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Air Handler transformers and caps burning out. Wiring issue?

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Last winter I installed a boiler and new controllers in my house with no C-wires. In order to get the Nest to work, I used this:

https://www.supplyhouse.com/Fast-Stat-FSCM00-Common-Maker-Thermostat-Wire-Extender-Adds-a-Common-C-Connection

And that worked well, but the Nest was complaining about the wiring for the AC, so for the winter, I just disconnected the AC wires and the heat worked great all winter.

Now it is Summer and I need the AC to work. My solution to the Nest complaining was to replace them with Honeywell smart thermostats. They worked for a bit. Then the AC went out.

I found that on two of my zones, both the 24v transformer and the starter cap was dead. These are from 1998, so I assumed they were just old. I replaced a transformer and cap, and the blower started and it worked for a day or so. Then it died.

I replaced that transformer again, and heard the new starter cap buzz, so that seems dead now again also. It seems likely my wiring is causing these to fail some how when heat and AC are both connected to the thermostats.

What could be wrong with my wiring that would make this happen?

Maybe I should disconnect the heater wires for now, get the AC working, this time get a C-wire from the C. Then plan on removing the "Common Maker" things before winter?

Comments

  • mattmia2
    mattmia2 Member Posts: 9,963
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    Was the transformer bad, did you test with with a voltmeter and ohmmeter? Poorly made caps that failed after a few weeks wouldn't surprise me if you weren't careful about what you bought or you perhaps didn't buy a cap with a high enough voltage dielectric.
  • mattmia2
    mattmia2 Member Posts: 9,963
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    If you have a transformer for the furnace and for the air handler you need to be careful to isolate them (or make sure they're connected in phase).
    ZmanMikeAmann
  • rsilvers
    rsilvers Member Posts: 182
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    Yes I used a multimeter on the transformers.

    It could be that the two transformers (one for the heat and one for the AC) were not isolated, and not in phase. How can one test for phase though? The output of the transformers are not labeled relative to the load and neutral going into to them.
  • mattmia2
    mattmia2 Member Posts: 9,963
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    Does the device you are using to extend to the t-stat have an Rh and Rc? Usually you only connect C to the equipment you want to draw power from and you have separate R for heat and cool. The only way to test the phasing is with a meter, if they are in phase if you connect one end of the transformers together the voltage between the other ends should be close to 0. if it is around 48 vac then they are out of phase.
  • HVACNUT
    HVACNUT Member Posts: 5,890
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    mattmia2 said:
    If you have a transformer for the furnace and for the air handler you need to be careful to isolate them (or make sure they're connected in phase).
    Yes, but none of that burns out run caps. The OP needs boots on the ground. Then maybe the Nest will stop complaining. 
  • 109A_5
    109A_5 Member Posts: 1,388
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    NEST infiltration issues and a possibility tired compressor may be two totally separate unrelated issues.
    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • mattmia2
    mattmia2 Member Posts: 9,963
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    HVACNUT said:


    mattmia2 said:

    If you have a transformer for the furnace and for the air handler you need to be careful to isolate them (or make sure they're connected in phase).

    Yes, but none of that burns out run caps. The OP needs boots on the ground. Then maybe the Nest will stop complaining. 

    they could have bought a crappy or wrong voltage cap
  • rsilvers
    rsilvers Member Posts: 182
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    mattmia2 said:

    Does the device you are using to extend to the t-stat have an Rh and Rc? Usually you only connect C to the equipment you want to draw power from and you have separate R for heat and cool. The only way to test the phasing is with a meter, if they are in phase if you connect one end of the transformers together the voltage between the other ends should be close to 0. if it is around 48 vac then they are out of phase.


    I have Rh hooked up to the boiler controller and I have Rc hooked up to the AC air handler.
    Currently I am getting the C wire from the boiler but using that extender system I mentioned because the boiler only has two wires run. I should probably remove that extender and get C from the air handler as those have 5 wires so it is easy to get C from there. That has the added benefit of being able to shut down the boiler in the summer and not lose the thermostat power.

    Ok on the phase.

    I think my next step is to disconnect the boiler wires and just work on the AC.

    I don't know how the caps are dying. I don't think they get power from the transformer or anything related to thermostat wiring.
  • mattmia2
    mattmia2 Member Posts: 9,963
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    you have r y g and c from the air handler? You can power it from that. Only connect Rh and W from the boiler. R and C from the air handler will provide power.
  • HVACNUT
    HVACNUT Member Posts: 5,890
    edited June 2023
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    Nice you can power the stats from the air handlers. 
    The capacitor situation is unrelated as far as the control wiring being a direct cause. I've never seen
    run capacitors continually go bad. Are you positive that's what it is? You've tested them for MFD?
    Can you see the ratings on the motor to know if you've got the right size capacitor? Is it a Rescue motor that was supposed to replace a 1/3 HP but is wired for 3/4 HP and using the incorrect capacitor?
    I think you said it does run for a while. Any chance you know the start and running amps?
    It's something stupid I can tell you that.
    Shut the power at the breaker and check all line volt connections. Switch, set screws, wire nuts. Make sure all ground connections are tight too.
    Without being there, I'm stumped. Maybe you can be on a future episode of @RayWohlfarth's "What The **** Just Happened?!"
  • mattmia2
    mattmia2 Member Posts: 9,963
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    There are a lot of extremely poor quality caps out there too. look for the made in mexico ones.
  • 109A_5
    109A_5 Member Posts: 1,388
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    Keep in mind the C wire(s) may actually be connected together between the heating equipment and the AC equipment with the Equipment Ground. If there is a switch or jumper that bonds Rh and Rc at the thermostat I would think it should be open or removed. Phasing correctly may help but the transformers and their loads should be independent.

    If you have not, you should probably wire it per the manual.
    https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Fast-Stat-FSCM00-Wiring-Guide.pdf

    One example


    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
    MikeAmann
  • rsilvers
    rsilvers Member Posts: 182
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    Thank you for that chart.

    One update - It's possible one of the caps didn't fail. I put in a new one and that one didn't start the motor until I bypassed the terminal block they are all plugged into. I need to find out what that is called and see if that is working properly.

    Also side question - this blower motor has a black, red, and white wire, which implies it is 220v, but only two of the wires are hooked up to 120 volts. Is that a normal thing to do? Or is that the wrong motor for a 120v feed system? Would running a 220v motor off 110v be much less blower power than a motor made for 120v? This zone has always been a bit weak on fan power.
  • mattmia2
    mattmia2 Member Posts: 9,963
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    Look at the ratings plat on the motor. Probably either different speeds or the connection for the capacitor.

    Are we talking about the air handler or the condenser(assuming this is a split system)?
  • ratio
    ratio Member Posts: 3,701
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    Sounds to me like a two speed 120 volt motor. If you applied 220 volts to a 120 volt motor, the smoke would come out pretty fast. If you energized both speed taps at the same time (or grounded the unused one), the smoke would again come out. If you applied 120 volts to a 220 volt motor, the smoke would still come out, but maybe not as quickly.

    The name of the terminals it's plugged in to will be a clue, as will the schematic. And the nameplate on the motor.

  • HVACNUT
    HVACNUT Member Posts: 5,890
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    rsilvers said:
    Thank you for that chart. One update - It's possible one of the caps didn't fail. I put in a new one and that one didn't start the motor until I bypassed the terminal block they are all plugged into. I need to find out what that is called and see if that is working properly. 
    Are you saying the motor leads had a molex connector and you cut it out and spliced the wires together color to color and now it works?

  • mattmia2
    mattmia2 Member Posts: 9,963
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    Pictures would be great here. Could be a contactor too.
  • rsilvers
    rsilvers Member Posts: 182
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    I took out the extenders and instead used the C wire that was on the cooling. I should have done that to begin with. Things are ok now with wiring.