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thermostate temp increasing but A/C unit blowing cool air

JayV Member Posts: 1
i have two trane A/C units - one upstairs and one downstairs.

each is controlled by separate honeywell digital round non-programmable thermostats - model T8775c.

my issue is simple:  i turn the thermonstat to "cool" and then "on" and finally set the desired temperature (ex: 78 degrees F).  the trane A/C units click on and cool air blows fine.  however, the room temperature on the honeywell digital round thermostat increases (ex: 80, 81, 82 degrees F) vs. decreases, therefore making it run continuously.

this is especially a problem at night when i turn it on and set it to a certain comfortable temperature, but instead it blows cool air all night and freezes us!

please advise.  all equipment is less than 5 years old and this has been an issue for one year now.

thank you.


  • JStar
    JStar Member Posts: 2,752

    ...switching the thermostat faces for a little while to see if it does the same thing. If it doesn't, you have a bad thermostat. If it does the same as before, you have another problem.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,578
    runaway A/C

    somehow the anticipator must be heating things up, as the system is calling for A/C. switching the thermostat faces is a good idea, as well as double checking the wiring at the thermostat, and the air-handler.-nbc
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669

    The problem is the Honeywell round mercury thermostat uses small metal balls as switch contacts and when they get old and worn they heat up - this causes the A/C to run constant since they heat up the thermostat.

    Replace the thermostat with a NEW Honeywell digital model and you will be real happy.

    Steve W.
  • Techman
    Techman Member Posts: 2,144
    edited July 2010
    run away ac

    Is the tstat on an outside wall?Is the hole in the wall where the tstat wirers come thru the wall, opened ,and not sealed off from the hot attic air,?
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