. I'm new to fixing HVAC stuff and it's been both rewarding and frustrating. I had a local HVAC guy come out when this started, and he saw the age of the unit and the bit of surface rust on the face of the heat exchanger and refused to go any further . I pulled the side of the unit off and inspected the heat exchanger closely and didn't find any holes or anything, so I'm going to try to use this unit for another year or two if I can.

Thanks again.

 

[PJB12]

Mine is a residential unit, single phase 240V unit. I confirmed it on the data plate. 48gx036090300. The 3 phase one has a 500 for the last digits of the model. 300 indicates 240v single phase. Sorry for the incomplete info, I was working from memory.

The 240v power comes into the unit and goes directly to the contactor. A 120V leg (constant) comes off of the contactor and feeds the transformer, which then feeds a 24V line to the control board. I have good 240v into the unit and a good 120V going into the transformer. I just went out to the unit yesterday and could not get any 24V at all out of the transformer. This intermittent problem just became a failure. I guess I caught the transformer on its way out. I have a new one on order and should be here in a few days. We'll see from there.

Thanks for the help! It's great to have another brain on the problem. I'm new to fixing HVAC stuff and it's been both rewarding and frustrating. I had a local HVAC guy come out when this started, and he saw the age of the unit and the bit of surface rust on the face of the heat exchanger and refused to go any further . I pulled the side of the unit off and inspected the heat exchanger closely and didn't find any holes or anything, so I'm going to try to use this unit for another year or two if I can.

Thanks again.

I think your approach is going to solve the problem. It's far from uncommon. But...

One puzzlement I have is that you mention that the unit has 120 supplying the transformer. Hopefully that's a typo.

If 120 transformer primary-
Where does it get the neutral? Does one come from the load center to the unit? It would be very unusual to have a neutral going to the unit. Also unusual would be a 120 x 24 volt transformer in a combined unit like yours.

Is there a 240 x 120 transformer ahead of the 120 x 24? Very unusual for a 240 unit. ( I apologize, but I could not find a schematic for your unit.)
If Not-

When you read across the two wires feeding the transformer primary right at the transformer, does your meter show 240 or 120?
If 120 and there is no neutral, then this brings up a different quandary.

If you have no neutral but have 120 reading across the transformer primary legs, you are missing a leg on the 240 supply.

(And why you get 120 and not zero is another mystery. There are many legitimate reasons, but much more diagnostic testing is required.)

I apologize but didn't know that the GX series came in single phase. Thanks for the update!
Paul