Hi all,

I've recently started looking after a site that has a lot of old R22 indoor fan coils connected to either VRV 407c or VRVIII annd VRVIV 410a outdoor units. Quite a few of the old 407c systems are constantly throwing up F3 faults (high discharge temp). We've looked at them with a service checker and, although some of them appear to be obviously short of gas by the condensing and evaporating temp reading, a lot of them don't but still generate the fault every couple of days or so.
The sytems are 2 pipe only and have 8 indoors per system. When I run all 8 indoors on heating they tick along ok with a discharge temp of roughly 90-95C and liquid injection coming on periodically when temp rises and bringing it down again ok. I then tried switching off 4 indoors and found that the discharge temp rises to over 100C and injection coming on more frequently. Two units only on raises temp further and then with only one indoor on the discharge raises again to the point of cutting out compressor, going into discharge temp retry and ultimately into an F3 fault.
The indoor sensor readings would indicate that all the LEVs are working ok in heating and cooling. There is no issue with discharge temp at all in cooling. The 4 way valves and hot gas bypass valves also appear to be operating correctly.
This problem is currently occuring with 8 systems and they all react the same way i.e. the fewer indoors running the higher the discharge temp annd the F3 fault only occurs when there are only 1 or 2 indoors calling for duty. This doesn't seem to support the idea of a shortage of refrigerant as they will run all day ok with all the indoors on.
Any help anyone can give with this would be greatly appreciated!

Cheers,

Markie.