Gas Hot Air Furnace / Blower Speed??

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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.
Tom
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Some of us won't use the term "FAG." Nothing personal, mind you....
Oh, your furnace was rated, but the AFUE rating wasn't that well known.

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Heatman - Tell me - Delta T????
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I looked on the ANSI table for the Delta sign, but couldn't find it.
Delta T and Delta P have one thing in common. It (Delta) means difference. Delta T is difference in tmp, Delta P means difference in pressure.
Kinda a head-slap, Homer Simpson "Doh!", huh?
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Tom,
Measure the return air temperature near the furnace with an accurate thermometer. Your thermometer should read degrees and tenths of degrees. Take at least 2 readings and get the average. Write it down.
Measure the supply air temperature near the furnace the same way, Take at least 4 readings and get the average. Make sure all readings are out of direct sight of the heat exchanger. Write that temperature down.
Subtract the return air temperature from the supply air temperature. That is the Temperature Rise or Temperature Difference (TD). Write it down. Get theBTU per hour OUTPUT rating from the furnace data plate. Write it down. Put these numbers in the following formula:
BTUs Output / (1.08 * TD) = CFM
This is the ACTUAL CFM of air flow. Note that each fan speed has a range of air flows. This is called a fan curve. It is usually shown as a graph or a table. For every pressure difference across the furnace, there will be a different air flow even though the blower speed connection has not changet. Each speed connection has a different fan curve. If you use all the fan speed connections and measure the air flows as shown above and plot them over the fan curves, you will get the system curve. The air flow will always be where the system curve crosses the current fan curve. A duct system with a higher resistance will result in a lower air flow. When you increase the resistance, the blower amps will go down! Try measuring the blower amps and partly covering the return with cardboard to demonstrate that.
If you cannot make the measurements and tests I outlined above, DO NOT try changing the blower speed connections. You should be able to measure the results of your efforts if you are going to make any changes.
Hope this helps.
Stretch
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Thanks Stretch!
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