Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Changing thermostat without cutting power - motor burned out

I had an HVAC technician at my house today and he changed out both my thermostats. he neglected to cut power to the 2nd one. At exactly the the same time, the motor on the 2nd unit burned out. He's claiming it was just "random timing" and wants to charge to replace the motor.

So my question - can neglecting to cut power while changing a thermostat short out / kill the AC motor?

Note: This is on a 6 year old system.

Thanks for any help


  • mattmia2
    mattmia2 Member Posts: 8,825
    Probably not but it could cause other things that would cause the motor to not run like blowing one of the line fuses if it is fuses rather than breakers.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,683
    I would talk to his boss. if he is the boss, then I would question his ethics.

    Although he is probably correct, there is the remote possibility that the on and off that might be associated with connecting a thermostat (24 volt system) maybe overloading the motor (line voltage system) by means of the relay(s) that power the motor. If for some reason both the heating speed to the motor and the cooling speed to the motor were powered at the same time, that would cause a motor failure (two windings powered at the same time should never happen).

    I would negotiate a lower price on the motor replacement. Maybe give up the retail markup on the part and reduce the labor for the motor replacement by 25% to 50%. This will teach him to turn off power when servicing thermostats. If course, agree to full price for the thermostat replacement.

    Just ask, See what happens?
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • mattmia2
    mattmia2 Member Posts: 8,825
    Oh, I was thinking line voltage t-stat for some reason. That makes a lot more sense and I can see where that could be the result of doing it powered. It isn't the recommended way to change a t-stat and it would be hard to prove that it didn't result from powering 2 speeds at once.
    HVACNUT Member Posts: 5,452
    6 years old. What brand? If it was registered it might be under warranty. 
    And are we sure he didn't just blow the 3 or 5 amp fuse if he cut across 24 volt power and common?
    It sounds like he's a thermostat replacer, not a service tech.
    Getting struck by lightning is "random timing" too. Fight the charge.