City Multi, indoor ducted unit unable to maintain target.

[e.mcl]

I have a city multi system comprised of 8 indoor ducted units (pefy-p25vmm-e) and one wall mounted mini-split unit serving a comms room. One of my ducted units is serving a room with a lot of natural light and as soon as the sun hits, the temperature runs away. Often a target of 19 will be set and the room will be sitting at 25 until long after the sun has gone down. During this time, the supply temp will match the room temp. It just won't cool. I don't feel this is due to too much demand because even with all other units turned off it is still unable to drop the supply temperature. The unit in the comms room will happily pump out -3 air even in the height of summer with all other units cooling as well. The room temp. thermistor is reporting correctly so it's not that.

[frank]

Not too up on mitsi, but I've never known an ac unit pump out -3C, or any other make to be honest. They are just not designed for sub zero temps.
Have you determined that the charge is ok?

[seanf]

Do a heat load calculation for that room and check it against the capacity of that rooms duct unit?
Have you stuck you head up at the duct unit to see whats happening pipe temp wise, and also if the evap is iced up or something simple like that?

Could it be unintentionally supplying -3C because its in a comms room with dry air?

[Peter_1]

Any possibility to measure SH over that server room coil? You then can exclude the EEV underfeeding for whatever reason the coil.
If SH is OK, then your heat load is ways bigger than this unit can extract. I bet that's the reason.

[e.mcl]

Thanks for the suggestions all, I'm actually an electrician. Just seeing if the issue was likely down to a faulty electronic component that i could replace myself but seems more likely to be poor system design or a refrigerant issue with the manifold unit which i beleive is called the EEV? - all a bit over my head i know. I'll be getting an HVAC engineer to look at the system.
The outdoor unit was replaced about 2 years ago and the issue existed before then. So seems unlikely it's an issue with improper gassing.

[Peter_1]

Electronic Expansion Valve