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  1. #1
    Join Date
    Dec 2004
    Posts
    21
    Dear Friends,
    I have 10 yrs old, Amana 80% efficiency gas furnace and it vents out through b-vent flue pipe. The b-vent pipe leaks in attic and basement due to condensation.

    I hired a HVAC pro to diagnose the problems. He used an analyser and found that the flue gas temperature is only around 225F and this low temperature results in condensation in the b-vent pipe. He mentioned that it should be around 300F for the proper venting of flue gases without condensation.

    He found that the furnace is also short cycling. He adjusted the gas supply which helped to minimize the short cycling to some extent. But, it didn't increase the flue gas termperature.

    He spent more time trying to get the flue temperature high. But, he was not successful.

    He finally recommended changing the furnace unit which I don't like to do.

    Please, help me with my situation now. Did he miss something in his diagnosis? How to increse the flue gas temperature? Any help is appreciated. Thanks a lot.

    Sanrishi.

    [Edited by sanrishi on 02-15-2005 at 02:28 PM]

  2. #2
    Join Date
    Jul 2002
    Location
    Zelienople, Pa
    Posts
    2,965
    Do a load calc.
    You may have no choice but to put in the proper size furnace or deal with the condensation.
    How tall are you Private???!!!!

  3. #3
    Join Date
    Dec 2004
    Posts
    21
    Hi BoltonNC,
    Thanks for your reply. What is load calc? My home is approx. 1500 sqft and we don't have any problems with the heating. We get sufficeint heat with this furnace. The only problem is condensation water leaks in attic and basement.

  4. #4
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,248
    Was the reason for the furnace short cycling ever determined?
    Did this condensation start after the short cycling problem appeared?
    Did this contractor measure the total external static pressure of the fan to determine how much air the furnace is moving?
    Did he measure the temperature rise of the furnace?
    If a furnace is moving too much air this will also cause the flue gas temperature to be low.
    Extreme care needs to be taken when adjusting gas pressure to increase the firing rate of the furnace.
    There is a fine line between a properly fired furnace & an overfired furnace.
    There are readings such as carbon monoxide,oxygen & draft that also need to be measured to insure the safety of the appliance.
    Sorry for the questions,just trying to get a place to start.

  5. #5
    Join Date
    Apr 2004
    Posts
    7,680
    He means you have too much heat availble and no place to use it. Your condensation problems will likely continue and you will sooner or later be replacing it anyway as that furnace doesnt like water.

    It may also be underfired by monkeying with the gas pressure but he sounds like he knew what he was talking about.

    Good luck, no quick fix here.

  6. #6
    Join Date
    Jul 2000
    Location
    NW IL.
    Posts
    3,935
    Was the vent system inspected for blockage. The inducer pressure switches on some of these furnaces are so outrageously high that they will continue to run with a partially blocked vent. This slows down the flow of vent gases through the furnace removing more heat and reducing stack temperature and causing condensation.

  7. #7
    Ok,

    The gas pressure was adjusted to solve the short cycle problem. Is it still short cycling ? because that in itself can cause the flue gases to condense if the vent is running through could areas in the house or adjacent to an outside wall. If I hear that the gas was adjusted to compensate for short cycling I'm thnkng he turned the pressure down not up.If he turned it down this will probably compound the moisture, I think he should of got a temp rise and clocked your meter to see what the actual input was to the furnace it sounds like your furnace is probably oversized and underfired or dealing with some extremely cold air in the basement . What was the BTU rating of the furnace and what region are you in? and what exactly is condensating is it the outside of the pipe or in the pipe?

  8. #8
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,657
    What temp do you keep your home set at?

    I am thinking you are moving too much air across the heat exchanger!
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

  9. #9
    Join Date
    Dec 2004
    Posts
    21
    Thank you all for your help. Some of the answers that I know. Please, help me.

    Freezeking2000:
    We set the temp at 66. My home is 1440 sqft.

    fat eddy:
    You are right. He turned down the pressue and it really helped to fix the shortcycling. Now, it is definitely running longer than it used to be. But, I am still collecting the same amount of water in my attic. So, turning down the gas pressure didn't increase or decrease the amount of water that I collect in my attic.

    My furnace is 67000 BTU, 80% effecient, serving 1440 sqft. I am in eastern Massachusetts.

    It is condensing inside the pipe. I have not seen it outside.

    My basement is not very cold. It is always around 55F. But. My attic really gets very cold. It is closer to outside temperature.

    MechAcc:
    He partially inspected the vent system and found no blockage. His main concern was the low flue gas temperature.

    docholiday:
    My furnace is 67000 BTU, 80% effecient, serving 1440 sqft. I am in eastern Massachusetts. Is it oversized?

    davidr:
    I think that gas pressure was causing the short cycling. He turned down the gas pressure and now it is running longer than it used to be. It also helped to reduce the initial CO level to within 400PPM( before he adjusted the gas pressure it was around 600PPM)
    I am not sure he measured the total external static pressure of the fan. But, he gave me the final stats of the analyser. It reads as follows,
    Efficiency-------- 81.7%
    Excess Air-------- 162.4%
    Primary Temp------ 51.0 F
    Stack Temp-------- 214 F
    O2---------------- 13.5%
    CO2--------------- 4.2%

    CO---------------- 24ppm
    CO AIR FREE------- 68ppm
    Draft------------- 0.17 WC

    He also measuerd the differential temp which is in the proper range(40F) as described in the furnace manual.

    Does the readings are okay? Is the flue gas temp too low? Should it always be around 300F?

    I hope that I answered all your questions. Please, help me.
    Thanks again for all your support. This forum is amazing!!

    [Edited by sanrishi on 02-16-2005 at 11:58 AM]

  10. #10
    Join Date
    Nov 2000
    Location
    Indiana
    Posts
    318
    No, those numbers are not ok.

    Oxygen number is huge. Should be down somewhere around 6% to 9%. Stack temp is low. Underfiring is contributing to the high O2 number, but other factors can figure in there - like the vent configuration. If the furnace is sized right, it should be fired right. We still don't know what the temp rise is. Another thought on short cycling may be the anticipator or cycle rate setting on the stat.

  11. #11
    Join Date
    Oct 2003
    Location
    west chester pa
    Posts
    343
    Originally posted by 7X
    No, those numbers are not ok.

    Oxygen number is huge. Should be down somewhere around 6% to 9%. Stack temp is low. Underfiring is contributing to the high O2 number, but other factors can figure in there - like the vent configuration. If the furnace is sized right, it should be fired right. We still don't know what the temp rise is. Another thought on short cycling may be the anticipator or cycle rate setting on the stat.
    the high o2 number could also be due to a cracked heat exchanger i haven't seen many gas units run a 6% o2 most of the time it is between 8-11%. what size is the flue pipe, could be oversized. I would change the furnace to a direct vent 90% save money on gas a do away with the chimney. How old is the heater anyway??

  12. #12
    Join Date
    Dec 2004
    Posts
    21
    7X:
    what do you mean by temp rise? Please, help me. I have programmable honeywell thermostat. I looked at the user manual and it seems that it doesn't have manual cycle rate setting.

    pipedope:
    The flue pipe is 5inch b-vent. The furnace is 10 yrs old. I bought this home on last August. I am not sure how long this problem is going on.

    what causes high oxygen number?

    thank you.

  13. #13
    Join Date
    Jan 2002
    Location
    New Hampshire
    Posts
    1,285

    High oxygen number

    That's caused by underfiring a furnace. It's the reason your fuel bills are going to go up.

    Temp rise is measured between the supply duct temperature and the return duct temperature, near the furnace. If it's not right, the duct size doesn't matcgh the furnace size someplace.

    This discussion should have happened BEFORE a furnace size was selected. WAaay back when...

    Noel

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