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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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My Honeywell Chromotherm III suddenly started going down to

Customer Question

My Honeywell Chromotherm III suddenly started going down to 45 degrees and blinking there. when I try to reset it I can get the A/C to come on but the temp still goes down to 45 and the system will not shut off unless I do it manually.
JA: I'll do all I can to help. Is there anything else the HVAC Technician should be aware of?
Customer: I also changed the batteries and got no change.
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the HVAC Technician about your situation and then connect you two.
Submitted: 1 year ago.
Category: HVAC
Customer: replied 1 year ago.
I already filled out the form
Expert:  Phil replied 1 year ago.
So far it sounds like the thermostat went bad and needs to be replaced, however we need to do some tests before replacing it. Tell me a lot about your technical skills background please, I can be more helpful that way.
Customer: replied 1 year ago.
I can probably replace it if it only needs to be attaching wires in the right places but I don't have a soldering iron.
Expert:  Phil replied 1 year ago.
Hello again, no soldering is needed, but we should do some tests first... the thermostat wires run from the thermostat to the furnace or air handler in the same compartment as the blower..... locate those, they are small 18 ga wires bundled together in brown or white sheath.... take a photo of those wire connections so that I can prescribe a few tests to both confirm the thermostat is bad, and to specify a workable replacement. The photo needs to show the wire terminal designations, typically R, Y, W, and G... and maybe an O or B. We can go from there.
Customer: replied 1 year ago.
I have do I get it to you?
Expert:  Phil replied 1 year ago.
Load the picture into your computer, then attach the picture file here using the paper clip icon at the right end of the tool bar, or by using the 'attach files' link to the left of your send button.
Customer: replied 1 year ago.
Hopefully the attached file will get to you
Expert:  Phil replied 1 year ago.
Thanks, ***** ***** the photo... you have we call a 'conventional system' easy to fit a new thermostat to, except that your blue and yellow wire colors are reversed, so we will leave them that way when fitting a new thermostat. However with the front part of the thermostat removed I cannot perform a test to see if the thermostat is defective or if there is some other issue. If you can trace the wires from the thermostat as shown, to where they attach inside the furnace... then with the front part of the thermostat re-installed, and set on cooling and down to 60F. remove the blue wire from its Y terminal inside the *furnace... then if the cooling stops running, it is the thermostat that was bad causing the problem. There is about a 1% chance that the red and blue thermostat wires inside the wall got shorted together with a metal staple..but we can ignore that for the time being.
Customer: replied 1 year ago.
Can I replace the front of the thermostat so that you can run your tests? I can't get into the furnace.
Expert:  Phil replied 1 year ago.
Hello again, if you can't get into the furnace it will not be possible to run the tests, because when the front of the thermostat is in place, the wiring is obscured. Can you tell me what the situation is with the furnace that is preventing access ?

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