There are many HVAC (Heating, Venting and Air Conditioning) contractors for you to choose from.  Many are very good and, as with all industries, some are not.  We recommend that if you are having a furnace service or certified that you find a HVAC contractor who provides you with more than just a visual inspection.

Let's put this in more familiar terms.  If you go to the doctor with some persisting medical issue and the doctor looks inside your mouth, nose, and ears but never draws any blood or performs and analytical tests, would you be satisfied with that doctors diagnosis?  The same is true with a service on your furnace.

There are many different types of diagnostic tools available for testing and analyzing furnaces.  Find an HVAC contractor that can provide you with quantitative, documented information.  This will help insure that the technician if performing thorough work.