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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8767
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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AC comes on and works fine, trouble is it won't shut off all

Customer Question

AC comes on and works fine, trouble is it won't shut off all the time. It cools way past the thermostat setting. Changed thermostat and problem still there. Relay?
Submitted: 2 years ago.
Category: HVAC
Expert:  Phil replied 2 years ago.
Hello,Please attach a photo of the printed circuit card where the thermostat wires attach... I can coach you on checkout procedure from there.Use the paper clip icon at the top right tool bar for that. .
Customer: replied 2 years ago.
I could not find the paper clip but put it on the add files.
Expert:  Phil replied 2 years ago.
Hello again, thanks for the pictures.I suspect that two of the thermostat wires inside the wall, where you cannot see them, are shorted across with one of the staples used to secure the wire. This is why I need to see a photo of the printed circuit board inside the *furnace or air handler where these wires from the thermostat terminate. That printed circuit card is much larger, and is generally mounted in the blower compartment. We can go from there in the morning..
Customer: replied 2 years ago.
The unit had been working fine until recently. It stays within the thermostat set parameters, just not all the time. When it does, I can turn the system off, then turn it back on and it will cycle through and turn on and off to the settings for a while, then eventually stays stuck on running again without turning off and continue cooling past the setting again. Isn't there a relay outside in the unit that may control that function. I will send a pic of what I think it may be. The handler is in the attic and I live in Phoenix, it's a million degrees up there right now! I did find some paper work and took pics of the schematics if it helps.
Expert:  Phil replied 2 years ago. if the indoor blower keeps running, the thermostat fan switch is set to 'fan on' or the problem is not that the contactor (relay) in the outside unit is sticking. Let me know what you can discover, we can go from there.-------------We are on the honor system here. Please remember to rate my service before you leave today. It is how I earn a living, and does not increase your charges. If you need anything else, just let me know. You can continue asking follow-up questions at no additional cost..
Customer: replied 2 years ago.
The fan is on "auto". It randomly continues to cool past the set temperature. For example I set the thermostats (both the new and the old one) on 78 and it will cycle a few times and shut off at 78, but then randomly it will continue to run to as low as 68 (it would probably run colder but that is when I caught it and turned it off)
Expert:  Phil replied 2 years ago.
Hello again,Get me the brand and model number of the outside unit please, and attach photo's of the wiring diagrams glued to the back of the electrical controls box on the unit outside. AND. the wiring diagram glued inside the blower access panel on the furnace or air handler. The photo's need to be taken straight on, in good light, and be very well focused, use the paper clip icon at the top right on the tool bar to attach them/ I can mark those up with trouble shooting directions if you are handy with that sort of thing. let me know about that as well. We can go from there.
Expert:  Phil replied 2 years ago.
I also need a photo of the printed circuit card inside the blower compartment of the furnace or air handler that shows where the thermostat wires connect.
Customer: replied 2 years ago.
Figured out it is the control panel (the one you mentioned earlier in the thread I think) that contains the relays etc., and is located in the handler. It is in the attic area above the top floor of the house, probably close to 120 degrees mid-day.That is where the wires terminate for the thermostat as you mentioned. Located the replacement and will attempt to replace tomorrow or next day. I will let you know how it went.
Expert:  Phil replied 2 years ago.
Hello again, that thing is called the 'printed circuit board'.. its expensive, they do fail with some regularity, but we have not done a diagnosis on it yet...I am not sure its the problem. If you want to do a diagnosis on it, send me the photo's I mentioned and we can do that. .
Customer: replied 2 years ago.
It was the time delay relay or fan relay. Replaced it and did some other maintenance on this unit and the unit that services the lower floor of the house. All seems to be working fine. The units are 22+ years old so a great deal of fin and stuff cleaning and blowing out of the condensation lines. Thanks, ***** ***** comments led me in the right direction.
Expert:  Phil replied 2 years ago.
Thats good work on your end, sorting these things out is never easy.If you care to rate my input I will appreciate it, it is how the company pays me, and it does not affect your charges. Thanks