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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?

Haney
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    Sounds like you're right in thinking that it's overheating. – Tester101 Jul 22 '15 at 19:03
  • @Tester101 to that point, it just rained here a little while ago, dropping the outside temperature from 103F to 99F, and now it's running just fine. – Haney Jul 22 '15 at 19:18
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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
  • Would form (at first) in between the fins and obstruct airflow. The fact that the inside fan keeps running is making me think "defrost?" and if that IS what the system is doing (it may or may not be) it should not build up to the point where it's on the outside of the coil. – Ecnerwal Jul 22 '15 at 19:42
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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
  • @Tester101 what's weird is that the thermostat knows about, and displays, the 5 min delay countdown, when the fan cuts out, yet keeps power to the blower going. Figure that one out. :P (Nest thermostat wired correctly) – Haney Jul 22 '15 at 21:30
  • Wait... Are you seeing the "5 min delay" message on the heat pump, or the thermostat? – Tester101 Jul 22 '15 at 22:03
  • @Tester101 both - 5 min delay code flashes on heat pump, and 5 min delay is displayed on Nest. Makes me wonder if Nest triggered the delay, but if it did, why is it still powering the air handler? – Haney Jul 22 '15 at 22:06
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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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    What's the make and model of the air handler? Maybe the system is smarter than usual. – Tester101 Jul 22 '15 at 23:59
  • @Tester101 I think you're about right re: thermostat cancelling cool call and blower delay. Model of air handler is Rheem RHLLHM6024JA – Haney Jul 23 '15 at 00:23
  • Is the Airwave™ feature active on your nest thermostat? – Tester101 Jul 23 '15 at 01:21
  • Yep, but it isn't Airwave when the blower continues in this scenario. You can trust me to know the difference between full-on blower being on (which is audible from my office, and the case here) vs Airwave (fan only, deadly silent). You can definitely rule out Airwave. – Haney Jul 23 '15 at 01:23
  • @Tester101 a thought I'm having - could it possibly be the bulb on the TXV? Would that cause this hot weather behavior? – Haney Jul 23 '15 at 01:26
  • Nest appears to have a "minimum on/off time, which is 5 minutes for A/C and heat pumps. – Tester101 Jul 23 '15 at 01:26
  • @Tester101 you're correct. But *why* is it going off? Another thing I noticed: loud compressor noise for 1.5 sec when it goes off like this. When it reaches the proper temp and turns off it's a silent shutdown of the heat pump. – Haney Jul 23 '15 at 01:27
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    Airwave™ turns off the condensing unit, and only runs the blower for the last 5-10 minutes of the cycle. It could be that Airwave™ shuts down the condensing unit, but since it's so hot out the set temp in no reached. The thermostat now has to turn the condensing unit back on, but has to wait for the minimum on/off. Just a guess. – Tester101 Jul 23 '15 at 01:31
  • @Tester101 an important fact I forgot to mention: when this scenario occurs, the air coming in quickly turns hot (from the outside but with condenser off). It definitely isn't Airwave as air handler is still drawing from outside, unaware of fact that condenser is off. – Haney Jul 23 '15 at 01:36
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    I'm kind of at a loss at this point. I'm not familiar with this new fangled equipment, and Rheem is not forthcoming with documentation. Without being able to poke around in the equipment, it's very difficult to troubleshoot. – Tester101 Jul 23 '15 at 01:43
  • When the heat pump (outdoor unit) shuts off, the air coming out of the vents gets **hot** or **not cool**? Is the system ever hitting the setpoint, or is it running continuously? – Tester101 Jul 23 '15 at 01:46
  • @Tester101 not cool for sure, but rapidly. Starts blowing warm at minute 2-3, but at minute 5 the heat pump kicks back in and it rapidly goes cold again. When this is happening it never even comes close to the set point. 105F outside, trying to hit 75F, cuts out around 79-85F trying to cool. In early morning and evening it operates normally and hits set point no problem. – Haney Jul 23 '15 at 01:56
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    If the heat pump shuts down, the refrigerant stops cycling. If the blower stays on, the refrigerant in the evaporator will heat up quickly. So you'll be cycling indoor air before you know it. If the heat pump is running continuously, it could be overheating, or running up against some duty cycle limiting feature. Without documentation from Rheem, I can't be sure what's happening. To confuse matters more, your smart thermostat could be outsmarting itself. You don't happen to have any documentation that came with the heat pump, do you? – Tester101 Jul 23 '15 at 02:10
  • There's no duty limit. That much I know. I've run it for 8 hours straight on prior days. Today and yesterday 2-3 hours in the AM before first cycle of this, then every 20-30 mins after that until about 6pm when it cools off outside. – Haney Jul 23 '15 at 02:31

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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

Haney
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