1:
I have access to a unit that was purchased new in 2004. The house it was servicing has been vacant since 2009. The house has slightly more square footage so I think if it operates properly, it should be sufficient. But. . . the house is about 60 miles away, so it would probably cost me several hundred dollars to get someone to pick up, deliver, and install–but it would be less expensive than purchasing a new unit. (I need at a minimum 3½ tons.) But my gut feeling is that it probably isn’t worth the risk, not knowing what condition it might be in. Opinions???
2:
I heard something at a mall ‘energy info center’ once upon a time that you shouldn’t get a bigger unit than you need because it can cause the unit to wear out quicker. That sounded odd to me. I can see not getting more than you need because of cost, but not because it might cause it to wear out quicker. Anything to it? With a big expenditure like this, I’d feel safer with a little cushion, and don’t want to find myself with something that is performing not quite up to what I need.
You want something that has been rusting away for a dozen years? Really??
Get a new one. Go with someone who knows what they are doing. Too large a unit will cool the air before removing the moisture–do you like cold dank rooms?
As it was explained to me when we got our A/C replaced a few months ago: a bigger than necessary unit (ha! :D) is inefficient. While it will cool your house very quickly, to maintain temperature it will turn on and off more frequently, never achieving full efficiency; think of what stop-and-go traffic does to your car. It takes an A/C unit about 8-10 minutes to reach full efficiency and also to start its dehumidifying effect.
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An AC unit that is too powerful for the space will cool it too quickly, causing rapid cycling of the unit, which is bad for the motor and also annoying. Additionally it may end up cooling the space so fast that not enough moisture is removed, which is very uncomfortable.
Regarding #2, I’m glad you mentioned that because I was going to bring it up since in #1 you mentioned twice that this unit might be oversized (slightly bigger house and needing at least 3.5 tons).
You don’t want to oversize your AC. Doing so will cause it to cycle on and off. Like DC, that’s not all that great for it. There’s lots of things going on when it starts and stops (motors, caps, pressures etc). But the bigger problem is that if it only runs for 5 minutes, then shuts off. Then 30 minutes later it runs for 5 minutes and so on all day, you’ll wind up with a cool damp house. OTOH. If it runs for 20 minutes, then off for an hour or something closer to that, it’ll pull the moisture out of the house as well.
You’ll even be able to set the temp a degree or two higher if the house’s humidity is lower since it won’t feel as warm if it isn’t muggy.
IOW, don’t oversize. Bigger isn’t better. Also, neither is smaller. If it never shuts off because it can’t keep up, that’s not good either.