Trane XL13i short cycling


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Old 06-20-16, 09:26 AM
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Trane XL13i short cycling

Hey everyone,

Been having some issues with the air conditioner (Trane XL13i) lately. Along with this, we were having issues with the Nest thermostat which was cutting power to the air conditioner and the furnace blower at 1 min 30 intervals, off for 15 seconds then repeat.

I disconnected the thermostat entirely and ran the air conditioner and furnace fan manually. The air conditioner starts fine and runs fine for about half an hour before both the condenser unit fan and the compressor suddenly stop for about 2 seconds (the furnace fan continues normally), sounds like pressure is escaping very briefly and I can best describe it as sounding very close to that of a turbo car's blowoff valve. The pump will then start up again for 2 seconds, then shut off. After that the pump will successfully start and run for a random number of minutes before repeating the same on-off-successful cycle.

I left the air conditioner off for about 20 hours to allow the pressures to equalize but I'm still having the same problem. Air out of the vents is cold at all times as the time the AC is off is very brief. The low pressure (large) line is always very cold.

To me this sounds like the high or low pressure switch is tripping, but I don't know why, because the system seems to run normally otherwise. If it was low freon charge, wouldn't the unit have problems starting right from the beginning rather than simply suddenly shutting off 30 mins in?

Before I call in a professional, is there anything apart from low or high charge that could cause symptoms like this?

Thanks!
 

Last edited by kk8d5; 06-20-16 at 10:25 AM.
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Old 06-20-16, 10:03 AM
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Welcome to the forums.

the fan and the pump suddenly stop for about 2 seconds,
What pump ?

There is a blower inside the house and a compressor and fan in the outside condensor.
It sounds like your compressor is overheating.

Have you been outside at the condensor when it happens ?
Is the fan stopping or slowing down ?
 
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Old 06-20-16, 10:24 AM
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Thanks for your response. Sorry, I wasn't clear on that part, I edited the post to correct it. I meant to say compressor when I said pump. The furnace fan works normally and isn't part of the problem.

What I meant to say is that the condenser unit fan and compressor stop running momentarily and simultaneously. It's like power is being cut to the whole air conditioner for these random short periods of time.

I opened the window that's located right next to the unit so I can see/hear when the fan and the compressor stop. The fan doesn't come to a complete stop because it's only off for 2 seconds at a time before turning back on, so in a sense it does slow down.

The compressor overheating sounds reasonable. When I'm testing the unit, how long can I run it for continuously? Is running it for 1 hour without stopping in 90 degree F heat too long?
 
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Old 06-20-16, 11:07 AM
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Here's a video of the issue:

https://www.youtube.com/watch?v=gtGazOYu4iQ In the first part of the video you can hear the compressor shut off. Note the ~2 min period in between in which the compressor and fan run normally. In the second part of the video, you can see the fan shut off and spin back up as the compressor clicks on and off.

Prior to making the video, the circuit breaker to the air conditioner was shut off for about an hour. The breaker was then turned on, and the air conditioner ran normally for about 7 minutes at which point the video begins. The condenser fan blows hard when it's on.
 
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Old 06-20-16, 12:14 PM
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From the noise the unit makes when it turn on, it sounds like the contactor is opening & closing. If you have a multimeter, you could very carefully (240VAC present) check the voltage across the contactor coil. It's normally the two side terminals. There should be ~24VAC between the side terminals when the thermostat is calling for cooling. If you're seeing the voltage alternate between 24VAC and 0 as the unit is cycling, the contactor is controlling the on/off cycling (as opposed to the thermal shutdown kicking in).

If the control voltage at the contactor is cycling, check the control voltage where it comes into the outside unit. The cable from the house usually connects to the internal wiring using wire nuts. If the control voltage (24VAC) is constant where it comes into the unit while the unit is cycling, then you probably have a pressure switch that's engaging, probably the low pressure switch, since a high pressure switch usually needs to be reset.
 
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Old 06-20-16, 12:27 PM
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AC systems are designed to run for hours. Sounds like the contactor is opening and closing. Pull the disconnect outside and take the cover off the electrical compartment. Then put the disconnect back in and video the electrical compartment while it's running.
 
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Old 06-20-16, 02:34 PM
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Thanks for the information about the contactor! That led me to open up the panel and inspect further.

The unit had been off for about 2 hours. When I went to turn it back on, it worked for a good hour or so without cycling, so it seems to be either heat or pressure related.

I did some testing and here's what I found:

Stable 24VAC at all times at the main input terminals from the house
Contactor responds properly to 24VAC applied (visible arcing, can see it engage)
0V at the contactor terminals when the air conditioner switches off randomly

This pressure switch, which is in series with the black 24VAC wire on the contactor and leads to the main input 24VAC wire, is responsible for the loss of 24VAC at the contactor. The blue wire from the contactor is directly connected to the second 24VAC wire from the house, so there are no failure points on this wire.

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Is this the low pressure switch, suggesting that the charge is low?
 
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Old 06-20-16, 02:55 PM
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It likely is the low pressure switch. The reason I say that is because most high pressure switches (not all systems have either or both) have a reset button (often red) that needs to be pushed to "reset" the switch should it trip. Since I don't see any reset button, I'm assuming it's a low pressure switch. If you have a schematic (wiring) diagram for the unit (often inside the cover), it should show whether you have a low pressure, high pressure or both present.

When the outside unit is running normally (not cycling), does it appear to be cooling normally? Often, if the system is low on refrigerant, the evaporator coil ("cold coil" inside your furnace) will freeze up causing a noticeable reduction in air flow (air can't pass through coil when it's iced up).
 
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Old 06-20-16, 03:19 PM
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I looked up the switch part number and it does appear to be the low pressure switch, as you said. Since that switch is the only one causing the unit to stop, I don't think that it would matter if there was another high-pressure switch installed.

When it's running normally, it appears to be cooling normally as I am getting noticeably cold air out of the vents and the large line is ice cold and sweating. It seems that the longer the unit is on, the more frequently that low pressure switch trips and the shorter the air conditioner is actually on, so it definitely diminishes the cooling performance and causes the air to become a bit humid, but cold. Perhaps the unit is just at the verge of low pressure, which is why it is intermittent, and slight fluctuations in temperature are causing it to trip? Or the switch is simply defective.

Here is the evaporator coil. It does not appear to be frosted over and there is some water pooled in the drain pan. Since the system is off most of the time since I discovered the issue, I would think that any built up frost would melt by the next time the condenser was turned on, but I could be wrong about that.

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Old 06-20-16, 03:26 PM
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Depending upon how "frozen over" the evaporator coil is, it could take 12 hours or more for the ice to melt. Since it appears that the evaporator coil isn't freezing over, it would appear that you're not too low on refrigerant. I'm not an A/C tech, just a homeowner, however I've heard of evaporator coils freezing over and the compressor was still running, meaning that the low pressure switch hadn't tripped. To determine whether you are low on refrigerant or just have a bad pressure switch, you'll have to have a technician come out. He will have the necessary equipment and expertise to determine which it is.
 
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Old 06-20-16, 03:39 PM
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Makes sense, thanks very much for your help. I'll have a technician come out, check the pressures and determine if the switch is the problem.
 
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Old 06-20-16, 09:11 PM
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Have it checked ASAP. That short cycling can blow the compressor up!
 
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Old 06-20-16, 09:27 PM
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Definitely, we have someone coming by tomorrow to check it out. In the meantime the power is disconnected. Unfortunately I don't know how long this has been happening... it could have been weeks or months.

If the pressure gauges check out okay and it's definitely a defective low pressure switch, do you suppose it's safe to just bypass the switch until the replacement switch is ordered and the refrigerant can be evacuated? Or is this just asking for trouble?
 
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Old 06-21-16, 12:40 AM
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Bypassing the low pressure switch is a gamble. If the system is actually dangerously low on refrigerant, running it for more than a few minutes could damage the compressor. Since you say that the system appears to be cooling normally (when its running), there's a reasonable chance that it's a defective pressure switch. However, since you have a technician coming out within a day's time, I would suggest that you don't run the system until it can be checked out. BTW, let us know what the diagnosis is.
 
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Old 06-21-16, 02:14 PM
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If your going from 24v to 0v then you have a loose connection or the stat has gone whacky.
 
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Old 06-21-16, 03:16 PM
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I thought I had read solid 24v to the condensor but losing the 24v at the contactor.
 
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Old 06-26-16, 09:41 AM
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Hey again,

PJmax, there is 24v constant coming from "the wall" but the 24v was being lost at the low pressure switch intermittently.

We had a technician come check out the unit just now and the refrigerant pressures check out fine, so there doesn't appear to be a leak.

It seems the compressor keeps going into thermal overload, which I believe is what's tripping the pressure switch since it's shutting down and losing pressure (??)

I bypassed the pressure switch and the unit ran fine (blowing very cold air) for a few hours has now quit running and the compressor doesn't start anymore (no noise at all), while the fan runs no problem. The tech says the thermal overload switch is preventing the unit from starting, so I'll leave it for a few hours before attempting to turn it on again.

The capacitor seemed to check out okay but there might be a chance it could use a replacement.

The fan blows constantly and doesn't cut out so that's not the problem. Could there be a plug in the system somewhere causing the unit to overheat, or is the compressor bad?
 
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Old 06-26-16, 10:03 AM
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So I powered the air conditioner back on after about 1 hour sitting and it starts and runs fine, so I'm positive it's going into thermal overload. The low pressure switch is not defective, evidently the compressor is causing the low pressure condition which made it seem like there was a low charge in the system.

I suppose we will need to call a different technician in order to diagnose the issue further because the first tech was in and out in a short period of time and simply diagnosed it as a defetive compressor causing thermal overload, which I don't believe is the true cause of the issue.
 
 

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