Old organic gas furnace?
Answers: Older equipment (and I mean OLDER!) can be derated, and it be quite adjectives to do it in the outdated days. The idea losing it was to customize the output, not increase helpfulness... efficiency will not devolution regardless of how much gas is burnt.
A 1981 furnace close to yours is not really that old and ought to fire at its rate input. There are several factors relying on the burner rate; devotee temperature switching and draft are only just a couple. Improperly derated equipment can actually end in flue condensation and premature rusting. Really old equipment have less sophisticated controls and be more flexible with good opinion to firing rates.
If your primary concern is efficiency you will be best served by replacing the entire furnace near a high helpfulness unit... the newer ones are approaching mid-90% ratings, where on earth your furnace might have be as low as 70% when new; that expected 30% of the heat generate was going up the chimney!
You are stuck. You must supply the gas tavern with satisfactory gas so it burns properly.
Keep your filter clean. Lower the therm to gather energy.
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