Mitsubishi electric city multi issues

[Ben93]

I’ve been asked to go look at something for a friend who has had a couple of people look at the system and had no joy in working out the fault and wanted some things to look at and opinions before I head over Thursday.

Basicaly it’s a PUMY-p140VKM5 condenser and has 4 indoor units not sure on indoor unit kw but been told it’s 20 meters to furthest indoor and a total of 32 meters liquid pipe additional charge of 3.2 kg 410A making a total charge of 8kg.

Basicaly it’s about 6-7 months old the install and was working fine over winter and In summer when they peope have used it for cooling after 3-4 hours it brings up a 1102 fault code (high discharge temp) the other people have weighed gas out and it’s the same 8kg as original installation. Tested it changed the discharge thermistor and stil happening after 3-4 hours running now in my opinion if it was a pcb it would fault instantly/ pretty quick not after 3-4 hours. I’ve been told the air offs of all 4 units in both heating and cooling are really good as well.

So yeah any suggestions would be appreciated thank you.

[frank]

Had similar on a Daikin vrv.
Turned out to be an oil return problem.
When system is cool, runs for a while and as compressor starts to heat up, lack of lubrication causes the compressor to overheat resulting in high discharge temp.

On my system, turned out to be pipework installed over design max distance and the oil was collecting in the fan coils at the furthest point. Due to the excessive pipe lengths, the gas velocities at this extreme length were insufficient to carry the oil back to the compressor

[Ben93]

Had similar on a Daikin vrv.
Turned out to be an oil return problem.
When system is cool, runs for a while and as compressor starts to heat up, lack of lubrication causes the compressor to overheat resulting in high discharge temp.

On my system, turned out to be pipework installed over design max distance and the oil was collecting in the fan coils at the furthest point. Due to the excessive pipe lengths, the gas velocities at this extreme length were insufficient to carry the oil back to the compressor

I’m going to just go from scratch measure liquid line and sizes and indoor units use the calculator to bring up charge correct that if it’s wrong check running pressures condensing temps and another one that I’ve had cause it is the lev inside bc box not operating properly causing a lack of refrigerant back through the suction preventing compressor cooling if all else fails will ring tech 😂

[Ben93]

The max allowable pipe length for this condenser is massive so shouldn’t be that issue

[kanchi]

It is most likely a defect in the discharge thermistor itself.
Discharge thermistors have normal resistance values at room temperature, but shift sharply at high temperatures.

[martinw58]

have you checked the addressing and that the port connections on the BC box are piped and addressed correctly

[martinw58]

it the ports are piped incorrect or address incorrectly as units shut down on temp system might run on low pressure causing compressor to over heat