For the standard residential system changing the filters regularly and cleaning the condenser/outside unit coil surfaces and fan blade regularly is probably all you need to do.

A few months ago, I did a maintenance check on all my house, new smoke alarm batteries, new CH/A filters, hose out the condenser, etc.   

They would qualify and set up appointments for Steve our office secretary would put a map package together so he could find the property he was a little directionally challenged.. so his day got condensed to 2 to 3 site visits and timber cruises.. working 1/3 the hours and first year we closed over 80 deals that had net revenue in the low 7 figures for the company.. 

I can tell you if it's already ducted if there is enough space between the plenum and furnace it mi gght be economic with a condenser. 

Then the air conditioner leaked due to not draining and cleaning some condensation thing on it, it leaked all over the ceiling and the ceiling collapsed on my tenants' stuff (thank God not my tenants weren't home).

I know it's more fun to assume someone did throw bull out and call them on it, but unfortunately in this case it wasn't bull, it was just a bad attempt at condensing a long answer.

YOur condenser unit getting stolen.. your foundation cracking and the house needs to be leveled..

Joseph Gozlan  I bet you are inadvertently buying stolen condenser units.. its a regional sport in the south and mid west..

I was doing a closing in Atlanta last year and was at the closing attornies office and asked why it was so hot.. ( my condenser units were stolen last night) and this was a nice part of Atlanta not the hood at all....We just don't have those issues by and large on the west coast.. but to be fair and balanced I did have my first unit stolen in Oregon last year..On a personal Note I lived at Silverado CC from 90 to 2001 and was and is one of the great places in the world to live..

The condensation from the line has been dripping and pooling underneath the house on the plastic covering the ground.