Page 1 of 2 12 LastLast
Results 1 to 13 of 21
  1. #1
    Join Date
    Jan 2020
    Location
    Colorado
    Posts
    10
    Post Likes

    Combustion Analysis

    Hello. I am having a trouble. I have dealt Goodman furnaces for many years and for the most part have been pleased with their performance and reliability. The issue that has me staying up all night, doing more and more research is the combustion analysis, specifically at higher altitudes (5000 Ft. +) . I have done combustion analysis on many brands of furnaces and Goodman seems to produce one of the higher amounts of CO in the exhaust gas. I know that anything below 100PPM undiluted is considered acceptable, but I am always concerned about our environment and over polluting it. I have talked to Goodman about this and they just tell me that as long as the furnace is installed below 7000 ft. there is no orifice change necessary. I can not get the undiluted PPM under 50 PPM on their 90+ furnaces while staying in their specifications for the manifold gas pressure of 3.2-3.8. Other brands CO exhaust levels are well below this. Almost all of the time lower than 30 PPM. Does anyone have any thoughts on this?

  2. #2
    Join Date
    Aug 2002
    Location
    Southold, NY
    Posts
    27,207
    Post Likes

  3. #3
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    11,842
    Post Likes
    Quote Originally Posted by hvasee6 View Post
    I can not get the undiluted PPM under 50 PPM on their 90+ furnaces while staying in their specifications for the manifold gas pressure of 3.2-3.8.

    Does anyone have any thoughts on this?
    What O2 percentages are you seeing at this 50+ppm?

    Have you tried adjusting the manifold pressure up (outside of the 3.2-3.8 range) to see if the CO drops?
    Instead of learning the tricks of the trade, learn the trade.

  4. #4
    Join Date
    Jan 2020
    Location
    Colorado
    Posts
    10
    Post Likes
    Thread Starter
    Hello rundawg. Thank you for replying. I just performed a combustion analysis on a Goodman GMVC960403 today. The O2% 5.3%, CO diluted 47 PPM, CO2% 8.8%, stack temp. 67 deg., excess air% 30.1%, CO undiluted 62 PPM. I have other tests that I have done but I will have to look them up and get back with you. I have adjusted manifold pressures many different times on many different furnaces. With Goodman and Amana usually I can not get the CO undiluted below 50 PPM unless I adjust the manifold gas pressure below 3.2 below this would be outside of manufacturers specifications. Goodman tech support and literature have told me multiple times DO NOT adjust out of specs. The only thing they say about it is that they are below the mandated CO levels, so the furnace is running within legal limits.

  5. #5
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    11,842
    Post Likes
    Quote Originally Posted by hvasee6 View Post
    I just performed a combustion analysis on a Goodman GMVC960403 today.

    The O2% 5.3%, CO diluted 47 PPM, stack temp. 67 deg.

    I have adjusted manifold pressures many different times on many different furnaces. With Goodman and Amana usually I can not get the CO undiluted below 50 PPM unless I adjust the manifold gas pressure below 3.2 below this would be outside of manufacturers specifications.

    Goodman tech support and literature have told me multiple times DO NOT adjust out of specs. The only thing they say about it is that they are below the mandated CO levels, so the furnace is running within legal limits.
    I have to admit that I haven’t done a lot of testing at high altitudes (> 2000’). I live and work at sea level.

    The number that stands out most to me is the low flue temp of 67ºf. I would want something in the 90º to 100ºf range. I have found that some condensing furnaces just wont get much higher than that.

    The O2 looks to be slightly over fired (< 6%), but not with that low flue temp.

    What kind of temp rise are you getting?

    What kind of supply air temp are you getting?

    How much airflow are you moving?

    Some basic testing info:
    CO undiluted is also called CO “air free”. This is a measurement that techs trained in combustion analysis don’t use. The reason is, this is a calculated measurement based on having no O2 in the flue gas sample.

    It is also based on the concept of stoichiometric combustion (perfect combustion), which is also an impossibility.

    The only numbers on your analyzer that matter, are the CO (as measured - diluted by O2), flue temp and O2. All other numbers are calculated based on the other actual “measured” numbers.

    Side note:
    Manufactures will always tell you not to adjust the manifold pressure outside their specs, so if you aren’t comfortable with that, you always have the option to change the orifice. They do allow that.

    Overall:
    Every furnace, and groups of furnaces (specific manufacturers), have their own personality. Not every one can be set up exactly the same.

    I wouldn't be the least bit concerned with a CO level at 90 ppm, if it was STABLE, and I had tuned the furnace to perform to the manufactures rated output.
    Last edited by rundawg; 01-06-2020 at 10:30 PM. Reason: typo
    Instead of learning the tricks of the trade, learn the trade.

  6. #6
    Join Date
    Jan 2020
    Location
    Colorado
    Posts
    10
    Post Likes
    Thread Starter
    Thank you for your input. The temp. rise was on the low side but within specs. 30-60 and it was 35. The return air was 68deg. and the supply air was 103deg. I did change the fan speed on tap lower which helped the temp rise a little. I did also end up changing the orifices to see what would happen. Almost all of the combustion numbers changed for the better, except he stack temp. Although it went up a couple of degrees , it was still only 70deg.

  7. #7
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    11,842
    Post Likes
    Quote Originally Posted by hvasee6 View Post
    Thank you for your input. The temp. rise was on the low side but within specs. 30-60 and it was 35. The return air was 68deg. and the supply air was 103deg. I did change the fan speed on tap lower which helped the temp rise a little. I did also end up changing the orifices to see what would happen. Almost all of the combustion numbers changed for the better, except he stack temp. Although it went up a couple of degrees , it was still only 70deg.
    So I am curious, what orifice did you change to (smaller or bigger), and what were the combustion numbers?

    The supply air still seem low, as does the flue temp.

    Did you actually measure static pressure to see what airflow you had, or are you just going by what it should be?

    I am kinda surprised the occupants of this home aren't complaining about the unit not heating correctly.

    Rating per product data manual:

    Rated for at Sea Level : 40,0000 (input) x .96 (efficiency) = 38,400 BTUH (output @ Sea Level)

    Rated for at 5000 feet: 40,0000 (input) x .96 (efficiency) x .83 (ADCF* @ 5000’) = 31,872 BTUH (output @ 5000’)

    *ADCF- Air density correction factor.

    BTUH formula:

    CFM x delta T x ADCF = BTUH output

    Your system:

    600 (CFM) x 35º (Delta T) x .83 (ADCF) = 17,430 output
    Instead of learning the tricks of the trade, learn the trade.

  8. #8
    Join Date
    Aug 2002
    Location
    Cincinnati, Ohio
    Posts
    3,048
    Post Likes
    Something I learned a long time ago is that 130 degrees at 5000 feet altitude is the same as it is at sea level. The air is less dense but temperatures don't change.
    The btus is gas varies at all places that are high altitude. The btus in gas in Colorado is as low as 696 btus per cubic foot to a maximum of 1003 (this is rare). Setting all furnaces or equipment at the same gas pressure is absolutely the wrong way to go.

    As previously stated, an ASHRAE study reported that below 7000 feet derating is not necessary, but manufacturers have apparently not read it. Most equipment is de-rated at sea level.

    Keeping CO below 50 ppm makes no sense. The legal level is 400 ppm, not that we ever want anything close to that, 100 ppm of CO is a very small amount in the flue gases. This means you are burning 99.99% of the fuel which is pretty good.

    Based on my testing and adjusting a furnace at high altitudes, the Delta T should still be around 60 degrees.

    I do not understand how your O2 is 5.3% and the temperatures are so low? The proper airflow is the one that produces the lowest flue temperature. We don't adjust airflow to increase flue temperatures. We adjust gas to increase Delta T. But CO must always be below 100 ppm and stable.
    captain CO

  9. #9
    Join Date
    Jan 2020
    Location
    Colorado
    Posts
    10
    Post Likes
    Thread Starter
    Hello rundawg. The orifice was smaller 45 switched to a 49. O2 8.5%, CO 3PPM, stack 72deg, excess air 61%. I did not check the static pressure. I will be going to do some more testing on it tomorrow. I will test static pressures and let you know. The lowest tap is rated for 800 CFM and it is a variable speed motor. Goodman says to derate to .80 at 5000 ft.

    Do you have experience clocking the gas meter? Is the final Btuh value at the end of the calculation the input rating of the furnace, or is it the output rating of the furnace after de-rating for altitude?

  10. #10
    Join Date
    Jan 2010
    Location
    Sea to Sky
    Posts
    3,659
    Post Likes
    Does that furnace require a "restrictor plate" in the combustion air pipe?

    A lot of small Carrier furnaces require the plate to restrict the volume of intake air due to the low but rating of a small furnace....

    Sent from my SM-G965W using Tapatalk

  11. #11
    Join Date
    Sep 2007
    Location
    State College, PA
    Posts
    2,985
    Post Likes
    Quote Originally Posted by BALloyd View Post
    Does that furnace require a "restrictor plate" in the combustion air pipe?

    A lot of small Carrier furnaces require the plate to restrict the volume of intake air due to the low but rating of a small furnace....

    Sent from my SM-G965W using Tapatalk
    I never heard of a gas furnace needing a plate to restrict air flow on the intake. I install lots of Payne condensing gas furnaces which is the same as Carrier and have not run across this info. Never had a problem with stack temp or combustion.

    Seems like that would mess with proper combustion.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


    Please view site rules: http://hvac-talk.com/vbb/showthread.php?t=1241

    Apply for Pro Membership: http://hvac-talk.com/vbb/showthread.php?t=116113

    Find your local HVAC-Talk Contractor: http://hvac-talk.com/aop

  12. #12
    Join Date
    Jan 2010
    Location
    Sea to Sky
    Posts
    3,659
    Post Likes
    Quote Originally Posted by joemach View Post
    I never heard of a gas furnace needing a plate to restrict air flow on the intake. I install lots of Payne condensing gas furnaces which is the same as Carrier and have not run across this info. Never had a problem with stack temp or combustion.

    Seems like that would mess with proper combustion.
    https://www.google.ca/url?sa=t&sourc...GV-NUImkldOt8Z


    Sent from my SM-G965W using Tapatalk

  13. #13
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    11,842
    Post Likes
    Quote Originally Posted by hvasee6 View Post
    Do you have experience clocking the gas meter? Is the final Btuh value at the end of the calculation the input rating of the furnace, or is it the output rating of the furnace after de-rating for altitude?
    Measuring input by clocking the gas meter can be very inaccurate. The primary reason is not knowing the ever changing BTUH content of the natural gas being delivered to the home.

    Add in gas meter dials sticking as they rotate, technicians ability to start and stop the timing accurately as the dial revolves, and ensuring no other gas appliance is operating during the test (including pilot lights on water heaters, fireplaces ect.)

    The only accurate way to measure in the field is by measuring the output of the furnace.
    The number I provided you at the end of post # 7 is actual output of the furnace based on altitude and a generic airflow number.
    Instead of learning the tricks of the trade, learn the trade.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Contracting Business
HPAC Engineering
EC&M
CONTRACTOR