Air conditioner runs constantly, doesn't cool much

We have a four-year-old central air conditioning system that ran fine until this summer, but now isn't doing so well. I don't believe the air is coming out as cool as it used to, and the system runs almost constantly but does not cool the house as much as it used to.
The company that installed it came out twice in one week to check it, and checked the attic components as well as the outside unit. They have found nothing wrong. However each time they said it needed "half a pound" of freon. Does that sound right?
We have replaced the filters. Today as a test I turned the thermostat way down (from mid-70s to mid-60s). The system ran constantly but the house only got a slight bit cooler.
Any idea what the service guys might want to check next?
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Possible. Maybe you need a better tech. Did they look for a leak?
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Al writes:

YOU should perform a basis performance test: check for air flow out of the vents, and measure the temperature difference from ambient. Should be 15 or 20 degrees if you're caught up on dehumidification.
This neat toy will save you time and money:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber778
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Al wrote:

I've had bad luck with A/C, so I keep a couple of dial thermometers stuck in the air vents to monitor the performance. After the system has run continuously for at least ten minutes, the air blowing out should be at least 15 deg. F. cooler than the temperature at the thermostat's thermometer..
It's not normal for home A/Cs to lose any Freon at all, but our old system developed thousands of pinholes in its evaporator coil when it was about six years old. Why can't the repair company find the leak, especially one that lets out 1/2 lb. every few days?
Good A/C web site with forums: www.hvac-talk.com
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On 29 Jul 2006 21:03:12 -0700, "Al"

This reminds me of the guy who was talking to the other guys at work about his wife:
He says, "Every morning my wife wants 50 dollars from me. Every day, day in and day out, week in and week out. Always the same, 50 dollars she wants, every day."
"What does she do with all that money?" another guy asks.
"I don't know. I never give it to her."
So, did you put the half pound in the first time he said you needed it? If not, it's not surprising they said the same thing later that week.

Where do you live? How hot is it outside? How hot is it during the day and in the middle of the night?
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Al wrote:

They must be selling refrigerant!
Print the second linked page and give it to the techie! - udarrell
--
Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
  Click to see the full signature.
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