How on earth will you know exactly which system to get? What size do you consider? What sort of features do you need? It can be hard to sort through all the new features are coming out each day.
If you are having issues with your HVAC, check the entire house before contacting someone to come for repairs. This will help the repairman determine where the problem is.
Know what you want done before you hire a contractor. It will be hard for a contractor to provide you a price estimate on the phone without knowing what your current setup is. It will be more difficult for him to assist you can’t figure out what they need to do.