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A/C started then stopped, thermostat went offline

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My AC started for the 1st time yesterday late afternoon when upstair temperature was over 77 degree. I heard the the whole system started for a few minutes then stopped. The thermostat screen went black. I visually checked the unit outside but see nothing unusual. The breaker did not trip. Note that the heating was good the whole winter.
I am wondering what went wrong? Any suggestion what to do next? Thanks!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,433
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    Is the unit really getting power? There may be another breaker or fuse -- sometimes in an outside disconnect near the outside unit.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Jon_blaney
    Jon_blaney Member Posts: 317
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    Is there a good battery in the thermostat?
    STEVEusaPA
  • pecmsg
    pecmsg Member Posts: 4,883
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    The T-stat is powered from the inside unit. 
    Check the air handler.  
    SuperTechtomtomtom
  • tomtomtom
    tomtomtom Member Posts: 35
    edited April 2023
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    Is there a good battery in the thermostat?

    I am not sure. The thermostat are new (Ecobee smart) and were replaced 6 months ago

    Is the unit really getting power? There may be another breaker or fuse -- sometimes in an outside disconnect near the outside unit.

    These are the photos of condenser unit (Carrier, 8 years old). It says no fuse I think







  • tomtomtom
    tomtomtom Member Posts: 35
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    pecmsg said:

    The T-stat is powered from the inside unit. 
    Check the air handler.  

    Thanks! What specific things I should check on the air handler. It's a gas furnace model 58PHY070
    https://resource.carrierenterprise.com/is/content/Watscocom/carrier_58phb070---01116_article_1428408421072_en_ss


  • pecmsg
    pecmsg Member Posts: 4,883
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    Follow the low voltage. 
  • SuperTech
    SuperTech Member Posts: 2,186
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    Should be a 3 or 5 Amp fuse on the circuit board in the furnace.  If that is blown then I would open up the electrical compartment in the outdoor unit and check for mouse nests or chewed up wires.
    HVACNUTtomtomtom
  • HVACNUT
    HVACNUT Member Posts: 5,863
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    The fuse in the furnace control board did what it was designed to do. 
    Check to see all the wiring is correct, then check for shorts.
    The heat works so it seems to be on the A/C circuit. 
    If you take the cover off the outdoor unit to check, pull the disconnect first.
    Any splices between the thermostat and control board?

    tomtomtom
  • tomtomtom
    tomtomtom Member Posts: 35
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    SuperTech said:

    Should be a 3 or 5 Amp fuse on the circuit board in the furnace.  If that is blown then I would open up the electrical compartment in the outdoor unit and check for mouse nests or chewed up wires.

    HVACNUT said:

    The fuse in the furnace control board did what it was designed to do. 
    Check to see all the wiring is correct, then check for shorts.
    The heat works so it seems to be on the A/C circuit. 
    If you take the cover off the outdoor unit to check, pull the disconnect first.
    Any splices between the thermostat and control board?

    Thank you! I opened the air handler, the 3 amp fuse was dead so I changed to a new one. However, as soon as I turned the AC on, the fuse was blown again. What should I do next?
    STEVEusaPA
  • pecmsg
    pecmsg Member Posts: 4,883
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    Do not install a larger fuse.

    Might be time to call a contractor.
    tomtomtom
  • tomtomtom
    tomtomtom Member Posts: 35
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    pecmsg said:

    Do not install a larger fuse.

    Might be time to call a contractor.

    Thanks for the reminder! I called a contractor in my area and was receive a quote for about $250 to replace the potential bad contactor. Is it reasonable?
  • tomtomtom
    tomtomtom Member Posts: 35
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    Some additional photos




  • mattmia2
    mattmia2 Member Posts: 9,740
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    Move that blue and red wire where they connect to the contactor, are they chaffed? It could be the contactor but I would suspect the wiring of shorting somewhere long before the contactor.
    tomtomtom
  • tomtomtom
    tomtomtom Member Posts: 35
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    mattmia2 said:

    Move that blue and red wire where they connect to the contactor, are they chaffed? It could be the contactor but I would suspect the wiring of shorting somewhere long before the contactor.

    Thanks let me try that. To check for wiring shorting, is there a special tool for it or you just follow the wires from the air handler to the condenser unit?
  • mattmia2
    mattmia2 Member Posts: 9,740
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    You can learn some things with an ohmmeter or a test light but it is mostly visual inspection.

    An ammeter can tell you what is going on with the contactor, if the coil is partially shorted. It could be a delay control or something like that too if there is one.
    tomtomtom
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
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    Get a qualified tech with the skills, and/or a pro short tool. It would've already been fixed by now.
    I've never just replaced a fuse and hoped it would work. If it were a board without a fuse you'd be out the price of 2 new boards.

    There was an error rendering this rich post.

    mattmia2tomtomtom
  • pecmsg
    pecmsg Member Posts: 4,883
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    tomtomtom said:

    pecmsg said:

    Do not install a larger fuse.

    Might be time to call a contractor.

    Thanks for the reminder! I called a contractor in my area and was receive a quote for about $250 to replace the potential bad contactor. Is it reasonable?
    Pull the wires off, don't let them lay on metal of touch themself and try again. If the fuse blows tell him Hes full of $hit!

    He should have done that to begin with!
    mattmia2tomtomtom
  • tomtomtom
    tomtomtom Member Posts: 35
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    mattmia2 said:

    You can learn some things with an ohmmeter or a test light but it is mostly visual inspection.

    An ammeter can tell you what is going on with the contactor, if the coil is partially shorted. It could be a delay control or something like that too if there is one.

    Get a qualified tech with the skills, and/or a pro short tool. It would've already been fixed by now.
    I've never just replaced a fuse and hoped it would work. If it were a board without a fuse you'd be out the price of 2 new boards.

    pecmsg said:

    tomtomtom said:

    pecmsg said:

    Do not install a larger fuse.

    Might be time to call a contractor.

    Thanks for the reminder! I called a contractor in my area and was receive a quote for about $250 to replace the potential bad contactor. Is it reasonable?
    Pull the wires off, don't let them lay on metal of touch themself and try again. If the fuse blows tell him Hes full of $hit!

    He should have done that to begin with!
    Thank you all! Pulling the wires out then manually pushing the contactor activated the condenser unit. I am going to try replacing the contactor (HN51KB024) myself. What is you guys go-to store to buy it?
  • mattmia2
    mattmia2 Member Posts: 9,740
    edited April 2023
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    Does the fuse blow with the wires disconnected from the contactor? Do you see the wires compromised either where they are crossing over each other or where they go through the panels of the condenser?

    You can check the resistance of the coil in the contactor with an ohmmeter, it is marked as being about 40 ohms, also check that it is open to the case of the condenser.
    tomtomtom
  • SuperTech
    SuperTech Member Posts: 2,186
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    Remove the blue wire from the right side of the contactor and try turning on the A/C at the thermostat again.  If the fuse doesn't blow then you have a bad contactor. I wouldn't worry about getting the exact same contactor, those particular ones are junk. Buy a Titan Pro 1.5 pole 40 Amp contactor with a 24 volt coil.
    tomtomtom
  • tomtomtom
    tomtomtom Member Posts: 35
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    Update: I was able to change the contactor and got the AC work again. Thank you all for your help!
  • pecmsg
    pecmsg Member Posts: 4,883
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    and what does the contactor have to do with the t-stat?
  • mattmia2
    mattmia2 Member Posts: 9,740
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    pecmsg said:

    and what does the contactor have to do with the t-stat?

    If the coil is shorted it will blow the fuse on the control transformer.
    SuperTechtomtomtom