Obviously, your 'first impression' means a lot when dealing with a potential customer. That alone will make the sale in many cases. It demonstrates to the customer that you have the ability to insure safe, reliable and efficient heating system operation. It also provides the homeowner some valuable information with regards to the most cost effective furnace replacement, for example. In other cases, combustion tests may identify the reason replacement is being considered in the first place and help troubleshoot and properly service the system or verify that replacement is indeed necessary.
Testing generates more work necessary to insure safe, efficient and reliable operation.
Test results shown to the customer help explain the situation and document that the additional expense was a necessary and worthwhile investment. When combustion reading are within the manufacturers' specifications, it assures two things. First, that the unit is safely and efficiently burning the fuel. Large commercial operations, in particular, can substantially reduce fuel costs. Second, the system is indeed operating as it was designed, engineered, manufactured, listed and approved.