Last month we called SCE&G about a free energy audit they were offering. We had been meaning to so something like this, but we were procrastinating and companies who do this professionally charge around $400 - so we opted for the free one. The inspectors came and looked around our attic and our crawl space. They inspected our A/C unit and water heater. We also discussed our windows.
The inspectors tested our A/C unit and I'm glad to say that it is working well, despite being the same age as the water heater. First they ran a test measuring the air temperature at the vents and comparing it to the air temperature at the return. The air should be a difference of 18 degrees between the two. I'm glad to say that ours met that standard. I was pretty surprised - I've worried about ductwork under the house since we moved in. We have a moisture issue under the house that affects part of the insulation and I was afraid that it would hurt the performance of our A/C - I was glad to hear that. I had the idea to move the ductwork in the attic, but they recommended against it. They said it gets too hot in the attic and that the crawl space is the best, because temperature is more consistent close to the ground. They noticed we were using a pleated air filter - one that pulls allergens, etc from the air. We have a small house and I thought it would be a good idea to filter the air well - it's an old house, too. He said that these restrict air flow, which reduces the efficiency of the system. It also doesn't work effectively as an air filter. He said if that's what we want, we should get a separate air purifier. You learn something every day.
We've got our last pleated air filter in there right now and I just bought some "green" fiberglass-free filters. I was amazed at the lack of choices - either they cost $3 for 4 30-day filters, or the pleated ones start at that price for one.