Pioneer mini split poor performance at night

[coolguy4]

Hello everyone, nearly 3 years ago I installed a Pioneer 12K BTU 22 SEER inverter driven mini split heat pump in my living room and overall the performance has been quite impressive for the price in both cooling and heating modes and it is extremely cheap to run. However, ever since I installed this unit, I noticed that it seems to perform better the hotter the outside temperature is.

Where I live, I see temps of up to nearly 100 degrees F with very high humidity. This unit has never had trouble keeping up during the hottest part of the day either, the temp in my living room rarely goes over 73 degrees F with the unit maxed out. Oddly enough, where the unit struggles is in the evening when the outside temperature starts to drop to below around 80 degrees F. The evaporator's delta T drops from up to around 26 degrees F in the afternoon heat down to around 16-17 degrees F when the outside temp drops and my living room sometimes gets up to around 78 degrees F- Not much cooler than outside temp, but certainly much drier.

So my question is what could cause the unit to perform well in hot weather and somewhat poorly in moderate-warm weather. As a test, one evening I tried partially blocking the condenser airflow with cardboard to raise head pressure, simulating hotter weather to see if the delta T would increase back to normal. Doing so lowered the delta T. I then wet the coil down with water, which slightly increased the delta T. This would seem to suggest that the issue isn't mechanical, but perhaps something with the controls.

I'm thinking the controls may have some type of issue or flaw that causes the compressor to slow down when the outside temp drops even when the inside temp is still much higher than the setpoint. I tried to test this theory by heating the ambient air temp sensor on the outdoor unit with a heat gun as an attempt to trick the unit into thinking it's hot outside to see if the compressor would speed up and start working better, but heating the sensor made no difference. However, since there are other sensors such as for compressor discharge temp and coil temp, my simulation wasn't perfect.

Anyways, I'm really looking forward to hearing your input on this one. Thanks in advance as always!

[frank]

Never heard of the 'Pioneer' make of A/C units, only the Pioneer HIFI here in the UK.

As ambient temps drop, head pressures are reduced by slowing fan speed and/or compressor capacity to prevent icing on the evaporator, where the system has a fixed metering device (captube). As you say, with other sensors in play, without knowing the control strategy it is difficult to say exactly what is happening.

[coolguy4]

Never heard of the 'Pioneer' make of A/C units, only the Pioneer HIFI here in the UK.

As ambient temps drop, head pressures are reduced by slowing fan speed and/or compressor capacity to prevent icing on the evaporator, where the system has a fixed metering device (captube). As you say, with other sensors in play, without knowing the control strategy it is difficult to say exactly what is happening.

It's not made by Pioneer electronics, here in America there's a company by the same name that makes lower cost mini split systems. Their support is surprisingly decent but they had no idea what was causing this issue. I asked him if this might be some issue with the control logic and he said no because it's not logical.

This unit does have the ability to modulate the condenser fan speed to control head pressure as it is rated to cool down to 5 degrees F, but the fan seems to run at full speed all the time unless the outside temp is in the 50's or less and cooling demand is low. The evaporator never ices up.

I would think when the room temperature is well above what the unit is set at and it's set to "TURBO" mode the compressor should be running at full speed no matter what the outside temp is, but I'm no engineer.

Also, for what it's worth, this unit uses an electronic expansion valve as the metering device.

[al]

It'd be interesting to get your gauges on it, is it 410a or 407c? I've noticed this myself at times with splits here, but i wonder is part of it psychological,putting it into turbo possibly only runs evap fan at full speed but if room is close to setpoint compressor may not ramp up?

[coolguy4]

It'd be interesting to get your gauges on it, is it 410a or 407c? I've noticed this myself at times with splits here, but i wonder is part of it psychological,putting it into turbo possibly only runs evap fan at full speed but if room is close to setpoint compressor may not ramp up?

Hmmm interesting. Which brand units did you notice that on?

From what Pioneer's support said, Turbo mode is supposed to force both the evaporator fan and compressor to full speed for 30 minutes regardless of the setpoint. I suppose it's possible that they are wrong though. It's hard to hear what speed the compressor is running at as it is so quiet it's hard to hear it over the fan.

This is a 410A unit and it only has one service port on the low side. Currently it's rainy here but next time we get another hot day I will check suction pressure as well as amp draw with the unit maxed out in the hot afternoon as well as in the evening once the outside temp cools off to try to get a better idea of what's going on and I will be sure to report back.

Also, this unit generally seems to have pretty poor control logic in several ways. For example, it will not always ramp itself up to full capacity when it's falling behind unless I manually turn the temp down. So if the unit is set at 67 and it's 75 degrees in the room and climbing, I need to turn it down as low as it goes (62 degrees) and activate turbo mode to get it to ramp up and start cooling. I would think when the indoor temp is 8 degrees above the setpoint and climbing the unit should already be running at full capacity without manually being told to, but I'm no engineer.

When I told Pioneer about that issue, they told me that I undersized it. However, if it's undersized, how does it keep up perfectly fine during the hottest part of the day as long as it's set at 62 degrees and running in turbo mode? My understanding was that AC units should be sized so they are basically maxed out while maintaining the desired temp on the hottest day of the year.

Keep in mind that this is my first mini split as well as my first variable capacity AC unit, so for all I know they may all operate in the same IMO annoying ways.

[Brian_UK]

<snip> I asked him if this might be some issue with the control logic and he said no because it's not logical.

It may not be logical because it operates under "fuzzy logic control". You can look that up and be none the wiser I'm afraid. I think that it means that the control idea moves around according to the conditions and is supposed to therefore be "clever". I was never convinced.