Hello- I am preparing to have my 3 ton Goodman a/c unit installed next week. This weekend I am going to pour the slab but I wanted to get some feedback first. The unit has a foot print of 29 x29. Can I dig 4" down into the soil without creating a form or does the slab have to be raised? Is a 4" slab thick enough? Will a 30" x 30" slab be large enough? The unit will be placed in a corner next to the house and a fence, how far should it be from the house?
The slab should be above grade. We use concrete sidewalk blocks and put them on 2" of crushed gravel and edging around the gravel so it does not wash away. I would allow at least 18" clearance on all sides for proper airflow and so a "healthy" sized tech can get in there to wash it and service it. Some units have louvers on the sides to remove and you need at least that much clearance.
"Cut it twice and it is still too short".
One consideration is how stable the soil is next to your house. I once built an addition and placed a site poured pad for the unit half way over the backfill and stable soil. Nine months later I got a call from the client saying the compressor had failed because the unit had tilted as the backfilled soil had compacted. The HVAC contractor said he had seen this many times before -- apparently keeping the unit level is crucial. ...and of course I paid for the repair.