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dal 03-19-2007 05:15 PM

What changes over the years?
This may be an odd question, but what happens to a gas forced-air furnace over the years to cause it to run at less than its rated efficiency? Is my 23 year old Rheem "80 Plus" still running at "80%" efficiency? I mean if the burners are kept clean and the flames appears to be burning cleanly, is it as efficient now as when it was new? If I were to buy a new 80% furnace, would I save on my GAS bill? Assuming the motor(s) of a new unit might be using less electricity- would a new furnace of the same rating really burn less gas? Just curious, I guess. I WANT to replace the old one, but unless I move up to the 90+%'ers (at big-time cost) how much will I save on my gas bill with a new 80%? What might cause a 23 year old furnace to use more gas than when it was new? You guys seem to know more about this stuff than the average homeowner. Any tips or input? Thank you.

harleyrider 03-19-2007 05:28 PM

What you will gain is reliabilaty, warrenty and heat exchangers have improved in there molecular make up. The way they move the air through the furnace has improved and well as most of the componants. I would stronglt suggest upgrading to a variable speed motor, and a 90 + % effecient furnace. One of the best units to come down the pike lately is the G61V by Lennox. Call you local Lennox dealer and set up an appointment with him to come out and show you the G61V.

dal 03-19-2007 05:44 PM

Thanks for the input, Harleyrider. Are you saying that a new 80% will burn less gas than my old one based on heat exchanger design?

hvactech 03-19-2007 06:08 PM

Efficiency is commonly misused term, a rating of 90% or 80% only means that under certain condition in a lab the furnace performed in such way, depending on a number of fact among the most relevant is proper install a new furnace may or may not reach its rating, I dare to say that most of the residential high efficiency units donít meet the rating and under perform, I wonder how you found about the efficiency of you old furnace? And if your old furnace is not giving you trouble donít change it, the energy savings are not that great.

dal 03-19-2007 06:28 PM

Thanks for the response....
As for the effiiciency, it's written on the front of the furnace and in the "manual" that came with it. In a homeowner understanding sort of way, I gathered that 20% of the gas used to run the beast goes up the flue, if all things are optimal. Or 20 cents of each dollar of my gas bill. Assuming this is correct for a brief moment, am I sending 20% up the flue now, or would that percentage be substantially higher since the furnace is so old? I guess my questions are kind of goofy; I just want to know if the gas valve wears out a bit and lets more gas through than is necessary or; do the orifice "holes" open up over time; or if the flames appear stiff and blue-- is it still running at its initial rating---whether it really was 80% or 79% or whatever. I'm not trying to be difficult; I just want to understand things.

hvactech 03-19-2007 09:11 PM

There are a few ways to determinate how much fuel your furnace is using or wasting, first you must figure how much is going in by clocking the gas meter this link explains how

the gas valve and orifices wont change over time, the heat exchanger will loose a bit of its thermal conductivity over the years increasing the amount of heat wasted, the only way that I know of maximazing furnaces efficiency is by performing a combustion anlysis and adjusting the fuel to air ratios, sadly very few contractor do such thing.

dal 03-20-2007 07:18 AM

Thank you
That link provides a lot of useful information. Thanks.

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