Don't buy a HVAC until you investigate the alternatives. The information provided here should help anyone decide.
Wikipedia states the fact that Ventilation, heating and air conditioning or HVAC units systems is really a key subdiscipline having to do with mechanical engineering. This is dependant the following: thermodynamics, heat exchange plus techniques regarding fluids.
HVAC coil replacement can put money in your pockets. Don't just run out and grab a new HVAC unit when it starts to break down. You can save an easy $1,000 on repairs and get many more years of service by just doing a simple repair or coil replacement.
The very first thing you need to do is give your HVAC unit a really good cleaning. Check to see if the coils of your unit are corroded. If they are really corroded you will need to replace those evaporator coils. However, if the coils are in good shape see if they have any leaks. If there are leaks simple brazing may mend them.
If they cannot be brazed because they are too corroded you may want to look at replacement coils. Replacement coils from the original manufacturer is expensive and their turn around rate is dismal most of the time. Fortunately there are all kinds of factories around that can manufacture these kinds of coils. Here is a Youtube video on how to measure your HAVC unit coils.
For more information and a good resource try replacement coils.