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I have a Rheem 15PJL48A01 heat pump which has been working correctly for years, and is maintained properly. It is hot here currently and so it is operating in cooling mode.

Yesterday and today, it has been very hot out. 105 °F yesterday and 103 °F today. The outdoor pump's fan has been cutting off every 20 minutes or so during the day. When it cuts off, it goes into a 5 minute delay cycle, then fires back up. During this time, the air handler (Rheem RHLLHM6024JA) keeps blowing air. There is power to the heat pump outdoor unit at all times. I removed the panel to investigate the shutdown code and it codes "5 min delay" and no other issue. Interestingly, at night when the ambient temperature cools off, the fan will run for hours without issue, blowing cold air through the home until the desired temperature is reached.

Here's the heat pump manual, specs, and troubleshooting guide

Here's a photo that shows the daytime shutoffs yesterday (Tuesday) between noon and 6pm as compared to the days prior. Notice that in the night and early morning it operates normally.

Tuesday weirdness

So what's happening? Is it overheating?

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    Sounds like you're right in thinking that it's overheating. – Tester101 Jul 22 '15 at 19:03
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    Could also be the inside coil icing up? – Ecnerwal Jul 22 '15 at 19:19
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    @Ecnerwal I'm a bit of a heat pump noob: I see no ice on it, would the ice be in it? – Haney Jul 22 '15 at 19:26
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    Defrost mode typically would happen in the winter, when the outdoor unit ices up. This is a common problem with heat pumps, so defrost is a "normal" mode. The indoor unit icing up is rare, and only happens when there's a problem with the system. Therefore it would be a fault condition, and one not many systems handle. The indoor fan continues to run, because these systems are fairly dumb and don't communicate with each other. The thermostat is calling for cool, so the blower fan keeps spinning. Even though the outdoor unit has overheated, and is not running. – Tester101 Jul 22 '15 at 20:47
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    The blower in the air handler likely has an off delay, which means it runs for a bit after it's told to turn off. Some heat pumps won't restart for a set period of time after they're turned off. So it could be that the thermostat is cancelling the cool call, then waiting 5 minutes to turn it back on. However, the blower is staying on due to the off delay. – Tester101 Jul 22 '15 at 23:59
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Actual, better answer:

The heat pump was definitely overheating. Since the heat wave has passed, there have been 0 issues. The ambient outside temperature is 90-95 °F this week, whereas it was 103 - 105 °F last week. This seems to make all the difference as things are purring along and the home is staying cool. Thanks for the help everyone!

Heat Pump Working Properly

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