Daikin vrv 3 u0 error

I've been looking at a daikin vrv 3 the system is bringing up a u0 error. I'm trying to work out if it's low on refrigerant or the expansion device is playing up.
I've had it running on test and the suction pressure varies depending on compressor speed but average is about 6-8 bar but the discharge is only getting to 17-20 bar at full whack. The air off the indoor units is poor about 2 degrees lower than ambient on each one.

Now if it was an expansion issue wouldn't the suction pressure be either really low or really high depending how it's stuck. Also there is tons of valves on this so it's hard to see what one I'm after. It's one of the vrv units with 2 compressors 2 fans etc 2 sets of boards.
The compressors are ridiculously hot and the suction line is warm to ambient not cool the discharge is scolding hot.



I think it's possibly low on gas as opposed to it being an expansion issue but what's the best way to go about checking. I'm having a really off day today normally this would be easy but I mainly work on split systems and not vrvs.

Get a copy of the VRV III service manual, downloadable, and follow the U0 fault finding scheme.

Just had a quick look going to have to look at it again tomorrow not feeling great today hence not being able to work it out. It's weird though 17 bar high side 7 low but warm suction pipe on both compressors.

Vrf's will run to reach a certain pressure as recorded by the pressure transducers. Looks like yours is short because head pressure is low, poor cooling inside.
I'm guessing compressor is only running slowly, not like they are ramping up that much.
Discharge shouldn't be over 100°C.

If it's SOG the inverter compressor (first called) will hang around 38hz and not move.

As Brian says, print out the piping schematic and follow the heat/cold around the pipework with your hand.

There are bypass valves that can go crook which can lead to a warm suction pipe

Put it through a test routine and see what codes come up

Was low on gas and yeah the compressors were not ramping up one was ticking over slowly even when asking for 19c in all units. I thought it would be that if it was a expansion issue there's all sorts of weird stuff that happens. All sorted now. Didnt appear to be low standing pressure or running but neither tell you much on a vrv as they will maintain certain pressures.

Tiny leak from a flare nut.

Thanks for the update.

Without the correct amount of refrigerant, they can't maintain certain pressures.