Replace A/C Unit or add refrigerant?

Question

I have a 20 year old A/C unit in my house here in Texas. Two weeks ago, the unit stopped cooling. A/C company couldn't come out until the next day and the house got pretty miserable. When the tech arrived early the next day he measured temps in the return plenum at 78, and supply plenum at 76. Obviously air was not being cooled. His diagnosis was that the evaporator coil was broken and needed to be replaced and that we should probably replace the whole system as well given the age.
Then we went outside to the condenser and he measured the R-22 levels. The readings were 58/196 (those numbers mean nothing to me, but he said it was pretty low) and the temperature in the system was 33 degrees, instead of 40 like it should be. He hooked everything back up and gave us quotes for $7200-$8600 to get a whole new system.
However, after hooking everything back up, the A/C began cooling just like it had before. My suspicion is that the line was either frozen (given the very close to freezing temperatures) or something else was preventing freon from moving through the system, and his measuring the levels caused it to start working again.
We obviously have an R-22 system, and the cost of adding R-22 is very expensive now, but it seems the rest of the system is still working fine (I just replaced a blower motor myself 2-3 months ago). I'd hate to dump $7k in a system that might have a year or more left in it before something really goes wrong. Would adding additional R-22 cause the temperature in the freon line to increase, and potentially prevent this from happening again for the short term? Would it be better to bite the bullet and just replace everything now?
Edit: After listening to the advice here, I added 2lbs of R-22 to the system. Unfortunately, everything stopped working again 8 days later and we couldn't get it to start cooling again after 36hours of turning it off and back on. We had to go ahead and replace the whole system. My assumption is that we had two problems: the R-22 was low, and the evaporator coil had gone bad and the initial problem was that the coil just hadn't completely died when the first service call happened.

Eric · Answer

The 58/196 pressures mentioned is the low (suction) side pressure and the high side pressure.  Those aren't horrible values since I believe 68/250 would be typical.  If the system was 33F outside, then that is cooling better than 40F, so you should be fine there.
Is there any chance that your evaporator inside the house froze?  That can happen if humidity is high and the air flow is too low.  The solution is to turn off the A/C and leave the fan running until the ice melts.
If it working again, check your airflow.  You mentioned that you replaced the blower motor -- did you happen to change the fan speed to a lower setting at the same time?

d.george · Answer

Continuing with @Eric's comments/answer, check the temperature difference between the air at the return and the air at the discharge or the temperature drop across the coil. You can purchase a probe type thermometer or use the temperature setting available on many multi-meters. The temperature difference should be between 15F & 20F degrees on a correctly charged A/C system. If it is more than 20F you do not have enough air movement if it is less than 15F it could be too much air movement. If there is not enough refrigerant in the system it could cause the coil to freeze up. The A/C may work OK at a lower outside temperature and stop cooling as the outside temp gets hotter. If the unit needs to be recharged once a year then there is a refrigerant leak. If the leak is easy to repair then that is what I would do and if the leak is in one of the coils then it is time to replace the unit. If you replace the unit, and noise is a problem, buy a higher SEER unit say 16 SEER or higher since these tend to be quieter than the standard units. The higher the SEER, the higher the cost (my 2 cents)

Answered by d.george on September 4, 2021

Jim Stewart · Answer

At this point I would call the tech back out to add R-22. When you call them ask what would be the cost to cone out and add R-22. Two or 3 lb might restore full function for several more years. The theory behind this is the system has a very slow leak.
I have a 3.5 ton R-22 Carrier system installed in June 1991. Looking at the sight glass. I would say the R-22 is a little low, but it is cooling so I am doing nothing.
EDIT
My 29 year old unit had bubbles in the sight glass and it stopped cooling a couple of times, but resumed after turning off and letting sit. This afternoon I called my HVAC service and they were out in less than 2 hr. I met the tech in the front yard and told him to come into the backyard with R-22. He quickly added 2 lb, told me he would charge it as a short visit and was out in less than 15 min. He said the coil was now at 43 deg F, sightglass clear and suction line noticeably cooler. Outside temp today was over 100 F and inside was 75 F.

Replace A/C Unit or add refrigerant?

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