Air Conditioning - Central Air Problem

I have an Armstrong 12 seer central air unit. The unit is making an unusual sound like it's struggling. When it's called by the thermostat, it runs lot longer than usual.

One a/c technician told me the compressor is shot. Another says the compressor is good and there is something wrong in the evaporator core. I had the evaporator core replaced 2 years ago. It's not low on freon and the house is staying relatively cool. Any suggestions? Thanks.

The system is 7 years old (put in when the house was built - not an add on). It's a 5-ton unit and producing an 18 - 20 degree drop.

Would have to have more information than "sounds like it is struggling". Post the ambient temp/high + low side pressures/superheat and subcooling info/compressor and condensor + blower fan amps/also note if this is TXV or fixed orfice type system and we can fix you up with a accurate diagnosis. Hopefully the techs that you had come out listed all the above info since they had to have based their diagnosis on something right?

No. neither technician provided any information to me. All either one did was quote parts and labor to "fix" the problem.

The second guy did say that it wasn't low on freon by apparently putting the gauges on it. I guess I can buy a set of gauges and report back the pressures and I can amp the blow motor.

Everything appears normal with the system, except for the tone of the compressor when it's running. If it were a centrifugal pump, it would sound like it was starting run dry or cavitate.

Ok thats a bit better description of the noise. If this is a copeland scroll and a later model it has a internal bypass mechanism the sound is fairly loud and once you have heard one you know exactly what it is. I think you are suffering froma refrigerant overcharge. A unit with this type of compressor is easily overcharged since the gauge readings lie to a hack oops I mean tech on the high side pressures, it is pretty common for these to get overcharged the only way to fix is to set the superheat and subcooling to the correct figures for your particular unit. I would advise doing that as soon as possible as your compressor life is being shortened with every cycle it runs. Just a guess here but without superheat/subcooling etc that is all anyone is doing including your hacks oops sorry I meant techs.

I don't know how long the unit has been making the noise. I would say only for a week or two. Do you think that it would stillbe the over charge problem. It's been two years since someone put freon in it. Can I get the superheat information from a set of gauges?

Also, a valve (spider) in the evaporator core went out two years ago. That's when I had the core replaced. The technician the original evaporator core had a non-replacable valve. Thanks for the information.

Well, unfortunately, if it has been two years what has probably happened is the unit was initailly overcharged when the coil was changed and compressor internal bypass was only periodically being activated probably during real high ambient outdoor and high indoor temps when the internal presures were real high during initial cooldown, but after tripping so many times it has probably weakened and is now releasing at a much lower head pressure. Only a good tech with proper temp probe clamps and a good set of gauges amp probe etc would have a shot at a proper diagnosis. I cannot help much sitting at a computer although it is fun to guess. Anyway it is probably time to start socking away some cash in the cookie jar for a new unit.

Thanks.

Compressor has a 10 year warranty. I'll just have to pay the labor and freon, etc. if it's that.