We have an a/c unit that is "settling" - the best way to level again?
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What is the AC unit set on. Usually HVAC techs set it on a lightweight concrete slab or plastic pad. If so, is the slab also settling? The AC unit is not that heavy and the slab/pad isn't also. You can use a crow bar or long piece of wood to gently leverage the slab and AC unit up a few inches at each corner and fill under with soil or gravel. Watch so you don't kink the copper lines. Check the level as you go along and that it isn't rocking.
After you fill in you can also bear down on the unit to settle it and check the level again, add more soil/gravel if needed.
You did ask for the "best" way. That is is to collect the freon and disconnect the unit. Move it to a area that is level and the soil finished settling and compacted. Place it on a thicker pad. (My personal tiff is the unit is so low I can not raise the soil level along the foundation for my flower garden.) If your home is more than a few years old and settling is occurring along the foundation then you need to find out why and correct that before moving your unit.
If you decide to lift and fill under the pad, consider using paver base. It is mixed to pack solid and forms an almost concrete surface after tamped down.