More importantly, how do I fix it? is. What are some of the main causes of a liquid line being so hot that you can only touch it for about a second, while the suction line is nice and cold? Unit is pretty old, but I don't know exact age. They simply have no working knowledge of how their AC manages to blow cool air on a hot day. to the best of my knowledge i beleive that as long as the suction temperature is low we can protect the compressor from overheat and abnormal head pressure. Now, last night, went to bed, system ran for hours without stopping. Just cleaned the filter. To prevent the suction line from heating up outdoors on its way to the compressor. Check the suction line and see if it feels cold and is sweating. The suction line is the part of the piping system of a ac/ref unit. is not cold to touch, but the thin line (high pressure line?) It was never started until now. Home a/c not cooling very good -- most likely cause? A couple times I've been evaluating a split residential system where the suction line temperature reads in the 70s but the supply air temperature is reading in the upper 50s. Not enough air flow means inability to dissipate cooling to the room, means freeze-up. This air conditioning repair article series discusses evaporator coil icing: the problems of ice and frost formation in air conditioning system air handler units, blower units, or AHU's, duct work, or other air conditioning system components. Temp out of vents inside house only coming out at 76 degrees. Our upstairs unit has been failing us this summer. If you do not know how the system was designed to operate, there is no need to hook up gauges. The system will not cool if it is low on refrigerant. suction line pressure:not sure because low side gauge was pegged out at 120,so i dont know if i am getting an accurate reading.i did not wait for that 120 to come down though.it was 120 psi as soon as compressor kicked on. There can be many reasons. The insulation has two functions: To prevent the suction line from sweating and dripping water inside the house. They simply have no working knowledge of how their AC manages to blow cool air on a hot day. But the experienced DIYer, who understands certain properties of the air conditioning system, can whittle down possible causes for problems.