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To check furnace efficiency, take a look at old fuel bills

Q. We have an old forced-air central heating system which uses a Holland Furnace No. 2208. While it was once a coal burner, it has since been converted to natural gas. How efficient is this furnace relative to contemporary gravity or forced-hot-air systems? Could I save enough money on fuel over the next five years to justify spending $1,000 for a new heating plant? Edmund Demers Waterloo, N.Y.

A. This being a technical question, we defer to an executive of Lennox Industries in Dallas. Here are some excerpts from his reply:

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''I can only talk in general terms and make a few suggestions. I am not familiar with (his) furnace; however, most furnaces of this design are rather open and rather inefficient even on coal.

''When converted to gas, I would suspect the equipment was no more than 30 to 40 percent efficient unless someone had taken a great deal of pains to try to install some internal baffling to improve the efficiency.

''As a starting place, I would suggest that a careful load calculation be made on the structure to determine the amount of heat needed.

''I would then go to the gas company and ask it to check my old bills against the degree days and against my current load calculations. In this way a determination can be made as to the efficiency of this furnace. It then becomes a simple matter to calculate the payback period on a newer and higher-efficiency furnace.''