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I haven't been able to find anyone online with quite this issue... I have a Carrier heat pump system, and through the winter it would often make that torpedo noise when heating the house (I later learned this is defrost mode to melt condensation that has frozen). It seems my AC system goes into defrost mode more than necessary, as I live in Florida and the temperatures are rarely below freezing, even at night. I do remember reading that humidity alters the freezing point somewhat. I also read there's a hack to get defrost mode to turn on only when necessary, rather than when the system "thinks" there is ice buildup.

Anyway, now that it's the hottest time of the year, I periodically hear a similar torpedo-shooting-like sound come from the outdoor unit. This happens several seconds (about 10 seconds) after the exhaust fan turns off. I have a Nest thermostat and so the air handler fan will run for a minute or so after the outdoor unit turns off. This noise is not made every time the outdoor unit shuts off, and seems to only happen at night, when my thermostat moves the temp from 78 to 74.

The only info I could find was that a spring was bad in the compressor, and it's a sealed unit so replacing it would mean replacing the whole outdoor unit. Is this a good use of my home warranty (it is covered)? Assuming this is something I can ignore, it is next to my bedroom window, and wakes me up at night.

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    What model is the outdoor unit? – ThreePhaseEel Aug 15 '19 at 3:00
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    Not sure about noise, but the high humidity in FL would condense and freeze inside the AC evaporator without a defrost cycle. – DrMoishe Pippik Aug 15 '19 at 4:48
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    Ac/heat pump units do not typically freeze because of the air temperature, they can freeze up when it is over 100 degrees out. The refrigerant in your system gets way below 32 degrees. If your unit is freezing up more often, you might need to get it serviced. – Gunner Aug 15 '19 at 11:01
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If there is a bad spring in the compressor , the compressor can be replaced, you do not have to replace everything. In cooling mode the outside unit should never need a defrost the condensing coils are actually hot, if there is any ice it would normally be on the suction line and is normally caused from a low charge but an over charge can also cause icing. As far as the torpedo noise I am not sure what you are describing but if non compressibles (air) has gotten into the system it can sound louder as the high side pressure drops once the compressor stops. With the noise and possible icing it is time to call a tech and make sure your unit has a proper Freon or refrigerant charge. As far as a home warranty if it is covered you should use it but at this point a service call is needed to identify what the problem is. You should get a recording of the noise so the tech can hear it it could be something like the reversing valve opening since you mentioned the temp set back but that is a guess, as It is really the only thing I can think of that would really dump the pressure and make a noise intermittently.

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  • Thanks for the reply. To clarify, it only goes into defrost mode in the winter, when the heat is on. My only concern with that is it does when the temp outside is in the 30s, but it also does it in the 40s and maybe even the 50s. I wouldn't expect ice with air temps that high but as mentioned, humidity supposedly changes that. – jspinella Aug 15 '19 at 13:22
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    When you are heating inside the outside unit is cooling it gets quite cold and will create ice even with the temps in 40-50 range. The higher temp you have set inside the harder it works outside or colder it gets. – Ed Beal Aug 15 '19 at 13:26
  • I can try to elaborate on the torpedo noise a bit. When it goes into defrost mode, you know switches the coolant direction, basically turns on cooling mode for a few minutes, that noise is what is described and shown here: arnoldservice.com/… A similar but not identical noise is made today, and that noise is made several seconds after the A/C turns off, rather than in the middle of it heating my home. It might be the defrost noise minus the fan spinning up. – jspinella Aug 15 '19 at 13:30
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    Ok that’s what I meant when I mentioned the reversing valve. The thermostat may be causing this since we don’t know the type of unit you have and it sounds like it happens with the temp setback. It should not be defrosting at all when in cooling mode. I would see if there is something in the thermostat programming that can be changed. If you can delay the defrost from starting until the unit has been idle for 3-5 minutes the pressure will have equalized and it won’t be so noisy. This would help in the winter. With a 4 degree swing it may be trying to change modes but that is a guess. – Ed Beal Aug 15 '19 at 14:06
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    Would appreciate an upvote or acceptance of the answer it doesn’t cost you any points and closes out the question as answered or calls t will keep coming up for review. – Ed Beal Aug 17 '19 at 5:32

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