How Would You Build Foundation for AC Pad?

[black00lightning]

I'm having the outside unit for the AC system installed soon and need to pour a 3' x 3' foundation. I want to have the pad 18" above grade, about a foot below the top of the workshop floor. I plan on removing what little topsoil there is (rocky soil) and laying a 4" base. I'm using 8"*16" concrete block in a 4 x 4 grid in the center. I'll build a osb form and fill with concrete. Any thoughts?

 

[Cyberbear]

We did something similar: put in the forms, filled it with soil, wet the soil, next day dug out the dirt for the footings and poured a short time later, worked fine.

 

[gungatim]

usually they just use a pre-cast pad. not much weight to worry about. have never seen one needing a foundation or poured in place, but maybe your area is different. are you saying you want it higher than grade or below?

 

[matt_i]

I don't get what the concrete blocks are for? Do they take up space to minimize the volume of concrete? Or are you using the voids in the center to run electrical or refrig tubing?

 

[reader2580]

If 18" above grade why not just build a wooden platform instead of trying to build a huge mound of dirt? You just put 4x4s into the dirt just like building a deck.

You could also use steel angle brackets attached to wall, but that might transmit noise into building.

 

[BD1]

I skipped the concrete idea and used steel. Built a angle iron frame that the outdoor unit fit in with angle iron legs to go into the ground. Post hole dug the four holes and added concrete. This was 30 years ago and still solid . I did have to modify when unit was replaced.
Another option a couple angle iron brackets mounted to concrete foundation wall.

 

[gregtwojeeps]

If 18" above grade why not just build a wooden platform instead of trying to build a huge mound of dirt? You just put 4x4s into the dirt just like building a deck.

You could also use steel angle brackets attached to wall, but that might transmit noise into building.

This ^^^^ ....If disturbed soil is going to be the pad's base it is going to eventually settle and allow the pad to drop. Most of the time really out of level, which is not good for the unit. jmo

 

[8man]

Rocky soil provides the base to do the wooden platform. If you are worried about termites or rot, do it in steel. The concrete you are planning is overkill, it'll work though.

 

[pmiranda]

Why 18" above grade? Do you get flooding at that spot? If so, I'd probably fab up a steel platform out of angle iron for the unit to sit on, and put that on top of a regular concrete pad a few inches above grade like normal. Otherwise a couple feet of concrete is really likely to settle unevenly.

 

[Kevin54]

They make a prefab fiberglass pad for air conditioning units. Way better than messing around with concrete. And they are colored gray to look like concrete.

This is a pic from Home Depot. Different places carry them and they range from $20-$30

 

[Kevin54]

I just saw where you want it above grade. Why not get a wall mount for it?

 

[QwikKotaTx]

I just saw where you want it above grade. Why not get a wall mount for it?

Just make sure not to bolt that to the wall outside of a bedroom. Would be vibration central unless it's one of the newer variable speed units that don't spin as fast.

 

[jd_1138]

They sell pads. AC units are not that heavy.

 

[ilovevocs]

I echo the above, get that side finish grade, throw the pad down and call it a day. Most HVAC contractors around here furnish the pad as part of the install.

 

[OCJohn]

This is a pic from Home Depot. Different places carry them and they range from $20-$30

Beat me to it.

 

[NUTTSGT]

Do you have a hill or drop off right outside the workshop ?

 

[black00lightning]

The lot slopes from the back to the front by 30 feet and the yard is graded to allow water to run off at the sides of the property line. Water flows on the back side of the shop which is where the AC will be installed. So basically I'm trying to keep it above the potential water line.

A height of 18" is not excessive as the house AC units were built on a poured 5 ' high foundation to match the house's floor elevation (house foundation is 1 foot above grade in back and 8 feet above grade at the front).

Concrete foundations for AC units are the norm here so I want maintain the "look".

 

[southalabama]

I built a wooden one for behind the office. Four by posts. Framed it like a deck and used 2/6 for the decking. I left the posts run long and framed it in and used slats to fence it in.

It's about 18" above grade. I wanted to avoid flash floods from the roadway behind the office.

 

[gregtwojeeps]

The lot slopes from the back to the front by 30 feet and the yard is graded to allow water to run off at the sides of the property line. Water flows on the back side of the shop which is where the AC will be installed. So basically I'm trying to keep it above the potential water line.

A height of 18" is not excessive as the house AC units were built on a poured 5 ' high foundation to match the house's floor elevation (house foundation is 1 foot above grade in back and 8 feet above grade at the front).

Concrete foundations for AC units are the norm here so I want maintain the "look".

If you are pulling up fresh soil that high to pour a slab on, you had better dig down below the original undisturbed soil some with posthole diggers and put two 6 in. " anchor posts " that will be part of the pour to anchor it. The disturbed soil as I said above, is going to settle over the years and you will have an out of level A/C condenser and that may even want to pull away from the structure...not good. Or, you may just shim the unit up until the slab finally settles down...either way works. All jmo

On edit: pics of my pre - cast condenser pad installed by the previous owner two years ago. It has the 4"x 8" x 16" solid blocks under it and basically little slope grade.

 

[Kev442]

I got sick of the chipmunks tunneling under my pad, so I went down 14" with solid concrete block. I am also above grade about 8-12", as my yard slopes also. Still level after two winters.

 

[NUTTSGT]

If you do have a water issue or that it could be a problem, I think you would be smart to get it up and out of the way right from the beginning. I'd rather do it with something more permanent like concrete, block or both.

 

[LS6 Tommy]

A lot of manufacturers have stand kits for use in coastal areas subject to possible flooding. Many beachfront areas require them by code.

Tommy

 

[thammel]

I've done 8 pads so far. I'm OCD and can't stand it when pads don't stay level. So I build them to last. Did down 3 feet and use an 8" builders tube with rebar tied to rebar in the pad. Build the pad as high as you want - I use 2x6 or 2x8 PT to frame it and leave that in place. If you want to elevate the outside unit/compressor, just buy some 6" or 8" snow feet. They are pretty cheap. My 2 cents.

Tom