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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.

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  • R-22 is no longer being produced. The only possible source is new old stock or recycling. (note that the law requires anyone draining an A/C system to recover the freon and return it to distributors for either re-use or annihiliation; that's why you can't DIY that). – Harper - Reinstate Monica Jul 26 '20 at 6:23
  • @Harper-ReinstateMonica I'm aware of the restrictions and thus the increased cost in obtaining it. Since my issue seems to be related to pressure in the system, I'm wondering if it's a good idea to just add more for now (even at an inflated cost). – Mordred Jul 26 '20 at 6:45
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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)

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  • If you decide to add R-22, you may also want to add a can of Stop-Leak as a temporary fix. But begin socking away bucks in a New AC account. – Phil Freedenberg Jul 26 '20 at 11:51
  • @d.george - good tip for the air handler tip - that will figure out if freeze-up is a possibility. – Eric Jul 26 '20 at 22:09
  • @Phil Freedenberg has a good point about a can of stop-leak to keep the system working for another 2 months while you save up for a new AC system. Tell the A/C guy that you don't have the money now to do the repair, but to let you know if the price changes. There is a good chance that when the cooling season is over but before heating season starts, he will be willing to do the job for less. – Eric Jul 26 '20 at 22:23
  • @Eric that's a very interesting idea. Might give that a go in the fall. – Mordred Jul 27 '20 at 2:10
  • Alternately, if you are up for a challenge, you could buy a new A/C system (condenser/compressor, line set, and indoor air handler) online (beware that this normally voids the warranty) and physically install everything yourself and then get the A/C technician to braze the connections, purge the lines, and top up the charge for $500. – Eric Jul 27 '20 at 9:31
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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?

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  • I did not change the fan speed. Motor should have been basically identical to the old one it replaced. It's certainly possible the evaporator froze. However, the unit was off for about 10-12 hours in a 100+ degree attic, and then I turned it on for 15 minutes before the repair guy arrived, and it wasn't cooling at all until after we checked the R-22 levels. Not sure if that sounds like a frozen evaporator or not. – Mordred Jul 26 '20 at 15:47
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    @Mordred - the not cooling part after being off for 10-12 hours doesn't seem like a freeze-up, but did you check that the compressor was running? I had a system that would go into an error mode when it froze during the night and wouldn't run again until the power was cycled. It took me a few weeks to figure out what was happening and check the refrigerant level. There is an R-22 pressure chart available at engineeringtoolbox.com/r22-properties-d_365.html. The boiling point at 68 PSIA is around 29F, so that would contribute to freeze up of the evaporator. – Eric Jul 26 '20 at 22:20
  • Do you mean condenser? That was running, but I'm not aware of any compressor that needed to be checked. Sorry, I'm a little unclear what that pressure chart means. At 68 PSIA, the temperature would be 29 in the line which makes it more likely the evaporator would freeze? In that case, adding R-22 would raise that temperature and then freezing is less likely, is that correct? – Mordred Jul 27 '20 at 2:09
  • The evaporator is in the air handler (inside your house). Liquid R22 goes through the small line at high pressure into the house and is expanded in the evaporator allowing it to essentially boil. It returns to the compressor and condenser through the large line as a vapor. You are correct about the table, the temperature is the boiling point at the specified pressure, so if the refrigerant level is low, the pressure and boiling point is lower than the freezing temperature of water which allows humidity in the air to freeze onto the evaporator. Adding R22 will raise that temperature. – Eric Jul 27 '20 at 9:19
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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.

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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.

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    That's what I ended up doing. We added 2lbs or R-22 to the system. Unfortunately, a week later it completely died and would not cool at all, so we had to replace the whole system. I believe we most likely had two issues: the R-22 was low, AND the evaporator coil was bad. – Mordred Aug 21 '20 at 15:22
  • Too bad. What was the total cost for adding the 2 lb of R-22? What did you replace the unit with? – Jim Stewart Aug 21 '20 at 17:46
  • They charged $140/lb + the service call, but I was able to get them to refund half of it on the recapture since they also replaced the unit. We went with a 15.5 SEER Trane dual-speed system. – Mordred Aug 21 '20 at 19:37
  • Tech added 2lb of R-22 and cleared the sight glass. The tech said a full charge on my system was probably 12 lb IIRC. – Jim Stewart Aug 30 '20 at 0:51
  • Yesterday a/c quit cooling and I quickly determined the compressor was trying to start and failing. Called tech and he tested the capacitor as barely OK but OK. Clamp meter showed 107 A. Diagnosis compressor locked. We are waiting for a new 4 ton system to arrive (NG furnace and 16 SEER coil and condensing unit). – Jim Stewart Sep 1 '20 at 23:13

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