Icing on a VRV condensing unit

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The recent cold temperatures reminded me of a situation that I encountered a few winters ago.  This is a VRV condensing unit in its first year of winter service.  Can you guess what the problem is here?  It isn’t a malfunctioning de-icing cycle.  The outside temps were around freezing when this happened, and it didn’t happen only once.  As soon as temperatures dropped, the condenser coils began icing.  Answer: the system was low on refrigerant.  This is a R-410A system, so the pressures are much higher than former refrigerants in use–cycles operate between 400-500 psi.  This system, as it turned out, was 70 pounds low on start-up.  Since we were starting up 9 systems at the same time, the low start-up pressure got lost in the shuffle of paperwork at job close-out.  This was a school job so we had only a few weeks of warm weather, then temperate weather, and then finally, in January, cold weather when the problem shook out.  VRV systems have many hundreds of feet of refrigerant piping per system and undergo a stringent start-up protocol to avoid leaks, which entails a vacuum held on the piping system for several days to test for integrity.  We were all hoping that a leak had not developed, which can be hard to detect.  Also, the precise amount of refrigerant to add on system start-up is calculated by the manufacturer and is based in part on pipe volume.  This system had some field pipe routing changes which had not made it back to the manufacturer for refrigerant volume re-calculation.  So fortunately, after the correct amount of refrigerant was added back into the system, we have had no further problems with the system, and it has now been in service for two years.

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