One of the hardest problems for HVAC Techs to diagnose is a 24 volt circuit problem in an HVAC system. There is one reason why they have never learned a low voltage systematic troubleshooting procedure.

I am sure there are many ways to find the problem in the control wiring of an HVAC system, but here is a sure fire way for you to Diagnose a 24 Volt Short:

SAFETY PRECAUTIONS: Make certain that all power is disconnected to the condenser and air handling/gas furnace unit. 

STEP-BY-STEP PROCEDURES
  1. Make certain that the power to the unit is off.
  2. Carefully remove the service panel(s) from the air handler or gas furnace that is(are) covering the blower compartment.
  3. Using a nut driver remove the service panel from the condensing unit.
  4. At the thermostat, air handler/gas furnace and condenser Inspect for shorted wires.
  5. Older thermostat wires may have split insulation.

Note – In heat pumps look for control voltage wires touching discharge refrigeration lines. This melts the insulation and shorts out the wiring to ground.

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Checking Wiring From Air Handler to Thermostat Sub-base

  1. Set your ohmmeter to the audible ohms (beep) setting.
  2. Alligator the  clip one ohmmeter lead to the common wire coming from the thermostat.
  3. One-by-one check:
  1. Common to red.
  2. Common to orange.
  3. Common to yellow.
  4. Common to green.
  5. Common to any other wire that may be connected (2-stage system)

10. Remove alligator clip from common and connect to the red wire.

11. One-by-one check:

  1.  Red to orange.
  2. Red to yellow.
  3. Red to green.
  4. Red to any other wire that may be connected (2-stage system)
  5. Red to cabinet (ground).

12. Remove alligator clip from red and connect to the orange wire.

13. One-by-one check:

  1. Orange to yellow.
  2. Orange to green.
  3. Orange to any other wire that may be connected (2-stage system)
  4. Orange to cabinet (ground).

14. Remove alligator clip from orange and connect to the yellow wire.

15. One-by-one check:

  1. Yellow to green.
  2. Yellow to any other wire that may be connected (2-stage system)
  3. Yellow to cabinet (ground).

16. Remove alligator clip from yellow and connect to the green wire.

 17. One-by-one check:

  1. Green to any other wire that may be connected (2-stage system)
  2. Green to cabinet (ground).
  3. Check any other wire to the cabinet (ground).

Checking Wiring From the Air Handler to the Condenser

18. Make the exact same check as listed in Steps 8-23 above, with the wires going out to the condensing unit.

Step 5 – Diagnosis

19. If your meter beeped in Step 8-17 the short is between the air handler and the thermostat.

  • Locate short – repair or replace wiring.
  • If your meter beeped in Step 18, the short is between the air handler and the condenser.
    • Locate short – repair or replace wiring.
  • If your meter DID NOT beep, the field installed control wiring is good. 
    • Check for factory wiring shorts in the air handler and condensing unit.
    • See heat pump note in Step 1.
  • The shorted wire(s) can be determined by noting which wire(s) beeped during steps 3 and 4.

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Ron Walker
Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.