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Thread: A/C thermostat shut off.

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    buddystore is offline New Member
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    A/C thermostat shut off.

    Howdy all, need some help with and A/C thermostat issue. My thermostat is losing power and shutting off then it comes back on. I have troubleshot the breaker and it is fine, both the fan and compressor work fine, the small line from the compressor is cool to the touch, and I washed the coils. What is happening is the thermostat turns on the A/C, runs sometimes for 15 minutes, sometimes 5 and quits. Thermostat shuts off no power. then it comes on by itself. I checked the 24vac after it shuts down and I am losing power to the thermostat. After about 2-3 minutes or so 24vac returns. I cycled the breaker outside and it turns the stat off and on as I flip the breaker. Anyone have any experience with this problem? Thanx Dave

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    paul52446m is offline Journeyman
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    Quote Originally Posted by buddystore View Post
    Howdy all, need some help with and A/C thermostat issue. My thermostat is losing power and shutting off then it comes back on. I have troubleshot the breaker and it is fine, both the fan and compressor work fine, the small line from the compressor is cool to the touch, and I washed the coils. What is happening is the thermostat turns on the A/C, runs sometimes for 15 minutes, sometimes 5 and quits. Thermostat shuts off no power. then it comes on by itself. I checked the 24vac after it shuts down and I am losing power to the thermostat. After about 2-3 minutes or so 24vac returns. I cycled the breaker outside and it turns the stat off and on as I flip the breaker. Anyone have any experience with this problem? Thanx Dave
    From what you have said i can't understand what is going on. First when the unit is running, the small line from the compressor would be warm, not cold. The large line would be cool.
    The next thing ,if you are losing power going to the thermostat, that the problem is not in the stat. The power going to the stat, comes from the air handler, or furnace, what ever you have. So you need to tell me what you have, furnace or air handler, and where and how you are checking this 24volt power. How old is your furnace? Paul

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    HayZee518 is offline Deity
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    if there is a high and low limit switch in your control circuit, this may be whats keeping your compressor from turning on. high - high side pressure will kick off the limit as will a low - low side pressure. I'm thinking that as the unit functions, either pressure switch activates and shuts off power to the thermostat. once the pressures equalize then the unit functions normally. Paul is correct, the smaller of the lines coming off the compressor should be warm or hot. the larger tubing is suction and is cool to cold at the touch.

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    buddystore is offline New Member
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    Thanx Paul and HayZee for the posts. The small line is in fact warm to the touch, and when operating the A/C cools fine. I checked upstairs and I have a ADP gas furnace, and the outside unit is a York affinity 8T. The furnace is 25 years old and the outside unit is about 4 years old. I am checking the 24vac at the R&W wires coming out of the wall into the thermostat. When the thermostat goes blank, I pulled it from the wall and read the voltage. No 24vac. But after about 4-5 minutes, power returns to the R&W wires that just read zero. This happens when I leave the thermostat hooked up also. The thermostat goes blank and the unit outside shuts down, and after some time like magic, it turns back on and fires off like a pro. The breaker outside is not tripped, and it shuts the power off and on when cycled. So in a nut shell, the thermostat and unit outside just shut down because I am losing the 24vac to the thermostat, and after some time the thermostat comes back online and fires up the unit outside. When running the A/C works like a champ. For some reason I am losing the 24vac intermittently.

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