Inverter mini split not cooling well at night

Hello everyone, I have been having this annoying problem with my Pioneer 12000 BTU 22 SEER inverter mini split ever since I installed it in 2018 and the manufacturer has not been much help in explaining what is going on or resolving it, so I'm hoping someone on here may have some ideas.

The problem is once the outside temperature starts to drop, the unit stops cooling properly. It still continues to run, but no longer cools my house adequately. During the middle of the hottest days we get here (around 95-100 degrees), this unit has always easily kept my living room at around 72 degrees. Where the problems start is in the evening when the outside temperature starts to drop while my attic, brick walls, etc are still heatsoaked from the sun. Once the unit starts to underperform in the evening, my living room gets as warm as 78 degrees or sometimes even higher.

Once the outside temperature drops, the unit seems to slow the compressor down even while the inside temperature is still well above its setpoint. Perhaps something could be going on with the electronic expansion valve not properly adjusting for changing conditions? Not sure.

Here's some data taken at different temperatures with the unit set to 62 degrees (as low as it goes) and "turbo" mode active (evaporator fan forced to full speed)

Outside temp: 96 degrees
Power consumption: ~950 watts
Return air temp: 80 degrees
Supply air temp: 48 degrees
Delta T: 32 degrees

Outside temp: 83 degrees
Power consumption: ~910 watts
Return air temp: 80 degrees
Supply air temp: 54 degrees
Delta T: 26 degrees

Outside temp: 78 degrees
Power consumption: ~440 watts
Return air temp: 82 degrees
Supply air temp: 58 degrees
Delta T: 24 degrees

I just don't understand what is going on here. I don't understand why the unit sometimes loses performance as the outside temp drops without power consumption changing much (indicating the compressor is still working hard) nor do I understand why it seems to slow the compressor down dramatically enough to cause a noticeable loss in performance while the inside temp is still WAY above its setpoint? Maybe some kind of stupid energy saving strategy to cheat the efficiency rating?

I asked Pioneer's support what could cause this several times and no one had any ideas. I asked if this is by design and was told that it is not, but they didn't seem to think there was anything wrong with the unit or its operation either, they said I just undersized it. But if it is so undersized, how does it always perform perfectly during the hottest part of the day and only struggle once the outside temp drops? Makes no sense.

Anyways, any ideas would be greatly appreciated. Thank you very much in advance!

There are a few things that could be creating your situation and without the actual model number I'm fishing a little bit here, but...

A slight shortage of refrigerant could cause unusual temperatures within the condensing unit outside and the unit will start going into protection mode to prevent overheating. If the leakage is more substantial the the unit should have stopped with an error code but it doesn't seem to have reached that point so maybe it is not the cause.

Try recording the compressor discharge pipe temperature to see if it is going out of range.

Sorry, getting ahead of myself here, I have been looking at a Pioneer service manual which may of may not include your model. Within the manual are the various check temperatures and sensor reading.

Also it could be that some of the temperature sensors are failing which can also create failsafe modes. The manual gives the resistance readings for all sensors used.

As I said this may not be your model but in general manufacturers tend to stick with the same sensors regardless of model or age.

Hope this is of some help.

Click for service manual (pdf)
https://www.google.com/url?sa=t&sour...Vzu5WdATa5BbWc

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Originally Posted by Brian_UK

There are a few things that could be creating your situation and without the actual model number I'm fishing a little bit here, but...

A slight shortage of refrigerant could cause unusual temperatures within the condensing unit outside and the unit will start going into protection mode to prevent overheating. If the leakage is more substantial the the unit should have stopped with an error code but it doesn't seem to have reached that point so maybe it is not the cause.

Try recording the compressor discharge pipe temperature to see if it is going out of range.

Sorry, getting ahead of myself here, I have been looking at a Pioneer service manual which may of may not include your model. Within the manual are the various check temperatures and sensor reading.

Also it could be that some of the temperature sensors are failing which can also create failsafe modes. The manual gives the resistance readings for all sensors used.

As I said this may not be your model but in general manufacturers tend to stick with the same sensors regardless of model or age.

Hope this is of some help.

Click for service manual (pdf)
https://www.google.com/url?sa=t&sour...Vzu5WdATa5BbWc

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Thank you for your help, great suggestions! I don't think low refrigerant is my issue because I actually had to have the system recharged about a year after I installed it when the defective schrader valve leaked the refrigerant out and I watched the tech precisely weigh the charge in accounting for the 25 foot lineset I used and the unit still didn't cool properly at night.

I think a defective sensor could be the problem, I'm sure one incorrect input could cause all sorts of odd behavior. I found the unit's service manual and I found how to get the unit to display potentially relevant information such as outside temperature, inside temperature, condenser temperature, compressor discharge temperature, EEV position, indoor and outdoor fan speeds, etc, so that will be very helpful in figuring out what's going on. Right now everything looks good, but I will keep an eye on that data and see if I notice anything changing or becoming obviously incorrect as the unit starts to not keep up.

I also found that it shuts the compressor off if the discharge temperature exceeds 115 degrees C, but unfortunately it doesn't say what (if anything) it does if the temperature gets close to the limit.

Here's the service manual. Unfortunately it left me with more questions then answers as I expected. https://www.pdhvac.com/site/downloads/WYS_SM.pdf

Do you think it's possible that slowing down the compressor when the outside temp drops regardless of inside temp or setpoint could be part of its control logic? Maybe that's just normal operation?

Well I caught the unit while it was slowing the compressor down despite being set to 62 degrees with an indoor temp of 77 degrees and the discharge temp, indoor temp, and condenser temp all looked fine. At 15 degrees above setpoint and all sensors within spec, I would think the compressor should be running at full speed trying to keep up even if the outside temperature dropped.

What I did notice is that the compressor frequency has dropped from 60 (94 degrees outside, system working well, about 950 watts power consumption) to 43 (81 degrees outside now, system struggling to keep up, around 500 watts power consumption) but I don't know why or what exactly that number means. It seems like maybe that number is Hz and 60(Hz)? is full speed while anything less than that is partial speed? Of course the mains power here is 60Hz, but I have no idea what the maximum frequency the inverter would produce is.

I also noticed that the reported evaporator temp has increased from 6 degrees C (43 degrees F) to 11 degrees C (52 degrees F) as the outside temp dropped, which doesn't surprise me since the compressor has slowed down.

The only way the unit knows the ambient temperature is the outdoor sensor. I would replace this and see if the issue goes away.

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Originally Posted by frank

The only way the unit knows the ambient temperature is the outdoor sensor. I would replace this and see if the issue goes away.

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Good point but so far I have no reason to believe the sensor is defective, it was reading spot on every time I checked it including when the compressor started to slow down last night.

But now I'm curious if the compressor would speed back up if I trick the system into thinking the outside temperature is hotter by slightly warming the outside temp sensor. That may just confuse the system since the condenser temperature would be lower than what the outside temperature reads, but it's worth trying.

Is the condenser fan slowing down when ambient is dropping, is it a low ambient unit?

coolguy4,
Maybe try warming up outdoor sensor to see what happens.
Don't know what they look like, or how, but simulate it if you can.

There was another thread some time in last year or two which problem no one could fix, works out it was an update to software program that fixed it.

I have an update. I put the outdoor temp sensor in a cup of warm water to trick the unit into thinking the outside temp is higher and the compressor sped up and started cooling properly. So it seems like the outside temperature is one of the inputs for compressor speed and what I'm seeing is probably by design. Who the heck designs a system that runs the compressor in some low speed mode while the inside temperature is 15 degrees higher than its setpoint because the outside temperature is "only" 85 degrees?!?!? The temperature inside is what I bought the system to control, not the temperature outside!

Another question. How could I possibly bypass this crap and allow the system to run at full capacity whenever needed without causing other problems? I don't know what exactly the outside temperature sensor is used for other than stupidly slowing the compressor down when I still need cooling. The service manual says it is used to control the condenser fan speed, but that (fortunately) seems to be controlled by the condenser temperature and maybe discharge temperature since spraying the condenser with water slows the fan down almost instantly.

If I replace the outside temp sensor with a resistor with a resistance that represents something like 95-100 degrees during cooling season, what kind of adverse effects could this have on the unit's operation? I would think the system could still appropriately regulate itself just off of inside temp, evaporator temp, condenser temp, and compressor discharge temp, but I don't want to cause any problems or damage anything. I don't like the idea of tricking the controls, but I'm not seeing any other way around this apparent design flaw.

Quote:

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Originally Posted by al

Is the condenser fan slowing down when ambient is dropping, is it a low ambient unit?

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The condenser fan usually runs at full speed since our lows usually aren't much below 70 degrees during cooling season, but I have seen it slow down when the outside temp is around 60-65 degrees or less and cooling demand is low.

Yes, it's the low ambient version. It's rated to cool down to 5 degrees F but I have no need to run in cool mode at anywhere near that low of a temperature. I just got the low ambient version for better heating performance.

One trick we have used is to push the outdoor sensor into the fins of the condenser coil so it reads hotter than ambient, thereby causing the fan to speed up. This was necessary on the old size 71 'F' series Daikin's when the outdoor pcb with 3 relays for fan speed control went wonky

Quote:

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Originally Posted by frank

One trick we have used is to push the outdoor sensor into the fins of the condenser coil so it reads hotter than ambient, thereby causing the fan to speed up. This was necessary on the old size 71 'F' series Daikin's when the outdoor pcb with 3 relays for fan speed control went wonky

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Beat me to it Frank, I was just thinking the same.

Looking at the time you posted Brian, it looks like you had a good lie-in this morning lol :D

Quote:

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Originally Posted by frank

Looking at the time you posted Brian, it looks like you had a good lie-in this morning lol :D

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Looks like my timezone is an hour out, reset now, hopefully the right way and not now 2 hours out.

Quote:

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Originally Posted by frank

One trick we have used is to push the outdoor sensor into the fins of the condenser coil so it reads hotter than ambient, thereby causing the fan to speed up. This was necessary on the old size 71 'F' series Daikin's when the outdoor pcb with 3 relays for fan speed control went wonky

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That's a great idea, I didn't think of that! Perhaps putting the sensor behind the condenser so it sees warmer than ambient air would work well too. I'll try that. It seems like as long as it sees 90+ degree air it will allow the compressor to run at full speed. What adverse effects could the outside temp sensor reading artificially high have on the system's operation?

Also, is it typical for variable speed inverter systems to slow the compressor down when the outside temperature drops even if the inside temperature is still high or is that specific to this unit? Maybe these kinds of flaws are why this system was so much cheaper than similar systems from Mitsubishi, Daikin, Fujitsu, etc.

No adverse effects as I can see in warm weather. Could lead to low liquid temp and indor evap frosting in cold weather on capillary systems though

Just reposition it during the heating season.

Awesome, huge thanks to everyone who responded Everyone's advice has been very helpful. I'm going to try moving the sensor later today or tomorrow and see how it does and I'll be sure to report back. My system has an electronic expansion valve and a temp sensor on the evaporator so I wouldn't think it would freeze up, it has never shown any signs of frost before of course with the exception of the normal frost in heating mode.

Now that I think about it, I think it's likely that the outdoor temp sensor is primarily used in heating mode for determining when defrost may be required and possibly for blocking out heating/cooling modes when the outside temperature is above/below the specified limits.

Edit: Moving the sensor to the condenser's fins added a few degrees to its reading and definitely delayed the compressor slowing down for a bit longer, but now that the outside temperature is down to 78 degrees, it slowed the compressor way down again. Seems like I need to move the sensor somewhere a bit warmer or maybe find some way to bypass it during cooling season such as a resistor. I wonder what just leaving the outside temp sensor unplugged would do. Hmmm, something else to try.

What temperatures are you seeing on your indoor coil in cooling mode? Not air temp but actual coil temp before the compressor slow down

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Originally Posted by frank

What temperatures are you seeing on your indoor coil in cooling mode? Not air temp but actual coil temp before the compressor slow down

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Right now while the system is working well, the outside temp is 94 degrees F and the evaporator temp is reading 41 degrees F. Once the compressor slows down, the evaporator temp increases up to around 52 degrees F. According to the service manual, the compressor will be slowed down if the evaporator temp drops below 36 degrees F to prevent freezing, but I never see it get nearly that cold.

The condenser temperature is always pretty similar to the outside temperature, usually a few degrees higher. The compressor discharge temperature always stays pretty low too. According to the service manual the compressor discharge temp limit is 115 degrees C or 239 degrees F, which seems ridiculously hot to me.

I moved the outside temp sensor behind the top of the condenser, which added about 20 degrees F to its reading. The good news is that this did indeed allow the compressor to run at full speed even after the outside temperature drops, the bad news is it seems to have had some unintended consequences.

First and most importantly, it doesn't seem to have gained me much capacity. After moving the sensor I measured a delta T of 21 degrees with an outdoor temp of 82 degrees, which really isn't significantly better than it otherwise would have been even though the power consumption was almost doubled. Discharge temp was also hotter.

Second, the supply line to the indoor unit was icing up. I'm not sure why this is. My best guess is maybe the EXV isn't being set to the correct position with an inaccurate outside temp reading. Not surprising that the system doesn't like the condenser temp reading much lower than the outside temp.

It's starting to look like there's not much I can do at this point other than just live with it. Unless anyone has other ideas?

My thoughts are SOG. Compressor backing off due to low evap temp.

First step would be to reclaim and check gas charge

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Originally Posted by frank

My thoughts are SOG. Compressor backing off due to low evap temp.

First step would be to reclaim and check gas charge

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I haven't seen any evidence of low evap temps other than after I moved the outside temp sensor behind the coil so the compressor would keep running at full speed and the supply line started icing up. But even then, the evap temp sensor was still reading well within its normal range.

The refrigerant charge is correct, I just had my HVAC tech friend recover and weigh the charge and it was spot on. He didn't have any other ideas, his suggestion is that I take steps to reduce heat intrusion such as improving my insulation. I ordered a roll of radiant barrier foil for my attic, which I am working on installing. I'm also in the process of improving my attic's ventilation. Seems logical to me. If I prevent the house/attic from heatsoaking so much, there won't be as much of a need for cooling overnight.