There are a lot of good contractors out there but having a license only means they know the answers and how to take and pass a test. It doesn't mean they are honest, dependable or capable. The certifications likely mean they KNOW how to do it and do it correctly, there are no guarantees they will. Your best option when choosing any type of contractor or licensed/certified installer is ask for references. If they balk at providing them, you should balk at hiring them.
My reasoning? Read on.....
A state-licensed contractor (in CA no less) could easily have killed my entire family with CO poisoning in '92 when he replaced a 12 year old gas-fired central HVAC unit. Because of a 360 sq. foot addition we chose a larger unit and it was correctly determined that a larger intake grill in the hallway was needed for the cold air return. While I was at work, the contractor dropped the old grill, sawed a larger rectangle in the sheetrock, cut the sheet metal out in the cold air return plenum, and tacked on a larger grill. In the process the idiot never re-formed or added sheet metal to the plenum so only air outside the closet and walls would be pulled in; it was also pulling air out of the closet, air which was furnished for natural gas combustion. Because the closet air was warm, the return air was not cool, heat was not removed from the exchanger so the closet temp rose and a safety thermo-disc thermostat would open the low voltage controls and shut off the gas because it thought the fan had frozen. More important, the cold air return plenum was pulling combustion air away from the flames, making the flames more yellow because of decreased oxygen which, in turn, increased the carbon monoxide. This carbon monoxide was being pushed throughout the house.
I found all this out when the thermo-disc kept opening up the control voltage and I pulled the return air grill off, looking for a reason and found how he butchered it and how the cold air return air got linked directly with the combustion air in the closet. He wanted to save so money and not do any sheet metal work. The lazy bastard even left all the sheetrock pieces and dust and debris that he cut out lie in the bottom of the cold air return plenum so the dust would be circulated through the house as well.
Needless to say, it's no small miracle that I am not penning this post from some California prison.
Are there good, licensed contractors out there? Sure! But having JUST the license means nothing to me. Nada. Zip.
I also want to see your workmanship, your work record and see and hear first hand the satisfaction of the owners where you have worked before.