The story

  1. We noticed the outdoor AC unit was running (i.e., fan was spinning) nonstop for > 18 hours and was relatively louder than usual -- there is now a humming vibration that can be heard while in the room on the other side of the wall. I can't tell if the compressor is presently running.

    • Note: cold air did blow periodically through indoor registers as expected when the thermostat temp reached threshold, but the outdoor fan continued to run even between moments of normal cool air blowing inside.

    • note: We did not go outside to check to see if compressor was running or not.

  2. We noticed the indoor refrigerant lines were dripping wet (soaked through the insulation). So I checked outside, and the compressor and surrounding refrigerant lines immediately outside the compressor had frozen with a layer of ice.

    • No noticeable ice on the outdoor coils or on the refrigerant line from the outdoor unit into the outside wall.

    • Note: a "shrrr"-ing sound could be heard through the refrigerant pipes when standing next to them near the evaporator.

  3. I shut off air-conditioning at the thermostat (i.e., set it to "off").

    • This did not shut off the fan outside. It continues to run > 1 hr after A/C was shut off at thermostat.

    • It does not sound like the compressor is running (but I am unsure).

  4. I turned the thermostat to "fan on" (while keeping A/C off). The compressor unfroze, the refrigerant lines warmed up, and water started leaking out of my plenum (suggesting the evaporator coil was [unsurprisingly] frozen as well).

    • indoor condensate pump is moving most of the water to the appropriate drain (it has run multiple times without issue). We put down towels to absorb any other water leaking down indoor unit.

    • air blowing throughout house is quite cold [~65F] despite AC being turned off (likely due to ice buildup?).

Some nuances:

  1. We put in a new air filter 3 days ago. (The last one was not especially dirty [i.e., about the same as usual] when I replaced it during normal scheduled replacement).

    • FYI: I just pulled out the new filter that was installed 3 days ago and checked it -- it's installed in the correct direction as usual, and no apparent issues presented themselves.
  2. I Checked registers and returns -- nothing seems blocked; air-flow feels normal throughout house.

    • Indoor blower I assume is working fine given that air is blowing through the registers without issue with "fan on" setting turned on.
  3. The furnace/AC units have not been serviced since we bought the house 1.5 years ago.


  • Indoor unit: Rheem model# RCBA-37656624 (serial# M2894) [no mfr date, but it's not new]

  • Outdoor unit: RUDD model# UAKA-030JAZ (serial# 5339 M1495 7649) [mfd: 04/95]


  1. Are my problems possibly related? (i.e., froze compressor/lines and the non-stop spinning outdoor fan)?

    • What would cause the outside fan to remain on? Is it the frozen lines, or is this likely caused by some other issue? (i.e., stuck relay, faulty contactor, etc.)

    • Can anyone point me to a trustworthy troubleshooting source for narrowing down the issue from possible causes?

  2. What is the best order of operations from this point?

  3. Should I shut down the outdoor fan manually?.

    • Can I / should I pull the fuse on the outdoor fan while the system is still melting/warming up? Would that be an issue for any reason?


  1. I shut off power to the outside unit (condenser/compressor) by pulling out the pullout switch in the disconnect box. That shut everything down.

  2. I waited 60 minutes. With the thermostat fan switch set to "auto" and AC control switch turned to "off", I put the pull-out switch back into the disconnect box. Both the condenser fan AND the compressor immediately kicked back on (despite the thermostat being set to "off").

    • "shrr"-ing sound continues in refrigerant pipes inside of house.
  3. I noticed ice [frost] started forming around the refrigerant pipe coming immediately out of compressor and at the base of the compressor itself within ~3 minutes of running. It continued to thicken as I let it run.

  4. After letting the unit run for about 10 minutes, I pulled the pull-out switch in the disconnect box to shut the system down again.


Ok, so it seems like low refrigerant could very likely be at play here (as suggested by RMDman in the comments). But what is still not clear to me is how that relates to the system automatically being "stuck" on when the thermostat is set to "off."

Do I just likely have two problems (1) Low freon and (2) an electrical issue elsewhere? Or do all my symptoms somehow relate?

  • What is the best/safest way to diagnose/troubleshoot the latter electrical issue? Or does stuck contactor sound about right?

  • could either of these issues (low refrigerant or electrical) have led to the other?

  • This YouTube video suggests that possibly a bad contactor can cause a fan to keep running which can cause the system to freeze. So, maybe all my issues are related to a bad contactor? Are these safe to replace myself? Commented May 27 at 14:51
  • This sounds like a classic case of low freon. A pro would be needed to check the system for leaks and do any needed repairs before replacing any freon.
    – RMDman
    Commented May 27 at 14:52
  • @RMDman thanks. How does that relate to the fan continuously running? OR do you think these are two separate issues? Commented May 27 at 14:53
  • Found OEM contactor at Repair Clinic. Is this worth pursuing, or can anyone suggest other issues to check for? Commented May 27 at 15:00
  • I'm not an HVAC pro. Some units can have a system to keep the compressor fan running. If the unit was NOT low on freon, it would run and not shut off, but not freeze. The frozen lines and evaporator are the indicators of low freon. This does not look like a DIY issue at this point. You may or may not need a new contactor. The "tell tale" would be the system freezing again.
    – RMDman
    Commented May 27 at 15:02

2 Answers 2


The system definitely has ice on the evaporator coil. This could take many hours to thaw on its own (30 to 60 minutes is nowhere near long enough). I'll return to advise on clearing this problem later.

As you know, the outdoor (condensing) unit is controlled by a contactor. This is a device that receives a 24-volt signal from the thermostat and connects/disconnects the power that runs the compressor and fan in the outdoor unit. Typically you'll find a cable with two small-gauge conductors taped to the refrigerant lines outside which runs into the wiring compartment of the outdoor unit.

Pull the handle, flip the switch, or otherwise operate the electrical disconnect at the outdoor unit to remove mains power. Open up the wiring compartment and find where those small-gauge thermostat conductors attach. Disconnect one of them and isolate it (use a spare wire-nut, wrap with a bit of electrical tape, etc). Restore mains power to the unit at the disconnect. Do the fan and compressor start up? If so, your contactor is stuck closed. It might be possible to un-jam it, but the recommended course of action is to replace it. Re-connect the thermostat wires disconnected earlier. Operate the disconnect again so that the outdoor unit is de-energized.

Now return to the problem of the ice in the evaporator. Its melting will be accelerated by running the system blower, but beware: the blower motor is cooled by the air passing through it; when that airflow is impeded by an iced-over evaporator, this can lead to the blower motor overheating. So, use the thermostat to turn on the blower and then check whether a reasonable amount of air flow is felt at one of the registers. If so, great, but if not, then turn the blower off and wait a while and try running the blower again later.

Some time after you've begun this process you'll notice first a few drops, then a trickle, then quite a flow of melt water at the condensate drain and pump. Keep an eye on this. When you've seen it go from nothing, to substantial flow, to nothing again, that's how you know the ice melting is completed.

With the evaporator de-iced, and the contactor operation checked and corrected if needed, you can operate the system normally. If you again notice symptoms of freeze-up, that's often caused by either of these:

  1. Not enough air flow - dirty filter, closed-off registers, under-sized ductwork, etc
  2. Possible low refrigerant charge
  • Thanks, Greg. Appreciate the response. 1 follow-up: RMDman in the commments above felt strongly that my described issues pointed to a refrigerant issue. Can you comment at all about your opinion on this and how I could be more sure of this? (or does it necessitate an HVAC tech to rule in favor of?). I assume since my filter is new, I checked la my registers, and this same unit has been operating for 30 years at this home, that I'm likely left with a refrigerant issue. My concern, of course, is that this system must be using r-22 freon, so I'm not sure how fixing this issue will play out... Commented May 27 at 20:46
  • Low refrigerant certainly can cause a freeze-up. But other conditions can too -- excessive running, especially with the indoor blower stopped, for example. Your mention that "It continues to run > 1 hr after A/C was shut off at thermostat" strongly suggests that the compressor was stuck running while the blower was stopped and that's why it made ice. That's why I made a point of testing the contactor. Considering its age, I'd suggest a person who has the means to opt for replacement if a repair estimate comes in higher than 15-20% of the cost of replacement.
    – Greg Hill
    Commented May 28 at 1:33

After opening the AC unit up, I determined the contactor was indeed stuck. I am trying to get a replacement part, but I cannot figure out which part is needed.

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