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airheatman, HVAC Technician
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Experience:  Furnace, A/C and heat pump specialist.
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I have a 3 ton Trane Condenser 4TTA3036B3000AA Serial #

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I have a 3 ton Trane Condenser 4TTA3036B3000AA Serial # 14491KY83F. This system uses 410A and was manufactured 12/2014. The condenser cycles on and off every 5 or 10 seconds. The sight glass is clear and when you first start the unit it will run for a few minutes during which time the suction line is cold.The system had been freezing up and about 2 weeks ago a HVAC service person found the pressures low and put in some 410A

Hello, one minute while I get some information together.

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Answer:

The unit is likely still very low on gas or possibly has a restriction in the sealed system those are the two things that would cause this issue,

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Customer: replied 1 month ago.
Please give this job to someone else as I previously had a bad experience with you.
Thanks

I will be happy to assist you today. It sounds like one of two things.WE have leaked back down on refrigerant or the metering device in indoor cool is sticking closed. Do you happen to have gauges to see what pressures are doing.

Customer: replied 1 month ago.
No gauges. Let me see if i can get some. I assume that they are different from automotive-i have a set of those.

Yes they are different.Hoses are different connectors.Let me know if you get them.Here is what we want to do. Connect the gauges before you turn on ac, then as it starts, watch the pressures. We should be running about a 130 suction and high 200s to low 300s head.If both are low, we are low on refrigerant.If head is normal or high and if suction goes to a vacuum the metering device is sticking.

Customer: replied 1 month ago.
The metering valve is the expansion valve? I will try to buy a set of gauges tomorrow and will get back to you. Thanks for your help!

That is correct.Look forward to helping you figure this out.

Customer: replied 1 month ago.
I just had an HVAC technician here. The pressures were 490 head and 110 suction. When he hooked up his gauges a little freon came out and his hand became black. The system kept running for the 10 or 15 minutes that he was working on the system, without cycling -probably because it was colder outside today (about 64F) Most of the condenser coil was caked with dirt. He said that there were enough unclogged coils that he was not concerned about this. He suggested that he should replace the expansion valve though he would not guarantee that this would fix the problem. I took the as much of the condenser housing off as I could manage, including the top fan and cleaned the coils with a hose-from both sides. I ordered a set of AC gauges that will arrive on Monday and I will check the pressures again. What is your opinion about this? Also, do you know how to remove the entire condenser housing on my Trane unit? One side unbolts but I couldn't figure out how to remove the rest of it. I couldn't figure out how the side housing unfastens from the bottom/
Thanks,
Peter
Your opinion?

I am with you.Lets get that condenser coil clean first.WE may be shutting down on high head instead of low suction. I have nor dismantled this particular unit.but I recommend removing top and backwashing from inside.Then rinse from outside.

Customer: replied 1 month ago.
Thanks for your response. I ordered the gauges and should have them latest on Monday. I will keep you posted.

Sounds good.Talk to you then

Customer: replied 1 month ago.
Received my gauges today. The ambient temperature here today is 47 degrees. On Thursday when the HVAC tech did the pressure test it probably was about 70 degrees. When I started the compressor the pressures were 350 and 100. After about 2 or 3 minutes they went down to 290 and 90. I did wash out the coils yesterday by removing the fan and washed them from the inside and then from the outside.
What do you think?
Peter

Peter are you in heat or cool mode today? And give me readings in both modes

Customer: replied 1 month ago.
This is a gas furnace with the evaporator coil on top of the furnace.

OK so we are in COOL mode. My bad

Really going to be hard to check refrigerant at 47 degrees as there is very little load on unit.

Is it warming back up tomorrow?

Customer: replied 1 month ago.
According to the weather guessers, it will be 58 degrees tomorrow and Monday after which it will get colder until next Saturday, when it will go to 61 degrees. I am not in a rush, so if you think I should wait until it gets warmer (tomorrow or next Saturday) I will.

WE really need a load on it to get decent pressures.

Customer: replied 1 month ago.
Is 61 degrees enough of a load? Would it make any sense to partially block off the fan? I have a piece of plywood that I cover the fan with during the winter

Lets do this if you can stand it.Turn on heat and get house up to about 80

Customer: replied 1 month ago.
Put on the heat until it is 80 degrees and then put on the AC and read the pressures?

Yes

Customer: replied 1 month ago.
I Just heated the store that we are dealing to 80 degrees and then put on the AC. The pressures went to 460/115 and then went down to 350/110 within about a minute. The outside temp is 51 degrees.FYI- About 2 + weeks ago an AC technician put in more 410A to try to stop the system from freezing. If I remember correctly his final pressure readings after putting in the freon was 300 or 350 (I don't remember) and 150 suction. The outside temp as about 70 degrees.This solved the freezing problem for about two weeks until the compressor started to cycle on and off.

I am going to say you have a refrigerant leak somewhere.WE should be up around at least 130 and 420

Customer: replied 1 month ago.
Do I assume correctly that the dirty coils caused the high readings last week and therefore the cycling of the compressor and that the expansion valve is no longer suspect?

AS long as those pressures stay as they are we can forget the TXV. Only problem there is TXVs can stick on occasion and work most of the time. If we could catch it when it is short cycling and see the suction drop down to below 20psig we would know the TXV was sticking and restricting flow.

Customer: replied 1 month ago.
Thanks, ***** ***** been great. Last question: Is there a chart that shows what the pressures are supposed to be at specific outside temps?

Let me see if I can put my hands on anything

Not really fining anything that will tell us both suction and liquid pressures at a given temperature.

Trane recommends charging by sub cooling.I am attaching the sub cooling chart with instructions on how to check.Somewhere on the unit it should tell you the recommended superheat for your unit.

Say the recommends sub cooling s 10,

1.Measure temp of liquid line

2.Locate that label (W are going to say 10)

3.Locate the value in the chart (Say liquid temp is 110, the pressure we are looking for is 418

I would still not rule out that TXV though.Watch it and see if it goes back to short cycling, connect those gauges and see if suction drops when it is doing it.

If you have further questions, please let me know.

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Just a follow up. I did a little more digging and found the chart below. Based on this chat, you my not be low on charge since it is so cold outdoors.AS mentioned previously it is really hard to charge with ambient temps this low.Actually the numbers you provided this morning are not that far off.

I am still thinking you may have a TXV that is occasionally sticking causing the short cycling.

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Category: HVAC
Satisfied Customers: 7732
Experience: Furnace, A/C and heat pump specialist.
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