Sometimes, at least to the seller and some parties, it can be harder to justify calling for professional service than it is at other times. For example, if the last service on the furnace is not established, but the unit is clean inside and running, there are those who think it is just fine. Now, the furnace in the photo below is an easy call and probably nobody will argue if you recommend service in that case. When a furnace has this kind of lint and debris, you want and need a professional to give it a whirl. That includes checking burners, carbon monoxide levels and the heat exchanger, any motors, switches, safety features. In my market the typical fee for this basic service, excluding parts if they are required, is $100 or less, so that is really quite a deal that nobody should balk at. 

Steven L. Smith

Bellingham WA Home Inspections