Air Conditioning - A/C copper line size

I had my a/c unit replaced last year. The copper supply lines are larger on the new unit (I am not sure if the smaller of the lines is larger too). They were not replaced because to do this would have meant cutting up the bedroom floor or opening up the first floor ceiling. The installer told me it would make no difference. the unit is just not cutting it.

An outside temp of 80 degrees or above, with the thermostat set at 75 degrees for the unit, the inside temp will actually rise to 77 to 79 degrees. The compressor runs constantly until later at night.

The temperature difference on a good day (75) between incoming (74) and outgoing (62) air is at best 11 to 12 degrees. The unit is properly charged. I have been told by a couple of people that 1) the unit will not work efficiently with smaller lines and 2) this will burn out the compressor in a few years.

The compressor unit is installed through the front wall of the house; it is not a free standing outside unit.

I could really use a professional opinion on this issue. I would also appreciate an explanation as to why the line size would effect the efficiency of the unit.

Thanks.

Your 10 to 11 degree air temperature difference sounds OK to me. As long as you are properly recirculating the room air through the cooling coils the room temperature should drop to the thermostat setpoint in a reasonable time. It sounds like you have return air issues after they redid your A/C unit.

As far as line set sizes go, the needed size depends on refrigerant flow rate and line length. If your old A/C was a smaller capacity unit then I would expect it to use smaller lines. The main idea is that smaller as well as longer lines produce a larger pressure drop than bigger or shorter ones. My guess is that if the lines are only 15 to 20 feet long the difference between line size won't be too significant.

Since smaller (or longer) lines produce a bigger pressure drop, the compressor sees a lower suction pressure and hence a bigger pressure differential between the suction and discharge side. A bigger pressure difference means that it will consume a little more electricity to raise the suction pressure to the much higher condensing pressure so it will be a little less efficient. Imagine trying to suck a milk shake through an extra small straw. Your A/C compressor has the same problem.

Line sizes should be designed for the worst case so I doubt that it's the bottom line issue in your mal-functioning A/C unit. Your compressor will use a little more power and it will run a little hotter with smaller line sizes. It sounds like your compressor is working hard as it is and it would be a good idea to get someone who really knows his stuff to address the issues you seem to have with your unit.

Most of the time across the coil you should get a 18o drop. Are we try for it. You dont say what make AC they put in. But all companys list what size line you need with what coil and compressor they made. Like jughead said how far apart the units are will call for a bigger line. Thats like some time back lennox had a set up on pressure drop. The liquid line was 50 ft and you had to use it coil it up, just put it some where dont cut it off.For the unit to work right.

ED ;)

wow, ed you do way back!!

i remember those, the line was actually the metering device, tonnage determined by length

you do not state unit tonnage or line size

i like 3/4 and 3/8 lines up to 3 tons, under 70 feet, if length is over 70, 7/8 and 3/8

3.5 to 5 tons i use 7/8 and 3/8 up to 70 ft, 1 1/8 and 3/8 over that

if the condenser is below the ahu by 3 or more stories, pressure loss on the smaller line may indicate a need for larger sizing, depending on tonnage and lift

i look for 16 to 20 degree drop across the coil, getting it at the register is not always good, as a duct issue will change your readings

if the condenser is more than 15 feet above the ahu, a trap may be needed to help the oil get back to the compressor

The unit is a "Nordyne" model GB3BM-024K-AB, 2.5 ton,unit on first floor, same as old unit and plenty for this house. There is about 24-25 ft. of line. I took the temp about 10 inches above the coil. Return air flow is good, best on the first floor right next to the blower.

Line sizes:

Old line, 5/8 and 1/4
Unit line, 3/4 and 3/8

I have the installer comming back tomorow night. I would like as much info as I can get in able to understand any possible problems.

You all have been a great help.