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We live in Northern Illinois (-20F in Winter + 1-2 ft snow to 100F summer) and have two a/c units which are now 30 years old and it is time to replace them.

They are located on the north side of our house which also has all of our neighbor's yard sloping toward it and we have continued to have water leakage into our basement from this in spite of draining all our gutters away from the house.

The current ac units sit on plastic footings set in a bed of gravel, problematic both because this conducts water to the nearby window well and because we end up having to "re-level" them every year. We are not sure if the gravel shifts, or the soil below it is settling, or what. We also have/had a sidewalk next to it which has settled/moved at least 1-2 inches over the 12 years we've lived here (due to those same water issues and/or chipmunks).

When we replace the units we are planning to get rid of the current gravel bed and replace that area with clay to try to reduce water infiltration to that side of the house and the nearby window well which results in basement leakage when we have hard rains (3" in a day).

What is the best solution for new A/C mounting?

Top considerations:

  1. Minimal/easy maintenance (re-leveling)
  2. Economical
  3. Quiet (AC units are right outside the home office)
  4. Water handling (allow water to run AWAY from the house, not act as a water reservoir right next to our house like the current gravel bed)

We've looked at the following options but aren't sure which is best:

1. Pour a new concrete pad

Pros: should be durable

Cons: May be expensive, hard to do, (and/or hard to find contractor to do) and if it still tilts/sinks (which we've seen with both the sidewalk and our driveway) may be expensive/hard to fix. Suggested methods would probably disturb soil and might add to water infiltration to basement.

2. Plastic pad

Pros: Cheap, light, easy install, seems to be favored by contractors

Cons: Not sure how durable these are - seems like it would be hard to replace if expected life is 10-15 years and it goes before we next intend to replace the AC units (which might be another 30 years). Also, may still tilt/sink if the soil below it moves and it's not clear how we'd fix the tilt.

3. Mount on wall of house

Pros: Should eliminate the possibility of tilting/moving

Cons: Not sure if allowed by code in Northern Illinois; Might be noisier since connected to house (but maybe there's a way to prevent noise transmission?); Not sure where to find materials to do or if contractors will be willing/competent to do it.

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    The ground moves as you can clearly attest to. Any pad that is not anchored on a footing that is below the frost level of your area is going to move. – Alaska Man Aug 23 at 18:18
  • It really comes down to opinion on the best method. Most of the split systems I have installed I have put on plastic pads 40$ delivered. In the past with larger outside units concrete pads were suggested putting one in after the unit is set will be difficult. if the unit is noisy mounting on the home is the worst option in my opinion as the noise will transfer through most walls even with vibration dampers. A proper sized plastic pad should distribute the load and reduce the shifting. As you found that even concrete can settle the pad might shift also but would be less than 4 small footings. – Ed Beal Aug 23 at 18:45
  • As the frost level in our area is >3ft and we did not want to be doing that much digging, we decided against the concrete pad. We ended up moving the AC condenser units to a different location, adding sufficient clay (tamped down with a hand tamper) sloped away from the house to ensure appropriate water flow, and covered with a raised bed of 2-4" of crushed limestone (mix of 3/4" gravel, 3/8" gravel, and screenings), contained in a box made from 6" high bricks supported by metal edging. On top of this our contractor put the plastic pads (3") and the condensers. – hoMEG Nov 11 at 17:18

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