I recently replaced some rotted insulation on the coolant pipe that runs out of my AC unit and into the evaporator in my house. When cleaning inside the actual AC unit itself I noticed that the coolant pipe running out of what I believe is the condenser (has a insulation blanket over it) is not insulated. Is there any reason not to insulate this coolant pipe? I traced the line and made sure that it is the same pipe that is insulated once it gets outside the AC unit.
This is the answer to your actual question:
"insulating the coolant pipe inside the actual AC unit itself."
As you correctly observed, if the outside pipe "should" be insulated (it should), then the inside pipe "should" also be insulated for all the same reasons everyone pointed out. However, the key is "should" vs "must." For argument's sake, say the outside pipe is 99 ft long and the inside pipe is 1 ft. Then the 99 ft pipe will absorb 99 times more heat from the outside air. So it's very important to insulate the outside pipe because it will absorb a lot of heat. It's not so important to insulate the inside pipe because it's so short it doesn't absorb much heat. Your AC would be slightly more efficient if you insulated the inside pipe -- but you get 99% of the benefit from insulating the outside, and only 1% of the benefit from insulating the inside pipe. Practically speaking, it's not enough benefit to make a difference.
The large, cold, low pressure line, carrying the evaporated refrigerant from the house, should be insulated to prevent condensation. The small, warm, high pressure line, carrying condensed refrigerant into the house, should not be insulated. I am not an air conditioning professional. However, en.allexperts.com, www.bobvila.com, and www.familyhandyman.com, all agree with me. See references below.
Explanation: The unit outside the house compresses the refrigerant from the large line. This makes the refrigerant very hot. Then the refrigerant is run through the condenser coils and the fan blows outside air over them to remove the heat and condense the refrigerant back to liquid. The small line now carries the warm liquid refrigerant back into the house. No insulation is needed on this line because it is still warmer than the outside air. If the air cools it some more, that only helps. In the house, the pressurized liquid refrigerant is allowed to rapidly decompress in the evaporator coil. The instant drop in pressure causes an instant drop in temperature, making the evaporator coil very cold. Interior air is blown over this coil, cooling the air and warming the coil and refrigerant. The evaporated refrigerant, which is still rather cold, now flows out of the house in the large pipe. It does not help us for this refrigerant to get warmer after it leaves the evaporator coils. It just makes it harder to compress and cool in the condenser coils. So we want to prevent condensation on this pipe. That is the reason for the insulation.
From this site:
"the skinny tube doesn't need insulation"
From another source:
"As you have seen, the large refrigerant line carries cold vapor while air conditioning, and on a humid day it sweats. The main reason we insulate the pipe is to prevent humid air from coming into contact with it." "For the small copper tube you do not need insulation. Just make sure that it does not contact masonry or other metal."
The following site provides good step by step maintenance instructions.
Suction Line Insulation
Insulating the suction line (larger pipe) is done for two reasons. First, it prevents condensation forming on the pipe. Condensation could drip from the pipe and cause damage to building materials, or create a slip hazard on the floor. The other reason to insulate the suction line, is to prevent the refrigerant in the line from picking up additional heat. The warmer the refrigerant is, the harder the compressor and condenser have to work. Minimizing the temperature of the refrigerant in the suction line, helps the condensing unit work more effectively.
Liquid Line Insulation
To avoid going off topic to explain metering, I'll just say that depending on where the metering takes place, you may or may not have to insulate the "liquid" line.
Metering in the outdoor unit
If the metering device is in the outdoor unit, you're going to want to insulate the line. The refrigerant in the line is ready to flash (boil), so adding too much heat could cause it to flash before it reaches the coil.
Metering in the indoor unit
If the liquid line is a true liquid line, and the metering device is inside near the coil, then there's no need to insulate it. A slight change in the temperature of the refrigerant, isn't going to make much of a difference. Consider that it takes 1 Btu to change one pound of 211° water to 212° water, but requires 970.4 Btu to change one pound of 212° water to 212° steam. As you can see, it's the change of state where the real cooling takes place. Because of this, changing the temperature of the refrigerant a few degrees in the liquid line doesn't make much difference.
If you are a diy'er like myself and not an air conditioning expert, realize that insulating external piping does allow for the most efficient operation and does not have to be complicated. On my outside unit I could not get regular split-form insulation to stay in place and not deteriorate rapidly where the unit is exposed to the sun. I took a serrated blade and cut a slot down the length of a few of those swimming pool float tubes so that I could slip them over the exposed metal parts of the condenser pipes. I then put a few turns of duct tape around the tube every 6 inches or so to close the slit in the tube and to prevent the whole thing from sliding around in the weather. The regular AC maintenance service checked things on its next visit and found no problem with it and I have had no problems with the piping since.
Any cool pipe in a basement will benefit from condensation reduction through insulation. Running a dehumidifier will also help. I have mine piped with a cut-off hose right over to the floor drain, and then connected to a right angle barb fitted with a 1" piece of hose going into the drain.
Hot water pipes will also benefit.
The closed cell (polyethylene) foam with the adhesive strips (covered by a pull-off protection strip) will last much longer and insulate better. You may still need an overlap wrap of tape at the butt joints. The open cell will work, but needs tape along length and is "crumblier".
I know of no reason NOT to cover the pipe with insulation. Some pro's state it does not matter however I know for a fact that if your unit has poor air flow due to a clogged air filter or your freon gets low that pipe will actually freeze solid if there is no insulation covering it causing the entire system to fail and potentially causing damage. My previous unit had a two inch thick block of ice surrounding that foot and a half copper pipe in the middle of summer and it was 104 degrees outside.
You can insulate the "liquid line" (the smaller one from the unit to the evaporator) as it carries the refrigerant that has been cooled and compressed and ready to absorb more heat. It might improve efficiency in extreme heat situations, but more than likely it wouldn't be drastically noticeable. You do not however want to insulate the big line or "suction" line. This carries Freon back to the compressor to have heat removed, and if you can lose any of that to the environment before the compressor that is just increased efficiency back to you.