Thermostat intermittantly unable to turn on A/C

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Old 07-05-17, 08:13 PM
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Thermostat intermittantly unable to turn on A/C

I have a somewhat strange problem with our home air conditioning unit. Sometimes the thermostat is unable to turn on air conditioner compressor. The thermostat indicates it is trying to turn the AC on, but it does not succeed. The A/C unit is 27 years old.

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I measured the voltage outside going to the compressor as switched by the thermostat. It reads 24V when the compressor turns on and when it will not turn on. I rigged up a thermostat bypass switch to control the air conditioner. It works 100% of the time. With the bypass switch I also measure 24V outside. I replaced the contactor thinking that might be the cause of the problem (perhaps requiring more current than could pass through the thermostat), but it was not. I replaced the thermostat, it wasn't the problem either. I can't think of what could possibly cause this problem.

I would greatly appreciate any suggestions.
 
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Old 07-05-17, 08:26 PM
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Welcome to the forums.

27 years is certainly a good run for an A/C system.

By confirming the 24vac AT the condensor you've proven the stat is ok.

There can be several items in the 24vac line inside the condensor......

Usually there is a low pressure switch that keeps the unit from running when low on refrigerant.

There could be a high pressure switch.
There could be a time delay relay.
 
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Old 07-06-17, 04:42 AM
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If I were a betting man, I would bet on a defective Time Delay Relay, particularly because the problem is intermittent. My system is also 20+ years old as well. Approximately 5 years after it was installed, it began to intermittently not turn on when the thermostat called for cooling. I opened up the outside unit and, after examining the wiring diagram, determined that the Time Delay Relay (TDR) was bad. When I went to the HVAC company that installed the system to buy a new TDR, I was told that the part frequently went bad. The purpose of the TDR is to prevent the compressor from coming on too soon after being turned off (high pressure will likely stall the compressor motor).

If you have a digital thermostat, you can bypass the TDR, since the digital thermostat has a 5 minute delay built into it (if A/C is running, you turn it off and then back on, it won't restart until 5 minutes have passed). The only condition that the thermostat won't protect against is a momentary power outage, since the thermostat won't know about that. If you have frequent momentary power outages, don't bypass the TDR. Otherwise, you can try bypassing it to see if that is the problem. Note, not all systems use a TDR, so there's no guarantee that you have one in your system.
 
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Old 07-09-17, 11:53 AM
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Thanks for all of the good suggestions. I haven't had a chance to investigate your ideas, but I will in the next bit and report back.
 
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Old 07-30-17, 12:15 PM
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I've attached pictures of the wiring diagram inside the condenser outside and the insides of the furnace. I don't think there is a TDR. When I use my thermostat bypass switch, the furnace will turn on/off at will with no delay. I also don't see anything that looks like a TDR. The red wires with a red/black wire inside are the ones that go outside to the condenser.

The only thing that is between the condensor/pump and the relay seems to be the thermostat control relay. I wonder if I should try replacing it. Thoughts? Am I missing anything?Name:  IMG_20170730_143143383.jpg
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Old 07-30-17, 12:29 PM
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Ok.... no TDR. So refer to my reply back in post 2.

It is most likely low on refrigerant.
 
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Old 08-23-17, 02:42 PM
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I haven't been able to identify a TDR, low pressure relay, etc. Any tips on how I would find and identify it?

Here's what I don't understand: My thermostat bypass switch literally connects to the same wires as the thermostat; the power line, AC control wire and fan wire. It's connected at same point at the thermostat wiring block (I have the thermostat hanging from the wall and wires connected to the back of it). When it's switch to "on" it works 100%. If there was a low pressure relay causing the issue shouldn't my bypass fail as well?

I tried replacing the thermostat control relay, but the problem still exists.

I'm getting to the point where I might call someone in to fix it. I am simply at a loss.
 
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Old 08-23-17, 03:21 PM
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I was actually a little hasty when I agreed there was no TDR. The TDR would be located in the outside condensor unit. It would be installed between the incoming thermostat wiring and the A/C contactor. In some cases the TDR can be considered optional equipment and is installed automatically.

I measured the voltage outside going to the compressor as switched by the thermostat. It reads 24V when the compressor turns on and when it will not turn on.
That's what you previously told us. IF that is true...... there is nothing you can do inside the house to make the condensor come on. Once the 24vac is present outside.... everything in the house is out of the equation.
 
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Old 08-23-17, 03:23 PM
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Can you post a pic of the "thermostat bypass switch" ?
 
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Old 08-23-17, 03:30 PM
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Several times you've mentioned that you rigged up a "thermostat bypass" switch. Where exactly is this switch, and how do you have it connected? In your first post you mentioned that you have 24VAC present at the outside unit both when it's working and when it won't turn on. If this is correct, that rules out the thermostat as being the problem since you're getting the 24VAC at the outside unit. Where are you measuring the 24VAC at the outside unit? Are you measuring it where the wires from the thermostat/house connect to the internal wiring of the outside unit (wire nuts)?

You're correct in that the wiring diagram does not show a TDR present. It does indicate that you may (optionally) have a low pressure, high pressure, or ambient temperature switch. Any such switch would be between where the 24VAC comes into the unit (wire nuts) and the coil on the contactor.
 
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Old 10-01-17, 03:40 PM
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I finally had time to work on this issue again! Please don't take my slow progress as an indication of a lack of interest, I really appreciate all of the responses.

I've attached pictures of the electrical wiring outside and where the coolant lines go into the outside unit. I measured the 24V at the contactor outside when activated (unsuccessfully) by the thermostat and when activated by the thermostat bypass switch.

I also attached a picture of the thermostat bypass switch. When on, it simply connects the AC and fan lines to power. I would say the issue is no longer intermittent. It only works with the bypass switch now. Heating still works with the thermostat with no issue.

I've followed the electrical and refrigerant lines from the furnace to outside and I can find no trace of a low pressure relay.
 
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Old 10-01-17, 03:55 PM
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You are forcing the contactor closed. IF IT IS closing and the condensor won't run.... it's a 240VAC line problem. Once you close the contactor manually.... all safeties are bypassed.

Without re-reading the entire thread.... confirm 240VAC TO the contactor and 240VAC OUT of the contactor.
 
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Old 10-04-17, 10:58 AM
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Here are my thoughts regarding 240V in/out, please let me know if I'm off base. The contactor is does not close when the thermostat attempts to turn on the AC, so there cannot be 240V coming out. The contactor closes when the bypass switch controls the contactor, so there must be 240v infor both scenarios and out with the bypass switch.

Originally I had theorized there was some kind of current problem going to the contactor when controlled by the thermostat as I measured 24V, but the contactor was still open. I replaced both the contractor and the thermostat control relay. I can't identify a LPR or TDR, so I don't know what else it could be?
 
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Old 10-04-17, 01:57 PM
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We're going around in circles.

When you use your "switch" to activate the contactor...... does it close and does the compressor and fan run ? If yes.... you have eliminated any 240v problem and have confirmed a working unit.

Now..... turn your "switch" off. Set the stat to cooling.... wait 5 minutes..... check the 24VAC where it enters the condensor. Leave the wiring connected.... just take off the wire nuts. Do you get 24vac here ? Did the contactor close ?

If you have the 24vac but the contactor did not close..... you'll need to trace those two wires from the incoming connection right to the contactor.
 
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