ABS/PVC connection on combustion air - but I consider that trivial (don’t know if ABS is permitted, we don’t use that crap here).
Looks like filter access may be obstructed - also trivial
CSST gas piping - just verify bonding and notify the client of fire risk
That’s about all I’m picking up, but I don’t regularly perform inspections from photos.
There is no return air ducting from anywhere and the return air is taken right near where the combustion takes place! There’s also a gas fired water heater nearby so this installation can be downright dangerous!
Add to that other points mentioned, the major one being condenser coil on the return side, etc. and you have a very deficient installation! As well, the 2 heat pump outside units do not have service disconnects!
That’s very different than what we are accustomed to seeing here in the states, at least the southern states for sure. It appears the R/A duct is the one on the top left down flowing to the evaporator coil then ducted over to the bottom of the up flowing furnace then out the top of the furnace to the supply duct. Like the others said this is a shot in the dark from one small photo and I am likely completely wrongg.
Section 705.7.2 Solvent Cement Joints (for PVC)
Where surfaces to be joined are cleaned and free of dirt, moisture, oil, and other foreign material, apply primer purple in color in accordance with ASMT F 656. – and – Solvent cements in accordance with ASTM D 2564 shall be applied to all joint surfaces.
So as long as you use the right purple primer, you can also glue PVC. You need to use a glue that meets ASTM D 2564, the standard for solvent cement for PVC. Of course.