Question: Can you explain the difference between the 90+ efficiency gas furnace and older units? Would you also give an explanation for having an induced fan blower? Please give a step-by-step procedure for setting one of these furnaces for proper operation.

-John West, Richmond, Va.

Answer: “Induced” draft furnaces incorporate a small blower wheel driven by a fractional horsepower motor to pull the products of combustion (burned gas) through the heat exchanger and into the flue pipe. The primary purpose of these induced draft blowers is to overcome internal heat exchanger resistance found in the new, smaller serpentine type designs.

The second question involves old style versus 90 percent annual fuel utilization efficiency (AFUE) furnaces. The primary difference is that a secondary heat exchanger (sometimes called a recuperative cell) is mounted in the furnace air stream and connected to a primary heat exchanger. The burned gas from the primary heat exchanger is drawn through this secondary cell instead of being discharged into the flue pipe as in the prior example.

As the flue gas passes through the secondary cell it is cooled by the air stream to extract additional heat and then enters the flue pipe at a much lower temperature. The effect is to increase the efficiency of the furnace, such as more heat from the same amount of gas. A secondary result is that the flue pipe can be made of polyvinyl chloride (PVC) instead of metal due to the lower flue gas temperatures.

-G.L. Jerry Stuck product manager, Trane Co.