LG Multisplit indoor unit stopping

[scottmcgur]

Hey all,

Brief background - I've inherited an LG multisplit system with my house and experiencing the following issue;

The system is a 4 split, but with only two ways connected. (C&D) There did used to be circuits on a and b but these units were upsized and as a result the odu became too small to handle the load.

As such I have a 7kw odu, connected to a 3.5kw ducted system on circuit c and a 3.5kw wall mount on circuit d.

If I call for heat on any of the units, all is well.

If however I call for heat on both the following happens;

Both units will initially heat - the odu ramps up.
Shortly afterwards circuit d drops into a preheat state and the odu ramps back down.
Circuit d does not restart until the unit on circuit c is shut down, nor does it appear to restart once circuit c has no demand.

Is there some sort of priority setting on these units? My second thought could be a gas shortage causing the odu to cut demand to keep pressures up elsewhere in the system.

I'm not familiar with the lg multi range so wanted to ask if there was something simple I've missed before I spend £££ and call out a service tech to do whatever needs be done.

Thanks!