What causes the suction line to freeze up periodically and has correct freon levels? The line freezes up maybe once a week. We have checked for leaks and there is no loss in freon.
The lines freeze when the cold air isn't being moved properly. This can be due to a few things, but the most common is low refrigerant. Dirty air filters, dirty coils, blocked or closed registers, blocked or closed returns can all contribute to low air flow, too, as can broken fan motors. You may not have noticed a leak in refrigerant, but did you have the pressure checked? It's likely that the pressure is low and refrigerant needs to be added. This is normally a closed system, so if some needs to be added, there is a leak somewhere - it may be a very small leak, but a leak nonetheless.
Well Freon pressure can be misleading. If non compressables are in the lines (air) the pressures may look correct but be way out. Was a temperature probe used to verify the the vapor pressure at a specific temp? This alone can have a 100 psi variation in pressure. I use super heat, and sub cooling to see if the system is operating correctly, another note what type of gauge or line set was used to measure the pressure? Most untrained folks making this kind of measurement use a standard gauge / line set that releases an amount of the Freon where many pros use a gauge on a nipple so there is no real loss. Next after opening the system (making a measurement is opening the system according to the EPA) was the system leaked checked? Schrader valves are a main point of leakage if not properly sealed and checked after servicing. All this requires a license in the U.S. and most of the rest of the world so it is not really a DIY project. Did you know there are fines up to 25,000.$ for working on HVAC systems with out a licence.