Several weeks ago, the high pressure pipe and the coils inside my interior blower frosted, I suspect because I had forgotten to change the filter so the air flow diffusing the coils was slow. Now cooling doesn't seem to work and the high pressure pipe, which always gets almost ice cold when it functions properly, is not cold. The interior fan and the compressor fan outside both work OK but, just like the high pressure pipe inside is not cold, the air being blown out of the compressor outside is not hot. Also, when I redirect the drain hose from the sewage into a bucket, there is no water coming out.

Could it be that the freezing cause damage and is there any other tests I can do?

  • a) Make/model of system? b) I'll let the experts (I don't know a lot about HVAC) answer, but it could be that something in the refrigerant system broke and the refrigerant leaked out - which would not be a good thing... May 7, 2023 at 18:59
  • Assuming the "drain hose" is condensate, that's a given - no cold = no condensate. May 7, 2023 at 19:16
  • 3
    The first sign of low refrigerant can be icing on the coils. If this is followed by a complete lack of cooling, it suggests a refrigerant leak.
    – MTA
    May 7, 2023 at 19:18
  • @MTA -- I had it topped when I had to replace my compressor less than two years ago
    – amphibient
    May 7, 2023 at 19:35
  • 1
    @amphibient, air conditioning systems are closed systems, so if everything is sealed properly, you should never loose refrigerant and need a recharge. So if you're down on refrigerant (or out, as your symptoms seem to indicate), you're very likely to have a refrigerant leak somewhere. Time to get a professional in to find the leak and see whether it's economically repairable or not.
    – Milwrdfan
    May 8, 2023 at 2:42

2 Answers 2


Let me start by sharing a little terminology. There are two tubes going between the compressor (or condenser) outdoors and the indoor coil (or evaporator). The smaller of the two is the "liquid line," so named because it carries liquified refrigerant from the condenser in to the evaporator. During operation this line is actually the one that's at higher pressure -- somewhere around 300 psi, as a rough figure. It's also warmer -- should be several degrees warmer than outdoor temperature. The larger of the two lines is the "suction line," so named because the compressor outside is sucking refrigerant from the evaporator through this line. Refrigerant in this line is in gaseous form; this line should be quite cool, and likely may get wet with water, but this line should never have frost or ice on it. A system is often set up to operate such that this line will be around 36-40 °F and pressure around 115 psi.

From an engineering standpoint these are relatively simple systems; there's not a lot that can go wrong. Based on the symptoms you shared, one might look for these problems:

  1. Not enough refrigerant - all the mechanical parts are working fine, but refrigerant charge is so low the system can't move much heat.
  2. Compressor not running - maybe the controls are not calling for it to turn on, or the contactor has failed, or the compressor has failed. If the compressor motor requires a capacitor to start or run, check that too.
  3. Refrigerant not flowing - there's always a metering device where the liquid line enters the evaporator coil indoors. It might be a simple "fixed orifice" or piston, or it might be a TXV (thermal expansion valve). Either way, this part has the job of controlling the flow of refrigerant. If it sticks closed then no refrigerant flows.

Many jurisdictions require a person handling refrigerants to be certified by a government agency such as the US EPA. That probably restricts you from doing much about #1 or #3.

Testing of the contactor is something that can be done by "anybody" with a volt meter and reasonable care. Briefly, one would confirm that voltage is present at the input side of the contactor, make sure that the contactor closes when the thermostat calls for cooling, and then verify that voltage is present at the output side of the contactor. Videos of the process could be found on YouTube, for example this one.

  • I had a technician come in to check and he is saying the capacitor is bad and unable to start the compressor -- is that a tenable hypothesis?
    – amphibient
    May 8, 2023 at 17:18
  • 2
    @amphibient Yeah, that's entirely plausible. I'll add it on the list of "compressor not running" causes.
    – Greg Hill
    May 8, 2023 at 17:26
  • he said the bad air filter choking the flow inside may have overloaded the capacitor
    – amphibient
    May 8, 2023 at 17:31
  • @amphibient, my experience with HVAC techs is that they know what they're doing. These systems aren't that complex, and they have the tools to troubleshoot the problems.
    – Tiger Guy
    May 8, 2023 at 17:54

Your filter would have to be very dirty to all of a sudden freeze up the coils. If you have a volt meter, check the line and load sides of the compressor contactor and the coil voltage of the contactor. Check for any loose wires. Manually, briefly turn on the contactor to see if the compressor runs and the high pressure pipe gets a little cold. If it does, then your compressor is probably OK and the problem is in the thermostat sending circuit. If it doesn't, then it could be a bad compressor capacitor or a bad compressor.

  • is the contactor inside or outside ?
    – amphibient
    May 7, 2023 at 19:57
  • 1
    The contactor normally is in the compressor section of the AC unit
    – JACK
    May 7, 2023 at 20:01
  • is it the electrical control board/manifold that wires go in ?
    – amphibient
    May 8, 2023 at 2:17
  • @amphibient No, it's a separate relay about 3" x 3" x 2".
    – JACK
    May 8, 2023 at 2:38

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