Excerpt from Consumer Guide to Home Energy Savings
Regular tune-ups of your heating system will cut heating costs and will most likely increase the lifetime and safety of the system. When a service technician performs a tune-up, he or she should test the efficiency of your heating system.
The technician should measure the efficiency of your system both before and after servicing it and provide you with a copy of the results. Combustion efficiency is determined indirectly, based on some of the following tests: 1) temperature of the flue (or chimney); 2) percent carbon dioxide or percent oxygen in the atmosphere; 3) presence of carbon monoxide in the atmosphere; and 4) draft. Incomplete combustion of fuel is the main contributor to low efficiency. If the technician cannot raise the combustion efficiency up to at least 75% after tuning your heating system, you should consider installing a new system or at least modifying your present system to increase its efficiency.
Adapted from Alex Wilson and John Morrill, Consumer Guide to Home Energy Savings. ©1993 by the American Council for an Energy-Efficient Economy.