We had the air conditioner running the other day and the air was not cold. I could hear the compressor cycling on and off outside, so I knew it was trying to work. I went in to the HVAC closet and looked at the copper lines that connect to the compressor. One of them was cold, with ice building up on it. I've played this game before, so I'm pretty sure the problem is that we are low on freon. Also we had it filled up about three years ago and the HVAC guy told us we had a small leak and would need to refill it now and then. The system is designed to compress freon at the compressor outside. Taking "stuff", in this case freon, and squeezing it into a smaller space makes the temperature go up. It runs through the outside coil to transfer this heat into the air outside the house, then through the high-pressure line to an expansion valve and into a heat exchanger coil inside the house. It expands, and since it has a certain amount of heat spread out over a greater volume the temperature goes down. I have a "central air" unit integrated with my furnace so the furnace fan blows air over the cold coil (and the air cools down as heat transfers from the house air into the freon) and the cool air goes on through the duct work and cools the inside of the house. The freon goes back through the low pressure line to the compressor outside, where it is compressed again. When the freon level is low the system can't keep the freon at high pressure in the high-pressure line. It expands before it gets to the expansion valve, so it cools the copper pipe instead of the coil. I am not an HVAC professional so some of my descriptions might not match the industry terms, but the theory is correct. I know this stuff because I'm a mechanical engineer, because I've had my share of troubles with this HVAC system over the twelve years I've had it, and because I can put those two things together with a little common sense.