When you say "older" ...does that mean you have a circuit board for fan or a control like this
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The control is like the one pictured
I am guessing the furnace is 30 years or older
under that cover is a dial. It has 3 settings...the first is blower off temp...next is blower on temp...and last is the high limit.
Try adjusting the pointer at 90 up to 110 first. This is a case oif the unit losing calibration over the years.
If it does not improve by turning up the dial...then the control will need to be replaced.
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