installation of condeser on slab

We have 3 A/C units outside on a slab at the back of the house, built 5 years ago. We have just replaced our 2nd compressor of the 3. The tech told us that the units are installed flush on the slab, and that has caused water to not be able to drain properly, causing the coils to all show extreme signs of rust. He showed us there are some "weep holes" that are essentially plugged by the slab.
I've called the original installation guys, but they can't tell me if these units were supposed to be raised off the slab or not. I've looked at the installation guide, and it looks like the units did in fact come with feet that would have raised the units a couple of inches off the slab.
In addition, the slab isn't sloped correctly (maybe it has shifted in the past years), and does not drain as well as it should.
So, in general, are these units supposed to be raised off the ground (HSXB15 Lennox) and if so, would this help cause the compressor problems we are having?
I am NOT an A/C guy, just trying to get information...
Thanks in advance...
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cdenker wrote:

Our unit is on a slab placed on a side walk. When they install the unit they brought a greenish looking slab saying made for the purpose. It acts like sort of cushion. No rust problem. I just had an annual check done and nothing mentioned by the tech. Brand is Carrier.
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The condensers should be a minimum of 18 inches apart and a minimum of 18 inches from the side of the house, with a minimum of 5 feet clear overhead. the slab should be level.
The "tech" is feeding you a line of BS, and failed compressors in only 5 years tells me that he doesn't know how to correctly balance the refrigerant charge.
Find another tech.
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