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Old 09-24-2012, 04:48 PM   #1
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Furnace Tune-up/ Efficiency test


In my attempt to keep my 21 + year old heating and cooling units running for at least 3-5 more years -now that the heating season has arrived in north east, I figured that I should make sure my Trane XE 70 - 2 burner gas furnace is in good working order

I have recently completed the following on the furnace myself:

Removed/cleaned blower wheel and housing
Cleaned/Oiled blower motor-verified no side to side wiggle in motor shaft
Cleaned reusable air filter
Cleaned A-Coil (actually done a few months ago- s/b ok, lots of air flow out vents)
Checked amps (using clamp meter) of blower motor - high draws 4.9 amps, med draws 3.9, motor rated at 6.7 amps
Checked Hot surface igniter - ohm'd out at 141.5, so I replaced it with honeywell glowfly hsi, 120 volt universal - 50.5 ohms.
Flame sensor - cleaned with green scotch brite pad

I then followed up by calling in a professional who did the following "tune up" :

Remove the burners - clean and blow out with nitrogen gas
Wire brushed Heat exchanger, vacuumed out chamber
Visual Inspected heat exchanger for cracks/corrosion etc.
Visual inspection of flames
Gas valve check
Verified heating system starts and ends properly
Combustion analysis performed with following results

79.5 % Efficency
stack temp 386F
Excess air 68 %
o2 9.0%
co2 6.7%
Co 12ppm
Co air free 21ppm.

Those were results after furnance on for about 5 minutes.

He then checked the supply air duct near furnace and was as follows:

Stack temp 138
02 20.7%
co 0 ppm

Just want to ask the pros on the site if these results seem ok - I am unsure of what "excess air" (nitrogen?) is and could not find on the web what the right range should be.

But I guess my main concerns were relieved that he did not find cracks in heat exchanger and flames seem to be burning correctly (blue, steady), and there is no carbon monoxide getting into the supply air. He stated that according to what he sees the system is performing very well.

I think I covered most of it - Is there anything I am missing that I should be checking for? Thanks.


Last edited by ea0680; 09-24-2012 at 07:43 PM. Reason: changed o2 21.7% to 20.7% correction
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Old 09-24-2012, 05:43 PM   #2
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Furnace Tune-up/ Efficiency test


Looks pretty good,a bit under fired though which isn't a bad thing ona 21 year old heat exchanger. I would have adjusted the gas pressure a bit to get the excess air down around 50% and increased the blower speed to get that temp rise a little lower.The CO2 levels would have went up to 7.1-7.2. You got a good guy for using an analyzer,most don't.

BTW excess air is a calculation the analyzer performs. Perfect combustion forms water and CO2 as a byproduct. We have that pesky nitrogen in the air so no way to get perfect combustion,hence excess air. The best we can do is adjust the fuel to air ratio to keep the O2 between 6 and 9,Co levels under 100 ppm(400 air free) and Co2 in the low 7's.


Last edited by Marty S.; 09-24-2012 at 05:58 PM.
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Old 09-24-2012, 06:05 PM   #3
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Furnace Tune-up/ Efficiency test


Looks ok, but his analyzer is out of calibration.
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Old 09-24-2012, 06:09 PM   #4
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Furnace Tune-up/ Efficiency test


Quote:
Originally Posted by beenthere View Post
Looks ok, but his analyzer is out of calibration.
I saw that 21.7
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Old 09-24-2012, 06:14 PM   #5
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Furnace Tune-up/ Efficiency test


His real O2 is probably close to 8%, and his CO2 probably closer to 7%, guessing CO is closer to 20%.
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Old 09-24-2012, 07:43 PM   #6
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Furnace Tune-up/ Efficiency test


Quote:
Originally Posted by Marty S. View Post
I saw that 21.7
The actual o2 in supply air was 20.7% on the printout, not 21.7, I made a error in typing, I am going back and editing it now, thanks for pointing it out.
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Old 09-24-2012, 07:49 PM   #7
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Furnace Tune-up/ Efficiency test


Quote:
Originally Posted by beenthere View Post
His real O2 is probably close to 8%, and his CO2 probably closer to 7%, guessing CO is closer to 20%.
CO is closer to 20 PPM or 20 %?
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Old 09-24-2012, 08:10 PM   #8
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Furnace Tune-up/ Efficiency test


Quote:
Originally Posted by Marty S. View Post
Looks pretty good,a bit under fired though which isn't a bad thing ona 21 year old heat exchanger. I would have adjusted the gas pressure a bit to get the excess air down around 50% and increased the blower speed to get that temp rise a little lower.The CO2 levels would have went up to 7.1-7.2. You got a good guy for using an analyzer,most don't.

BTW excess air is a calculation the analyzer performs. Perfect combustion forms water and CO2 as a byproduct. We have that pesky nitrogen in the air so no way to get perfect combustion,hence excess air. The best we can do is adjust the fuel to air ratio to keep the O2 between 6 and 9,Co levels under 100 ppm(400 air free) and Co2 in the low 7's.

Thanks for your response - I did ask the guy if he needed to adjust the gas pressure based on the results- but he did not seem like he thought it was worth possibly messing it up (my words) as they are "factory set" (his words). Perhaps next year I will see if I can find a guy who will tweak it correctly.

Another question, I just want to be sure, when the combustion analyzer says 79.5 % efficient - and its a original "80% efficient" unit - are we talking about the same thing? after 21 years, its basically the same efficiency? seems hard to think thats the case and maybe talking about 2 different things.
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Old 09-24-2012, 09:17 PM   #9
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Furnace Tune-up/ Efficiency test


Furnace AFUE ratings and combustion eff are not the same. Your 79.5 was combustion eff. AFUE wouldn't change over the years so long as the heat exchanger condition and air flow remained the same. Dirty heat exchangers don't transfer the heat as well so more of it goes out the flue.
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Old 09-24-2012, 09:35 PM   #10
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Quote:
Originally Posted by ea0680 View Post
CO is closer to 20 PPM or 20 %?
No, 20% would be enough that it would kill anything that got a whiff of the exhaust fumes.

Think of a barrel with 1,000,000 drops of water in it. Remove 20 drops and now put in 20 drops of oil. You have 999,980 drops of water, and 20 drops of oil.
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Old 09-24-2012, 09:48 PM   #11
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Furnace Tune-up/ Efficiency test


Quote:
Originally Posted by beenthere View Post
No, 20% would be enough that it would kill anything that got a whiff of the exhaust fumes.

Think of a barrel with 1,000,000 drops of water in it. Remove 20 drops and now put in 20 drops of oil. You have 999,980 drops of water, and 20 drops of oil.

Thats what I thought ) thanks for the response...

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