Heatman, you're right, I read the heat rise off the furnace manual, not
a thermometer. This Sears FAG furnace (105,000BTU / 84,630 BTU) was
installed 1987, prior to the AFUE rating system. No changes have been
made from the (125 degree ON and 90 degree OFF) factory settings and
none are contemplated - they're fine.
The blower is now set to run Med. Lo. (1,100 +/- cu ft/min).
The original question was if any BTU efficiency would be gained if the
blower speed were raised to say, Med. Hi. (1,315 cu ft/min).
Our No. Calif. utility is charging $1.79375 / Therm (above a Baseline
quantity) for NG so the monthly bill is focusing my attention on this
subject. Last winter we used about 1,020 Therms.