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I live in Phoenix, AZ and have an old Trane heat pump circa 1993. Over the past two weeks, several times the unit has stopped cooling yet the inside and outside units kept running. When this happens, I switch off the unit at the thermostat, wait a while, and switch the unit back on to see if it starts cooling again. If it doesn't start cooling right away, I switch it off, wait a while longer, and try again. Eventually it starts working again, but a couple times it took around 4 hours before the unit started cooling again.

Twice when the unit stopped cooling, I noticed the air handler unit started to hiss and whistle for a short time right before it stopped cooling. I touched the smaller copper line and it was crazy hot--like hot enough to burn my hand. This got me wondering if some kind of thermal cutoff is happening to prevent things from getting overheated.

I recognize that I'm already on borrowed time with such an old unit, but do these symptoms suggest anything in particular?

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  • Have you checked the inside coil for icing when it stops cooling?
    – Ecnerwal
    Jul 30, 2023 at 22:20
  • No. Can you explain how to check for icing?
    – Kirby
    Jul 30, 2023 at 22:23
  • I found the cover on the air handler to access the evaporator coil. No ice.
    – Kirby
    Jul 30, 2023 at 22:48
  • Why? Because "circa 1993". Honestly, you've gotten 30 years out of it! Ours was on its last legs when we replaced it last summer after 27 years.
    – FreeMan
    Jul 31, 2023 at 0:43

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When you write "inside and outside units kept running" I take that to mean that the blowers/fans kept running. Those are the most easy to observe. However, there's also a motor in the compressor in the outdoor unit.

Motors nearly always have built-in thermal protection to shut them down when they overheat. Frequently this protection has an auto-reset characteristic, which leads to motors "randomly" re-starting after a time. The compressor is cooled by the refrigerant flowing through it; a compressor that got hot enough to hit its thermal cut-out may take a rather long time to cool, especially considering your recent weather.

The temperature of that liquid line should be a bit above outdoor ambient temperature. Given Phoenix has been flirting with 110 degrees F lately, that liquid line you touched should be maybe 120 F or even a little hotter. That is hot enough to cause a burn.

You can infer whether the compressor is running by checking the temperature of the two copper tubes, either indoors or out. If the small one is about as hot as outdoor ambient and the large one is significantly cooler then things are somewhat normal. If there's little temperature difference then the compressor is likely not running.

It's possible there could be some debris intermittently plugging up the flow of refrigerant and causing the compressor to overheat, but this isn't something that can be checked DIY.

Make sure the coils on your outdoor unit are clear of obstructions and clean. Consider shading it from direct sun exposure (though this may not help much). Beyond that.. start thinking about replacement.

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  • Thanks for the detailed response, Greg, Yes, I meant the fans in both the indoor and outdoor units continued to run. During times when the indoor and outdoor fans are running yet the system isn't cooling, I've observed that the the copper lines are the same temperature, which suggests the compressor isn't running.
    – Kirby
    Jul 31, 2023 at 6:13
  • I have a technician scheduled to check the unit in a couple days. I'm hopeful he can keep the unit chugging along until the worst of the summer heat is over. Then, I'll give the old unit a distinguished service award and put it out to pasture.
    – Kirby
    Jul 31, 2023 at 6:23
  • Given the present cost of circa 1993 banned (no longer made) refrigerants, only available via old stock and what's recovered/recycled from systems being decommissioned, that may be an expensive visit.
    – Ecnerwal
    Jul 31, 2023 at 13:43
  • @Ecnerwal You might be right. I think the local rate for R-22 is roughly 200 USD per pound.
    – Kirby
    Jul 31, 2023 at 19:41
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Brief update: After a couple service visits from an HVAC technician, the root cause finally revealed itself: the fan motor (original equipment) on the outdoor unit was intermittently seizing up when it got hot. As a result, the refrigerant couldn't be cooled properly and a safety circuit would halt the compressor.

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