Today I had a quick look at a system which is reported to trip regularly when the weather is cold.
The outdoor unit is an old(ish) Airedale R407c cuhs heat pump. It's matched to a custom made coil within a ahu.
Fairly simple system really, each end has a tev and all controls are electro mechanical.

Took some readings today, head pressure 180psi and suction 50psi when in heating.
They seem a bit low to me?
The system has a llsg which shows clear with no flashing, although obviously in heating it's the liquid return to the outdoor unit from the indoor dx coil.

I found some old paperwork from previous work which shows it has a charge of 3kg. I looked at the manufacturers spec, measured pipework etc and it would appear it should have 5kg.
Spec shows 0.6kg base charge (up to 7m) then 28g per metre. Total run was 23m

The lp switch trips at 0.5 bar. I'm kinda assuming that when the weather is cold, the reduced temp over the outdoor coil is possibly causing the suction to drop to cut out point?

Would you agree I'm on the right track here with what may be the cause of the trips? I've not looked at this before and today it was 10c and it did not trip, so can't even confirm it was tripping on lp, that's just my guess at this time. I can't test it in cooling, as it supplies a temp critical area.

The thing that's niggling me, is that a total charge of 5kg seems a lot for what is a fairly small 5kw system.

Advice appreciated as always.