Do I Need a New AC Unit?

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SAS

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We have an 11 year old Trane XL15i 48,000 BTU unit. We moved into the house before the start of last summer. The AC unit seemed OK until mid-summer when it was unable to effectively cool the house. We had a technician come. He could not find a refrigerant leak, but he confirmed that the unit was low on refrigerant and added 6 pounds of R410a. It worked fine until today. We were away for 5 days and had the unit off during a heat wave of daily 95+ degree temperatures. I turned the unit on remotely before I headed home. The inside temperature at that time, 8 am, was about 90 degrees. Today was another 95 degree day, and when we arrive home at 4 pm, the inside temperature was around 80 degrees. The unit has been running continuously, but it cannot get the temperature under 80 degrees. The air temperature at the output vents is 68.2 degrees, and the input temperature is 80.1 degrees.

If the problem is once again a lack of R410a, I'm curious as to why it suddenly happened. Or does it gradually decrease until it passes a critical threshold? And if the problem is once again a leak, do I try to have someone find it again? In a prior house we had a leak that no one could find, and I'm concerned that I'll waste a lot of money only to be told that they can't find the leak. In that case do I try to find someone to add refrigerant again? It was $600 last time, and if it's going to happen every year, I guess it's probably time to replace the unit.
 
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breplum

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Broadly, yes to your supposition 'it gradually decrease until it passes a critical threshold'.
Obviously you do have a slowish leak and obviously the refrigerant is low with that low temp. differential stated.
Finding leaks can be tricky and there are some varieties of leak detection (I think). I've been an installer for decades but never purchased the leak detection products.
 

Fitter30

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6 lbs is a big leak. Probably the serviceperson didn't pull the evaporator coil cabinet apart to check the coil. UV dye should be installed and come back the next day and use a UV light to find it. 18 - 24° difference between and return temp.
 

SAS

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6 lbs is a big leak. Probably the serviceperson didn't pull the evaporator coil cabinet apart to check the coil. UV dye should be installed and come back the next day and use a UV light to find it. 18 - 24° difference between and return temp.
So far I've spoken to two contractors. The first one said that it did not pay to try and find the leak, since it was like looking for a needle in a haystack. If it lasted from last year until now, it would have to be so small that you would just waste time and money looking for it. The second one said that they are 100% successful in finding leaks, but sometimes they are in places that cannot be repaired. They charge $160/hour to look for the leak and it could take 1-4 hours.

And yes, you are correct that the technician did not check the coil. In fairness to him, he told us that finding the leak required equipment that he didn't have with him and would require another service call. He also expressed doubt that they would be able to find and repair the leak.

So any suggestions on where to go from here?
 
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Fitter30

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Don't know how small contractors work but another service call is bs if he had the equipment he would of at least there would of been a attempt.
 

WorthFlorida

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A slow leak is on the low pressure side but as written by Fritter30, 6 lbs is a lot and it gets expensive. Other than using a die, a sniffer tool is used and all AC techs should have one. Good ones are not cheap but herein Florida, service companies charge extra to use the sniffer tool, which of course is crazy. Like a doctor charging extra to use his stethoscope. The most common area for leaks is the evaporator coil. A good sniffer tool (well over $300 and not Harbor Freight quality or Amazon) will find the leak in seconds as what happened to me. A few years ago my eight year old Carrier unit coil leaked and a month later the same at my son's home built at the same time, same AC unit and same builder. It turned out replacing both air handler and condenser was about $4K (2014) where as just changing out the coil was just $2,500.

You have a good model but an all new unit will be more efficient and will have at least 5 or 10 year warranty on parts, etc. but not labor. All residential units are made just about the same by all manufactures. They'll buy from the same sources. Other than LG compressors, they are all Copland Scroll compressors and use ECM motors for the fans. Higher SEER models will have different controller boards from the manufactures.

If the cost is no object, I would go with a new unit all around. If the coil is replaced, everything else is 11 years old and no warranty except maybe the work to install the coil. The hard part is knowing of a good company. I've used a one man owner company and a large company with a fleet of trucks and never had problems with either, but the larger companies do seem to charge more.

In Florida we live and die with central air as in CT heating is more important. I can call on about 100 AC companies here in Central Florida, and the work must be permitted and inspected to protect everyone. 32 years in Florida I've dealt with quite a few AC techs and never came across anyone not knowing what they are doing. It seem you had one that did not know what he was talking about.
 
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